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| author | nfenwick <nfenwick@pglaf.org> | 2025-01-22 00:11:17 -0800 |
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| committer | nfenwick <nfenwick@pglaf.org> | 2025-01-22 00:11:17 -0800 |
| commit | 11766cf5d1e4128b9613a03ccabf5f980f47cce9 (patch) | |
| tree | 736f5060a6d0e2e3241eb9d1162747a07abf75ab | |
| parent | 4a775c63722564b87538cdbec15f750a8bce7b96 (diff) | |
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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..32acfb1 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #67873 (https://www.gutenberg.org/ebooks/67873) diff --git a/old/67873-0.txt b/old/67873-0.txt deleted file mode 100644 index 39d17dd..0000000 --- a/old/67873-0.txt +++ /dev/null @@ -1,10894 +0,0 @@ -The Project Gutenberg eBook of Volcanoes: What They are and What They -Teach, by John Wesley Judd - -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you -will have to check the laws of the country where you are located before -using this eBook. - -Title: Volcanoes: What They are and What They Teach - -Author: John Wesley Judd - -Release Date: April 18, 2022 [eBook #67873] - -Language: English - -Produced by: Tom Cosmas compiled from materials made availbe at The - Internet Archive and placed in the Public Domain. - -*** START OF THE PROJECT GUTENBERG EBOOK VOLCANOES: WHAT THEY ARE AND -WHAT THEY TEACH *** - - - - - - - -Transcriber Note - -Text emphasis denoted as _Italics_. - - - - -THE - -International Scientific Series - -VOL. XXXV. - -_Frontispiece._ - -[Illustration] - -Sections of Igneous Rocks, illustrating the passage from the glassy to -the crystalline structure. - - 1. Vitreous Rock. 2. Semi-Vitreous Rock. 3. Vitreous Rock with - Sphærulites. 4. Rock with Crypto-crystalline Base. 5. Rock with - Micro-crystalline Base. 6. Rock of Granite Structure built up - entirely of Crystals. - -[_See pp._ 63-68. - - - - - VOLCANOES - - WHAT THEY ARE and WHAT THEY TEACH - - - BY - - JOHN W. JUDD, F.R.S. - - PROFESSOR OF GEOLOGY IN THE ROYAL SCHOOL OF MINES - - - - _WITH 96 ILLUSTRATIONS_ - - - - SIXTH EDITION - - - - LONDON - - KEGAN PAUL, TRENCH, TRÜBNER & CO. Ltd. - - PATERNOSTER HOUSE, CHARING CROSS ROAD - - 1903 - - -(_The rights of translation and of reproduction are reserved._) - - - - -PREFACE. - - -In preparing this work, I have aimed at carrying out a design suggested -to me by the late Mr. Poulett Scrope, the accomplishment of which has -been unfortunately delayed, longer than I could have wished, by many -pressing duties. - -Mr. Scrope's well-known works, 'Volcanoes' and 'The Geology and Extinct -Volcanoes of Central France'--which passed through several editions -in this country, and have been translated into the principal European -languages--embody the results of much careful observation and acute -reasoning upon the questions which the author made the study of his -life. In the first of these works the phenomena of volcanic activity -are described, and its causes discussed; in the second it is shown that -much insight concerning these problems may be obtained by a study of -the ruined and denuded relics of the volcanoes of former geological -periods. The appearance of these works, in the years 1825 and 1827 -respectively, did much to prepare the minds of the earlier cultivators -of science for the reception of those doctrines of geological -uniformity and continuity, which were shortly afterwards so ably -advocated by Lyell in his 'Principles of Geology.' - -Since the date of the appearance of the last editions of Scrope's -works, inquiry and speculation concerning the nature and origin of -volcanoes have been alike active, and many of the problems which were -discussed by him, now present themselves under aspects entirely new and -different from those in which he was accustomed to regard them. No one -was ever more ready to welcome original views or to submit to having -long-cherished principles exposed to the ordeal of free criticism than -was Scrope; and few men retained to so advanced an age the power of -subjecting novel theories to the test of a rigorous comparison with -ascertained facts. - -But this eminent geologist was not content with the devotion of his -own time and energies to the advancement of his favourite science, for -as increasing age and growing infirmities rendered travel and personal -research impossible, he found a new source of pleasure in seeking -out the younger workers in those fields of inquiry which he had so -long and successfully cultivated, and in furthering their efforts by -his judicious advice and kindly aid. Among the chosen disciples of -this distinguished man, who will ever be regarded as one of the chief -pioneers of geological thought, I had the good fortune to be numbered, -and when he committed to me the task of preparing a popular exposition -of the present condition of our knowledge on volcanoes, I felt that I -had been greatly honoured. - -In order to keep the work within the prescribed limits, and to avoid -unnecessary repetitions, I have confined myself to the examination of -such selected examples of volcanoes as could be shown to be really -typical of all the various classes which exist upon the globe; and -I have endeavoured from the study of these to deduce those general -laws which appear to govern volcanic action. But it has, at the -same time, been my aim to approach the question from a somewhat new -standpoint, and to give an account of those investigations which have -in recent times thrown so much fresh light upon the whole problem. -In this way I have been led to dwell at some length upon subjects -which might not at first sight appear to be germane to the question -under discussion;--such as the characters of lavas revealed to us -by microscopic examination; the nature and movements of the liquids -enclosed in the crystals of igneous rocks; the relations of minerals -occurring in some volcanic products to those found in meteorites; the -nature and origin of the remarkable iron-masses found at Ovifak in -Greenland; and the indications which have been discovered of analogies -between the composition and dynamics of our earth and those of other -members of the family of worlds to which it belongs. While not evading -the discussion of theoretical questions, I have endeavoured to keep -such discussions in strict subordination to that presentation of the -results attained by observation and experiment, which constitutes the -principal object of the work. - -The woodcuts which illustrate the volume are in some cases prepared -from photographs, and I am indebted to Mr. Cooper for the skill with -which he has carried out my wishes concerning their reproduction. -Others among the engravings are copies of sketches which I made in -Italy, Hungary, Bohemia, and other volcanic districts. The whole of the -wood-blocks employed by Mr. Poulett Scrope in his work on Volcanoes -were placed at my disposal before his death, and such of them as -were useful for my purpose I have freely employed. To Captain S. P. -Oliver, R.A., I am obliged for a beautiful drawing made in the Island -of Bourbon, and to Mr. Norman Lockyer and his publishers, Messrs. -Macmillan & Co., for the use of several wood-blocks illustrating -sun-spots and solar prominences. - - J. W. J. - -London: _May 1881_. - - - - - CONTENTS. - - - CHAPTER I. - - PAGE - - INTRODUCTORY: NATURE OF THE ENQUIRY - - 1 - - CHAPTER II. - - THE NATURE OF VOLCANIC ACTION - - 7 - - CHAPTER III. - - THE PRODUCTS OF VOLCANIC ACTION - - 39 - - CHAPTER IV. - - THE DISTRIBUTION OF THE MATERIALS EJECTED FROM VOLCANIC VENTS - - 67 - - CHAPTER V. - - THE INTERNAL STRUCTURE OF VOLCANIC MOUNTAINS - - 112 - - CHAPTER VI. - - THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC VENTS - - 161 - - CHAPTER VII. - - THE SUCCESSION OF OPERATIONS TAKING PLACE AT VOLCANIC CENTRES - - 186 - - CHAPTER VIII. - - THE DISTRIBUTION OF VOLCANOES UPON THE SURFACE OF THE GLOBE - - 224 - - CHAPTER IX. - - VOLCANIC ACTION AT DIFFERENT PERIODS OF THE EARTH'S HISTORY - - 247 - - CHAPTER X. - - THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF NATURE - - 281 - - CHAPTER XI. - - WHAT VOLCANOES TEACH US CONCERNING THE NATURE - OF THE EARTH'S INTERIOR - - 307 - - CHAPTER XII. - - THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN - THE CAUSES OF VOLCANIC ACTION - - 331 - - INDEX - - 371 - - - - -ILLUSTRATIONS. - -Sections of igneous rocks illustrating the passage from the -glassy to the crystalline structure - - _Frontispiece_ - - Fig. Page - - 1. Stromboli, viewed from the north-west, April 1874 _to face p._ 10 - - 2. Map of the Island of Stromboli 11 - - 3. Section through the Island of Stromboli from north-west to - south-east 13 - - 4. The crater of Stromboli as viewed from the side of the - Sciarra during an eruption on the morning of April 24, - 1874. 14 - - 5. Vesuvius in eruption, as seen from Naples, April 26, 1872. - (_From a photograph_) _to face p._ 24 - - 6. View of Vulcano, with Vulcanello in the foreground--taken - from the south end of the Island of Lipari 43 - - 7. Minute cavities, containing liquids, in the crystals of rocks. - (_After Zirkel_) _to face p._ 60 - - 8. Minute liquid-cavity in a crystal, with a moving bubble. - (_After Hartley_) 63 - - 9. Cavity in crystal, containing carbonic-acid gas at a - temperature of 86° F., and passing from the liquid to - the gaseous condition. (_After Hartley_) 64 - - 10. Monte Nuovo (440 ft high) on the shores of the Bay of Naples. - (_After Scrope_) 76 - - 11. Map of the district around Naples, showing Monte Nuovo and the - surrounding volcanoes of older date 78 - - 12. Outlines of the summit of Vesuvius during the eruption of - 1767. (_After Sir W. Hamilton_) _to face p._ 80 - - 13. Crater of Vesuvius formed during the eruption of 1822 - (_After Scrope_) 82 - - 14. Crater of Vesuvius in 1756, from a drawing made on the spot. - (_After Sir W. Hamilton_) 84 - - 15. The summit of Vesuvius in 1767, from an original drawing. - (_After Sir W, Hamilton_) 85 - - 16. Summit of Vesuvius in 1843 86 - - 17. Outlines of Vesuvius, showing its form at different periods - of its history 87 - - 18. Cascade of lava tumbling over a cliff in the Island of - Bourbon. (_After Capt. S. P. Oliver, R.A._) 93 - - 19. Lava-stream (obsidian) in the Island of Vulcano, showing - the imperfect liquidity of the mass 95 - - 20. Interior of a rhyolitic lava-stream in the Island of Lipari, - showing broad, sigmoidal folds, produced by the slow - movements of the mass 96 - - 21. Interior of a rhyolitic lava-stream in the Island of Lipari, - showing the complicated crumplings and puckerings, - produced by the slow movements of the mass 96 - - 22. Vesuvian lava-stream of 1858, exhibiting the peculiar - 'ropy' surfaces of slowly-moving currents. - (_From a photograph_) _to face p._ 98 - - 23. Vesuvian lava-stream of 1872, exhibiting the rough cindery - surfaces characteristic of rapidly flowing currents. - (_From a photograph_) _to face p._ 96 - - 24. Concentric folds on mass of cooled lava. (_After Heaphy_) 100 - - 25. Mass of cooled lava formed over a spiracle on the slopes - of Hawaii. (_After Dana_) 100 - - 26. Group of small cones thrown up on the Vesuvian lava-current - of 1855. (_After Schmidt_) 101 - - 27. Natural section of a lava-stream in the Island of Vulcano, - showing the compact central portion and the scoriaceous - upper and under surfaces 104 - - 28. Section of a lava-stream exposed on the side of the river - Ardèche, in the south-west of France. (_After Scrope_) 106 - - 29. Portion of a basaltic column from the Giant's Causeway, - exhibiting both the ball-and-socket and the - tenon-and-mortise structure 107 - - 30. Vein of green pitchstone at Chiaja di Luna, in the Island - of Ponza, breaking up into regular columns and into - spherical masses with a concentric series of joints. - (_After Scrope_) 108 - - 31. Illustration of the 'perlitic structure' in glassy rocks 109 - - 32. Transverse section of a lava-stream 111 - - 33. The Kammerbühl, or Kammerberg, Bohemia (as seen from - the south-west) 113 - - 34. Section of the Kammerbühl in Bohemia 114 - - 35. Natural section of a volcanic cone in the Island of Vulcano 116 - - 36. Section in the side of the Kammerbühl, Bohemia 118 - - 37. Experimental illustration of the mode of formation of - volcanic cones, composed of fragmental materials 120 - - 38. Natural section of a tuff-cone, forming the Cape of Misenum, - and exhibiting the peculiar internal arrangement, - characteristic of volcanoes composed of fragmentary - materials. (_After Scrope_) 121 - - 39. Section of a small scoria-cone formed within the crater of - Vesuvius in the year 1835, illustrating the filling up of - the central vent of the cone by subsequent ejections. - (_After Abich_) 122 - - 40. Volcanic cones composed of scoriæ, and breached on one - side by the outflow of lava-currents. (_After Scrope_) 128 - - 41. Campo Bianco, in the Island of Lipari. A pumice-cone - breached by the outflow of an obsidian lava-stream - _to face p._ 124 - - 42. Volcanic cones in Auvergne, which have suffered to some - extent from atmospheric denudation. (_After Scrope_) 124 - - 43. Experimental illustration of the mode of formation of - volcanic cones composed of viscid lavas. (_After Reyer_) 126 - - 44. The Grand Puy of Sarcoui, composed of trachyte, rising - between two breached scoria-cones (Auvergne). (_After - Scrope_) 126 - - 45. Volcanic cone (Mamelon) composed of very viscid lava - (Island of Bourbon). (_After Bory de St. Vincent_) 127 - - 46. Another Mamelon in the Island of Bourbon, with a crater - at its summit. (_After Bory de St. Vincent_) 127 - - 47. Cliff-section in the Island of Madeira, showing how a - composite volcano is built up of lava-streams, beds of - scoriæ, and dykes. (_After Lyell_) 125 - - 48. Section seen at the cascade, Bains du Mont Dore. (_After - Scrope_) 130 - - 49. Section in the Island of Ventotienne, showing a great - stream of andesitic lava overlying stratified tuffs. - (_After Scrope_) 130 - - 50. Cliff on the south side of the Island of San Stephano 131 - - 51. The headland of Monte della Guardia, in the Island of Ponza 131 - - 52. Western side of the same headland, as seen from the north - side of Luna Bay 132 - - 53. Sea-cliff at Il Capo, the north-east point of Salina, - showing stratified agglomerates traversed by numerous - dykes, the whole being unconformably overlaid by - stratified, aqueous deposits 137 - - 54. Section observed in the Val del Bove, Etna, showing a - basaltic dyke, from the upper part of which a - lava-current has flowed 138 - - 55. Basaltic dykes projecting from masses of stratified scoriæ - in the sides of the Val del Bove, Etna 134 - - 56. Sheets of igneous rock (basalt) intruded between beds of - sandstone, clay, and limestone (Island of Skye) 137 - - 57. Plan of the dissected volcano of Mull in the Inner - Hebrides _to face p._ 142 - - 58. Section of the volcano of Mull along the line A B " 142 - - 59. Summit of the volcano of Monte Sant' Angelo, in Lipari, - exhibiting a crater with walls worn down by denudation 158 - - 60. Outlines of lava-cones 160 - - 61. Diagram illustrating the formation of parasitic cones along - lines of fissure formed on the flanks of a great volcanic - mountain 162 - - 62. Outline of Etna, as seen from Catania 162 - - 63. Outline of Etna, as seen from the Val del Bronte 163 - - 64. Plan of the volcano forming the Island of Ischia 163 - - 65. A primary parasitic cone, with a secondary one at its - base--Ischia 164 - - 66. Scoria-cone near Auckland, New Zealand, with a lava-current - flowing from it. (_After Heaphy_) 165 - - 67. Section of rocks below the ancient triassic volcano of - Predazzo in the Tyrol 165 - - 68. Cotopaxi, as seen from a distance of ninety miles. (_After - Humboldt_) 168 - - 69. Citlaltepetl, or the Pic d'Orizaba, in Mexico, as seen from - the Forest of Xalapa. (_After Humboldt_) 169 - - 70. Lac Paven, in the Auvergne. (_After Scrope_) 171 - - 71. The crater-lake called Lago del Bagno, in Ischia, converted - into a harbour 172 - - 72. Lake of Gustavila, in Mexico. (_After Humboldt_) 172 - - 73. Peak of Teneriffe, surrounded by great crater-rings. (_After - Piazzi-Smyth_) 175 - - 74. The volcano of Bourbon, rising in the midst of a crater-ring - four miles in diameter. (_After Bory de St. Vincent_) 176 - - 75. The volcano of Bourbon, as seen from another point of - view, with three concentric crater-rings encircling its - base. (_After Bory de St. Vincent_) 176 - - 76. Vesuvius as seen from Sorrento, half encircled by the - crater-ring of Somma 177 - - 77. Outlines of various volcanoes illustrating the different - relations of the craters to cones _to face p._ 178 - - 78. Island thrown up In the Mediterranean Sea in July and - August, 1831. (_After the Prince de Joinville_) 179 - - 79. Sinter-cones surrounding the orifices of geysers 183 - - 80. Diagram illustrating the mode of formation of travertine- - and sinter-terraces on the sides of a hill of tuff 185 - - 81. Map of the volcanic group of the Lipari Islands, illustrating - the position of the lines of fissure upon which - the volcanoes have been built up 192 - - 82. The Puy de Pariou, in the Auvergne, illustrating the shifting - of eruption along a line of fissures 193 - - 83. Ideal section of the Puy de Pariou 194 - - 84. Fissure formed on the flanks of Etna during the emotion - of 1865. (_After Silvestri_) 194 - - 85. Plan of the Island of Vulcano, based on the map of the - Italian Government 196 - - 86. Vulcanello, with its three craters 197 - - 87. Section of basalt from Ovifak, Greenland, with particles of - metallic iron diffused through its mass 319 - - 88. Diagram illustrating the relations between the terrestrial - and the extra-terrestrial rocks _to face p._ 322 - - 89. A group of sun-spots. (_After Secchi_) 362 - - 90. A sun-spot, showing the great masses of incandescent - vapour rising or falling within it. (_After Secchi_) 363 - - 91. The edge of a sun-spot, showing a portion of the prominent - masses of incandescent gas (A) which detached itself - at B and floated into the midst of the cavity. - (_After Norman Lockyer_) 363 - - 92. Drawing of a solar prominence made by Mr. Norman - Lockyer, March 14, 1869, at 11 h. 5 m. A.M. 364 - - 93. The same object, as seen at 11 h. 15 m. on the same day. - (_After Norman Lockyer_) 365 - - 94. Drawings of a solar prominence at four different periods - on September 7, 1871. (_After Young_) 366 - - 95. A group of Lunar craters (Maurolycus, Barocius, &c.), the - largest being more than sixty miles in diameter 368 - - - - -VOLCANOES. - - - - -CHAPTER I. - -INTRODUCTORY: NATURE OF THE INQUIRY. - - -'What is a volcano?' This is a familiar question, often addressed to us -in our youth, which 'Catechisms of Universal Knowledge,' and similar -school manuals, have taught us to reply to in some such terms as the -following: 'A volcano is a burning mountain, from the summit of which -issue smoke and flames.' Such a statement as this, it is probable, does -not unfairly represent the ideas which are, even at the present day, -popularly entertained upon the subject. - -But in this, as in so many other cases, our first step towards the -acquirement of scientific or exact knowledge, must be the unlearning of -what we have before been led to regard as true. The description which -we have quoted is not merely incomplete and inadequate as a whole, -but each individual proposition of which it is made up is grossly -inaccurate, and, what is worse, perversely misleading. In the first -place, the action which takes place at volcanoes is not 'burning,' or -combustion, and bears, indeed, no relation whatever to that well-known -process. Nor are volcanoes necessarily 'mountains' at all; essentially, -they are just the reverse--namely, holes in the earth's crust, or -outer portion, by means of which a communication is kept up between -the surface and the interior of our globe. When mountains do exist at -centres of volcanic activity, they are simply the heaps of materials -thrown out of these holes, and must therefore be regarded not as the -causes but as the consequences of the volcanic action. Neither does -this action always take place at the 'summits' of volcanic mountains, -when such exist, for eruptions occur quite as frequently on their sides -or at their base. That, too, which popular fancy regards as 'smoke' -is really condensing steam or watery vapour, and the supposed raging -'flames' are nothing more than the glowing light of a mass of molten -material reflected from these vapour clouds. - -It is not difficult to understand how these false notions on the -subject of volcanic action have come to be so generally prevalent. -In the earlier stages of its development, the human mind is much -more congenially employed in drinking in that which is marvellous -than in searching for that which is true. It must be admitted, too, -that the grand and striking phenomena displayed by volcanoes are -especially calculated to inspire terror and to excite superstition, -and such feelings most operate in preventing those close and accurate -observations which alone can form the basis of scientific reasoning. - -[Sidenote: IDEAS OF THE ANCIENTS.] - -The ancients were acquainted only with the four or five active -volcanoes in the Mediterranean area; the term 'volcano' being the -name of one of these (Vulcano, or Volcano, in the Lipari Islands), -which has come to be applied to all similar phenomena. It is only in -comparatively modern times that it has become a known £act that many -hundreds of volcanoes exist upon the globe, and are scattered over -almost every part of its surface. Classical mythology appropriated -Vulcano as the forge of Hephæstus, and his Roman representative Vulcan, -while Etna was regarded as formed by the mountains under which a -vengeful deity had buried the rebellious Typhon; it may be imagined, -therefore, that any endeavour to more closely investigate the phenomena -displayed at these localities would be regarded, not simply as an act -of temerity, but as one of actual impiety. In mediæval times similar -feelings would operate with not less force in the same direction, for -the popular belief identified the subterranean fires with a place of -everlasting torment; Vulcano was regarded as the place of punishment -of the Arian Emperor Theodosius, while Etna was assigned to poor Anne -Boleyn, the perverter of faith in the person of its stoutest defender. -That such feelings of superstitious terror in connection with volcanoes -are, even at the present day, far from being extinct, will be attested -by every traveller who, in carrying on investigations about volcanic -centres, has had to avail himself of the assistance of guides and -attendants from among the common people. - -Among the great writers of antiquity we find several who had so far -emancipated their minds from the popular superstitions as to be able -to enunciate just and rational views upon the subject of volcanoes. -Until quite recent times, however, their teaching was quite forgotten -or neglected, and the modern science of Vulcanology may be said to have -entirely grown up within the last one hundred years. - -The great pioneer in this important branch of research was the -illustrious Italian naturalist Spallanzani, who, in the year 1788, -visited the several volcanoes of his native land, and published an -account of the numerous valuable and original observations which he -had made upon them. About the same time the French geologist Dolomieu -showed how much light might be thrown on the nature of volcanic action -by a study of the various materials which are ejected from volcanic -vents; while our own countryman. Sir William Hamilton, was engaged in -a systematic study of the changes in form of volcanic mountains, and -of the causes which determine their growth. At a somewhat later date -the three German naturalists. Von Buch, Humboldt, and Abich, greatly -extended our knowledge of volcanoes by their travels in different -portions of the globe. - -[Sidenote: CHARACTER OF MODERN RESEARCHES.] - -The first attempt, however, to frame a satisfactory theory of volcanic -action, and to show the part which volcanoes have played in the past -history of our globe, together with their place in its present economy, -was made in 1825, by Poulett Scrope, whose great work, 'Considerations -on Volcanoes,' may be regarded as the earliest systematic treatise on -Vulcanology. Since the publication of this work, many new lines of -inquiry have been opened up in connection with the subject, and fresh -methods of research have been devised and applied to it. More exact -observations of travellers over wider areas have greatly multiplied -the facts upon which we may reason and speculate, and many erroneous -hypotheses which had grown up in connection with the subject have been -removed by patient and critical inquiry. - -We propose in the following pages to give an outline of the present -state of knowledge upon the subject, and to indicate the bearings -of those conclusions which have already been arrived at, upon the -great questions of the history of our globe and the relations which -it bears to the other portions of the universe. In attempting this -task we cannot do better than take up the several lines of inquiry -in the order in which they have been seized upon and worked out by -the original investigators; for never, perhaps, is the development -of thought in the individual mind so natural in its methods, and so -permanent in its effects, as when it obeys those laws which determined -its growth in the collective mind of the race. In our minds, as in our -bodies, development in the individual is an epitome, or microcosmic -reproduction, of evolution in the species. - - - - -CHAPTER II. - -THE NATURE OF VOLCANIC ACTION. - - -The dose investigation of what goes on within a volcanic vent may -appear at first sight to be a task beset with so many difficulties and -dangers that we may be tempted to abandon it as altogether hopeless. At -the first recorded eruption of Vesuvius the elder Pliny lost his life -in an attempt to approach the mountain and examine the action which -was taking place there; and during the last great outburst of the same -volcano a band of Neapolitan students, whose curiosity was greater than -their prudence, shared the same fate. - -But in both these cases the inquirers paid the penalty of having -adopted a wrong method. If we wish to examine the mode of working of a -complicated steam-engine, it will be of little avail for us to watch -the machinery when the full blast of steam is turned on, and the rapid -movements of levers, pinions, and slides baffle all attempts to follow -them, and render hopeless every effort to trace their connection with -one another. But if some friendly hand turn off the greater part of -the steam-supply, then, as the rods move slowly backwards and forwards, -as the wheels make their measured revolutions, and the valves axe -seen gradually opening and shutting, we may have an opportunity of -determining the relations of the several parts of the machine to one -another, and of arriving at just conclusions concerning the plan on -which it is constructed. Nor can we doubt that the parts of the machine -bear the same relation to one another, and that their movements take -place in precisely the same order, when the supply of steam is large as -when it is small. - -Now, as we shall show in the sequel, a volcano is a kind of great -natural steam-engine, and our best method of investigating its action -is to watch it when a part of the steam-supply is cut off. It is -true that we cannot at will control the source of supply of steam to -a volcano, as we can in a steam-engine, but as some volcanoes have -usually only a small steam-supply, and nearly all volcanoes vary -greatly in the intensity of their action at different periods, we can, -by a careful selection of the object or the time of our study, gain all -those advantages which would be obtained by regulating its action for -ourselves. - -Spallanzani appears to have been the first to perceive the important -fact, that the nature of volcanic action remains the same, however -its intensity may vary. Taking advantage of the circumstance that -there exists in the Mediterranean Sea a volcano--Stromboli--which -for at least 2,000 years has been in a constant and regular, but not -in a violent or dangerous, state of activity, he visited the spot, -and made the series of careful observations which laid the foundation -of our knowledge of the 'physiology of volcanoes.' Since the time -of Spallanzani, many other investigators have visited the crater -of Stromboli, and they have been able to confirm and extend the -observations of the great Italian naturalist, as to the character of -the action which is constantly taking place within it. We cannot better -illustrate the nature of volcanic action than by describing what has -been witnessed by numerous observers within the crater of Stromboli, -where it is possible to watch the series of operations going on by the -hour together, and to do so without having our judgment warped either -by an excited imagination or the sense of danger. - -[Sidenote: APPEARANCE OF STROMBOLI FROM A DISTANCE.] - -In the sketch, fig. 1, which was made on April 20, 1874, I have shown -the appearance which this interesting volcano usually presents, when -viewed from a distance. The island is of rudely circular outline, and -conical form, and rises to the height of 3,090 feet above the level of -the Mediterranean. From a point on the side of the mountain, masses -of vapour are seen to issue, and these unite to form a cloud over -the mountain, the outline of this vapour-cloud varying continually -according to the hygrometric state of the atmosphere, and the direction -and force of the wind. At the time when this sketch was made, the -vapour-cloud was spread in a great horizontal stratum overshadowing -the whole island, but it was clearly seen to be made up of a number -of globular masses, each of which, as we shall hereafter see, is the -product of a distinct outburst of the volcanic forces. - -Viewed at night-time, Stromboli presents a far more striking and -singular spectacle. The mountain, with its vapour canopy, is visible -over an area having a radius of more than 100 miles. When watched from -the deck of a vessel anywhere within this area, a glow of red light is -seen to make its appearance from time to time above the summit of the -mountain; this glow of light may be observed to increase gradually in -intensity, and then as gradually to die away. After a short interval -the same appearances are repeated, and this goes on till the increasing -light of the dawn causes the phenomenon to be no longer visible. The -resemblance presented by Stromboli to a 'flashing light' on a most -gigantic scale is very striking, and the mountain has long been known -as 'the lighthouse of the Mediterranean.' - -It must be pointed out, however, that in two very important particulars -the appearances presented by Stromboli differ markedly from those -rhythmical gleams exhibited by the 'flashing-lights' of our coasts. -In the first place, the intervals between successive flashes are very -unequal, varying from less than one minute to twenty minutes, or even -more; and in the second place, the duration and intensity of the red -glow above the mountain are subject to like variation, being sometimes -a momentary scarcely visible gleam, and at others a vivid burst of -light which illuminates the sky to a considerable distance round. - -[Illustration: Fig. 1.--Stromboli, viewed from the North-west, April -1874.] - -[Illustration: Fig. 2.--Map of tub Island of Stromboli. (Scale about -two inches to a mile.)] - -[Sidenote: GENERAL FEATURES OF THE MOUNTAIN.] - -Let us now draw near and examine this wonderful phenomenon of a -mountain which seemingly ever burns with fire, and yet is not consumed. -The general form of the Island of Stromboli will be gathered from an -inspection of the plan, fig. 2, which is copied from a map published by -the Italian Government. When we land upon the island, we find that it -is entirely built up of such materials as we know to be ejected from -volcanoes; indeed, it resembles on a gigantic scale the surroundings -of an iron furnace, with its heaps of cinders and masses of slag. The -irregularity in the form of the island is at once seen to be due to the -action of the wind, the rain, and the waves of the surrounding sea, -which have removed the loose, cindery materials at some points, and -left the hard, slaggy masses standing up prominently at others. - -This great heap of cindery and slaggy materials rises, as we have said, -to a height of more than 3,000 feet above the sea-level, but even this -measurement does not give a just idea of its vast bulk. Soundings in -the sea surrounding the island show that the bottom gradually shelves -around the shores to the depth of nearly 600 fathoms, so that Stromboli -is a great conical mass of cinders and slaggy materials, having a -height of over 6,000 feet, and a base whose diameter exceeds four miles. - -The general form and proportions of this mass will be better understood -by an examination of the section, fig. 3, which is also constructed -from the materials furnished by the map of the island issued by the -Italian Government. The same section, and the map, fig. 2, will serve -to make clear the position and relations of the point on the mountain -at which the volcanic activity takes place. At a spot on the north-west -slope of the mountain, about 1,000 feet below its summit, and 2,000 -feet above the level of the sea, there exists a circular depression, -the present active 'crater' of the volcano; and leading down from this -to the sea there is a flat slope making an angle of about 35° with the -horizon, and known as the 'Sciarra.' The Sciarra is bounded by steep -cliffs, as shown in the sketch fig. 1, and the plan fig. 2. - -[Illustration: Fig. 3.--Section through the Island of Stromboli from -n.w. to s.e. - -_a._ Highest summit of the mountain, _c._ Cratère del Fossa, _b._ Point -overlooking the crater, _d._ Steep slope known as the Sciarra del -Fuoco. _e._ Continuation of the same slope beneath the level of the -sea. _f._ Steep cliffs of the Punta dell' Omo.] - -[Sidenote: FORM AND FUNCTION OF THE CRATER.] - -If we climb up to this scene of volcanic activity, we shall be able to -watch narrowly the operations which are going on there. On the morning -of the 24th of April, 1874, I paid a visit to this interesting spot in -order to get a near view of what was taking place. On reaching a point -upon the side of the Sciarra, from which the crater was in full view -before me, I witnessed, and made a sketch of, an outburst which then -took place, and this sketch has been reproduced in fig. 4. Before the -outburst, numerous light curling wreaths of vapour were seen ascending -from fissures on the sides and bottom of the crater. Suddenly, and -without the slightest warning, a sound was heard like that produced -when a locomotive blows off its steam at a railway-station; a great -volume of watery vapour was at the same time thrown violently into -the atmosphere, and with it there were hurled upwards a number of -dark fragments, which rose to the height of 400 or 500 feet above the -crater, describing curves in their course, and then falling back upon -the mountain. Most of these fragments tumbled into the crater with a -loud, rattling noise, but some of them fell outside the crater, and a -few rolled down the steep slope of the Sciarra into the sea. Some of -these falling fragments were found to be still hot and glowing, and in -a semi-molten condition, so that they readily received the impression -of a coin thrust into them. - -[Illustration: Fig. 4.--The Crates of Stromboli as viewed from the side -of the Sciarra during an eruption on the morning of April 24, 1874.] - -[Sidenote: APERTURES AT THE BOTTOM OF THE CRATER.] - -But on the upper side of the crater, at the point marked 6, on the -section fig. 3, there exists a spot from which we can look down upon -the bottom of the crater, and view the operations taking place there. -This is the place where Spallanzani and other later investigators have -carried on their observations, and, when the wind is blowing from -the spectator towards the crater, he may sit for hours watching the -wonderful scene displayed before him. The black slaggy bottom of the -crater is seen to be traversed by many fissures or cracks, from most -of which curling jets of vapour issue quietly, and gradually mingle -with and disappear in the atmosphere. But besides these smaller cracks -at the bottom of the crater, several larger openings are seen, which -vary in number and position at different periods; sometimes only one of -these apertures is visible, at others as many as six or seven, and the -phenomena presented at these larger apertures are especially worthy of -careful investigation. - -These larger apertures, if we study the nature of the action taking -place at them, may be divided into three classes. From those of the -first class, steam is emitted with loud, snorting puffs, like those -produced by a locomotive-engine, but far less regular and rhythmical -in their succession. In the second class of apertures masses of molten -material are seen welling out, and, if the position of the aperture -be favourable, flowing outside the crater; from this liquid molten -mass steam is seen to escape, sometimes in considerable quantities. -The openings of the third class present still more interesting -appearances. Within the walls of the aperture a viscid or semi-liquid -substance is seen slowly heaving up and down. As we watch the seething -mass the agitation within it is observed to increase gradually, and at -last a gigantic bubble is formed which violently bursts, when a great -rush of steam takes place, carrying fragments of the scum-like surface -of the liquid high into the atmosphere. - -If we visit the crater by night, the appearances presented are found to -be still more striking and suggestive. The smaller cracks and larger -openings glow with a ruddy light. The liquid matter is seen to be red- -or even white-hot, while the scum or crust which forms upon it is of a -dull red colour. Every time a bubble bursts and the crust is broken up -by the escape of steam, a fresh, glowing surface of the incandescent -material is exposed. If at these moments we look up at the vapour-cloud -covering the mountain, we shall at once understand the cause of the -singular appearances presented by Stromboli when viewed from a distance -at night, for the great masses of vapour are seen to be lit up with a -vivid, ruddy glow, like that produced when an engine-driver opens the -door of the furnace and illuminates the stream of vapour issuing from -the funnel of his locomotive. - -Let us now endeavour to analyse the phenomena so admirably displayed -before us in the crater of Stromboli. The three essential conditions -on which the production of these phenomena seems to depend are the -following: first, the existence of certain apertures or cracks -communicating between the interior and the surface of the earth; -secondly, the presence of matter in a highly heated condition beneath -the surface; and thirdly, the existence of great quantities of water -imprisoned in the subterranean regions--which water, escaping as steam, -gives rise to all those active phenomena we have been describing. - -[Sidenote: CAUSE OF THE GLOWING LIGHT.] - -We have said, at the outset, that there exists no analogy whatever -between the action which takes place in volcanoes and the operation of -burning or combustion. Occasionally, it is true, certain inflammable -substances are formed by the action going on within the volcano, and -these inflammable substances, taking fire, produce real flames. Such -flames are, however, in almost all cases only feebly luminous, and do -not give rise to any conspicuous appearances. What is usually taken for -flame during volcanic eruptions is simply, as we have already pointed -out, the glowing red-hot surface of a mass of molten rock, reflected -from a vapour-cloud hanging over it. The red glow observed over a -volcano in eruption is indeed precisely similar in its nature and -origin to that which is seen above London during a night of heavy fog, -and which is produced by the reflection of the gas-lights of the city -from the innumerable particles of water-vapour diffused through the -atmosphere. Fires, of course, occur when the molten and incandescent -materials poured out from a volcano come in contact with inflammable -substances, such as forests and houses, but in these cases the -combustion is quite a secondary phenomenon. - -There is another popular delusion concerning volcanic action, which it -may be necessary to refer to and to combat. From the well-known fact -that sulphur or brimstone is found abundantly in volcanic regions, the -popular belief has arisen that this highly inflammable substance has -something to do with the production of the eruptions of volcanoes. In -school-books which were, until comparatively recent years, in constant -use in this country, the statement may be found that by burying certain -quantities of sulphur, iron-pyrites, and charcoal in a hole in the -ground, we may form a miniature volcano, and produce all the essential -phenomena of a volcanic eruption. No greater mistake could possibly be -made. The chemical reactions which take place when sulphur and other -substances are made to act upon each other differ entirely from the -phenomena of volcanic action. The sulphur which is found in volcanic -regions is the result and not the cause of volcanic action. Among the -most common substances emitted from volcanic vents along with the steam -are the two gases, sulphurous acid and sulphuretted hydrogen. When -these two gases come into contact with one another, chemical action -takes place, and the elements contained in them--oxygen, hydrogen, -and sulphur--are free to group themselves together in an entirely new -fashion; the consequence of this is that water and sulphuric acid (oil -of vitriol) are formed, and a certain quantity of sulphur is set free. -The water escapes into the atmosphere, the sulphuric acid combines with -lime, iron, or other substances contained in the surrounding rocks, -and the sulphur builds up crystals in any cavities which may happen to -exist in these rocks. - -[Sidenote: VOLCANIC ACTION RESEMBLES BOILING.] - -If, however, careful and exact observations, like those carried on -at Stromboli, compel us to reject the popular notions concerning -the supposed resemblance between volcanic action and the combustion -of sulphur or other substances, they nevertheless suggest analogies -with certain other simple and well-known operations. And in pursuing -these analogies, we are led to the recognition of some admirable -illustrations both of the attendant phenomena and of the true cause of -volcanic outbursts. - -No one can look down on the mass of seething material in violent -agitation within the fissures at the bottom of the crater of Stromboli, -without being forcibly reminded of the appearances presented by liquids -in a state of boiling or ebullition. The glowing material seems to be -agitated by two kinds of movements, the one whirling or rotatory, the -other vertical or up-and-down in its direction. The fluid mass in this -way appears to be gradually impelled upwards, till it approaches the -lips of the aperture, when vast bubbles are formed upon its surface, -and to the sudden bursting of these the phenomena of the eruption are -due. - -Now if we take a tall narrow vessel and fill it with porridge or -some similar substance of imperfect fluidity, we shall be able, by -placing it over a fire, to imitate very closely indeed the appearances -presented in the crater of Stromboli. As the temperature of the mass -rises, steam is generated within it, and in the efforts of this steam -to escape, the substance is set in violent movement. These movements of -the mass are partly rotatory and partly vertical in their direction; as -fresh steam is generated in the mass its surface is gradually raised, -while an escape of the steam is immediately followed by a fall of the -surface. Thus an up-and-down movement of the liquid is maintained, but -as the generation of steam goes on faster than it can escape through -the viscid mass, there is a constant tendency in the latter to rise -towards the mouth of the vessel. At last, as we know, if heat continues -to be applied to the vessel, the fluid contents will be forced up to -its edge and a catastrophe will occur; the steam being suddenly and -violently liberated from the bubbles formed on the surface of the mass, -and a considerable quantity of the material forcibly expelled from -the vessel. The suddenness and violence of this catastrophe is easily -accounted for, if we bear in mind that the escaping steam acts after -the manner of a compressed spring which is suddenly released. Steam -is first formed at the bottom of the vessel which is in contact with -the fire; but here it is under the pressure of the whole mass of the -liquid, and moreover, the viscidity of the substance tends to retard -the union of the steam bubbles and their rise to the surface of the -mass. But when the pressure is relieved by the bursting of bubbles at -the surface, the whole of the generated steam tends to escape suddenly. - -[Sidenote: ESCAPE OF STEAM-BUBBLES FROM LAVA.] - - -Now within the crater of Stromboli we have precisely the necessary -conditions for the display of the same series of operations. In the -apertures at the bottom there exists a quantity of imperfectly fluid -materials at a higher temperature, containing water entangled in its -mass. As this water passes into the state of steam it tends to escape, -and in so doing puts the whole mass into violent movement. When the -steam rises to the surface, bubbles are formed, and the formation of -these bubbles is promoted by the circumstance that the liquid mass, -where exposed to the atmosphere, becomes chilled, and thereby rendered -less perfectly fluid. By the bursting of these bubbles the pressure is -partially relieved, and a violent escape of the pent-up steam takes -place through the whole mass. Equilibrium being thus restored, there -follows a longer or shorter interval of quiescence, during which steam -is being generated and collected within the mass, and the series of -operations which we have described then recommences. - -There is one other consideration which must be borne in mind in -connection with this subject. It is well known that if water be -subjected to sufficiently great pressure it may be raised to a very -high temperature and still retain its liquid condition. When this -pressure is removed, however, the whole mass passes at once into the -condition of steam or water-gas; and the gas thus formed at high -temperatures has a proportionably high tension. In a Papin's digester -water confined in a strong vessel is raised to temperatures far above -its ordinary boiling-point, and from any opening in such a vessel the -steam escapes with prodigious violence. Now, at considerable depths -beneath the earth's surface, and under the pressure of many hundreds -or thousands of feet of solid rock, water still retaining its liquid -condition may become intensely heated. When the pressure is relieved by -the formation of a crack or fissure in the superincumbent mass of rock, -the escape of the superheated steam will be of very violent character, -and may be attended with the most striking and destructive results. In -the existence of high temperatures beneath the earth's surface, and the -presence in the same regions of imprisoned water capable of passing -into the highly elastic gas which we call steam, we have a cause fully -competent to produce all the phenomena which we have described as -occurring at Stromboli. - -It may at first sight appear that the grand and terrible displays -of violence witnessed during a great volcanic eruption differ -fundamentally in their character and their origin from those feeble -outbursts which we are able to examine closely and analyse rigorously -at Stromboli. But that such is not the case a few simple considerations -will soon convince us. - -[Sidenote: STROMBOLI COMPARED WITH VESUVIUS.] - -Although Stromboli usually displays the subdued and moderate activity -which we have been describing, yet the intensity of the action going -on within it is subject to considerable variation. Occasionally the -violence of the outbursts is greatly increased--the roaring of the -steam-jets may be heard for many miles around, considerable streams of -incandescent liquefied rock flow down the Sciarra into the sea, and the -explosions in the crater are far more frequent and energetic, cinders -and fragments of rock being scattered all over the island and the -surrounding seas. - -On the other hand, volcanoes like Vesuvius, which are sometimes the -scene of eruptions on the very grandest scale, at others subside into a -temporary state of moderate activity quite similar in character to that -which is the normal condition of Stromboli. Thus, shortly before the -great eruption of Vesuvius in April 1872, a small cone was formed near -the edge of the crater, and during some months observers could watch, -in ease and safety, a series of small explosions taking place, quite -similar in their character and attendant phenomena to those which we -have described as occurring at Stromboli. French geologists are in the -habit of defining the condition of activity in a volcano by speaking -of the more quiet and, regular state as the 'Strombolian stage,' and -the more violent and paroxysmal as the 'Vesuvian stage'; but the two -conditions are, as we have seen, presented by the same volcano at -different periods, and pass into one another by the most insensible -gradations. - -We must now proceed to compare the grand and terrible appearances -presented during a great eruption with those more feeble displays which -we have been describing, to show that in all their essential features -these different kinds of outbursts are identical with one another, and -must be referred to the action of similar causes. - -The volcanic eruption which has been most carefully studied in recent -times is that which we have already referred to as occurring at -Vesuvius, in the month of April 1872. With the exception, perhaps, of -that which took place in October 1822, this eruption was the grandest -which has broken out at Vesuvius during the present century. Owing to -the circumstance of its proximity to the great city of Naples, Vesuvius -has always been the most carefully watched of all volcanoes, and in -recent years the erection of an observatory, provided with instruments -for recording the smallest subterranean tremors affecting the mountain, -has facilitated the carrying on of those continuous and minute -observations which are so necessary for exact scientific inquiry. - -[Illustration: Fig 5. Vesuvius in Eruption, as seen from Naples, April -26, 1872. (_From a photograph_)] - -[Sidenote: VESUVIUS ERUPTION OF 1872.] - -On the occasion of this outburst, the aid of instantaneous photography -was first made available for obtaining a permanent record of the -appearances displayed at volcanic eruptions. In fig. 5 we have one of -these photographs, which was taken at 5 o'clock P.M. on April 26, 1872, -transferred to a wood-block and engraved. In examining it we feel sure -that we are not being misled by any exaggeration or error on the part -of the artist. Vesuvius rises to the height of nearly 4,000 feet above -the level of the sea, and an inspection of the photograph proves that -the vapours and rock-fragments were thrown to the enormous height of -20,000 feet, or nearly four miles, into the atmosphere. - -The main features of this terrifying outburst were as follows. For -more than a twelvemonth before, the activity of the forces at work -within the mountain appeared to be gradually increasing, and the great -eruption commenced on April 24, attained its climax on the 26th, and -began to die out on the following day. During the eruption the bottom -of the crater was entirely broken up, and the sides of the mountain -were rent by fissures in all directions. So numerous were these -fissures and cracks that liquid matter appeared to be oozing from every -part of its surface, and, as Professor Palmieri, who witnessed the -outburst from the observatory, expressed it, 'Vesuvius sweated fire.' -One of the fissures was of enormous size, extending from the summit to -far beyond the base of the cone; the scar left by this gigantic rent -being plainly visible at the present day. - -From the great opening or crater at the summit, and from some of the -fissures on the sides of the mountain, enormous volumes of steam rushed -out with a prodigious roaring sound, the noise being so terrific that -the inhabitants of Naples, five miles off, fled from their houses -and spent the night in the open streets. Although this roaring sound -appeared at a distance to be continuous, yet those upon the mountain -could perceive that it was produced by detonations or explosions -rapidly following one another. Each of these explosions was accompanied -by the formation of a great globe of white vapour, which, rising into -the atmosphere, swelled the bulk of the vast cloud overhanging the -mountain. An inspection of the photographs (see fig. 5) shows that the -great vapour-cloud over Vesuvius was made up of the globular masses -ejected at successive explosions. Each of these explosive upward -rushes of steam carried along with it a considerable quantity of solid -fragments, and these fell in great numbers all over the surface of -the mountain, breaking the windows of the observatory, and making it -dangerous to be out of doors. - -We have said that lava, or molten rock, appeared to be issuing from -the very numerous cracks formed all over the flanks of the mountain. -But at three points this molten rock issued in such quantities as to -form great, fiery floods, which rushed down the sides of the mountain, -and flowed to a considerable distance beyond its base. The largest -of these lava-floods overwhelmed and destroyed the two villages of -Massa di Somma and San Sebastiano, besides many country houses in the -neighbourhood. - -[Sidenote: STEAM EMITTED FROM LAVA-CURRENT.] - -A very marked and interesting feature exhibited by these three -lava-floods was the quantity of watery vapour which they gave off -during their flow. All along their course, enormous volumes of steam -were evolved from them, as will be seen by an inspection of the -photograph. Indeed, such was the abundance and tension of the steam -thus escaping from the surfaces of the lava-currents that it forced -the congealing rock up into great bubbles and blisters, and gave rise -to the formation of innumerable miniature volcanoes, varying in size -from a beehive to a cottage, some of which remained in a state of -independent activity for a considerable time. - -So far, what we have described as taking place at Vesuvius, in April -1872, has been only the repetition on a £Eur grander scale of the -three kinds of action which we have shown to be constantly taking -place at Stromboli; namely, the formation of cracks or fissures in the -earth's surface, the escape of steam with explosive violence from these -openings, often propelling rock-fragments into the atmosphere, and the -outwelling, under the influence of this compressed steam, of masses of -molten materials. - -There were some other appearances presented at the great outburst at -Vesuvius, which do not seem at first sight to find any analogies in -the manifestations of the more feeble action continually going on at -Stromboli. - -Before and during the great outbreak of April 1872, Vesuvius itself -and the whole country round were visited with earthquake-shocks, or -tremblings of the ground. The sensitive instruments in the Vesuvian -Observatory showed the mountain daring the eruption to be in a constant -state of tremor. These earthquakes are not, as is commonly supposed, -actual upheavings of the earth's surface, but are vibrations propagated -through the solid materials of which the earth is built up. We cannot -stamp our feet upon the ground without giving rise to such vibrations, -though our senses may not be sufficiently acute to perceive them. -The explosive escape of steam from a crack is a cause sufficiently -powerful to produce a shock which is propagated and may be felt for -a considerable distance round. Even on Stromboli an observer at the -edge of the crater may notice that each explosive outburst of steam -is accompanied by a perceptible tremor of the ground, and in the case -of Vesuvius the violent shocks produced by the escape of far larger -volumes of steam give rise to proportionately stronger vibrations. The -nature and origin of those far more terrible and destructive shocks -which sometimes accompany, and more frequently precede, great volcanic -eruptions, we shall consider in the sequel. - -[Sidenote: CAUSE OF LIGHTNING DURING ERUPTIONS.] - -Another striking phenomenon which was exhibited in the great eruption -of Vesuvius in 1872 was the vivid display of lightning accompanied -by thunder. The uprushing current of steam and rock-fragments forms -a vertical column, but as the steam condenses it spreads out into a -great horizontal cloud which is seen to be made up of the great globes -of vapour emitted at successive explosions. When there is little or -no wind the vertical column with a horizontal cloud above it bears a -striking resemblance to the stone-pine trees which form so conspicuous -a feature in every Neapolitan landscape. Around this column of vapour -the most vivid lightning constantly plays and adds not a little to the -grand and awful character of the spectacle of a volcanic eruption, -especially when it is viewed by night. - -In the eruption of 1872 a strong wind blowing from the north-west -destroyed the usual regular appearance of this 'pine-tree appendage' to -the mountain, which is so well known to, and dreaded by the inhabitants -of Naples; the cloud, as will be seen from the photograph (fig. 5, -_facing_ p. 24), was blown on one side, and most of the falling -fragments took the same direction. - -It is well known that when high-pressure steam is allowed to escape -through an orifice, electricity is abundantly generated by the -friction, and Sir William Armstrong's hydro-electric machine is -constructed on this principle. Every volcano in violent eruption is -a very efficient hydro-electric machine, and the uprushing column -is in a condition of intense electrical excitation. This result is -probably aided by the friction of the solid particles as they are -propelled upwards and fall back into the crater. The restoration of -the condition of electrical stability between this column and the -surrounding atmosphere is attended with the production of frequent -lightning-flashes and thunder-claps, the found of the latter being -usually, however, drowned in the still louder roar of the uprushing -steam-column. - -The discharge of Buch large quantities of steam into the atmosphere -soon causes the latter to be saturated with watery vapour, and there -follows an excessive rainfall; long-continued rain and floods were an -accompaniment of the great Vesuvian outbreak of 1872, as they have -been of almost all great volcanic eruptions. The Italians, indeed, -dread the floods which follow an eruption more than the fiery streams -of lava which accompany it--for they have found the mud-streams (_lave -di fango_), formed by rain-water sweeping along the loose volcanic -materials, to be more widely destructive in their effects than the -currents of molten rock (_lave di fuoco_). - -Besides the phenomena which we have now described as accompanying a -great volcanic outburst, many others have undoubtedly been recorded -by apparently trustworthy authorities. But, in dealing with the -descriptions of these grand and terrible events, we must always be on -our guard against accepting as literal facts, the statements made by -witnesses, often writing at some distance from the scene of action, and -almost always under the influence of violent excitement and terror. -The desire to administer to the universal love of the marvellous, and -the tendency to exaggeration, will usually account for many of the -wonderful statements contained in such records; and, even where the -witness is accurately relating events which he thinks passed before his -eyes, we must remember that it is probable he may have had neither the -opportunity nor the capacity for exact observation. - -The more carefully we sift the accounts which have been preserved of -great volcanic outbursts, the more are we struck by the fact that the -appearances described can be resolved into a few simple operations, the -true character of which has been distorted or disguised by the want of -accurate observation on the part of the witnesses. - -[Sidenote: SIMILARITY OF FEEBLE AND VIOLENT ERUPTIONS.] - -We are thus led to the conclusion that the grand and terrible -appearances displayed at Vesuvius and other volcanoes in a state of -violent eruption do not differ in any essential respect from the -phenomena which we have witnessed accompanying the miniature outbursts -of Stromboli. And we are convinced, by the same considerations, that -the forces which give rise to the feeble displays in the latter case -would produce, if acting with greater intensity and violence, all the -magnificent spectacles presented in the former. - -In Vesuvius and Stromboli alike, the active cause of all the phenomena -exhibited is found to be the escape of steam from the midst of -masses of incandescent liquefied rock. The violence, and therefore -the grandeur and destructive effects of an eruption, depend upon the -abundance and tension of this escaping steam. - -There is one respect in which volcanic phenomena are especially -calculated to excite the fear and wonder of beholders--namely, in the -sudden and apparently spontaneous character of their manifestations. -Eclipses were regarded as equally portentous with volcanic eruptions -till astronomers learned not only to explain the causes which gave -rise to them, but even to predict to the second the times of their -occurrence. If we were able in like manner to warn the inhabitants of -volcanic regions of the approach of a grand eruption, the fear and -superstition with which these events are now regarded would doubtless -be in great part dispelled. The power of prediction is alike the -crucial test and the crowning triumph of a scientific theory. - -But, although natural philosophers are able to assign the causes to -which the grand operations of volcanoes are due, and also to explain -all the varied appearances which accompany them, they have not as yet -so far mastered the laws which govern volcanic action as to be able to -predict the periods of their manifestation. - -That these operations, like all others going on upon the globe, are -governed by great natural laws we cannot for a moment doubt. And that, -in all probability, more careful and exact observation and reasoning -will at some future time lead us to the recognition of these laws, -every student of nature is sanguine. But at the present time, it must -be confessed, we are very far indeed from being able to afford that -crowning proof of the truth of our theories of volcanic action which is -implied in the power of predicting the period and degree of intensity -of their manifestations. - -[Sidenote: ERUPTIONS AND THE INTERVALS BETWEEN THEM.] - -There are, however, some observations which lead us to hope that the -time may not be far distant when we shall have so £Eur obtained a -knowledge of the conditions on which volcanic action depends as to be -able to form some judgment as to its manifestations in the future at -any particular locality. But we must recollect that these conditions -axe very numerous and complicated, and that some of them may lie almost -entirely outside our sphere of observation; hence hasty attempts in -this direction, such as have recently been made, are to be deprecated -by every true lover of science. - -Concerning the eruptions that have taken place at those volcanic -centres which have been known from a remote antiquity, we have records -from which we can determine the intervals separating these outbursts -and their relative violence. A critical examination of these records -leads to the following conclusions:-- - -(1.) A long period of quiescence is generally followed by an eruption -which is either of long duration or of great violence. - -(2.) A long-continued, or very violent eruption is usually followed by -a prolonged period of repose. - -(3.) Feeble and short eruptions usually succeed one another at brief -intervals. - -(4.) As a general rule, the violence of a great eruption is inversely -proportional to its duration. - -It will be seen that these general conclusions are in perfect harmony -with the theory that volcanic outbursts are due to the accumulation -of steam at volcanic centres, and that the tension of this imprisoned -gas eventually overcomes the repressing forces which tend to prevent -its manifestation. Before astronomers had learnt to determine all -the conditions on which the production of eclipses depends, they had -found that these phenomena succeed one another at regular intervals. -The discovery of such astronomical cycles was a great advance in our -knowledge of the heavenly bodies, and in the same way the determination -of these general relations between the intensity and duration of -volcanic outbursts and the intervals of time which separate them may -be regarded as the first step towards the discovery of the laws which -govern volcanic activity. - -In the actual determination of the conditions upon which the occurrence -of volcanic eruptions depends, it must be confessed, however, that -very little has as yet been done. This is in part due to the fact that -some at least of these conditions lie beyond the limits of direct -observation. But it must also be admitted, on the other hand, that -little has been as yet accomplished towards the careful and systematic -observation of those phenomena which may, and probably do, exert an -influence in bringing about volcanic outbursts. - -[Sidenote: INFLUENCE OF ATMOSPHERIC CONDITIONS.] - -In the Lipari Islands there has prevailed a belief, from the very -earliest period of history, that the feeble eruptions of Stromboli -are in some way dependent upon the condition of the atmosphere. -These islands were known to the ancients as the Æolian Isles, from -the fact that they were once ruled over by a king of the name of -Æolus. It seems not improbable that Æolus was gifted with natural -powers of observation and reasoning far in advance of those of his -contemporaries. A careful study of the vapour-cloud which covers -Stromboli would certainly afford him information concerning the -hygrometric condition of the atmosphere; the form and position -assumed by this vapour-cloud would be a no less perfect index of the -direction and force of the wind; and, if the popular belief be well -founded, the frequence and violence of the explosions taking place -from the crater would indicate the barometric pressure. From these -data an acute observer would be able to issue 'storm-warnings' and -weather-prognostics of considerable value. In the vulgar mind, the -idea of the prediction of natural events is closely bound up with that -of their production; and the shrewd weather-prophet of Lipari was -after his death raised to the rank of a god, and invested with the -sovereignty of the winds. - -Whether the popular idea that the outbursts of Stromboli are regulated -by atmospheric conditions has any foundation is still open to grave -doubt. It seems to be certain, however, that during autumn and winter -the more violent paroxysms of the volcano occur, and that in summer -the action which takes place is far more regular and equable. It would -be of the greatest benefit to science if an observatory were erected -beside the crater of Stromboli, where a careful record might be kept of -all atmospheric changes, and of the synchronous manifestations of the -volcanic forces. - -A little consideration will show that it is a by no means unreasonable -supposition that variations in atmospheric pressure may exercise a very -important influence in bringing about volcanic outbursts. Changes in -the barometer to the extent of two inches within a very short period -are not uncommon occurrences. A very simple calculation will show that -the fall of the mercury in the barometer to the extent of two inches -indicates the removal of a weight of two millions of tons from each -square mile of the earth's surface where this change takes place. Now, -if we suppose, as we have good ground for doing, that under volcanic -areas vast quantities of superheated water are only prevented from -flashing into steam by the superincumbent pressure, a relief of this -pressure to the extent of two millions of tons on every square mile -could scarcely fail to produce very marked effects. The way in which -explosions in fiery coal-mines generally follow closely upon sudden -falls in the atmospheric pressure is now well known; and coal-mine -explosions and volcanic outbursts have this in common, that both -result from the sudden and violent liberation of subterranean gases. -There are not a few apparently well-authenticated accounts of volcanic -and earthquake phenomena following closely on peculiar atmospheric -conditions, and the whole question of the relation of the volcanic -forces to atmospheric pressure, as Spallanzani himself so long ago -pointed out, is deserving of a most careful and rigorous investigation. - -[Sidenote: SUPPOSED TIDAL EFFECTS.] - -There is one other consideration which has frequently been urged as -worthy of especial attention, in dealing with the question of the -exciting causes of volcanic outbursts. If volcanoes were, as was at one -time almost universally supposed, in direct communication with a great -central mass of liquefied materials, or even if any large reservoirs -of such liquids existed beneath volcanic districts, as others have -imagined, then the different mobility of the solid and liquid portions -of the earth's mass would give rise to tidal effects similar to those -occurring in the surface waters of the globe. Under such circumstances, -volcanic outbursts, like the tides, would be determined by the relative -positions of the sun and moon to our globe. It is certain, however, -that no very direct relation has yet been established between the -lunar periods and those of volcanic outbursts, though recent close -observations upon the crater of Vesuvius, by Professor Palmieri, do -seem to lend support to the view that such relations may exist. - -At the present time, therefore, it must be admitted that vulcanologists -have only just commenced those series of exact and continuous -observations which are necessary to determine the conditions that -regulate the appearance of volcanic phenomena. The study of the laws -of volcanic action is yet in its infancy. But the establishment of -observatories on Vesuvius and Etna 18 fall of promise for the future, -and when we consider the advances which have been made, during the last -one hundred years, in our knowledge of the true nature of volcanic -action, we need not despair that the extension of the same methods -of inquiry will lead to equally important results concerning the -conditions which determine and the laws which govern it. - -In the meanwhile, it is no small gain to have established the fact that -volcanic phenomena, divested of all those wonderful attributes with -which superstition and the love of the marvellous have surrounded them, -are operations of nature obeying definite laws, which laws we may hope -by careful observation and accurate reasoning to determine; and that -the varied appearances, presented alike in the grandest and feeblest -outbursts, can all be referred to one simple cause--namely, the escape, -from the midst of masses of molten materials, of imprisoned steam or -water-gas. - - - - -CHAPTER III. - -THE PRODUCTS OF VOLCANIC ACTION. - - -While Spallanzani was engaged in investigating the nature of the action -going on at Stromboli and other Italian volcanoes, his contemporary -Dolomieu was laying the foundation of another important branch of -vulcanology by studying the characters of the different materials of -which volcanoes are built up. Since the publication of Dolomieu's -admirable works on the rocks of the Lipari and Ponza Islands, science -has advanced with prodigious strides. The chemist has taught us how -to split up a rock into its constituent elements and to determine the -proportions of these to one another with mathematical precision; the -mineralogist has done much in the investigation of the characters and -mode of origin of the crystalline minerals which occur in these rocks; -and the microscopist has shown how the minute internal structure of -these rocks may be made clearly manifest. We shall proceed to give a -sketch of the present state of knowledge obtained by these different -kinds of investigations, concerning the materials which are ejected -from volcanic vents. - -The most abundant of the substances which are ejected from volcanoes -is steam or water-gas, which, as we have seen, issues in prodigious -quantities during every eruption. But with the steam a great number -of other volatile materials frequently make their appearance. The -chief among these are the add gases known as hydrochloric acid, -sulphurous acid, sulphuretted hydrogen, carbonic add, and boracic acid; -and with these acid gases there issue hydrogen, nitrogen, ammonia, -the volatile metals arsenic, antimony, and mercury, and some other -substances. In considering the nature of the products which issue from -volcanic fissures, it must be remembered that many substances which -under ordinary circumstances do not exhibit marked volatility are -nevertheless easily carried away in fine particles when a current of -steam is passed over them. As we shall have to point out in the sequel, -different volatile substances have a tendency to make their appearance -at volcanic vents according as the intensity of the action going on -within it varies. - -The volatile substances issuing from volcanic fissures at high -temperatures react upon one another, and many new compounds are thus -formed. We have already seen how, by the action of sulphurous acid -and sulphuretted hydrogen on each other, the sulphur so common in -volcanic districts has been separated and deposited. The hydrochloric -acid acts very energetically on the rocks around the vents, uniting -with the iron in them to form the yellow ferric-chloride. The rocks -all round a volcanic vent are not unfrequently found coated with this -yellow substance, which is almost always mistaken by casual observers -for sulphur. In many volcanoes the constant passage through the rocks -of the various acid gases has caused nearly the whole of the iron, -lime, and alkaline materials of the rocks to be converted into soluble -compounds known as sulphates, chlorides, carbonates, and borates; and, -on the removal of these by the rain, there remains a white, powdery -substance, resembling chalk in outward appearance, but composed of -almost pure silica. There are certain cases in which travellers have -visited volcanic islands where chemical action of this kind has gone on -to such an extent, that they have been led to describe the islands as -composed entirely of chalk. - -[Sidenote: GASES EMITTED FROM VOLCANOES.] - -Some of the substances issuing from volcanic vents, such as hydrogen -and sulphuretted hydrogen, are inflammable, and when they issue at a -high temperature, these gases burst into flame the moment that they -come into contact with the air. Hence, when volcanic fissures axe -watched at night, faint lambent flames are frequently seen playing over -them, and sometimes these flames are brilliantly coloured, through the -presence of small quantities of certain metallic oxides. Such volcanic -flames, however, are scarcely ever strongly luminous and, as we have -already seen, the red, glowing light which is observed over volcanic -mountains in eruption is due to quite another cause. The study by the -aid of the spectroscope of the flames which issue from volcanic vents -promises to throw much new light on the rarer materials ejected by -volcanoes. Spectroscopic observations of this kind have already been -commenced by Janssen, at Stromboli and Santorin. - -Some of the volatile substances issuing from volcanic vents, are at -once deposited when they come in contact with the cool atmosphere, -others form new compounds with one another and the constituents of the -atmosphere, while others again attack the materials of the surrounding -rocks and form fresh chemical compounds with some of their ingredients. -Thus, there are continually accumulating on the sides and lips of -volcanic fissures deposits of sulphates, chlorides, and borates of -the alkalies and alkaline earths, with sal-ammoniac, sulphur, and the -oxides and sulphides of certain metals. The lips of the fissures from -which steam and acid gases issue in volcanoes are constantly seen to -be coated with yellow and reddish-brown incrustations, consisting of -mixtures, in varying proportions, of these different materials, and -these sometimes assume the form of stalactites and pendent masses. - -[Sidenote: DEPOSITS AROUND VOLCANIC VENTS.] - -Some of these products of volcanic action are of considerable -commercial value. At Vulcano regular chemical works have been -established in the crater of the volcano, by an enterprising Scotch -firm, a great number of workmen being engaged in collecting the -materials which are deposited around the fissures, and are renewed -by the volcanic action almost as soon as they are removed. In fig. -6, I have given a sketch of this singular spot, taken from the high -ground of the neighbouring Island of Lipari. From the village at -the foot of the volcano, where the workmen live, a zig-zag road has -been constructed leading up the side, and down into the crater of -the volcano. On this road, workmen and mules, laden with the various -volcanic materials, may be seen constantly passing up and down. - -[Illustration: Fig. 6.--View of Vulcano, with Vulcanello in the -foreground taken from the south end of the Island of Lipari.] - -Vulcano appears to have been frequently in a state of violent eruption -during the past 2,000 years--the last great outburst having taken place -in 1786. In 1873 the activity in the crater of Vulcano suddenly became -more pronounced in character, and the workmen hastened to escape from -the dangerous spot, but, before they could do so, several of them -were severely injured by the explosions. After this outburst, which -did not prove to be of very violent character, the quantity of gases -issuing from the fissures in the crater was for a time much greater -than before, and the productiveness of these great natural chemical -works was proportionately increased: but eventually the action died out -almost entirely. The chief products of Vulcano which are of commercial -value, are sal-ammoniac, sulphur, and boracic acid. At one time it was -even contemplated that great leaden chambers should be erected over the -principal fissures at the bottom of the crater of Vulcano, in which -chambers the volatile materials might be condensed and collected. The -change in the condition of the volcano has unfortunately prevented the -carrying out of this bold project. - -Besides the volatile substances which issue from volcanic vents, -mingling with the atmosphere or condensing upon their sides, there are -also many solid materials ejected, and these may accumulate around the -orifices, till they build up mountains of vast dimensions, like Etna, -Teneriffe, and Chimborazo. Some of these solid materials are evidently -fragments of the rock-masses, through which the volcanic fissure has -been rent; these fragments have been carried upwards by the force of -the steam-blast and scattered over the sides of the volcano. But the -principal portion of the solid materials ejected from volcanic orifices -consists of matter which has been extruded from sources far beneath the -surface, in a highly-heated and fluid or semi-fluid condition. - -[Sidenote: EJECTED ROCK-FRAGMENTS.] - -The fragments torn from the sides of volcanic fissures consist of the -rocks through which the eruptive forces may happen to have opened their -way; pieces of sandstone, limestone, slate, granite, &c., are thus -frequently found in considerable numbers among materials which build up -volcanic mountains. Thus, some of the volcanic cones in the Eifel are -very largely made up of fragments of slate, which have been torn from -the sides of the vents by the uprushing currents of steam. At Vesuvius -masses of limestone are frequently ejected, and may be picked up all -over the slopes of the mountains. These limestone-fragments frequently -contain fossils, and Professor Guiscardi, of Naples, has been able to -collect several hundred species of shells, transported thus by volcanic -action from the rock-masses which form the foundation of the volcano -of Vesuvius. The action of water at a high temperature, and under -such enormous pressure as must exist beneath volcanic mountains, has -often produced changes in the rocks of which fragments are ejected -from volcanic vents. The so-called 'lava' ornaments, which are so -extensively sold at Naples, are not made from the materials to which -geologists apply that name, but from the fragments of altered limestone -that have been torn from the rocks beneath the mountain, and scattered -by the eruptive forces all over its sides. The chemical action of the -superheated and highly-compressed steam on the rocks beneath volcanoes -frequently results in the formation of beautifully crystallised -minerals. Such crystallised minerals abound in the rock-fragments -scattered over the sides of Vesuvius and other volcanoes, both active -and extinct. They have been formed in the great chemical laboratories -which exist beneath the volcano, and have been brought to the surface -by the action of the steam-jets issuing from its fissures. - -Of still greater interest are those materials which issue from volcanic -orifices in an incandescent, and often in a molten, condition, and -which are evidently derived from sources far below the earth's surface. -It is to these materials that the name of 'lavas' is properly applied. - -Lavas present a general resemblance to the slags and clinkers which -are formed in our furnaces and brick-kilns, and consist, like them, of -various stony substances which have been more or less perfectly fused. -When we come to study the chemical composition and the microscopical -structure of lavas, however, we shall find that there are many respects -in which they differ entirely from these artificial products. - -Let us first consider the facts which are taught us concerning the -nature and origin of lavas, by a chemical analysis of them. - -[Sidenote: CHEMICAL COMPOSITION OF LAVAS.] - -Of the sixty-five or seventy chemical elements, only a very small -number occur at all commonly in lavas. Eight elements, indeed, make -up the great mass of all lavas--these are oxygen, silicon, aluminium, -magnesium, calcium, iron, sodium, and potassium. But even these -eight elements are present in very unequal proportions. Oxygen makes -up nearly one-half the weight of all lavas. Almost all the other -elements found in lavas exist in combination with oxygen, so that -lavas consist entirely of what chemists call 'oxides.' This is a most -remarkable circumstance, which, as we shall presently see, is of great -significance. The metalloid silicon makes up about one-fourth of the -weight of most lavas, and the metal aluminium about one-tenth. The -other five elements vary greatly in their relative proportions in -different lavas. - -In all lavas the substance which forms the greatest part of the mass -is the compound of oxygen and silicon, known as silica or silicic -acid. In its pure form, this substance is familiar to us as quartz, or -rock-crystal and flint. Silica is present in all lavas in proportions -which vary from one-half to four-fifths of the whole mass. Now, this -substance, silica, has the property of forming more complex compounds -by uniting with the other oxides present in lavas--namely, the oxides -of aluminium, magnesium, calcium, iron, potassium, and sodium. Silica -is called by chemists an _acid_, the other oxides in lavas are termed -_bases_, and the compounds of silica with the bases are known as -_silicates_. Hence we see that lavas are composed of a number of -different silicates--the silicates of aluminium, magnesium, calcium, -iron, potassium, and sodium. - -The above statements will perhaps be made clearer by the accompanying -table from which it will be seen that lavas are compounds in varying -proportions of six kinds of salts--namely, the silicates of alumina, -magnesia, lime, iron, potash, and soda. - -Composition of Lavas. - - Elements Binary Compounds Salts - Acid Bases - { Silicon Silica--+ - { | - O { Aluminum +--Alumina Silicate of Alumina - x { | - y { Magnesium +--Magnesia " " Magnesia - g { | - e { Calcium +--Lime " " Lime - n { | - { Iron +--Iron " " Iron - { | - { Potassium +--Potash " " Potash - { | - { Sodium +--Soda " " Soda - - -Now, in some lavas the acid constituent, or silica, is present in much -larger proportions than in others. Those lavas with a large proportion -of silica are called 'acid lavas,' those with a lower percentage of -silica, and therefore a higher proportion of the bases, are known as -the 'basic lavas.' It is convenient to employ the term 'intermediate -lavas' for those in which the proportion of silica is lower than in the -acid lavas, and the proportion of the bases is lower than in the basic -lavas. - -The acid lavas contain from 66 to 80 per cent, of silica; they are poor -in lime, magnesia, and oxide of iron, but rich in potash and soda. The -basic lavas contain from 45 to 55 per cent, of silica; they are rich in -magnesia, lime, and oxide of iron, but poor in soda and potash. In the -intermediate lavas the proportion of silica varies from 55 to 66 per -cent. - -As the basic-lavas contain a larger proportion of oxide of iron and -other heavy oxides than the acid-lavas, the former have usually a -higher specific gravity than the latter; it is, indeed, possible in -most cases to distinguish between these different varieties by simply -weighing them in water and in air. - -[Sidenote: DIFFERENT KINDS OF LAVA.] - -The basic lavas are usually of much darker colour than the add -lavas--the terms acid lavas, intermediate lavas, and basic lavas -correspond indeed pretty closely with the names trachytes, greystones -and basalt, which were given to the varieties of lavas by the older -writers on volcanoes, at a time when their chemical constitution had -not been accurately studied. Fresh lavas of acid composition are -usually nearly white in colour, intermediate lavas are of various -tints of grey, and basic lavas nearly black. It must be remembered, -however, that colour is one of the least persistent, and therefore -one of the least valuable, characters by means of which rocks can -be discriminated, and also that by exposure to the influence of the -atmospheric moisture the iron present in all lavas is affected, and -the lavas belonging to all classes, when weathered, assume reddish and -reddish-brown tints. - -Geologists have devised a great number of names for the various kinds -of lava which have been found occurring round volcanic vents in -different parts of the world, and the study of these varieties is full -of interest. For our present purpose, however, it will be sufficient -to state that they nearly all fall into five great groups, known as -the Rhyolites, the Trachytes, the Andesites, the Phonolites, and the -Basalts. The Rhyolites are acid lavas, the Basalts are basic lavas, -and the Trachytes, Andesites, and Phonolites, different kinds of -intermediate lavas, distinguished by the particular minerals which they -contain. - -Before we part from this subject of the classification of lavas -according to their chemical composition, it will be well to point -out that there exists a small group of lavas which stand quite by -themselves, and cannot be referred to either of the classes we have -indicated. They contain a smaller proportion of silica, and a much -larger proportion of magnesia and oxide of iron than the other lavas, -and may be made to constitute a small sub-group, to which we may apply -the term of 'ultra-basic lavas.' Although much less widely distributed -than the other varieties, they are, in some respects, as we shall -presently have to point out, of far greater interest to the geologist -than all the other kinds of lavas. - -[Sidenote: MINUTE STRUCTURE OF LAVAS.] - -We will now proceed to consider the facts which are brought to light -concerning the nature of lavas, when they are studied by the aid -of the microscope. Although most lavas appear at first sight to be -opaque substances, yet it is easy to prepare slices of them which are -sufficiently thin to transmit light. In such thin transparent slices -we are able to make out, by the aid of the microscope, certain very -interesting details of structure, which afford new and important -evidence bearing on the mode of origin of these rocks. - -Host lavas are capable of being melted by the heat of our furnaces; -but the different kinds of lava vary greatly in the degree of their -fusibility. The basic lavas, or those with the smallest proportion of -silica, are usually much more easily fusible than those which contain a -high percentage of silica, the add lavas. - -Now, it is a very noteworthy circumstance, that when a lava is -artificially fused it assumes on cooling very different physical -characters to those which were presented by the original rock. - -If we examine the freshly-broken surface of a piece of lava, we -shall, in most cases, find that it contains a great number of those -regular-shaped bodies which we call crystals; in some cases these -crystals are so small as to be scarcely visible to the naked eye, in -others they may be an inch or more in length. Most lavas are thus seen -to be largely made up of crystals of different minerals. The minerals -which are usually contained in lavas are quartz, the various kinds of -felspar, augite, hornblende, the different kinds of mica, olivine, and -magnetite. - -But when a piece of lava is melted in a furnace, all these crystalline -minerals disappear, and the resulting product is the homogeneous -substance which we call glass. If, as many suppose, lavas acquire the -fluidity which they possess when issuing from volcanic vents as the -result of simple fusion it is strange that artificially fused lavas do -not agree more closely in character with the natural products. - -A careful examination of different kinds of lavas, however, will -show that they vary very greatly in character among themselves. Some -lavas are as perfectly glassy in structure as those which have been -artificially fused, while others contain great numbers of crystals, -which may sometimes be of very large size. - -If we prepare thin transparent slices of these different kinds of -lavas, and examine them by the aid of the microscope, we shall find -that lavas are made up of two kinds of materials, a base or groundmass -of a glassy character, and distinct crystals of different minerals, -which are irregularly distributed through this glassy base, like the -raisins, currants, and pieces of candied peel in a cake. In some -cases the glassy base makes up the whole mass of the rock; in others, -smaller or larger numbers of crystals are seen to be scattered through -a glassy base; while in others again the crystals are so numerous that -the presence of an intervening glassy base or groundmass can only be -detected by the aid of the microscope. - -[Sidenote: STUDY OF LAVAS WITH THE MICROSCOPE.] - -If thin slices of the glassy materials of lavas be examined with high -magnifying powers, new and interesting facts are revealed. Through -the midst of the clear glassy substance cloudy patches are seen to be -diffused; and, if we examine them with a still higher power, these -cloudy patches resolve themselves into innumerable particles, some -transparent and others opaque, having very definite outlines. At the -same time fresh cloudy patches are brought into view, which can only -be resolved by yet higher powers of the microscope. In examining these -natural glasses by the aid of the microscope, we are forcibly reminded -of what occurs when the 'Milky Way' and some other parts of the heavens -are studied with a telescope. As the power of the instrument is -increased the nebulous patches are resolved into distinct stars, but -fresh nebulous masses come into view, which are in turn resolved into -stars, when higher powers of the instrument are employed. - -In the Frontispiece, No. 1 illustrates the appearance presented by -these volcanic glasses when examined with a high power of a microscope. -Through a glassy base is seen a number of diffused nebulous patches, -which are in places resolved into definite particles. - -These minute particles of definite form, which the microscope has -revealed in the midst of the glassy portions of lava, have received the -name of microliths, or crystallites. The study of the characters and -mode of arrangement of these microliths or crystallites has in recent -years thrown much new light on the interesting problems presented by -lavas. - -In some glassy lavas the microliths or crystallites, instead of being -indiscriminately diffused through the mass of the base or groundmass, -are found to be collected together into groups of very definite form. -In No. 2 of the Frontispiece we have a section of a glassy rock in -which the crystallites have united together, so as to build up groups -presenting the most striking resemblance to fronds of ferns. Around -these groups spaces of dear glass have been left by the gathering up -of the crystallites, which in other parts of the mass are seen to be -equally diffused through it. In this formation of groups of microliths -we cannot but recognise the action of those crystalline forces, which -on frosty mornings cover our windows with a mimic vegetation composed -of icy particles. - -In other cases, again, the crystallites scattered through the glassy -portions of lavas unite in radial groups about certain centres, and -thus build up globular masses to which the name of 'sphærulites' has -been given. No. 3 in the Frontispiece illustrates the formation of -these sphærulites. - -Now, a careful study of the microliths or crystallites has proved that -they are the minute elements of which those wonderfully beautiful -objects which we call crystals are built up. In some cases we can -see that the crystallites are becoming united together in positions -determined by mathematical laws, and the group is gradually assuming -the outward form and internal structure of a crystal. In other cases -crystals may be found which are undergoing a disintegrating action, -and are then seen to be made up of minute elements similar to the -crystallites or microliths of glassy rocks. - -[Sidenote: CRYSTALLITES AND CRYSTALS.] - -The conclusion is confirmed by the fact that if we take an artificially -fused lava and allow it to cool slowly, it will be found that the -glassy mass into which it has resolved itself contains numerous -crystallites. If the cooling process be still further prolonged, these -crystallites will be found to have united themselves into definite -groups, and sometimes distinct crystals are formed in the mass; under -these circumstances the rock frequently loses its glassy appearance and -assumes a stony character. - -In connection with this subject, it may be mentioned that some years -ago a very ingenious invention was submitted to trial in the Works -of the Messrs. Chance, of Birmingham. It had been suggested that if -certain lavas of easy fusibility were melted and poured into moulds, -we might thus obtain elaborately ornamented stone-work, composed of -the hardest material, without the labour of the mason. The molten rock -when quickly cooled was found to assume the form of a black glass, but -when very slowly cooled passed into a stony material. Unfortunately, -it was found that this material did not withstand the weather like -ordinary building stones, and, in consequence, the manufacture had to -be abandoned. - -Now, the study of the products of volcanoes has led geologists to -recognise the true relations between glassy and crystalline rocks. - -In the amorphous mixture of various silicates which compose a glass, -chemical affinity causes the separation of certain portions of -definite composition, and these form the microliths or elements of -which different crystalline minerals are built up. Under the influence -of the crystalline forces, there is a great shaking or agitation in -the mass, and the microliths of similar kind come together and become -united, like the fragments in Ezekiel's valley of dry bones. - -Although we cannot see this process taking place under our eyes, -in a mass of lava, yet we may study specimens in which the action -has been arrested in its different stages. In order to understand -the development of an acorn into an oak-tree, it is not necessary -to watch the whole series of changes in a particular case. A visit -to an oak-thicket, in which illustrations of every stage of the -transformation may be found, will afford us equally certain information -on the subject. - -In the same way by the examination of such a series of rock-sections -as that represented in the Frontispiece, we may understand how, in the -midst of a mass of mixed silicates constituting a natural glass, the -separation of microliths takes place; these unite into groups which are -the skeletons of crystals, and finally, by the filling up of the empty -spaces in these skeletons, complete crystals are built up. The series -of operations may, however, be interrupted at any stage, and this stage -we may have the chance of studying. - -[Sidenote: GLASSY AND CRYSTALLINE LAVAS.] - -We are able, as we shall show in a future chapter, to examine many -rock-masses that have evidently formed the reservoirs from which -volcanoes have been supplied, and others that fill up the ducts which -constituted the means of communication between these subterranean -reservoirs, and the surface of the earth. Now in these subterranean -regions the lavas have been placed under conditions especially -favourable for the action of the crystalline forces--they must have -cooled with extreme slowness, and they must have been under an enormous -pressure, produced in part by the weight of the superincumbent rocks, -and in part by the expansive force of the imprisoned steam. We are not, -therefore, surprised to find that in these subterranean regions, the -lavas, while retaining the same chemical composition, have assumed a -much more perfectly crystalline condition. In some cases, indeed, the -whole rock has become a mass of crystals without any base or groundmass -at all. - -An examination of the Frontispiece will illustrate this perfect -gradation from the glassy to the crystalline condition of lavas. No. -1 represents a glass through which microliths or crystallites of -different dimensions and character are diffused. In Nos. 2 and 3, -these crystallites have united to form regular groups. In No. 4, which -may be taken as typical of the features presented by most lavas, we -have a glassy groundmass containing microliths (a 'crypto-crystalline -base'), through which distinct crystals are distributed. Nos. 5 and 6 -illustrate the characters presented by lavas which have consolidated -at considerable depths beneath the surface; in the former we have -a mans of small crystals (a 'micro-crystalline base') with larger -crystals scattered through it; while the latter is entirely made up of -large crystals without any trace of a base or groundmass. - -Now, as all lavas are found sometimes assuming the glassy condition at -the surface, so when seen in the masses which have consolidated with -extreme slowness, and under great pressure, in subterranean regions, -the same materials are found in the condition of a rock which is built -up entirely of crystals. Chemists have found that artificial mixtures -of silicates in which soda and potash are present in considerable -quantities, have a great tendency to assume the glassy condition on -cooling from a state of fusion, and glass manufacturers are always -careful to use considerable proportions of the alkalis as ingredients, -in making glass. It is found, in like manner, that those lavas which -contain the largest portion of the silicates of soda and potash (the -'acid lavas') most frequently assume the condition of a natural glass. - -Geologists have given distinct names to the glassy and the perfectly -crystalline conditions of the different kinds of lavas, the glassy -varieties being found in masses which have cooled rapidly near the -surface, and the crystalline varieties in masses which have cooled -slowly at great depths. The names of these two conditions of the five -great classes into which we have divided lavas are as follows:-- - -[Sidenote: HIGHLY CRYSTALLINE IGNEOUS ROCKS.] - - - _Crystalline Forms._ _Lavas._ _Glassy Forms._ - - Granite Rhyolite } - Syenite Trachyte } Obsidian. - Diorite Andesite } - Miascite Phonolite } - Gabbro Basalt Tachylyte. - -As vitreous rocks have little in their general appearance to -distinguish them from one another, the glassy forms of the first -four classes of lava have not hitherto received distinct names, but -have been confounded together under the name of obsidian. If we -determine the specific gravities of rocks having the same composition -but different structures, we shall find that they become heavier in -proportion as the crystalline structure is developed in them. Thus -gabbro is heavier, but tachylyte is lighter than basalt, bulk for bulk, -though all have the same chemical composition. - -Nor are the crystals contained in lavas less worthy of careful study, -by the aid of the microscope, than the more or less glassy groundmass -in which they are embedded. Mr. Sorby has shown that the crystals found -in lavas, exhibit many interesting points of difference from those -which separate out in the midst of a mass of the same rock, when it has -been artificially melted and slowly cooled. There are other facts which -also point to the conclusion that, while the glassy groundmass of lavas -may have been formed by cooling from a state of fusion, the larger and -well-formed crystals in these lavas must have been formed under other -and very different conditions. - -The larger crystals in lavas exhibit evidence of having been slowly -built up in the midst of a glassy mass, containing crystallites and -small crystals. We can frequently detect evidence of the interruptions -which have occurred in the growth of these crystals in the concentric -zones of different colour or texture which they exhibit; and portions -of the glassy base or groundmass are often found to have been caught up -and enclosed in these crystals during their growth. - -But when we find, as in the porphyritic pitchstones, a glassy base -containing only minute crystallites, through which large and perfectly -formed crystals are distributed, we can scarcely doubt that the minute -crystallites and the larger crystals have separated from the base under -very different conditions. This is indicated by the bet that we detect -in these cases no connecting links between the embryo microliths and -the perfect crystals; and a confirmation of the conclusion is seen in -the circumstance that many of the crystals are found to have suffered -injury as if from transport, their edges and angles being rounded and -abraded, and portions being occasionally broken off from them. - -Hence we are led to conclude that the larger crystals in lavas were -probably separated from the amorphous mass in the subterranean -reservoirs beneath the volcano, and were carried up to the surface in -the midst of the liquefied glassy material which forms the groundmass -of lavas. When we come to examine these crystals more closely, we find -that certain very curious phenomena are exhibited by them which lend -powerful support to this conclusion. - -[Illustration: Fig. 7.--Minute Cavities, containing Liquids, in the -Crystals of Rocks.] - -[Sidenote: LIQUID CAVITIES IN CRYSTALS.] - -It is found convenient by geologists to designate those rocks which -have consolidated in deep-seated portions of the earth's crust -as Platonic Rocks, confining the name of Volcanic rocks to those -consolidating At the surface; but Plutonic and Volcanic Rocks shade -into one another by the most insensible gradations. - -When the crystals embedded in granitic rocks, and in some lavas, are -examined with the higher powers of the microscope, they are frequently -seen to contain great numbers of excessively minute cavities. Each of -these cavities resembles a small spirit-level, having a quantity of -liquid and a bubble of gas within it. In fig. 7 we have given a series -of drawings of these cavities in crystals as seen under a high power -of the microscope. In No. 1 a group of such cavities is represented, -one of which is full of liquid, while two others are quite empty; the -remaining cavities all contain a liquid with a moving bubble of gas. -In No. 2 two larger cavities are shown, containing a liquid and a -bubble of gas; and it will be seen from these how varied in form these -cavities sometimes are. In Nos. 3, 4 and 6 the liquid in the cavities -contains, besides the bubbles, several, minute crystals; and in No. 6 -we have a cavity containing two liquids and a bubble. - -In the largest of such cavities the bubble is seen to change its -place so as always to lie at the upper side of the cavity, when the -position of the latter is altered, just as in a spirit-level. But in -the smallest cavities the bubbles appear to be endowed with a power of -spontaneous movement; like imprisoned creatures trying to escape, these -bubbles are seen continually oscillating from side to side and from end -to end of the cavities which enclose them. In fig. 8 a minute cavity -containing a liquid and bubble is shown, the path pursued by the latter -in its wonderful gyrations being indicated by the dark line. These -cavities are exceedingly minute, and so numerous that in some crystals -there must be millions of them present; indeed, in certain cases, as -we increase the magnifying power of our microscopes, new and smaller -cavities continually become visible. It has been estimated that in some -instances the number of these minute liquid-cavities in the crystals of -rocks amounts to from one thousand millions to ten thousand millions in -a cubic inch of space. - -[Illustration: Fig. 8--Minute Liquid-cavity in a Crystal, with a moving -Bubble. (The path of the bubble is indicated by the dark line.)] - -[Sidenote: NATURE OF LIQUIDS IN CAVITIES.] - -What is the nature of the liquids which are thus imprisoned in these -cavities contained in the crystals of lavas and granites? Careful -experiments have given a conclusive answer to this question. In many -cases the liquid is water, usually containing considerable quantities -of saline matter dissolved in it. Sometimes the saline matters are -present in such abundance that they cannot all pass into solution, but -crystallise out, as in fig. 7--Nos. 3, 4, 5--where cubic crystals of -the chlorides of sodium and potassium are seen floating in the liquid; -in other cases the liquid is a hydrocarbon like the mineral oil which -is present in great abundance in deep-seated rocks in many parts of the -globe. But in some other cases the liquid contained in the cavities -of crystals is found to be one which could scarcely be anticipated -to occur under such circumstances--the gas known as carbonic add, -which under extreme pressure can be reduced to a liquid condition. In -cavities containing liquefied carbonic acid, if the rock be warmed up -to 86° or 90° Fahrenheit the bubble suddenly vanishes, sometimes with -an appearance like ebullition or boiling, as represented in fig. 9. -Now the temperature which we have indicated is the 'critical point' of -carbonic acid, and above that temperature it cannot exist in a liquid -condition, however great may be the pressure to which it is subjected. -The liquid has been converted into a gas which completely fills the -cavity. The carbonic acid in the cavities of crystals has frequently -been isolated and its nature placed beyond doubt by spectroscopic and -ordinary chemical tests. - -The presence of these liquids in the cavities of crystals clearly -proves that the latter must have been formed under enormous pressure--a -pressure sufficiently great to reduce, not only steam, but also -volatile hydrocarbons and even gaseous carbonic acid, to the bulk of a -liquid. - -[Illustration: Fig. 9.--Cavity in Crystal containing Carbonic-Acid Gas -at a temperature of 86° F., and passing from the liquid to the gaseous -condition.] - -Such conditions of enormous pressure we may infer to exist in the -deep-seated reservoirs beneath volcanoes, where, besides the weight -of the superincumbent rock-masses, we have the compressing force of -great quantities of elastic vapour held in confinement. The crystals of -which granitic rocks are entirely built up exhibit clear evidence of -having been all formed under these conditions of enormous pressure. The -glassy base or groundmass of lavas, on the other hand, presents all the -characters of materials that have cooled from a state of fusion. Most -lavas consist in part of crystals, exhibiting fluid-cavities like those -present in granite, and in part of a base, which has evidently been -formed by the cooling of a fused mass. We are therefore justified in -concluding that the crystals have been formed in subterranean recesses, -and that the groundmass or base has consolidated at the surface. The -bearing of these conclusions upon some of the great problems presented -by volcanoes we shall have occasion to point out in the sequel. - -[Sidenote: CAUSE OF MOVEMENT OF BUBBLES.] - -One of the most interesting inquiries suggested by the study of -the liquid-cavities in volcanic rocks is that of the cause of the -apparently spontaneous movement of the bubbles which we have described -as taking place in some of the smaller of them. The ingenious -experiments of Mr. Noel Hartley have suggested to Professor Stokes -an explanation which is probably the true one. It appears that these -minute globes of vapour are in such a state of unstable equilibrium as -to be affected by the smallest changes of temperature, and that the -variations in the heat of the atmosphere, due to currents of air and -the movement of warm or cold bodies through it, are sufficient to cause -the oscillation of these sensitively poised bubbles. - -The short account which we have been able to give in the foregoing -pages of the researches that have been carried on concerning the nature -of the materials ejected from volcanoes will serve to show that these -investigations have already made known many facts of great interest, -and that the farther pursuit of them is full of the highest promise. -To the scientific worker no subject is too vast for his research, no -object so minute as to be unworthy of his most patient study. In some -of our future inquiries concerning the nature of volcanic action, we -shall be led to an investigation of the phenomena displayed in the sun, -moon, comets and other great bodies of the universe; but another road -to truths of the same grandeur and importance is found, as we have -seen, in an examination of the mode of development of crystallites, and -a study of the materials contained in the microscopic cavities of the -minutest crystals. - - - - -CHAPTER IV. - -THE DISTRIBUTION OF THE MATERIALS EJECTED FROM VOLCANIC VENTS. - - -The escape of great quantities of steam and other gases from the midst -of a mass of fluid or semi-fluid lava gives rise to the formation of -vast quantities of froth or foam upon its surface. This froth or foam, -which is formed upon the surface of lava by the escape of gaseous -matters from within it, is made up of portions of the lava distended -into vesicles, in the same way that bubbles are formed on the surface -of water. It bears precisely the same relation to the liquid mass of -lava that the white crest of foam upon an advancing wave does to the -sea-water, from the bubbles of which it is formed. - -This froth upon the surface of lavas varies greatly in character -according to the nature of the material from which it is formed. In -the majority of cases the lavas consist, as we have seen, of a mass -of crystals floating in a liquid magma, and the distension of such a -mass by the escape of steam from its midst gives rise to the formation -of the rough cindery-looking material to which the name of 'scoria' -is applied. But when the lava contains no ready-formed crystals, but -consists entirely of a glassy substance in a more or less perfect state -of fusion, the liberation of steam gives rise to the formation of the -beautiful material known as 'pumice.' Pumice consists of a mass of -minute glass bubbles; these bubbles have not usually, however, retained -their globular form, but have been elongated in one direction through -the movement of the mass while it was still in a plastic state. - -The steam frequently escapes from lava with such violence that the -froth or scum on its surface is broken up and scattered in all -directions, as the foam crests of waves are dispersed by the wind -during a storm. In this way fragments of scoria or pumice are often -thrown to the height of many hundreds or thousands of feet into the -atmosphere, as we have seen is the case at Stromboli and Vesuvius. -Indeed, during violent eruptions, a continuous upward discharge of -these fragments is maintained, the ragged cindery masses hurtling one -another in the atmosphere, as they are shot perpendicularly upwards -to an enormous height and fall back into the vent; or they may rise -obliquely and describe curves so as to descend outside the orifice from -which they were ejected. - -[Sidenote: FINENESS OF VOLCANIC DUST.] - -During their upward discharge and downward fall, the cindery fragments -are by attrition continually reduced to smaller dimensions. The noise -made by these fragments, as they strike against one another in -the air during their rise and fall, is one of the most noteworthy -accompaniments of volcanic eruptions. It has been noticed that in many -cases there is a constant diminution in the size of the fragments -ejected during a volcanic outburst, this being doubtless due to the -friction of the masses as they are ejected and re-ejected from the -vent. Thus it is related by Mr. Poulett Scrope, who watched the -Vesuvian eruption of 1822, which lasted for nearly a month, that -during the earlier stages of the outburst fragments of enormous size -were thrown out of the crater, but by constant re-ejection these were -gradually reduced in size, till at last only the most impalpable dust -issued from the vent. This dust filled the atmosphere, producing in -the city of Naples 'a darkness that might be felt,' and so excessively -finely divided was it, that it penetrated into all drawers, boxes, and -the most closely fastened receptacles, filling them completely. Mr. -Whymper relates that, while standing on the summit of Chimborazo, he -witnessed an eruption of Cotopaxi, which is distant more than fifty -miles from the former mountain. The fine volcanic dust fell in great -quantities around him, and he estimated that no less than two millions -of tons must have been ejected during this slight outburst. Professor -Bonney has examined this volcanic dust from Cotopaxi, and calculates -that it would take from 4,000 to 25,000 particles to make up a grain in -weight. - -Various names have been given by geologists to the fragments ejected -from volcanic vents, which, as we have seen, differ greatly in their -dimensions and other characters. Sometimes masses of more or less -fluid lava are flung bodily to a great height in the atmosphere. -During their rise and fall these masses are caused to rotate, and in -consequence assume a globular or spheroidal form. The water imprisoned -in these masses, during their passage through the atmosphere, tends to -expand into steam, and they become more or less completely distended -with bubbles. Such masses, which sometimes assume very regular and -striking forms, are known as 'volcanic bombs.' Many volcanic bombs have -a solid nucleus of refractory materials. The large, rough, angular, -cindery-looking fragments are termed 'scoriæ.' When reduced to the -dimensions of a marble or pea they are usually called by the Italian -name of 'lapilli.' The still finer materials are known as volcanic sand -and dust. - -There are, however, two names which are frequently applied to these -fragmentary materials ejected from volcanoes, which are perhaps -liable to give rise to misconception. These are the terms 'cinders' -and 'ashes.' It must be remembered that the scoriæ or cindery-looking -masses are not, like the cinders of our fires, the product of the -partial combustion of a material containing inflammable gases, but are, -like the clinkers of furnaces and brick-kilns, portions of partially -vitrified and fused rock distended by gases. So, too, volcanic ashes -only resemble the ashes of our grates in being very finely divided; -they are not, like the latter, the incombustible residue of a mass -which has been burnt. - -[Sidenote: VOLCANIC BOMBS AND PELE'S HAIR.] - -The glassy lavas, when distended by escaping gases, give rise to the -formation of pumice, the white colour of which, as in the case of the -foam of a wave, is due to the reflection of a portion of the light in -its frequent passage from one medium to another--in this case from -air to glass, and from glass to air. The volcanic bombs formed from -glassy lavas are often of especially beautiful and regular forms. -Sometimes the passage of steam through a mass of molten glass produces -large quantities of a material resembling spun glass. Small particles -or shots of the glass are carried into the air and leave behind them -thin, glassy filaments like a tail. At the volcano of Kilauea in Hawaii -this filamentous volcanic glass is abundantly produced, and is known -as 'Pele's Hair'--Pele being the name of the goddess of the mountain. -Birds' nests are sometimes found composed of this beautiful material. -In recent years an artificial substance similar to this Pele's hair -has been extensively manufactured by passing jets of steam through the -molten slag of iron-furnaces; it resembles cotton-wool, but is made up -of fine threads of glass, and is employed for the packing of boilers -and other purposes. - -The very finely-divided volcanic dust is often borne to enormous -distances from the volcano out of which it has been ejected. The force -of the steam-current carrying the fragments into the atmosphere is -often so great that they rise to the height of several miles above -the mountain. Here they may actually pass into the upper currents of -the atmosphere and be borne away to the distance of many hundreds -or thousands of miles. Hence it is not an unusual circumstance for -vessels at sea to encounter at great distances from land falling -showers of this finely divided, volcanic dust. We sometimes meet with -this far-travelled, volcanic dust under very unexpected circumstances. -Thus, in the spring of 1875 I had occasion to visit Prof. Vom Rath of -Bonn, who showed me a quantity of fine volcanic dust which had during -the past winter fallen in considerable quantities in certain parts -of Norway. This dust, upon microscopic examination, proved to be so -similar to what was known to be frequently ejected from the Icelandic -volcanoes that a strong presumption was raised that volcanic outbursts -had been going on in that island. On returning to England I found that -the first steamer of the season had just reached Leith from Iceland, -bringing the intelligence that very violent eruptions had taken place -during the preceding months. - -[Sidenote: DISPERSION OF PUMICE AND VOLCANIC DUST.] - -This finely-divided volcanic dust is thus carried by the winds and -spread over every part of the ocean. Everyone is familiar with the -fact that pumice floats upon water; this it does, not because it is a -material specifically lighter than water, but because cavities filled -with air make up a great part of its bulk. If we pulverise pumice, we -find the powder sinks readily in water, but the rock in its natural -condition floats for the same reason that an iron ship does--because -of the air-chambers which it encloses. When this pumice is ejected from -a volcano and falls into a river or the ocean, it floats for a long -time, till decomposition causes the breaking down of the thin glassy -partitions between the air chambers, and causes the admission of water -into the latter, by which means the whole mass gets water-logged. Near -the Liparis and other volcanic islands the sea is sometimes covered -with fragments of pumice to such an extent that it is difficult for a -boat to make progress through it, and the same substance is frequently -found floating in the open ocean and is cast up on every shore. - -During the year 1878 masses of floating pumice were reported as -existing in the vicinity of the Solomon Isles, and covering the surface -of the sea to such extent that it took ships three days to force their -way through them. Sometimes these masses of pumice accumulate in such -quantities along coasts that it is difficult to determine the position -of the shore within a mile or two, as we may land and walk about on -the great floating raft of pumice. Now, recent deep-sea soundings, -carried on in the 'Challenger' and other vessels, have shown that the -bottom of the deepest portion of the ocean, far away from the land, is -covered with these volcanic materials which have been carried through -the air or floated on the surface of the ocean. To these deeper parts -of the ocean no sediments carried down by the rivers are borne, and the -remains of calcareous organisms are, in these abysses, soon dissolved; -under such conditions, therefore, almost the only material accumulating -on the sea bottom is the ubiquitous wind- and wave-borne volcanic -products. These particles of volcanic dust and fragments of pumice by -their disintegration give rise to a clayey material, and the oxidation -of the magnetite, which all lavas contain, communicates to the mass a -reddish tint. This appears to be the true origin of those masses of -'red-clay' which, according to recent researches, are found to cover -all the deeper parts of the ocean, but which probably attain to no -great thickness. - -But while some portion of the materials ejected from volcanoes may -thus be carried by winds and waves, so as to be dispersed over every -part of the land and the ocean-bed, another, and in most cases by far -the largest, portion of these ejections falls around the volcanic vent -itself. It is by the constant accumulation of these ejected materials -that such great mountain masses as Etna, Teneriffe, Fusiyama, and -Chimborazo have been gradually built up around centres of volcanic -action. - -There are cases in which the formation of volcanic mountains on a small -scale has actually been observed by trustworthy witnesses. There are -other cases in which volcanic mountains of larger size can be shown -to have increased in height and bulk by the fall upon their sides and -summits of fragmentary materials ejected from the volcanic vent. In all -cases the examination of these mountain-masses leads to the conclusion -that they are entirely built up of just such materials as we constantly -see thrown out of volcanoes during eruption. - -[Sidenote: FORMATION OF VOLCANIC MOUNTAINS.] - -Thus we are led to the conclusion that all volcanic mountains are -nothing but heaps of materials ejected from fissures in the earth's -crust, the smaller ones having been formed during a single volcanic -outburst, the larger ones being the result of repeated eruptions from -the same orifice which may, in some cases, have continued in action for -tens or hundreds of thousands of years. - -No observer has done such useful work in connection with the study of -the mode of formation of volcanic mountains as our countryman, Sir -William Hamilton, who was ambassador at Naples from 1764 to 1800, and -made the best possible use of his opportunities for examining the -numerous volcanoes in Southern Italy. - -A little to the west of the town of Puzzuoli on the Bay of Naples there -stands a conical hill rising to the height of 440 feet above the level -of the Mediterranean, and covering an area more than half a mile in -diameter. Now we have the most conclusive evidence that in ancient -times no such hill existed on this site, which was partly occupied by -the Lucrine Lake, and the fact is recognised in the name which the hill -bears, that of Monte Nuovo, or the 'New Mountain.' See fig. 10. - -Sir William Hamilton rendered admirable service to science by -collecting all the contemporary records relating to this interesting -case, and he was able to prove, by the testimony of several -intelligent and trustworthy witnesses, that during the week following -the 29th of September, 1538, this hill had gradually been formed of -materials ejected from a volcanic vent which had opened upon this site. - -[Illustration: Fig. 10. Monte Nuovo (440 ft. high) on the shores of the -Bay of Naples.] - -[Sidenote: HISTORY OF THE FORMATION OF MONTE NUOVO.] - -The records collected by Hamilton with others which have been -discovered since his death prove most conclusively the following -facts. During more than two years, the country round was affected -by earthquakes, which gradually increased in intensity and attained -their climax in the month of September 1538; on the 27th and 28th of -that month these earthquake shocks are said to have been felt almost -continuously day and night. About 8 o'clock on the morning of the 29th, -a depression of the ground was noticed on the site of the future hill, -and from this depression, water, which was at first cold and afterwards -tepid, began to issue. Four hours afterwards the ground was seen to -swell up and open, forming a gaping fissure, within which incandescent -matter was visible. From this fissure numerous masses of stone, some of -them 'as large as an ox,' with vast quantities of pumice and mud, were -thrown: up to a great height, and these falling upon the sides of the -vent formed a great mound. This violent ejection of materials continued -for two days and nights, and on the third day a very considerable hill -was seen to have been built up by the falling fragments, and this hill -was climbed by some of the eye-witnesses of the eruption. The next day -the ejections were resumed, and many persons who had ventured on the -hill were injured, and several killed by the falling stones. The later -ejections were however of less violence than the earlier ones, and seem -to have died out on the seventh or eighth day after the beginning of -the outburst. The great mass of this considerable hill would appear, -according to the accounts which have been preserved, to have been built -up by the materials which were ejected during two days and nights. - -Monte Nuovo is a hill of truncated conical form, which rises to the -height of 440 feet above the waters of the Mediterranean, and is now -covered with thickets of stone-pine. The hill is entirely made up -of volcanic scoriæ, lapilli, and dust, and the sloping sides have -evidently been produced by these fragmentary materials sliding over one -another till they attained the angle of rest; just as happens with the -earth and stones tipped from railway-waggons during the construction of -an embankment. In the centre of this conical hill is a vast circular -depression, with steeply sloping sides, which is of such depth that its -bottom is but little above the sea-level. This cup-shaped depression is -the 'crater' of the volcano, and it has evidently been formed by the -explosive action which has thrown out the materials immediately above -the vent, and caused them to be accumulated around it. - -[Illustration: Fig. 11.--Map of the district around Naples, showing -Monte Nuovo and the surrounding volcanoes of older date.] - -The district lying to the west of Naples, in which the Monte Nuovo is -situated, contains a great number of hills, all of which present a most -striking similarity to that volcano. All these hills are truncated -cones, with larger or smaller circular depressions at their summits, -and they axe entirely composed of volcanic scoriæ, lapilli, and dust. -Some of these hills are of considerably larger dimensions than the -Monte Nuovo, while others are of smaller size, as shown in the annexed -map, fig. 11. No stranger visiting the district, without previous -information upon the subject, would ever suspect the fact that, while -all the other hills of the district have existed from time immemorial, -and are constantly mentioned in the works of Greek and Roman writers, -this particular hill of Monte Nuovo came into existence less than 350 -years ago. - -[Sidenote: OLDER VOLCANOES OF THE CAMPI PHLEGRÆI.] - -The evidently fused condition of the materials of which these hills are -built up is a dear sign of the volcanic action which has taken place -in it; and this feet was so fully recognised by the ancients that they -called the district the Campi Phlegræi, or 'the Burning Fields,' and -regarded one of the circular depressions in it as the entrance to Hades. - -It is impossible for anyone to examine this district without being -convinced that all the numerous cones and craters which cover it -have been formed by the same agency as that by which Monte Nuovo was -produced. We have shown that there is the most satisfactory historical -evidence as to what that agency was. - -Now volcanic cones with craters in their centres occur in great numbers -in many parts of the earth's surface. In some districts, like the -Auvergne, the Catacecaumene in Asia Minor, and certain parts of New -Zealand, these volcanic cones occur by hundreds and thousands. In some -instances, these volcanic cones have been formed in historic times, -but in the great majority of cases we can only infer their mode of -origin from their similarity to others of which the formation has been -witnessed. - -Most of the smaller volcanic hills, with their craters, have been -thrown up during a single eruption from a volcanic fissure; but, as -Hamilton conclusively proved, the grandest volcanic mountains must -have been produced by frequent repetitions of similar operations upon -the same site. For not only are these great volcanic piles found to be -entirely composed of materials which have evidently been ejected from -volcanic vents, but, when carefully watched, such mountains are found -undergoing continual changes in form, by the addition of materials -thrown out from the vent, and falling upon their sides. - -This fact will be well illustrated by a comparison of the series of -drawings of the summit of Vesuvius which were made by Sir William -Hamilton in 1767, and which we have copied in fig. 12. During the -earlier months of that year the summit of the mountain was seen to be -of truncated form, a great crater having been originated by the violent -outbursts of the preceding year. This condition of the mountain-top -is represented in the first figure of the series. The drawing made by -Hamilton, on July 8, shows that not only was the outer rim of the great -crater being modified in form by the fall of materials upon it, but -that in the centre of the crater a small cone was being gradually built -up by the quiet ejections which were taking place. - -[Illustration: Fig. 12.--Outlines of the Summit of Vesuvius during the -Eruption of 1767.] - -[Sidenote: CHANGES IN FORM OF VESUVIUS.] - -If we compare the drawings made at successive dates, we shall find that -the constant showers of falling materials were not only raising the -edge of the great crater but were at the same time increasing the size -of the small cone inside the crater. By the end of October the small -cone had grown to such an extent that its sides were confluent with -those of the principal cone, which had thus entirely lost its truncated -form and been raised to a much greater height. The comparison of these -drawings will be facilitated by the dotted lines, which represent -the outline of the top of the mountain at the preceding observation; -so that the space between the dotted and the continuous line in each -drawing shows the extent to which the bulk of the cone had increased in -the interval between two observations. - -But, although the general tendency of the action going on at volcanic -mountains is to increase their height and bulk by the materials falling -upon their summits and aides, it must be remembered that this action -does not take place by any means continuously and regularly. Not only -are there periods of rest in the activity of the volcano, during which -the rain and winds may accomplish a great deal in the way of crumbling -down the loose materials of which volcanic mountains are largely built -up, but sudden and violent eruptions may in a very short time undo the -slow work of years by blowing away the whole summit of the mountain at -once. Thus, before the great eruption of 1822, the cone of Vesuvius, by -the almost constant ejection of ashes during several years, had been -raised to the height of more than 4,000 feet above the level of the -sea; but by the terrible outburst which then took place the cone was -reduced in height by 400 feet, and a vast crater, which had a diameter -of nearly a mile, and a depth of nearly 1,000 feet (see fig. 13), was -formed at the top of the mountain. The enormous quantity of material -thus removed was either distributed over the flanks of the mountain, -or, when reduced to a finely comminuted condition, was carried by the -wind to the distance of many miles, darkening the air, and coating the -surface of the ground with a thick covering of dust. - -[Illustration: Fig. 13.--Crater of Vesuvius formed during the eruption -in 1822. (It was nearly 1 mile in diameter and 1,000 ft. deep.)] - -[Sidenote: EARLY HISTORY OF VESUVIUS.] - -The volcano of Vesuvius, although of somewhat insignificant dimensions -when compared with the grander volcanic mountains of the globe, -possesses great interest for the student of Vulcanology, inasmuch as -being situated in the midst of a thickly populated district and in -close proximity to the city of Naples, it has attracted much attention -during past times, and there is no other volcano concerning which we -have so complete a series of historical records. The present cone of -Vesuvius, which rises within the great encircling crater-ring of Somma, -has a height of about 1,000 feet. But there is undoubted evidence that -this cone, to the top of which a railway has recently been constructed -for the convenience of tourists, has been entirely built up during the -last 1,800 years, and, what is more, that during this period it has -been many times almost wholly destroyed and reconstructed. - -Nothing is more certain than the bet that the Vesuvius upon which the -ancient Romans and the Greek settlers of Southern Italy looked, was a -mountain differing entirely in its form and appearance from that with -which we are familiar. The Vesuvius known to the ancients was a great -truncated cone, having a diameter at its base of eight or nine miles, -and a height of about 4,000 feet. The summit of this mountain was -formed by a circular depressed plain, nearly three miles in diameter, -within which the gladiator Spartacus, with his followers, were -besieged by a Roman army. There is no evidence that at this time the -volcanic character of the mountain was generally recognised, and its -slopes are described by the ancient geographers as being clothed with -fertile fields and vineyards, while the hollow at the top was a waste -overgrown with wild vines. - -[Illustration: Fig. 14.--Crater of Vesuvius in 1756. (From a drawing -made on the spot)] - -But in the year 79 a terrible and unexpected eruption occurred, by -which a vast, crateral hollow was formed in the midst of Vesuvius, and -all the southern side of the great rim surrounding this crater was -broken down. Under the materials ejected during this eruption, the -cities of Pompeii, Herculaneum, and Stabiæ were overwhelmed and buried. - -Numerous descriptions and drawings enable us to understand how in the -midst of the vast crater formed in the year 79 the modern cone has -gradually been built up. Fresh eruptions are continually increasing the -bulk, or raising the height of the Vesuvian cone. - -The accompanying drawings made by Sir William Hamilton enable us to -understand the nature of the changes which have been continually -taking place at the summit of Vesuvius. The drawing fig. 14 shows the -appearance presented by the crater in the year 1756. - -[Illustration: Fig. 15.--The Summit of Vesuvius in 1767. (From an -original drawing.)] - -[Sidenote: VESUVIUS IN MODERN TIMES.] - -At this time we see that inside the crater a series of cones had been -built up one within the other from which lava issued, filling the -bottom of the crater and finding its way through a breach in its walls, -down the side of the cone. It is evident that the ejected materials -falling on the sides of the innermost cone would tend to enlarge the -latter till its sides became confluent with the cone surrounding it, -and if this action went on long enough, the crater would be entirely -filled up and a perfect cone with only a small aperture at the top -would be produced. But from time to time, grand and paroxysmal -outbursts have occurred at Vesuvius, which have truncated the cone, -and sometimes formed great, cup-shaped cavities, reaching almost to its -base, like that shown in fig. 13. - -In 1767 the crater of Vesuvius, as shown in fig. 15, contained a single -small cone in a state of constant spasmodic outburst, like that of -Stromboli. - -[Illustration: Fig. 16.--Summit of Vesuvius in 1848.] - -In 1843, we find that the crater of Vesuvius contained three such small -cones arranged in a line along its bottom as depicted in fig. 16. - -These drawings of the summit of Vesuvius give a fair notion of the -changes which have been continually going on there during the whole of -the historical period. Ever and anon a grand outburst, like that of -1822, has produced a vast and deep crater such as is represented in -fig. 13, and then a long continuance of quiet and regular ejections -has built up within the crater small cones like those shown in figs. -14, 15 and 16, till at last the great crater has been completely filled -up, and the cone reconstructed. - -[Illustration: Fig. 17.--Outlines of Vesuvius, showing its Form at -different periods of its history.] - -[Sidenote: CHANGES IN OUTLINE OF VESUVIUS.] - -In the series of outlines in fig. 17, we have endeavoured to illustrate -the succession of changes which has taken place in Vesuvius during -historical times. In the year 79 one side of the crater-wall of the -vast mountain-mass was blown away. Subsequent ejections built up the -present cone of Vesuvius within the great encircling crater-wall of -Somma, and the form of this cone and the crater at its summit have -been undergoing continual changes during the successive eruptions of -eighteen centuries. - -What its _future_ history may be we can only conjecture from analogy. -It may be that a long continuance of eruptions of moderate energy may -gradually raise the central cone till its sides are confluent with -those of the original mountain; or it may be that some violent paroxysm -will entirely destroy the modern cone, reducing the mountain to the -condition in which it was after the great outburst of 79. On the other -hand, if the volcanic forces under Vesuvius are gradually becoming -extinct (but of this we have certainly no evidence at present), the -mountain may gradually sink into a state of quiescence, retaining its -existing form. - -The series of changes in the shape of Vesuvius, which are proved by -documentary evidence to have been going on during the last 2,000 years, -probably find their parallel in all active volcanoes. In all of these, -as we shall hereafter show, the activity of the vents undergoes great -vicissitudes. Periods of continuous moderate activity alternate with -short and violent paroxysmal outbursts and intervals of complete rest, -which may in some cases last for hundreds or even thousands of years. -During the periods of continuous moderate activity, the crater of the -volcano is slowly filled up by the growth of smaller cones within it; -and the height of the mountain is raised. By the terrible paroxysmal -outbursts the mountain is often completely gutted and its summit blown -away; but the materials thus removed from the top and centre of the -mass are for the most part spread over its aides, so that its bulk -and the area of its base are thereby increased. During the intervals -of rest, the sides of the mountain which are so largely composed of -loose and pulverulent materials are washed downwards by rains and -driven about by winds. Thus all volcanoes in a state of activity are -continually growing in size every ejection, except in the case of those -where the materials are in the finest state of subdivision, adding to -their bulk; the area of their bases being increased during paroxysmal -outbursts, and their height during long-continued moderate eruptions. - -[Sidenote: DEVIATIONS FROM CONICAL FORM.] - -We have pointed out that the conical form of volcanic mountains is -due to the slipping of the falling materials over one another till -they attain the angle at which they can rest. There are, however, some -deviations from this regular conical form of volcanoes which it may be -well to refer to. - -The quantity of rain which falls during volcanic eruptions is often -enormous, owing to the condensation of the great volumes of steam -emitted from the vent. Consequently the falling lapilli and dust often -descend upon the mountain, not in a dry state but in the condition -of a muddy paste. Many volcanic mountains have evidently been built -up by the flow of successive masses of such muddy paste over their -surfaces. Some volcanic materials when mixed with water have the -property of rapidly 'setting' like concrete. The ancient Romans and -modern Italians, well acquainted with this property of certain kinds of -volcanic dust and lapilli, have in all ages employed this 'puzzolana,' -as it is called, as mortar for building. The volcanic muds have often -set in their natural positions, so as to form a rock, which, though -light and porous, is of tolerably firm consistency. To this kind of -rock, of which Naples and many other cities are built, the name of -'tuff' or 'tufa' is applied. A similar material is known in Northern -Germany as 'trass.' - -The cause of the 'setting' of puzzolana and tufa is that rain-water -containing a small proportion of carbonic acid acts on the lime in the -volcanic fragments, and these become cemented together by the carbonate -of lime and the free silica, which are thus produced in the mass. - -When a strong wind is blowing during a volcanic outburst, the materials -may be driven to one side of the vent, and accumulate there more -rapidly than on the other. Thus lop-sided cones are formed, such -as may frequently be observed in some volcanic districts. In areas -where constant currents of air, like the trade-winds, prevail, all -the scoria-cones of the district may thus be found to be unequally -developed on opposite sides, being lowest on those from which the -prevalent winds blow, and highest on the sides towards which these -winds blow. - -[Sidenote: ANGLE OF SLOPE IN VOLCANIC CONES.] - -The examination of any careful drawing, or better still of the -photograph, of a volcanic cone, will prove that the profile of such -cones is not formed by straight lines, but by curves often of a -delicate and beautiful character. The delineations of the sacred -volcano of Fusiyama, which are so constantly found in the productions -of Japanese artists, must have familiarised everyone with the elegant -curved lines exhibited by the profiles of volcanoes. The upper slope -of the mountain is comparatively steep, often exhibiting angles of -30° to 35°, but this steepness of slope gradually diminishes, till it -eventually merges in the surrounding plains. The cause of this elegant -form assumed by most volcanic mountains is probably two-fold. In the -first place we have to remember that the materials falling upon the -flanks of the mountain differ in size and shape, and some will rest -on a steeper slope than others. Thus, while some of the materials -remain on the upper part of the mountains, others are rolling outwards -and downwards. Hence we find that those cones which are composed of -uniform materials have straight sides. But in some cases, we shall -see hereafter, there has certainly been a central subsidence of the -mountain mass, and it is this subsidence which has probably given rise -to the curvature of its flanks. - -We have hitherto considered only the methods by which the froth or -foam, which accumulates on the surface of fluid lava, is dispersed. -But in many cases not only is this scum of the lava ejected from the -volcanic vent by the escaping steam, but the fluid lava itself is -extruded forcibly, and often in enormous quantities. - -The lava in a volcanic vent is always in a highly heated, usually -incandescent, condition. Seen by night, its freshly exposed surface -is glowing red, sometimes apparently white-hot. But by exposure to -the atmosphere the surface is rapidly chilled, appearing dull red by -night, and black by day. Many persons are surprised to find that a -flowing stream of lava presents the appearance of a great mass of rough -cinders, rolling along with a rattling sound, owing to the striking of -the clinker-like fragments against each other. When viewed by night, -the gleaming, red light between these rough, cindery masses betrays the -presence of incandescent materials below the chilled surface of the -lava-stream. - -No fact in connection with lavas is more striking than the varying -degrees of liquidity presented by them in different cases. While some -lava-streams seem to resemble rivers, the material flowing rapidly -along, filling every channel in its course, and deluging the whole -country around, others would be more fitly compared to glaciers, -creeping along at so slow a rate that the fact of their movement can -only be demonstrated by the most careful observation. Even when falling -over a precipice such lavas, owing to their imperfect liquidity, form -heavy, pendent masses like a 'guttering' candle, as is shown by fig. -18, which is taken from a drawing kindly furnished to me by Capt. S. P. -Oliver, R.A. The causes of these differences in the rate of motion of -lava-streams we must proceed to consider. - -[Illustration: Fig. 18.--Cascade of Lava tumbling over a cliff in the -Island of Bourbon.] - -[Sidenote: TEMPERATURE OF LAVA-STREAMS.] - -There can be no doubt that the temperature of lavas varies greatly in -different cases. This is shown by the fact that while some lavas are in -a state of complete fusion, similar to that of the slags of furnaces, -and like the latter, such lavas on cooling form a glassy mass, others -consist of a liquid magma in which a larger or smaller number of -crystals are found floating. In these latter cases the temperature of -the magma must be below the fusing-point of the minerals which exist -in a crystalline condition in its midst. It has indeed been suggested -that the whole of the crystals in lavas are formed during the cooling -down of a completely fused mass; but no one can imagine that the -enclosed crystals of quartz, felspar, leucite, olivine, &c., have -been so formed, such crystals being sometimes more than an inch in -diameter. The microscopic examination of lavas usually enables us to -discriminate between those complete crystals which have been formed at -great depths and carried up to the surface, and the minute crystalline -particles and microliths which have been developed in the glassy mass -during cooling. Crystals of the former class, indeed, exhibit abundant -evidence, in their liquid cavities and other peculiarities, that they -have not been formed by simple cooling from a state of fusion, but -under the combined action of heat, the presence of water and various -gases, and intense pressure. - -As we have already seen, the different lavas vary greatly in their -degrees of fusibility. The basic lavas, containing a low percentage -of silica, are much more fusible than the acid lavas, which contain -a high percentage of silica. When the basic lavas are reduced to a -complete state of fusion their liquidity is sometimes very perfect, as -is the case at Kilauea in Hawaii, where the lava is thrown up into jets -and fountains, falling in minute drops, and being drawn out into fine -glassy threads. On the other hand, the less fusible acid lavas appear -to be usually only reduced to the viscous or pasty condition, which -artificial glasses assume long before their complete fusion. Of this -fact I have found many proofs in the Lipari Islands, where such glassy, -acid lavas abound. In fig. 6 (page 43) a lava-stream is represented on -the side of the cone of Vulcano. - -[Sidenote: IMPERFECTLY FLUID LAVAS.] - -This lava is an obsidian--that is to say, it is of the add type and -completely glassy--but its liquidity must have been very imperfect, -seeing that the stream has come to a standstill before reaching the -bottom of a steep slope of about 35°. In fig. 19 there is given a side -view of the same stream of obsidian, from which it will be seen that -it has flowed slowly down a steep slope and heaped itself up at the -bottom, as its fluidity was not complete enough to enable it to move on -a slighter incline. An examination of the interior of such imperfectly -fluid lavas affords fresh proofs of the slow and tortuous movements -of the mass. Everywhere we find that the bands of crystallites and -sphærulites are, by the movement of the mass, folded and crumpled and -puckered in the most remarkable manner, as is illustrated in figs. 20 -and 21. Similar appearances occur again and again among the vitreous -and semi-vitreous acid lavas of Hungary. - -[Illustration: Fig. 19.--Lava-stream (obsidian) in the Island of -Vulcano showing the imperfect liquidity of the mass.] - -[Illustration: Fig. 20.--Interior of a Rhyolitic Lava-stream in the -Island of Lipari, showing broad sigmoidal folds produced by the slow -movements of the mass.] - -[Illustration: Fig. 21.--Interior of a Rhyolitic Lava-stream in the -Island of Lipari, showing the complicated crumplings and puckerings -produced by the slow movements of the mass.] - -[Sidenote: RATE OF MOVEMENT OF VESUVIAN LAVAS.] - -But, although the temperature of lava-streams and the fusibility of -their materials may in some cases account for their condition of -either perfect liquidity or viscidity, it is clear that in other -instances there must be some other cause for this difference. Thus -it has been found that at Vesuvius the lavas erupted in modern times -have all a striking similarity to one another in chemical composition, -in the minerals which they contain, and in their structure. They are -all basic lavas, which when examined by the microscope are seen to -consist of a more or less glassy magma, in the midst of which numerous -crystals of augite, leucite, olivine, magnetite, and other minerals -are scattered. Yet nothing can be more strikingly different than the -behaviour of the lavas poured out from Vesuvius at various periods. In -some cases the lava appears to be in such a perfectly liquid condition -that, issuing from the crater, it has been described as rushing down -the slope of the cone like a stream of water, and such exceedingly -liquid lavas have in some cases flowed to the distance of several -miles from the base of the mountain in a very short time. But other -Vesuvian lavas have been in such a viscid condition that their rate -of movement has been so extremely slow as to be almost imperceptible. -Such lava-streams have continued in movement during many years, but the -progress has been so slow (often only a few inches in a day) that it -could only be proved by means of careful measurements. - -If we examine some of these Vesuvian lavas which have exhibited such -striking differences in their rate of flow, we shall find that they -present equally marked differences in the character of their surfaces. -The lava-current of 1858 was a remarkable example of a slow-flowing -stream, and its surface, as will be seen in fig. 22, which is taken -from a photograph, has a very marked and peculiar character. A -tenacious crust seems to have formed on the surface, and by the further -motion of the mass this crust or scum has been wrinkled and folded -in a very remarkable manner. Sometimes this folded and twisted crust -presents a striking resemblance to coils of rope. Precisely similar -appearances may be observed on the surface of many artificial slags -when they flow from furnaces, and are seen to be due to the same -cause, namely, the wrinkling up of the chilled surface-crust by the -movement of the liquid mass below. Lavas which present this appearance -are frequently called 'ropy lavas'; an admirable example of them is -afforded in the lava-cascade of the Island of Bourbon represented in -fig. 18 (page 93). - -But lavas in which the rate of flow has been very rapid, exhibit -quite a different kind of surface to that of the ropy lavas. The -Vesuvian lava-stream of 1872 was remarkable for the rapidity of its -flow, and its surface presents a remarkable contrast to that of the -slow-moving lava of 1858. The surface of the lava-current of 1872 is -covered with rough cindery masses, often of enormous dimensions, and -it is exceedingly difficult to traverse it, as the ragged projecting -fragments tear the boots and lacerate the skin. The appearance -presented by this lava-stream is illustrated by fig. 23, which is also -taken from a photograph. - -[Illustration: Fig. 22.--Vesuvian Lava-stream of 1858, exhibiting the -peculiar 'Ropy' Surfaces of Slowly Moving Currents. - -(_From a Photograph._)] - -[Illustration: Fig. 23.--Vesuvian Lava-stream of 1872, exhibiting the -Rough Cindery Surfaces characteristic of Rapidly Flowing Currents. - -(_From a Photograph._)] - -[Sidenote: VESUVIAN LAVA-STREAM OF 1872.] - -Now it is found that those lava-streams which move slowly and present -ropy surfaces give off but little steam during their flow, while those -lava-streams which flow more rapidly and present a rough and cindery -appearance give off vast quantities of steam. The extraordinary amount -of vapour given off from the lava-streams which flowed from Vesuvius -in 1872 is illustrated in the photograph copied in fig. 5 (facing page -24), in which the three lava-currents are each seen to be surmounted -by enormous vapour-clouds rising to the height of several thousands -of feet above them, and mingling with the column that issued from -the central vent. By the escape of this enormous quantity of steam -the surface of the lava was thrown into rugged cindery projections, -and in some places little cones were formed upon it, which threw out -small scoriæ and dust. The quantity of vapour was, in fact, so great, -that little parasitical volcanoes were formed on the surface of the -lava-stream. Some of these miniature volcanoes were of such small -dimensions that they were carried away on boards to be employed as -illustrations in the lecture-rooms of the University of Naples. - -The arrangement of the materials forced out from fissures on the -surfaces of lava-streams by the disengaged vapours and gases depends -on the degree of fluidity of the lava, and the force of the escaping -steam-jets. In very viscous lavas the materials may issue quietly, -forming great concentric masses like coils of rope; such were described -by Mr. Heaphy as occurring in New Zealand (see fig. 24). - -[Illustration: Fig. 24.--Concentric Folds on mass of cooled Lava.] - -In other cases the lava, if somewhat more liquid, may in issuing -quietly without great outbursts of steam, accumulate in great -bottle-shaped masses, which have been compared to 'petrified -fountains.' Cases of this kind have been described by Professor Dana as -occurring on the slopes of Hawaii (see fig. 25). - -[Illustration: Fig. 25.--Mass of cooled Lava formed over a spiracle on -the slopes of Hawaii.] - -[Sidenote: MINIATURE CONES ON LAVA-STREAMS.] - -When the steam escapes with explosive violence from a spiracle -('bocca') on the surface of a lava-stream, minute cinder cones, -like those described as being formed in 1872, are the result. Fig. -26 represents a group of miniature cones thrown up on the Vesuvian -lava-stream of 1855: it is taken from a drawing by Schmidt. - -[Illustration: Fig. 26.--Group of small Cones thrown up on the Vesuvian -Lava-current of 1856.] - -Some of these appear like burst blisters or bubbles, while others -are built up of scoriaceous masses which have been ejected from the -aperture and have become united while in a semi-fluid condition. Other -examples of these spiracles or bocche on the surfaces of lava-currents -may be seen in the figs. 22 and 23, which are copied from photographs. - -The facts we have described all point to the conclusion that the -presence of large quantities of water imprisoned in a mass of lava -contributes greatly to its mobility. And this conclusion is supported -by so many other considerations that it is now very generally accepted -by geologists. The condition of this imprisoned water in lavas is one -which demands further investigation at the hands of physicists. It has -been suggested, with some show of reason, that the water may exist -in the midst of the red-hot lava as minute particles in the curious -'spheroidal condition' of Boutigny, and that these flash into steam as -the lava flows along. - -Lava, when extruded from a volcanic crater in a more or less completely -fluid state, flows down the side of the cone, and then finds its way -along any channel or valley that may lie in its course, obeying in its -movements all the laws of fluid bodies. The lava-currents thus formed -are sometimes of enormous dimensions, and may flood the whole country -for many miles around the vent. - -Lava-streams have been described, which have flowed for a distance of -from fifty to a hundred miles from their source, and which have had -a breadth varying from ten to twenty miles. Some lava-streams have a -thickness of 500 feet, or even more. These measures will give some -idea of the enormous quantities of material brought from the earth's -interior by volcanic action and distributed over its surface. The mass -of lava which flowed out during an eruption off Reykjanes in Iceland, -in the year 1783, has been calculated to be equal in bulk to Mont Blanc. - -There are many parts of the earth's surface, such as the Western Isles -of Scotland and the North-east of Ireland, the Deccan of India, and -large tracts in the Rocky Mountains, where successive lava-sheets have -been piled upon one another to the height of several thousands of feet, -and cover areas of many hundreds or even thousands of square miles. - -[Sidenote: FEATURES OF LAVA-STREAMS.] - -The more fusible basic lavas are as a general rule more liquid in -character than any others, and it is these very liquid lavas that are -usually found forming plateaux built up of successive lava-streams. The -less liquid lavas, like those of Hungary and Bohemia, are not usually -found flowing to such distances from the vent, but form dome-shaped -mountain-masses. - -Lava-streams usually exhibit in their upper and under surfaces a -scoriaceous texture due to the escape of steam from the upper surface, -portions of the cindery masses so formed falling off from the end of -the stream, and being rolled over by the stream so as to form its -base. The thickness of this scoriaceous upper and lower part of a -lava-stream varies according to the quantity of steam imprisoned in -it; but all thick lava-streams have a compact central portion which -is composed of hard, solid rock. Very good examples of the internal -structure of lava-streams may sometimes be examined in the sea-cliffs -of volcanic islands. In fig. 27 we have given a copy of a drawing made -while sailing round the shores of Vulcano. The scoriaceous portions of -lava-streams are sometimes employed, as at Volvic in the Auvergne, as -a building material, or as at Neidermendig in the Eifel and in Hungary -for mill-stones; the compact portions are employed for building and -paving, and for road metal. The rock of some of the modern lava-streams -of Vesuvius is largely quarried for paving the streets of Naples. - -This solid portion of the lava-streams in slowly cooling down from its -highly-heated condition undergoes contraction, and in consequence is -rent asunder by a number of cracks. Sometimes these cracks assume a -wonderfully regular arrangement, and the rock may be broken up into -very symmetrical masses. - -[Illustration: Fig. 27.--Natural section of a Lava-stream in the Island -of Vulcano, showing the compact central portion and the scoriaceous -upper and under surfaces.] - -[Sidenote: COLUMNAR STRUCTURE OF LAVAS.] - -If we imagine a great sheet of heated material, like a lava-stream, -slowly cooling down, it is evident that the contraction which must -take place in it will tend to produce fissures breaking up the mass -into prisms. A little consideration will convince us what the form of -these prisms must be. There are only three regular figures into which -a surface can be divided, namely, equilateral triangles, squares, and -regular hexagons; the first being produced by the intersection of -sets of six lines radiating at angles of 60° from certain centres; -the second by the intersection of sets of _four_ lines radiating from -centres at angles of 90°; and the third from sets of _three_ lines -radiating from centres at an angle of 120°. It is evident that a less -amount of contractile force will be required to produce the sets of -_three_ cracks rather than those of four or six cracks; or, in other -words, the contractile force in a mass will be competent to produce the -cracks which give rise to hexagons rather than those which form squares -or triangles. This is no doubt the reason why the prisms formed by the -cooling of lava, as well as those produced during the drying of starch -or clay, are hexagonal in form. - -The hexagonal prisms or columns formed by contraction during the -consolidation of lavas vary greatly in size, according to the rate of -cooling, the nature of the materials, and the conditions affecting the -mass. Sometimes such columns may be found having a diameter of eight -or ten feet and a length of five hundred feet, as in the Shiant Isles -lying to the north of the Island of Skye; in other cases, as in certain -volcanic glasses, minute columns, an inch or two in length and scarcely -thicker than a needle, are formed; and examples of almost every -intermediate grade between these two extremes may sometimes be found. -The largest columns are those which are formed in very slowly cooling -masses. - -The columnar structure is exhibited by all kinds of lava, and indeed -in other rock-masses which have been heated by contact with igneous -masses and gradually cooled. The rocks which display the structure in -greatest perfection, however, are the basalts. - -Mr. Scrope first called attention to the fact that the upper and lower -portions of lava-streams sometimes cool in very different ways, and -hence produce columns of dissimilar character. The lower portion of the -mass parts with its heat very slowly, by conduction to the underlying -rocks, while the upper portions radiate heat more irregularly into -the surrounding atmosphere. Hence we often find the lower portions of -thick lava-streams to be formed of stout, vertical columns of great -regularity; while the upper part is made up of smaller and less regular -columns, as shown in fig. 28. - -[Illustration: Fig. 28.--Section of a Lava-stream exposed on the side -of the river Ardèche, in the south-west of France.] - -The remarkable grotto known as Fingal's Cave in the Island of Staffa -has been formed in the midst of a lava-stream such as we have been -describing; the thick vertical columns, which rise from beneath the -level of the sea, are divided by joints and have been broken away by -the action of the sea; in this way a great cavern has been produced, -the sides of which are formed by vertical columns, while the roof is -made up of smaller and interlacing ones. The whole structure bears some -resemblance to a Gothic cathedral; the sea finding access to its floor -of broken columns, and permitting the entrance of a boat during fine -weather. Similar, though perhaps less striking, structures are found in -many other parts of the globe wherever basaltic and other lava-streams -exhibit the remarkable columnar structure as the result of their slow -cooling. Portions of basaltic columns are often employed for posts -by the road-sides, as in Central Germany and Bohemia, or for paving -stones, as in Pompeii and at the Monte Albano near Rome. - -[Illustration: Fig. 29.--Portion of a Basaltic Column from the Giant's -Causeway, exhibiting both the ball-and-socket and the tenon-and-mortise -structures.] - -[Sidenote: OTHER JOINT-STRUCTURES IN LAVAS.] - -Occasionally basaltic lava-streams exhibit other curious structures in -addition to the columnar. Thus some basaltic columns are found divided -into regular joints by equidistant, curved surfaces, the joints thus -fitting into one another by a kind of ball-and-socket arrangement. -Sometimes we find processes projecting from the angles of the curved -joint-surfaces, which cause the blocks to fit together as with a tenon -and mortise. This kind of structure is admirably displayed at the -Giant's Causeway, Co. Antrim, in the North of Ireland. A portion of a -basaltic column from this locality is represented in fig. 29. - -[Illustration: Fig. 30.--Vein of green Pitchstone, at Chiaja di Luna -in the Island of Ponza, breaking up into regular columns, and into -spherical masses with a concentric series of joints.] - -While the ordinary columnar structures are very common in basalts, -the ball-and-socket and tenon-and-mortise structures are exceedingly -rare. The question of the mode of origin of these remarkable structures -has given rise to much discussion, and the opinions of geologists and -physicists are by no means unanimous upon the subject. - -Sometimes we find masses of lava traversed by curved joints, and -occasionally we find curious combinations of curved and plane joints, -giving rise to appearances scarcely less remarkable than those -presented by the columns of the Giant's Causeway. Some of the more -striking examples of this kind have been described and explained by -Professor Bonney. - -[Illustration: Fig. 31.--Illustration of the 'Perlitic structure' in -glassy Rocks. - - a. Perlltic structure, as seen in a lava from Hungary. - b. The same structure, artificially produced in Canada Balsam - during cooling. -] - -[Sidenote: PERLITIC-STRUCTURE IN LAVAS.] - -In the Ponza Islands there occurs a remarkable example of a columnar -pitchstone, which is also traversed by a member of curved concentric -joints, causing the rock to break up into pieces like the coats of an -onion. This remarkable rock-mass is represented in fig. 30. - -A very similar structure is often seen in certain glassy lavas, when -they are examined in thin sections under the microscope. Such glassy -lavas exhibit the peculiar lustre of mother-of-pearly doubtless in -consequence of the interference of light along the cracks. Lavas -exhibiting this character are known to geologists as 'perlites.' The -perlitic structure has been produced artificially by Mr. Grenville Cole -in Canada Balsam, and by MM. Fonqué and Michel Lévy, in chemically -deposited silica. See fig. 31. - -A thick lava-stream must take an enormous period to cool down--probably -many hundreds or even thousands of years. It is possible to walk over -lava-streams in which at a few inches below the surface the rock is -still red-hot, so that a piece of stick is lighted if thrust into a -crack. Lava is a very bad conductor of heat, and loose scoriæ and -dust are still worse conductors. During the eruption of Vesuvius in -1872, masses of snow which were covered with a thick layer of scoriæ, -and afterwards by a stream of lava, were found three years afterwards -consolidated into ice, but not melted. The city of Catania is -constantly supplied with ice from masses of snow which have been buried -under the ejections of Etna. - -During the cooling down of lavas, the escape of steam and various -gases gives rise to the deposition of many beautiful crystalline -substances in the cavities and on the surfaces of the lava. Deposits -of sulphur, specular-iron, tridymite, and many other substances are -often thus produced, and the colour and appearance of the rock-masses -are sometimes completely disguised by these surface incrustations, or -by the decomposition of the materials of the lava by the action of the -add gases, and vapours upon it. - -[Sidenote: SINKING OF SURFACES OF LAVA-STREAMS.] - -Very frequently the surface of a lava-stream becomes solid, while the -deeper portions retain their fluid condition; under such circumstances -the central portions may flow away, leaving a great hollow chamber or -cavern. In consequence of this action, we not unfrequently find the -upper surface of a lava-current exhibiting a depression, due to the -falling in of the solidified upper portions when the liquid lava has -flowed away and left it unsupported, as in fig. 32. - -[Illustration: Fig. 32.--Transverse section of a Lava stream. (The -dotted line indicates the original surface.)] - - - - -CHAPTER V - -THE INTERNAL STRUCTURE OF VOLCANIC MOUNTAINS. - - -Near the high-road which passes between the towns of Eger and -Franzenbad in Bohemia, there rises a small hill known as the Kammerbühl -(see fig. 33), which has attracted to itself an amount of interest -and attention quite out of proportion to its magnitude or importance. -During the latter part of the last century and the earlier years of -the present one, the fiercest controversies were waged between the -partisans of rival schools of cosmogony over this insignificant hill; -some maintaining that it originated in the combustion of a bed of coal, -others that its materials were entirely formed by some kind of 'aqueous -precipitation,' and others again that the hill was the relic of a small -volcanic cone. - -Among those who took a very active part in this controversy was -the poet Goethe, who stoutly maintained the volcanic origin of the -Kammerbühl, styling it 'a pocket edition of a volcano.' To Goethe -belongs the merit of having suggested a Very simple method by which the -controversies concerning this hill might be set at rest: he proposed -that a series of excavations should be undertaken around the hill, and -a tunnel driven right under its centre. - -[Illustration: Fig. 33.--The Kammerbühl of Kammerberg, Bohemia. - -(As seen from the south-west)] - -[Sidenote: THE KAMMERBÜHL.] - -The poet's friend, Count Caspar von Sternberg, determined to put this -project into execution. This series of excavations, which was completed -in 1837, has for ever set at rest all doubts as to the volcanic origin -of the Kammerbühl. A plug of basalt was found filling the centre of -the mass, and connected with a small lava-stream flowing down the side -of the hill; while the bulk of the hill was shown to be composed of -volcanic scoriæ and lapilli. The section fig. 34 will illustrate the -structure of the hill as revealed by these interesting excavations. - -[Illustration: Fig. 34.--Section of the Kammerbühl, in Bohemia. - -_a a._ Metamorphic rocks. _b._ Basaltic scoriæ. _c._ Solid plug -of basalt rising through the centre of the volcanic pile, _d d._ -Lava-stream composed of the same rock. _e e._ Alluvial matter -surrounding the old volcano. - -(The dotted lines indicate the probable former outline of the volcano.)] - -[Sidenote: VOLCANOES DISSECTED BY DENUDATION.] - -It can of course very seldom happen that actual mining operations, like -those undertaken in the case of the Kammerbühl, will be resorted to -in order to determine the structure of volcanic mountains. Geologists -have usually to avail themselves of less direct, but by no means less -certain, methods than that of making artificial excavations in order -to investigate the earth's crust. Fortunately it happens that what we -cannot accomplish ourselves, nature does for us. The action which we -call 'denudation' serves as a scalpel to dissect volcanic mountains -for us, and to expose their inner recesses to our view. Many portions -of the earth's surface are complete museums crowded with volcanic -'subjects,' exhibiting every stage of the process of dissection. In -some, rains and winds have stripped off the loose covering of cinders -and dust, and exposed the harder and more solid parts--the skeleton of -the mountain. In others, the work of destruction has proceeded still -further, and slowly wearing rivers or the waves of the sea may have cut -perfect, vertical sections of the mountain-mass. Sometimes the removal -of the materials of the volcanic mountain has gone on to such an extent -that its base and ground-plan are fully exposed. It only requires the -necessary skill in piecing together our observations on these dissected -volcanoes, in order to arrive at just views concerning the 'comparative -anatomy' of volcanoes. As the knowledge of the structure of animals -remained in the most rudimentary condition until the practice of -dissection was commenced, so our knowledge of volcanoes was likewise -exceedingly imperfect till geologists availed themselves of the -opportunities afforded to them of studying naturally dissected volcanic -mountains. - -In some cases we may find that the sea has encroached on the base -of a volcanic hill, till one half of it has been washed away, and -the structure of the mass to its very centre is exposed to our view. -Thus in fig. 6 (page 43), it will be seen that there lies in front of -Vulcano a peninsula called Vulcanello, consisting of three volcanic -cones, united at their base, with the lava-streams which have flowed -from them. One half of the cone on the left-hand side of the picture -has been completely washed away by the sea, and a perfect section -of the internal structure of the cone is exposed. The appearances -presented in this section are shown in the sketch, fig. 35. Some -portions of the face of this section are concealed by the heaps of -fragments which have fallen from it, but enough is visible to convince -us that three kinds of structures go to make up the cone. In the first -place, we have the loose scoriæ and lapilli, which in falling through -the air have arranged themselves in tolerably regular layers upon the -sides of the cone. - -[Illustration: Fig. 35.--Natural section or a Volcanic Cone in the -Island of Vulcano. - -_a._ Crater. _b b._ Lava-streams. _c._ Dykes which have clearly formed -the ducts, through which the lava has risen to the crater. _d d._ -Stratified volcanic scoriæ. _e._ Talus of fallen materials.] - -In the second place, we have lava-streams which have been ejected from -the crater or from fissures on the flanks of the cone, and flowed down -its sides. And thirdly, we find masses of lava filling up cracks in -the cone; these latter are called 'dykes.' Of these three kinds of -structures most volcanic mountains are built up, but in different -cases the part played by these several elements may be very unequal. -Sometimes volcanoes consist entirely of fragmentary materials, at -others they are made up of lavas only, while in the majority of -cases they have been formed by alternations of fragmentary and fluid -ejections, the whole being bound together by dykes, which are masses of -lava injected into the cracks formed from time to time in the sides of -the growing cone. - -If we direct our attention in the first place to the fragmentary -ejections, we shall find that they affect a very marked and peculiar -arrangement, which is best exhibited in those volcanic cones composed -entirely of such materials. - -[Sidenote: INTERNAL STRUCTURE OF VOLCANIC CONES.] - -Everyone who examines volcanoes for the first time will probably be -struck by the regular stratification of materials of which they are -composed. Thus the tuffs covering the city of Pompeii are found to -consist of numerous thin layers of lapilli and volcanic dust, perfectly -distinct from one another, and assuming even the arrangement which we -usually regard as characteristic of materials that have been deposited -from a state of suspension in water. The fragmentary materials in -falling through the air are sorted, the finer particles being carried -farther from the vent than the larger and heavier ones. The force -of different volcanic outbursts also varies greatly, and sometimes -materials of different character are thrown out during successive -ejections. These facts will be illustrated by fig. 36, which is a -drawing of a section exposed in a quarry opened in the side of the -Kammerbühl. In this section we see that the falling scoriæ have been -arranged in rudely parallel beds, but the regular deposition of these -has been interrupted by the ejection of masses of burnt slate torn from -the side of the vent, probably during some more than usually violent -paroxysm of the volcano. In those volcanoes which are built up of tuffs -and materials which have fallen in the condition of a muddy paste, the -perfect stratification of the mass is often very striking indeed, and -large cones are found built up of thin uniformly-spread layers of more -or less finely-divided materials, disposed in parallel succession. -Such finely-stratified tuff-cones abound in the district of the Campi -Phlegræi. - -[Illustration: Fig. 36.--Section in the side of the Kammerbühl, Bohemia. - -_a a._ stratified basaltic scoriæ. _b b._ Bands made up of fragments -of burnt slate. _c._ Stratified basaltic scoriæ. _d d._ Pseudo-dykes -occupying lines of fault.] - -[Sidenote: ARRANGEMENT OF FRAGMENTAL MATERIALS.] - -If, in consequence of any subterranean movements, fissures are produced -in the sides of the cones formed of fragmentary materials, these often -become gradually filled with loose fragments from the sides of the -fissure, and in this manner 'pseudo-dykes' are formed. An example of -such pseudo-dykes is represented in fig. 36, where the beds composing -the volcanic cone of the Kammerbühl are seen to have been broken across -or faulted, and the fissures produced in the mass have been gradually -filled with loose fragments. - -It is not difficult to imitate, on a small scale, the conditions which -exist at those volcanic vents from which only fragmentary materials -are ejected. If we take a board having a hole in its centre, into -which a pipe is inserted conveying a strong air-blast, we shall, by -introducing some light material like bran or sawdust into this pipe -cause an ejection of fragments, which will, when the board is placed -horizontally, fall around the orifice of the pipe and accumulate there -in a conical heap (fig. 37). It will be found necessary, as was shown -by Mr. Woodward, who performed the experiment before the Physical -Society, to adopt some contrivance, such as a screw, for forcing the -material into the air-pipe. If we alternately introduce materials of -different colours, like mahogany- and deal-sawdust into the pipe, these -materials will be arranged in layers which can be easily recognised, -and the mode of accumulation of the mass will be evident. By means of -a sheet of tin or cardboard we may divide this miniature volcanic cone -vertically into two portions, and if we sweep one of these away the -internal structure of the other half will be clearly displayed before -our eyes. - -In this way we shall find that the conical heap of sawdust with the -hole in its centre has a very peculiar and definite arrangement of its -materials. It is made up of a number of layers each of which slopes in -opposite directions, towards the centre of ejection and away from that -centre. These layers are thickest along the line of the circle where -the change in slope takes place, and they thin away in the direction of -the two opposite slopes. - -[Illustration: Fig. 37.--Experimental illustration of the mode of -Formation of Volcanic Cones composed of fragmental materials.] - -[Sidenote: CAUSE OF THIS ARRANGEMENT.] - -The cause of this peculiar arrangement of the materials is evident. -The sawdust thrown up by the air blast descends in a shower and tends -to accumulate in a circular heap around the orifice, the area of this -circular heap being determined by the force of the blast. Within this -circular area, however, the quantity of falling fragments is not -everywhere the same; along a circle surrounding the vent at a certain -distance, the maximum number of falling fragments will be found to -descend, and here the thickest deposit will take place. As this goes -on, a circular ridge will be formed, with slopes towards and away -from the centre of injection. As the ridge increases in height, the -materials will tend to roll down either one slope or the other, and -gradually a structure of the form shown in the figure will be piled up. -The materials sliding down the outer slope will tend to increase the -area of the base of the cone, while those which find their way down the -inner slope will fall into the vent to be again ejected. - -[Illustration: Fig. 38.--Natural section of a Tuff-cone forming the -Cape of Misenum, and exhibiting the peculiar internal Arrangement -characteristic of volcanoes composed of fragmentary materials.] - -Volcanic cones composed of scoriæ, dust, &c. are found to have exactly -the same internal structure as is exhibited by the miniature cone of -sawdust. The more or less regular layers of which they are made up dip -in opposite directions, away from and towards the vent, and thin out -in the direction of their dip (see fig. 38). In small cones the crater -or central cavity is of considerable size in proportion to the whole -mass, but as the cone grows upwards and outwards, the dimensions of -the crater remain the same, while the area of the base and the height -of the cone are continually increasing. This is the normal structure -of volcanic cones formed of fragmentary materials, though, as we shall -hereafter show, many irregularities are often produced by local and -temporary causes. - -[Illustration: Fig. 39.--Section of a small Scoria-cone formed within -the crater of Vesuvius in the year 1885, illustrating the filling-up of -the central tent of the cone by subsequent ejections.] - -In some cases the central vent of a volcanic scoria-cone may be -filled up by subsequent ejections. A beautiful example of this kind -was observed by Abich, in the case of a small cone formed within the -crater of Vesuvius in 1835, and is represented in fig. 39. - -[Illustration: Fig. 40.--Volcanic Cones composed of Scoriæ, and -breached on one side by the outflow of lava-currents.] - -[Sidenote: BREACHED CONES.] - -Many cones formed in the first instance of scoriæ, tuff, and pumice may -give rise to streams of lava, before the vent which they surround sinks -into a state of quiescence. In these cases, the liquid lava in the vent -gives off such quantities of steam that masses of froth or scoriæ are -formed, which are ejected and accumulate around the orifice. When the -force of the explosive action is exhausted, the lava rises bodily in -the crater, which it more or less completely fills. But, eventually, -the weaker side of the crater-wall yields beneath the pressure of the -liquid mass, and this part of the crater and cone is swept away before -the advancing lava-stream. Examples of such 'breached cones' abound in -Auvergne and many other volcanic districts (see fig. 40). A beautiful -example of a cone formed of pumice, which has been breached by the -outflow of a lava-stream of obsidian, occurs in the Lipari Islands, at -the Rocche Rosse. It is this locality which supplies the whole world -with pumice (see fig. 41). - -[Illustration: Fig. 41.--Campo Bianco, in the Island of Lipari. A -Pumice-cone breached by the Outflow of an Obsidian Lava-current.] - -It is often surprising to find how volcanic cones composed of loose -materials, such as tuffs, scoriæ, or pumice, retain their distinctive -forms, and even the sharpness of their outlines, during enormous -periods of time. Thus, in the scoria-cones which abound in the -Auvergne, and were, in all probability, formed before the historical -period, the sharp edges of the craters appear to have suffered scarcely -any erosion, and the cones are as perfect in their outlines as though -formed but yesterday. It is probable that the facility with which these -cindery heaps are penetrated by the rain which falls upon them is the -cause why they are not more frequently washed away. - -[Illustration: Fig. 42.--Volcanic Cones in Auvergne which have suffered -to some extent from atmospheric denudation.] - -Sometimes, however, scoria-cones are found reduced by atmospheric -waste to mere heaps of cinders, in which the position of the crater is -indicated only by a slight depression, as in fig. 42. - -[Sidenote: CONES COMPOSED OF LAVA.] - -When but little explosive action takes place at the volcanic vent, and -only fluid lava is ejected, mountains are formed differing very greatly -in character from the cones composed of fragmentary materials. - -If the lavas be of very perfect liquidity, like those erupted in the -Sandwich Islands, they flow outwards around the vent to enormous -distances. By the accumulation of materials during successive -outbursts, a conical mass is built up which has but a slight elevation -in proportion to the area of its base. Thus in Hawaii we find great -volcanic cones, composed of very fluid lavas, which have a height of -nearly 14,000 feet with a diameter of base of seventy miles. In these -Hawaiian mountains the slope of the sides rarely exceeds 6° to 8°. - -But if, on the other hand, the lavas be of much more viscid -consistency, the character of the volcanic cones which are produced by -their extrusion will be very different. The outwelling material will -tend to accumulate and heap itself up around the vent. By successive -ejections the first-formed shell is forced upwards and outwards, and -a steep-sided protuberant mass is formed, exhibiting in its interior -a marked concentric arrangement. Dr. Ed. Reyer, of Grätz, has devised -a very ingenious method for reproducing on a miniature scale the -characteristic features of these eruptions of viscid lavas. He takes a -quantity of plaster of Paris reduced to a pasty consistence, which he -forces through a hole in a board. The plaster accumulates in a great -rounded boss about the orifice through which it has been forced. If the -plaster have some colouring matter introduced into it, the mass, on -being cut across, will exhibit in the disposition of its colour-bands -the kind of action which has gone on during its extrusion, fig. 43. - -[Illustration: Fig. 43.--Experimental illustration of the Mode of -Formation of volcanic cones composed of viscid lavas.] - -[Illustration: Fig. 44.--The Grand Puy of Sarcoui, composed of -trachyte, rising between two breached scoria-cones (Auvergne).] - -There are many volcanic cones which exhibit clear evidence of having -thus been formed by the extrusion of a viscid mass of lava through -a volcanic fissure. Among such we may mention the domitic Puys of -Auvergne, fig. 44, many andesitic volcanoes in Hungary, the phonolite -hills of Bohemia, and the so-called 'mamelons' of the Island of -Bourbon. See figs. 45 and 46. When the interior of these masses is -exposed by natural or artificial sections, they are all found to -exhibit the onion-like structure which occurs in the plaster models. - -[Sidenote: INTERNAL STRUCTURE OF LAVA-CONES.] - -[Illustration: Fig. 45.--Volcanic Cone (Mamelon) composed of very -viscid lava. (Island of Bourbon.)] - -[Illustration: Fig. 46.--Another Mamelon in the Island of Bourbon, with -a crater at its summit.] - -But while some volcanoes are composed entirely of the fragmentary -ejections and others are wholly formed by successive outflows of lava, -the majority of volcanoes, especially those of larger dimensions, are -built up of alternations of these different kinds of materials. - -[Illustration: Fig. 47.--Cliff-section in the Island of Madeira, -showing how a composite volcano is built up of lava-streams, beds of -scoriæ, and dykes.] - -[Sidenote: NATURAL SECTIONS OF CONES.] - -The structure of these composite cones may be understood by an -inspection of the accompanying fig. 47, which shows the appearances -presented in a cliff on the coast of the Island of Madeira. We see -that the mass is made up of numerous layers of volcanic scoriæ, -alternating with sheets of lava. The latter, which are represented -in transverse section in the drawing, are seen to thin out on either -side, and to vary greatly in breadth. Besides the alternating masses -of scoriæ and the lava-sheets, there are seen in the section, bands -of a bright-red colour, which are represented in the drawing by black -lines. These are layers of soil, or volcanic dust, which, by the -passage of a lava-stream over their surface, have been burnt so as -to acquire a brick-red colour. These bands of red material, to which -the name of 'laterite' has been frequently applied, very commonly -occur in sections of composite volcanic cones. Crossing the whole of -the horizontally-disposed masses in the section, we find a number of -'dykes,' which are evidently great cracks filled with lava from below. -Some of these run vertically through the cliffs, others obliquely. In -some cases the lava, rising to fill a dyke, has flowed as a lava-stream -at the surface. Last of all, we must call attention to the fact that -the section exhibits evidence of great movements having taken place -subsequently to the accumulation of the whole of the materials. A -great crack has been produced, on one side of which the whole mass has -subsided bodily, giving rise to the phenomenon which geologists call a -'fault.' - -In the section, fig. 27, p. 104, copied from a drawing of a sea-cliff -in the Island of Vulcano, a transverse section of a lava-stream is -represented on a somewhat larger scale. The upper and under surface of -the lava-stream is seen to have a scoriaceous structure, but the thick -central mass is compact, and divided by regular joint-planes. This -section also illustrates the fact that, before the lava-stream flowed -down the sides of the mountain, a valley had been cut by meteoric -agencies on the flanks of the volcano, the dykes which traverse the -lower beds of tuff being abruptly truncated. - -In mountain ravines, upon the slopes of ancient volcanoes, and in the -cliffs of volcanic islands, we are often able to study the way in which -these great mountain masses are built up of alternating lava-currents, -beds of volcanic agglomerate, scoriæ, tuff and dust, and intersecting -dykes. In fig. 48, the features above described are illustrated by a -section in the sides of the great volcano of Mont Dore. - -[Illustration: Fig. 48.--Section seen at the cascade. Bains du Mont -Dore.] - -[Illustration: Fig. 49.--Section in the Island of Ventotienne, showing -a great stream of andesitic lava overlying stratified tuffs.] - -[Sidenote: SECTIONS IN THE PONZA ISLANDS.] - -In figs. 49, 50, 51, and 52, we have given drawings of portions of the -sea-cliffs in several of the Ponza Islands, a small volcanic group off -the Italian coast. - -[Illustration: Fig. 50.--Cliff on the south side of the Island of San -Stephano. - -_a._ Trachyte lava-stream, with a scoriaceous upper surface overlaid by -stratified tuffs, _b_.] - -[Illustration: Fig. 51.--The headland of Monte della Guardia, in the -Island of Ponza. - -_a._ Columnar trachyte. _b._ Stratified tuffs. _c._ Pumiceous -agglomerates. _d._ Dyke of rhyolite.] - -[Illustration: Fig. 52.--Western side of the same headland, as seen -from the north side of Luna Bay. - -_a._ Trachyte lava. _b._ Stratified tuffs. _c._ Dykes of rhyolite, with -their edges passing into pitchstone. _d._ Pumiceous agglomerate.] - -[Illustration: Fig. 53.--Sea-cliff at Il Capo, the north-east point of -Salina showing stratified agglomerates traversed by numerous dykes, the -whole being unconformably overlaid by stratified aqueous deposits.] - -Fig, 53 represents a cliff-section in the island of Salina, one of the -Liparis, exhibiting evidence that a series of volcanic agglomerates -traversed by dykes of Andesite have been denuded and covered by a -recent stratified deposit. - -[Sidenote: PART PLAYED BY DYKES IN CONE-BUILDING.] - -In the formation of these great composite cones, a minor but by no -means insignificant part is played by the dykes, or lava-filled -fissures, which are seen traversing the mass in all directions. That -dyke-fissures often reach the surface of a volcanic cone, and that -the material which injects them then issues as a lava-stream, is -illustrated by fig. 54. The formation of these cracks in a volcanic -cone, and their injection by liquid lava, must of course distend -the mountainous mass and increase its volume. If we visit the great -crater-walls of Somma in Vesuvius, and of the Val del Bove in Etna, -we shall find that the dykes are so numerous that they make up a -considerable portion of the mass. When the loose scoriæ and tuffs are -removed by denudation, these hard dykes often stand up prominently like -great walls, as represented in fig. 55. Even in such cases as these, -however, it is doubtful whether the bulk of all the dykes put together -exceeds one-tenth of that of the lavas and fragmentary materials. - -[Illustration: Fig. 54.--Section observed in the Val del Bove, Etna, -showing a basaltic dyke, from the upper part of which a lava-current -has flowed.] - -[Illustration: Fig. 55.--Basaltic Dykes projecting from masses of -stratified scoriæ in the sides of the Val del Bove, Etna.] - -Hence we are led by an examination of the internal structure of -volcanic mountains to conclude that scoriæ- and tuff-cones, and cones -formed of very liquid lavas, increase by an _exogenous_ mode of growth, -all new materials being added to them from without; in the cones formed -of very viscid lavas, on the other hand, the growth is _endogenous_, -taking place by successive accretions within it. The composite cones -owe their origin to both the _exogenous_ and the _endogenous_ modes of -growth, but in a much greater degree to the former than the latter. The -layers of scoriæ, tuff, and dust, and the successive lava-streams are -added to the mass from without, and the lava forming the dykes from -within it. - -[Sidenote: THEORY OF ELEVATION CRATERS.] - -There are doubtless cases in which, when a tuff-cone is formed, a mass -of very viscid lavas is extruded into its interior, and the mass is -distended like a gigantic bubble. But inasmuch as the very viscid lavas -do not appear to give rise to scoriæ to anything like the same extent -as the more liquid kinds, such 'cupolas,' as they have been called -by some German geologists, are probably not very numerous, and may -be regarded as constituting the exception rather than the rule. The -idea which was formerly entertained by some geologists that all great -volcanic mountains were formed of masses originally deposited in a -horizontal position, and subsequently blown up into a conical form, has -been effectually disposed of by the observations of Lyell and Scrope. - -The condition of the great fluid masses which underlie volcanic vents -is another point on which much light has been thrown by the study -of naturally-dissected volcanoes. In some cases, as was shown by -Hochstetter during his admirable researches among the New Zealand -volcanoes, the rising lavas form a great chamber for themselves in the -midst of a volcanic cinder-cone, taking the place of loose materials -which are re-ejected from the vent, or have been re-fused and absorbed -into the mass of lava itself. From this central reservoir of lava, -eruptions are kept up for some time, but when the volcano sinks -into a state of quiescence the lava slowly consolidates. In such -slowly solidified masses of lava, very beautiful groups of radiating -columns are often exhibited Northern Germany abounds with examples -of such basaltic masses, which have once formed the centres of great -cinder-cones; but in consequence of the removal of the loose materials -and the surrounding strata by denudation, these central reservoirs of -the volcanoes have been left standing above the surface, and exhibit -the peculiar arrangements of the columns formed in them during the -process of cooling. - -[Sidenote: INTRUSIVE LAVA-SHEETS.] - -But in the majority of the more solidly-built composite volcanoes no -such liquid reservoir can be formed within the volcanic cone itself. -Under these circumstances, the lavas, especially those of more liquid -character, tend to force passages for themselves among the rocks -through which they are extruded. Wherever a weak point exists, there -such lavas will find their way, and as the planes of stratification -in sedimentary rocks constitute such weak places, we constantly find -sheets of lava thus inserted between beds of aqueous origin. The areas -over which these intrusive sheets of rock sometimes extend may be very -great, but the more fusible, basic lavas (basalt, &c.) usually form -much more widely-spreading sheets than the less fusible, acid lavas. -In some cases these great intrusive sheets are found extending to a -distance of twenty or thirty miles from the centre at which they were -ejected, and they often follow the bedding of the strata with which -they are intercalated in so regular a manner, that it is difficult for -an observer to believe at first sight that they can have been formed in -the way which we have described. A closer examination will generally -reveal the fact that while these intrusive lava-sheets retain their -parallelism with the strata among which they have been intruded, over -considerable areas, yet they sometimes break across, or send offshoots -into them, as shown in fig. 56. In all cases, too, the rocks lying -above and below such sheets will be found to be more or less baked and -altered, and this affords a very convincing evidence of the intrusion -of the igneous mass between the strata so altered. - -[Illustration: Fig. 56.--Sheets of Igneous Rock (Basalt) intruded -between beds of sandstone, clay, and limestone. (Island of Skye.)] - -That in the case of most great volcanic mountains, or systems of -mountains, vast reservoirs of liquid lava must exist in the earth's -crust far below the surface, there can be little room for doubt. -Whether such fluid masses are in direct or indirect communication with -a great central reservoir, even supposing such to exist, is a totally -different question. In many cases the outburst of volcanoes in more or -less close proximity has been observed to take place simultaneously, -while in others the commencement of the eruption of one volcano has -coincided with the lapse into quiescence of another in its vicinity. -On the other hand, the remarkable case of the volcanoes of Hawaii -seems to indicate that two vents in close proximity may be supplied -from perfectly distinct reservoirs of lava. The active craters of -Mauna Loa and Kilauea are situated at the heights of 14,000 and 4,000 -feet respectively above the sea level; yet the former is sometimes -in a state of violent activity, with which the latter shows no signs -of sympathy whatever. We shall, in a future chapter, adduce evidence -that the liquid lavas in underground reservoirs may undergo various -stages of change in the enormous periods of time during which habitual -volcanic vents are supplied from them. - -We have already shown that the character assumed by a mass of fused -material in cooling varies greatly according as the cooling takes place -rapidly at the surface or slowly under enormous pressure. In the former -case a glassy base is formed containing a greater or smaller number -of crystallites or embryo crystals, in the latter the whole rock is -converted into a mass of fully-developed crystals. - -[Sidenote: CONSOLIDATION OF LAVAS AT GREAT DEPTHS.] - -The lavas which are poured out at the surface consist, as we have -seen, of a glassy magma in which a greater or smaller number of -crystals are found which have been borne up from below. The great -dykes and intrusive sheets consist for the most part of a mass of -small or imperfectly developed crystals in which a number of large and -perfectly formed crystals are embedded. Such rocks are said to have a -'porphyritic' structure. The rocks formed by the consolidation of the -liquid masses in the underground reservoirs are found to be perfectly -crystallised, the crystals impressing one another on every side and -making up the whole mass to the exclusion of any paste or magma between -them. The crystals in those rocks which have consolidated at these -vast depths exhibit evidence, in their enclosed watery solutions and -liquefied carbonic acid, of the enormous pressures under which they -must have been consolidated. The lavas, the more or less porphyritic -rocks of the dykes and sheets, and the perfectly crystalline (granitic) -rocks of the underground reservoirs pass into one another, however, by -the most insensible gradations. - -We sometimes find examples of volcanoes which, by the action of -denuding forces, have had their very foundations exposed to our view. -Such examples occur in the Western Isles of Scotland, in the Euganean -Hills near Padua in Northern Italy, and in many other parts of the -earth's surface. In these cases we are able to trace the ground-plan of -the volcanic pile, and to study the materials which have consolidated -deep beneath the surface in the very heart of the mountain. - -In studying these 'basal wrecks' of old volcanoes it is always -necessary to bear in mind that the appearance and general characters of -a volcanic rock may be completely disguised by chemical changes going -on within it. It is through want of attention to this fact that so many -mistakes were made by the Wernerian school of geologists who declared -that they could find no analogy between the basaltic rocks of the globe -and the products of active volcanoes, and were hence led to refer the -origin of the former to some kind of 'aqueous precipitation.' - -Many of the hard and crystalline marbles which are employed as -ornamental stones were originally loose masses of shells and corals, -as we easily perceive when we examine the polished faces. But these -incoherent heaps of organic _débris_ have been converted into a compact -and solid rock in consequence of the mass being penetrated by water -containing carbonate of lime in solution. Crystals of this substance -were deposited in every cavity and interstice of the mass, and thus -the accumulation of separate organisms was gradually transformed to a -material of great solidity and hardness. - -[Sidenote: FORMATION OF AMYGDALOIDS.] - -In precisely the same way loose heaps of scoriæ, lapilli, or pumice -may, by the passage through them of water containing various substances -in solution, have their vesicles filled with crystals, and thus be -converted into the hardest and most solid of rock-masses. Similarly -the scoriaceous portions of lava-streams have their vesicles filled -with crystalline substances deposited from a state of solution, and -are thus converted into a solid mass which may at first sight appear -to offer but little resemblance to the vesicular materials of recent -lava-streams. To these vesicular rocks which have their cavities filled -with crystalline substances geologists apply the name of amygdaloids -(_L. amygdalus_, an almond). The cavities in lava-rocks are usually -more or less elongated, owing to the movement of the mass while in -a still plastic state, and the crystalline materials filling these -cavities take the almond-like shape; hence the name. - -When the amygdaloids and altered fragmentary ejections of volcanoes -are studied microscopically, their true character is at once made -manifest. The exposure of faces of these altered volcanic rocks to the -weathering influences of the atmosphere, in many cases also causes -their true nature to be revealed, the crystalline materials filling -the interstices and vesicles of the mass are dissolved away by the -rain-water containing carbonic acid, and the rock regains its original -cavernous structure and appearance. But this repeated passage of water -through volcanic rock-masses may result in the removal of so large a -portion of their materials that the remainder crumbles down into the -condition of a clay or mud. - -In the basal wrecks of volcanoes, of which we have spoken, we usually -find only small and fragmentary remains of the great accumulations -of loose and scoriaceous materials which originally constituted the -bulk of the mountain mass. In the centre of the ground-plan of such -a denuded volcano we find great masses of highly crystalline or -granitic rock, which evidently occupy vast fissures broken through -the sedimentary or other rocks upon which the volcanic pile has been -reared. These highly crystalline rocks exhibit, as we have shown, -clear evidence of having been consolidated from a state of fusion -with extreme slowness and under enormous pressure, but their ultimate -chemical composition is identical with that of the lavas which have -been ejected from the volcano. - -When, as frequently happens, the volcano, after pouring out one kind -of lava for a certain period, has changed the nature of its ejections, -and given rise to materials of different composition, we find clear -evidence of the fact in studying the basal wreck or ground-plan of -the volcano. A great intrusive crystalline mass, of the same chemical -composition as the first-extruded lava, is found to be rent asunder and -penetrated by a similarly crystalline mass having the composition of -the lavas of the second period. Thus, in the volcanoes of the Western -Isles of Scotland, which are reduced by the action of denudation to -this condition of basal wrecks, we find that rhyolites, trachytes, and -andesites were ejected during the earlier periods of their history, and -basalts during the later periods. - -[Illustration: Fig. 57.--Plan of the Dissected Volcano of Mull, in the -Inner Hebrides.] - -[Illustration: - - _a_ Rocks on which the Volcano has been built up. - _b_ Great intrusive masses of acid and intermediate rocks. - _c_ Lara currents of basalt which have flowed from _d_. - _d_ Intrusive masses of gabbros & dolerite. - _e_ Lava currents which have flowed from _b_. - _f_ Volcanic tuffs and agglomerates. - -Fig. 58.--Section of the Volcano along the line _A B_.] - -[Sidenote: ANCIENT VOLCANO OF MULL.] - -We perceive on studying the ground-plan of these volcanoes that -great masses of granite, syenite, and diorite--the crystalline -representatives of the first-extruded lavas--are penetrated by -intrusions of gabbro--the granitic form of the later-ejected lavas. -These features are admirably illustrated by the ruined volcano now -constituting the Island of Mull, one of the Inner Hebrides, a plan -of which is given in fig. 57, and a section in fig. 58. This volcano -probably had a diameter at its base of nearly thirty miles, and a -height of from 10,000 to 12,000 feet, but is now reduced to a group of -hills few of which exceed 3,000 feet in height. - -From these great intrusive masses of highly crystalline rocks there -proceed in every direction great spurs or dykes, which are evidently -the radiating fissures formed during the outwelling of igneous -materials from below, injected by these fluid substances. The rock -forming these dykes is often less perfectly crystalline than that which -constitutes the centre of the mass, and we may indeed detect among the -materials of these dykes examples of every variety of structure, from -the perfectly crystalline granite to the more or less glassy substance -of lavas. Besides the vertical or oblique dykes we also find horizontal -sheets, which, passing from these central masses, have penetrated -between the surrounding strata, often, as we have seen, to enormous -distances. - -For the sake of simplicity, we have spoken of these ground-plans, or -basal wrecks of volcanoes, as constituting a flat plain; as a matter of -fact, however, the unequal hardness of the materials composing volcanic -mountains causes them to assume, under the influence of denuding -agencies, a very rugged and uneven surface. The hard crystalline -materials filling the central vent stand up as great mountain groups; -each large dyke, by the removal of the surrounding softer materials, is -left as a huge wall-like mass, while the remnants of lava-streams are -seen constituting a number of isolated plateaux. - -The great Island of Skye is the basal wreck of another volcano which -was also in eruption during Tertiary times; probably, many millions -of years ago. This immense volcano had originally a diameter at its -base of about thirty miles, and a height of 12,000 to 15,000 feet, -and must have been comparable to Etna or Teneriffe in its dimensions. -At the present time, there is nothing left of this vast pile but the -highly crystalline granites and gabbros filling up the great fissures -through which the eruption of igneous materials took place. These, worn -by denudation into rounded dome-like masses and wild rugged peaks, -constitute the Red Mountains and Coolin Hills of Skye, which rise to -the height of more than 3,000 feet above the sea-level. From these -great, central masses of crystalline rocks, innumerable radiating dykes -may be found rising through the surrounding rock-masses, with isolated -patches of the scoriæ and lapilli ejected from the volcano, which have -here and there escaped removal by denudation. Along what were the -outskirts of this great mountain-mass are found flat-topped hills, -built up of lava-streams, only small portions of which have escaped -removal by denudation. - -[Sidenote: RESERVOIRS BENEATH VOLCANOES.] - -But this wearing away of the structure of a volcanic cone by the -denuding forces may proceed even one stage farther, and we may then -have revealed for our inspection and study the mass of originally fluid -materials, from which one or more volcanoes have been fed, cooled and -consolidated in their original reservoir. There are many examples of -masses of granitic or highly crystalline rocks, having precisely the -same composition as the different varieties of lavas, which are found -lying in the midst of the sedimentary rocks, and sending off into -these rocks veins and dykes of the same composition with themselves. -No one who has carefully studied the appearances presented by volcanic -mountains in different stages of dissection, by the action of denuding -forces, can avoid recognising these great granitic masses as the cooled -reservoirs from which volcanoes have in all probability been supplied -during earlier periods of the earth's history. - -The eruption of these great masses of incandescent rock, impregnated -with water and acid gases, through strata of limestone, sandstone, -clay, coal, &c., may be expected to produce striking and wonderful -chemical changes in the latter. Nor are we disappointed in these -anticipations. Whenever we examine the sedimentary materials around -volcanic vents, we find that, in contact with the once-fused materials, -they everywhere exhibit remarkable evidences of the chemical action -to which they have been subjected. The limestones are converted into -statuary marble, the sandstones pass into quartzite, the days assume -the hardness and lustre of porcelain, while the coals have lost their -volatile ingredients and assumed a form like coke or graphite. And -these changes are found to extend in many cases to the distance of many -hundreds of yards from the planes of junction between the igneous and -the sedimentary materials. - -Among the most interesting effects resulting from the extrusion of -masses of incandescent rock, charged with water and various gases, -through beds of limestone, clay, sandstone, &c., we may mention the -production of those beautiful crystalline minerals which adorn our -museums and are so highly prized as gems. By far the larger part of -these beautiful minerals have been formed, directly or indirectly, by -volcanic agencies. - -These gems and beautiful minerals are, for the most part, substances -of every-day occurrence, which entirely owe their beauty to the -crystalline forms they have assumed. The diamond is crystallised -carbon, the ruby and sapphire are crystallised alumina, the amethyst -and a host of other gems are crystallised silica; and in almost all -cases the materials of gems are common and widely diffused, it is only -in their finely crystalline condition that they are rare and therefore -valuable. - -[Sidenote: FORMATION OF VOLCANIC MINERALS.] - -Crystals are formed during the slow deposition of a substance, either -by the evaporation of a liquid in which it is dissolved, by its -volatilisation, or its cooling from a state of fusion. In many cases it -can be shown that the formation of large and regular crystals is aided -if the work goes on with extreme slowness and under great pressure. -By sealing up various substances in tubes containing water which can -be kept at a high temperature, minute crystals of many well-known -minerals have been artificially formed by chemists. Part of the water -converted into steam has formed a powerful spring, which, reacting -upon the remainder of the liquid in the tube, has subjected it to -enormous pressure, and under these conditions of extreme pressure and -temperature, chemical actions take place of which we have no experience -under ordinary circumstances. The experiments of Mr. Hannay seem to -prove that when carbon is separated from certain organic substances -at a high temperature and under great pressure, it may crystallise in -the form of the diamond. And the recent discovery of diamonds in the -midst of materials filling old volcanic vents in South Africa seems -to show that this was in many cases the mode in which the gem was -originated. Even under the conditions which prevail at the earth's -surface, however, minute and unnoticed chemical actions taking place -during long periods of time, produce most remarkable results. This has -been well illustrated by M. Daubrée, who has shown that in the midst of -masses of concrete which the Romans built up around the hot springs of -Plombières and other localities, many crystalline minerals have been -formed, in the course of 2,000 years, by the action of the waters upon -the ingredients of the concrete. - -But most of the crystals of minerals which have been thus artificially -formed are of minute, indeed often of microscopic, dimensions. In the -underground reservoirs beneath volcanoes, however, we have all the -necessary conditions for the formation of crystals of minerals on a -far grander scale. High temperatures, pressures far greater than any -we can command at the earth's surface, the action of superheated steam -and many acid gases on the various constituents of both igneous and -sedimentary rocks, and, above all, time of almost unlimited duration; -these constitute such a set of conditions as may fairly be expected -to result in the formation of crystals, similar to those artificially -produced but of far greater size and beauty. - -If we visit those parts of the earth's surface where great masses of -fused volcanic rock have slowly cooled down in contact with sedimentary -materials, we shall not be disappointed in our expectations. Diamonds, -rubies, sapphires, emeralds, topazes, garnets, and a host of equally -beautiful, if less highly prized, crystalline substances, are found in -such situations, lying in the subterranean chemical laboratories in -which they have been formed, but now, by the action of denuding forces, -revealed to our view. - -In some cases it is not necessary to penetrate to these subterranean -laboratories in order to find these beautiful gems and other -crystallised minerals; for the steam jets which issue from volcanic -fissures carry up fragments of rock torn from the side of the vent, -and in the cavities and fissures of such ejected masses beautiful -crystallised products are often found. Such rock-fragments containing -minerals finely crystallised are found abundantly on the flanks of -Vesuvius and other active volcanoes, and among the materials of the -Laacher See and other extinct volcanoes. - -[Sidenote: FORMATION OF MINERAL-VEINS.] - -But it is not only the finely crystallised minerals and gems which -we owe to volcanic action. The various metallic minerals have nearly -all been brought from deep-seated portions of the earth's crust and -deposited upon the sides of rock-fissures by the agency of the same -volcanic forces. It is these forces which have, in the first instance, -opened the cracks through the solid rock masses; and, in the second -place, have brought the metallic sulphides, oxides, and salts--either -in fusion, in solution, or in a vaporised condition--from the -deep-seated masses within the earth, causing them to crystallise upon -the sides of the fissures, and thus form those metallic lodes and veins -which are within reach of our mining operations. - -There is still one other important class of minerals which owe the -existence, though indirectly, to volcanic agencies. The cavities of -igneous rocks, especially the vesicles formed by the escape of steam, -constitute, when filled with water, laboratories in which complicated -chemical reactions take place. The materials of the lava are gradually -dissolved and re-crystallised in new combinations. By this means the -most beautiful examples of such minerals as the agates, the onyxes, -the rock-crystals, the Iceland-spars, and the numerous beautiful -crystals classed together as 'Zeolites' have been formed. No one can -visit a large collection of crystalline minerals without being struck -with the large number of beautiful substances which have thus been -formed as secondary products from volcanic materials. - - - - -CHAPTER VI. - -THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC VENTS - - -From what has been said in the preceding chapters it will be seen that -while some of the materials ejected from volcanic vents are, by the -movements of the air and ocean, distributed over every part of the -face of the globe, another, and by far the larger, part of the matter -so ejected, accumulates in the immediate vicinity of the vent itself. -By this accumulation of erupted materials, various structures are -built up around the orifices from which the ejections take place, and -the size and character of these structures vary greatly in different -cases, according to the quantity and nature of the ejected materials, -and the intensity of the eruptive forces by which they were thrown from -the orifice. We shall proceed in the present chapter to notice the -chief varieties in the forms and characters of the heaps of materials -accumulated round volcanic vents. - -These heaps of materials vary in size from masses no bigger than a -mole-heap up to mountains like Etna, Teneriffe, and Chimborazo. The -size of volcanic mountains is principally determined by the conditions -of the eruptive action at the vent around which they are formed. If -this action exhausts itself in a single effort, very considerable -volcanic cones, like the Monte Nuovo with many similar hills in its -vicinity, and the Puys of Auvergne, may be formed; but if repeated -eruptions take place at longer or shorter intervals from the same vent, -there appears to be scarcely any limit to the size of the structures -which may, under such conditions, be formed. It is by this repeated -action from the same volcanic vent going on for thousands or even -millions of years, that the grandest volcanic mountains of the globe -have been built up. Such volcanoes have sometimes a diameter at their -base of from 30 to 100 miles, and an elevation of from 10,000 to 25,000 -feet. - -The _form_ of volcanic mountains is determined in part by the nature -of the materials ejected, and in part by the character of the eruptive -action. - -From what has been said in the preceding chapter, it will be gathered -that the volcanoes built up by ejections of fragmentary materials -differ in many striking particulars from those formed by the outwelling -of lavas from volcanic vents. In a less degree, the volcanoes composed -of the same kind of volcanic materials also vary among themselves. - -[Sidenote: CHARACTERS OF SCORIA-CONES.] - -When masses of scoriæ in a semi-fluid condition are thrown to only a -little distance above the volcanic vent, so that they have not time to -assume a perfectly solid condition before they fall round the vent, the -rugged masses of lava unite to form heaps of most irregular shape. In -such cases, the falling fragments being in a semi-plastic state, stick -to the masses below, and do not tend to roll down the sides of the -heap. Irregular heaps of such volcanic scoriæ abound on the surfaces -of lava-streams, being piled up around each 'bocca' or vent which the -steam-jets escaping from the lava-currents form at their surfaces. -Such irregular accumulations of scoriæ were observed on the lavas of -Vesuvius during the eruptions of 1822, 1855, and 1872, and have also -been described in the case of many other volcanoes. In fig. 26 (p. 101) -we have given representations of a group of such irregular scoria-cones -which was observed by Schmidt on the Vesuvian lava of 1855. It will -be seen from this drawing that there is scarcely any limit to the -steepness of the sides of such scoria-heaps, in which the materials are -in an imperfectly solidified condition when they reach the ground. - -But in the majority of cases, the scoriæ ejected from volcanic vents -are thrown to a great height, and are in a more or less perfectly -solidified condition when they fell to the ground again. In such cases -the fragments obey the ordinary mechanical laws of falling bodies, -rolling and sliding over one another, till they acquire a slope which -varies according to the size and form of the fragments. In this way -the great conical mounds are formed which are known as 'cinder-cones,' -or more properly as 'scoria-cones.' Scoria-cones usually vary in the -slope of their sides from 35° to 40° and may differ in size from mere -monticules to hills a thousand feet or more in height. Scoria-cones -of this character abound in many volcanic districts, as the Auvergne, -where they may be numbered by thousands. The materials forming such -scoria-cones vary in size from that of a nut to masses as large as a -man's head, and fragments of even larger dimensions are by no means -uncommon. - -When the lava in a volcanic vent is perfectly glassy, instead of being -partially crystalline in structure, we find not scoriæ but pumice -ejected. In such cases, as in the Lipari Islands for example, we see -cones entirely built up of pumice. Such pumice-cones resemble in -the angle of their slope (see fig. 41, facing p. 124), the ordinary -scoria-cones, but are of a brilliant white colour, appearing as if -covered with snow. - -[Sidenote: PRESERVATION OF SCORIA-CONES.] - -Ordinary scoriæ are usually of a black colour when first ejected, but -after a short time the black oxide of iron (magnetite) which they -contain, attracts the oxygen of the air and moisture, and assumes the -reddish-brown colour of iron-rust. Under such circumstances the heaps -of black material gradually acquire the red-brown colour which is -characteristic of so many of the scoria-cones around Etna, and in the -Auvergne and the Eifel. The moisture of the air, and the rain falling -upon these loose cindery heaps, cause them to decompose upon their -surfaces; the action is facilitated by the growth of the lower forms -of vegetation, such as mosses and lichens, and thus at last a soil is -produced on the surfaces of these conical piles of loose materials -which may support an abundant vegetation. Cinder- or scoria-cones are -not uncommonly found retaining in a most perfect manner their regular, -conical form, the lips of their craters being sharp and unbroken as if -the cone were formed but yesterday, while their slopes may nevertheless -be covered with a rich soil supporting abundant grass and forest-trees. -It may at first sight seem difficult to understand how a loose mass of -scoriæ could have so long withstood the action of the rain and floods, -retaining so perfectly its even slopes and sharp ridges. A little -consideration will, however, convince us that it is the very loose and -pervious nature of the materials of which scoria-cones are composed, -which tends to their perfect preservation. The rain at once sinks into -their mass, before it has time to form rivulets and streams which would -wear away their surfaces and destroy the regularity of their outlines. - -Scoria- and pumice-cones are frequently found to be acted upon by acid -vapours to such an extent that the whole of the materials is reduced -to a white pulverulent mass. In these cases the oxides of iron and the -alkalis have united with the sulphuric or hydrochloric or carbonic -acids, the compounds being carried away in solution by the rain-water -falling on the mass; the materials left are silica, the hydrated -silicate of alumina, and hydrated sulphate of lime (gypsum), all of -which are of a white colour. - -Cinder- or scoria-cones, and pumice-cones, are often found raised -by the action of winds to a greater elevation on one side than the -other, in the manner already described. One side of the cone is often -seen to be more or less completely swept away by an outwelling stream -of lava, and thus breached cones are formed (see fig. 40, p. 123). -Not unfrequently we find a number of cones which are united more or -less completely at their bases, as in Vulcanello (fig. 6, p. 43), the -several vents being so near together that their ejections have mingled -with one another. Cones composed entirely of fragmentary materials -often show an approach to the beautifully curved slopes which we have -described as being so characteristic of volcanoes, as may be seen in -fig. 41, facing p. 124. In the case of scoria- and pumice-cones this -curvature is probably due to the rolling downnwards and outwards of the -larger fragments. - -We have already pointed out that with the scoriæ there are often -ejected fragments torn from the sides of the volcanic vents. Sometimes -such fragments are so numerous as to make up a considerable portion of -the mass of the volcanic cones. Thus in the Eifel we find hills, of -by no means insignificant size, completely built up of small scoriæ -and broken fragments of slate torn from the rocks through which the -volcanic fissures have been opened. Occasionally we see that few or no -scoriæ have been ejected, and the volcanic vents are surrounded simply -by heaps of burnt slate. - -The smaller fragmentary materials ejected from volcanic vents--such -as lapilli and dust--rest in heaps, having a different angle of slope -from those formed by scoriæ. In many cases, as we have seen, such -finely-divided materials descend in the condition of mud, which flows -evenly over the surface of the growing cone and consolidates in beds of -very regularly stratified 'tufa' or 'tuff.' - -[Sidenote: CHARACTERS OF TUFF-CONES.] - -The 'tuff-cones' thus formed differ in many important respects from the -scoria-cones already described. The slope of their sides varies from -15° to 30°, and is almost always considerably less than in scoria- and -pumice-cones. The tuff-cones undergo much more rapid degradation from -rain and moisture than do the scoria-cones; for, though the materials -of the former 'set,' as we have seen, into a substance of considerable -hardness, yet this substance, being much less pervious to water than -the loose scoria heaps, permits of the formation of surface-streams -which furrow and wear away the sides of the cones. Sometimes the sides -of the crater are found to be almost wholly removed by atmospheric -denudation, and only a shallow depression is found occupying the -site of the crater; such a case is represented in fig. 59. We not -unfrequently find the whole slopes of such cones to be traversed by a -series of radiating grooves passing from the summit to the base of the -mountains, these channels being formed by water, which has collected -into streams, flowing down the slopes of the mountains. The volcanic -cone, under these circumstances, frequently presents the appearance of -a partially opened umbrella. Owing to the impervious character of the -materials composing tuff-cones, their craters are frequently found to -be occupied by lakes. - -[Illustration: Fig. 59.--Summit of the volcano of Monte Sant' Angelo in -Lipari exhibiting a crater with walls worn down by denudatioh.] - -Tufas have usually a white or yellowish-brown colour, and these are the -colours exhibited by the cones composed of this material before they -become covered by vegetation. Tufas scoriæ, and lavas usually crumble -down to form a very rich soil, and many of the choicest wines are -produced from grapes grown on the fertile slopes of volcanic mountains. -When, however, as not unfrequently happens, the materials are finely -divided and incoherent, they are so easily driven about by the winds -that cultivation of any kind is rendered almost impossible. In the -Islands of Stromboli and Vulcano the gardens have to be surrounded by -high fences to prevent them from being overwhelmed by the ever-shifting -masses of volcanic sand. - -[Sidenote: CHARACTERS OF LAVA-CONES.] - -There are some cones which are composed in part of scoriæ and in part -of tufa. Hence we are sometimes at a loss whether to group them with -the one class of cones or the other. But in the majority of cases, -scoria- and tuff-cones present the sufficiently well-marked and -distinctive characters which we have described. - -Lava-cones differ quite as greatly in their forms as do the cones -composed of fragmentary materials, the variations being principally -determined by the degree of liquidity of the lavas. - -We sometimes find that outwelling masses of lava, when issuing in small -quantities from a vent, accumulate in cauliflower-shaped masses, or -sometimes in the form of a column, or bottle. Professor J. D. Dana -describes many such fantastically-formed masses of lava as being found -in Hawaii, one of which is represented in fig. 25 (p. 100). - -When the lava issues from the vent in great quantities it tends to -flow on all sides of it, and to build up a great conical heap above -the orifice. If the lava be very liquid it flows to great distances, -resting at a very slight slope. Thus we find that the volcanoes of -Hawaii have been built up of successive ejections of very liquid lava, -which have formed cones having a slope of only 6° to 8°, but of such -enormous dimensions that the diameter of their bases is seventy miles -and their height 14,000 feet. - -[Illustration: Fig. 60.--Outlines of Lava-cones. - -1. Mauna Loa, in Hawaii. Composed of very fluid lava. 3. The -Schlossberg of Teplitz, Bohemia. Composed of very imperfectly fluid or -viscid lava.] - -If, on the other hand, the lava be viscid, or very imperfectly liquid -in character, it tends to accumulate immediately around the vent; fresh -ejections force the first extruded matter outwards, in the manner so -well illustrated by Dr. Reyer's experiments, and at last a more or less -steep-sided bulbous mass is formed over the vent. Such bulbous masses, -composed of imperfectly fluid lavas, occur in many volcanic districts, -and constitute hills of considerable size. From the tendency of matters -thus extruded to choke up the vents, however, these volcanoes composed -of viscid lavas cannot be expected to attain the vast dimensions -reached by some of those composed of very liquid lavas. The difference -in the forms of lava-cones composed of very fluid or of somewhat -viscid materials is illustrated in fig. 58. When the interior of such -steep-sided volcanic mountains composed of viscid materials is exposed -by the action of denuding forces, the peculiar internal structure we -have described is displayed by them. In the Chodi-Berg of Hungary, -a great bulbous mass of andesitic rock, this endogenous structure is -admirably displayed. It is also well seen in the excavation of the hill -of the Grand Sarcoui, a similar mass, composed of altered trachyte, -which has been erupted in the midst of a scoria-cone in the Auvergne. -See fig. 44 (p. 126). - -[Sidenote: CHARACTERS OF COMPOSITE CONES.] - -Most of the great volcanic mountains of the globe belong to the -class of 'composite cones,' and are built up by alternate ejections -of fluid lava and fragmentary materials. The slope of the sides in -such composite cones is subject to a wide range of variation, being -determined in part by the degree of liquidity of the lavas, in part -by the nature of the fragmentary materials ejected, and in part by -the proportions which the fragmentary and lava-ejections bear to one -another. - -But there is another set of causes which tends to modify the form -and character of these composite, volcanic cones. As we have already -pointed out, the sides of such cones are liable to be rent asunder from -time to time, and the fissures so produced are injected with masses of -liquid lava from below. These fissures, rent in the sides of volcanic -cones, often reach the surface and eruptive action takes place, giving -rise to the formation of a cone, or series of cones, upon the line -of the fissure (fig. 61). Such small cones thrown up on the flanks -of a great volcanic mountain are known as 'parasitic cones'; though -subordinate to the great mountain mass, they may be in themselves of -considerable dimensions. Among the hundreds of parasitic cones which -stud the flanks of Etna, there are some which are nearly 800 feet in -height. - -[Illustration: Fig. 61.--Diagram illustrating the formation of -Parasitic Cones along lines of fissure formed on the flanks of a great -volcanic mountain.] - -[Illustration: Fig. 62.--Outline of Etna, as seen from Catania.] - -[Sidenote: FORMATION OF PARASITIC CONES.] - -The building up of parasitic cones upon the flanks of a volcanic -mountain tends, of course, to destroy its regular conical form. This -may be well seen in Etna, which, by the accumulation of materials -upon its flanks, has become a remarkably 'round-shouldered' mountain. -(See figs. 62 and 63.) At the same time it must be remembered that -materials erupted from the central vent tend to fill up the hollows -between these parasitic cones, and thus to restore to the mountain its -regularly conical form. - -[Illustration: Fig. 63.--Outline of Etna, as seen from the Val del -Bronte.] - -[Illustration: Fig. 64.--Plan of the Volcano forming the Island of -Ischia. - - _a, a, a._ The semi-circular crater-ring of Epomeo. - _b, c, d._ Lava-currents which have flowed from the principal crater. - _e, f, g, h._ Plateaux formed by ancient lava-currents. - _k._ Montagnone. } - _l._ Monte Rotaro. } - _m._ Monte Tabor. } Parasitic cones and craters on the slopes - _n._ Castiglione. } of the mountain. - _o._ Lago di Bagno. } - _p._ The Cremate. } - _r._ Lava-stream of the Arso, which flowed from the Cremate in 1301. - _x, x, x._ Raised beaches on the shores of the island, showing that it - has recently undergone elevation. -] - -The Island of Ischia is a good example of a great volcanic cone the -flanks of which are covered with numerous small parasitic cones. While -the great central volcano has evidently been long extinct, and one -side of its crater-wall is completely broken down, some of the small -parasitic cones around its base have been formed within the historical -period--one of them as recently as the year 1302. Fig. 64 is a plan of -the Island of Ischia, showing the numerous parasitic cones scattered -over the slopes of the principal cone. - -[Illustration: Fig, 65.--A primary Parasitic Cone with a secondary one -at its base--Ischia. - -_a._ Monte Rotaro. _b._ Monte Tabor. _c._ Lava-stream flowing from the -latter.] - -In one case we find that a parasitic cone, the Monte Rotaro, has itself -a similar smaller cone, which is parasitic to it, at its foot; this -secondary parasitic cone gives off a small lava-stream of trachyte, -which has flowed down to the sea. (See fig. 65.) - -[Illustration: Fig. 66.--Scoria-cone near Auckland, New Zealand, with a -lava-current flowing from it. - -The strata beneath the volcanic cone are exposed in the sea-cliff, and -exhibit proofs of depression having taken place.] - -[Illustration: Fig. 67.--Section of rocks below the ancient triassic -volcano of Predazzo in the Tyrol. - -The position of the strata _a b c_, etc., indicates a central -subsidence.] - -[Sidenote: SUBSIDENCE BENEATH VOLCANIC VENTS.] - -Most great volcanic mountains exhibit a tendency towards a subsidence -of their central portions, which may take place either during or -subsequently to their period of activity. When we examine the strata -upon which a volcano has been built up, but which are now exposed to -our study by denuding forces, we usually find that they incline towards -the centre of the eruptive activity. (See figs. 66 and 67.) Two causes -may contribute to bring about this result. In the first instance, -we must remark that the piling up of materials around the volcanic -vent causes the subjacent strata to be subjected to a degree of -pressure far is excess of that which acts upon the surrounding rocks. -And secondly, it must be borne in mind that the continual removal -of material from below the mountain must tend to the production of -hollows, into which the overlying strata will sink. The effect of this -central subsidence is to give to the flanks of volcanic cones those -beautifully curved outlines which constitute so striking a feature in -Vesuvius (see fig. 17, p. 87), Fusiyama (see fig. 77, No. 1, facing p. -178), and many other volcanic mountains. - -There seems, at first sight, to be scarcely any limit to the dimensions -which these great composite volcanic cones may attain: the lateral -eruptions tending to enlarge the area of the base of the mountain, -and, by the injection of the fissures, to knit together and strengthen -its structure, while the central eruptions continually increase the -elevation of the mass. Great, however, as is the force which is -concerned in the production of our terrestrial volcanoes, it has its -limits; and, at last, the piling up of materials will have gone on -to such an extent, that the active forces beneath the volcano are no -longer competent either to raise materials to the elevated summit of -the mountain or to tear asunder its strengthened and fortified flanks. -Under these circumstances, the volcanic forces, if they have not -already exhausted themselves, will be compelled to find weak places in -the district surrounding the volcano, at which fissures may be produced -and the phenomena of eruption displayed. - -[Sidenote: SHIFTING OF VOLCANIC FOCI.] - -Some volcanic cones exhibit evidence that during the series of -eruptions by which they have gradually been built up, the centre of -volcanic action has shifted to another point within the mountain. Thus -Lyell has shown, in the case of Etna, that during the earlier periods -in the history of the mountain the piling up of materials went on -around a centre which is now situated at a distance of nearly four -miles from the present focus of eruptive activity. Some of our old -British volcanoes, of which the denuded wrecks exist in the Western -Isles of Scotland, show similar evidence of a shifting of the axis of -eruption. - -One of the most conspicuous features of a volcanic cone is the great -depression or crater found at its summit. In describing the internal -structure of volcanic cones, we have seen how these craters are -produced and acquire their inverted conical form, by the slipping and -rolling back of materials towards the centre of eruptive action. - -Almost all volcanic cones exhibit craters, but in those which are -formed entirely by the outwelling of viscid lavas the central -depression is often slight and inconspicuous, and occasionally -altogether wanting. It frequently happens, however, that eruptive -action has ceased at the centre of a volcano, and its summit-crater -may by denudation be entirely destroyed, while new and active craters -are formed upon its flanks. Stromboli furnishes us with an admirable -example of this kind (see fig. 1, facing p. 10). Other volcanoes may -exhibit several craters, one at the summit of the mountain and others -upon its flanks. Of this we find a good example in Vulcano (fig. 6, p. -43). - -[Illustration: Fig. 68.--Cotopaxi (19,600 feet), as seen from a -distance of ninety miles.] - -When a volcano has been built up by regular and continuous eruptions -from the same volcanic vent, the size of the crater remains the same, -while the volcano continually grows in height and in the diameter of -its base. The size of the crater will be determined by the eruptive -force at the volcanic centre, the size of the mountain by the duration -of the volcanic activity and the quantity of material ejected. In the -earliest stage of its history, such a volcano will resemble Monte -Nuovo, which has a crater reaching down almost to the base of the -mountain; in the later stages of its history, such a volcano will -resemble Cotopaxi (fig. 68) and Citlaltepetl (fig. 69), in which the -crater, though of far greater absolute dimensions than that of Monte -Nuovo, bears but a small proportion to the vast cone at the summit of -which it is situated. - -[Illustration: Fig. 69.--Citlaltepetl, or the Pic d'Orizaba, in Mexico -(17,370 feet), as seem from the forest of Xalapa.] - -[Sidenote: ORIGIN OF VOLCANIC CRATERS.] - -In the great majority of volcanoes, however, eruptive action does not -go on by any means regularly and continuously, but terrible paroxysmal -outbursts occur, which suddenly enlarge the dimensions of the crater to -an enormous extent. - -In the year 1772, there occurred a volcanic eruption in the Island -of Java, which is perhaps the most violent and terrible that has -happened within the historical period. A lofty volcanic cone, called -Papandayang, 9,000 feet high, burst into eruption, and, in a single -night, 30,000,000,000 cubic feet of materials were thrown into the -atmosphere, falling upon the country around the mountain where no less -than forty villages were buried. After the eruption, the volcano was -found to have been reduced in height from 9,000 to 5,000 feet, and -to present a vast crater in its midst, which had been formed by the -ejection of the enormous mass of materials. - -Many similar cases might be cited of the removal of a great part of a -mountain-mass by a sudden, paroxysmal outburst. In some cases, indeed, -the whole mass of a mountain has been blown away during a terrific -eruption, and the site of the mountain is now occupied by a lake. This -is said to have been the case with the Island of Timor, where an active -volcano, which was visible from a distance of 300 miles at sea, has -entirely disappeared. - -The removal of the central portion of great volcanic mountains by -explosive action, gives rise to the formation of those vast, circular, -crater-rings of which such remarkable examples occur in many volcanic -districts. These crater-rings present a wall with an outer slope -agreeing with that of the volcanic cone of which they originally formed -a part, but with steep inner cliffs, which exhibit good sections of the -beds of tuff, ash, and lava with the intersecting dykes of which the -original volcano was built up. Near Naples, one of these crater-rings, -with sloping outer sides and steep inner ones, is employed to form the -royal game-preserve of Astroni, the only entrance to the crater being -closed by gates. - -[Sidenote: FORMATION OF CRATER-LAKES.] - -As these crater-rings are usually composed of materials more or -less impervious to water, they often become the site of lakes. The -beautiful circular lake of Laach, in the Rhine Provinces, with the -numerous similar examples of Central Italy--Albano, Nemi, Bracciano, -and Bolsena--the lakes of the Campi Phlegræi (Agnano, Avernus, &c.), -and some similar lakes in the Auvergne, may be adduced as examples of -crater-rings which have become the site of lakes. - -[Illustration: Fig. 70.--Lac Paven, in the Auvergne. - -_a._ Scoriæ. _b._ Basalt.] - -One of the most beautiful of the crater-lakes in the Auvergne is -Lac Paven (fig. 70), which lies at the foot of a scoria-cone, Mont -Chalme, and is itself surrounded by masses of ejected materials. The -crater-lake of Bagno, in Ischia (fig. 71), has had a channel cut -between it and the sea, so that it serves as a natural harbour. The -lake of Gustavila, in Mexico (fig. 72), is an example of a crater-lake -on a much larger scale. - -In many of these crater-rings the diameter of the circular space -enclosed by them is often very great indeed as compared with the height -of the walls. - -[Illustration: Fig. 71.--The crater-lake called Lago del Bagno, in -Ischia, converted into a harbour.] - -[Illustration: Fig. 72.--Lake of Gustavila, in Mexico. - -(The terraces round the lake have been artificially formed.)] - -[Sidenote: DIMENSIONS OF CRATER-LAKES.] - -Two of the largest crater-rings in the world are found in Central -Italy, and are both occupied by lakes, the circular forms of which -must strike every observer. The Lago di Bracciano, which lies to -the north-west of Rome, is a circular lake six and a half miles in -diameter, surrounded by hills which at their highest point rise to the -height of 1,486 feet above the sea, while the surface of the waters -of the lake is 640 feet above the sea-level. The Lago di Bolsena is -somewhat less perfectly circular in outline than the Lago di Bracciano; -it has a length from north to south of ten-and-a-quarter miles and a -breadth from east to west of nine miles; the surface of the waters of -this lake is 962 feet above that of the waters of the Mediterranean. -The lake of Bolsena, like that of Bracciano, is surrounded by hills -composed of volcanic materials; the highest points of this ring of -hills rise to elevations of 684, 780, and 985 feet respectively above -the waters of the lake. - -In these great circular lakes of Bolsena and Bracciano, as well as in -the smaller ones of Albano, Nemi, and the lakes of Frascati in the same -district, the vast circular spaces enclosed by them, the gradual outer -slope of the ring, and the inner precipices which bound the lake, all -afford evidence of the explosive action to which they owe their origin. - -But while the vast crater-rings we have mentioned are frequently found -to be occupied by lakes, there are many other similar crater-rings -which remain dry, either from the materials of which they are composed -being of more pervious character, or from rivers having cut a channel -through the walls of the crater, in this way draining off its waters. - -Thus in the Campi Phlegræi, while we have the craters of Agnano and -Avernus forming complete circular lakes, Astroni has only a few -insignificant lakelets on its floor, and the Pianura, the Piano di -Quarto, which have each a diameter of three or four miles, with many -others, remain perfectly dry. In the vicinity of the great crater-lakes -of Central Italy we find the crater-ring of the Vallariccia, which has -evidently once been a lake but is now drained, its floor being covered -with villages and vineyards. - -[Sidenote: CRATER-RINGS SURROUNDING CONES.] - -A comparison of these vast crater-rings leads us to the conclusion that -in the majority of cases, if not in every instance, they are composed -almost entirely of volcanic tuff and dust. In the case of the more -solidly-built composite volcanic cones, the volcanic forces, as we have -seen, produce fissures in the mass, and along these fissures parasitic -cones are thrown up, the tension of the mass of imprisoned vapours -below the mountain being thus from time to time relieved. But in the -case of a volcanic cone composed of loose fragmentary materials, such -temporary relief is impossible. The cracks, as soon as they originate, -will be filled up and choked by the falling in of materials from above -and at their sides. In this way the eruptive action will be continually -repressed, till at last the imprisoned vapours acquire such a high -state of tension that the outburst, when it occurs, is of the most -terrible character, and the whole central mass of the volcano is blown -into the air. It may often seem surprising that the ejection of such -vast masses of material from the centre of a volcanic cone does not -effect more in the way of raising the height of the crater-walls. But -it must be remembered that, in the case of craters of such vast area, -the majority of the ejected materials must fall back again within -its circumference. By repeated ejections these materials will at last -be reduced to such an extreme state of comminution that they can be -borne away by the winds, and spread over the country to the distance of -hundreds or thousands of miles. After great volcanic outbursts enormous -areas are thus found covered with fine volcanic dust to the depth of -many inches or feet. - -[Illustration: Fig. 73.--Peak of Teneriffe in the Canary Islands -(12,182 ft.), surrounded by great crater-rings.] - -Sometimes, as in the case of the Lago di Bracciano, the eruptive forces -appear to have entirely exhausted themselves in the prodigious outburst -by which the great crater was produced. But in other cases, as in that -of the Lago di Bolsena, the eruptive action was resumed at a later -date, and small tuff-cones were thrown up upon the floor of the crater; -these now rise as islands above the surface of the lake. In other -cases, again, the eruptive action was resumed after the formation of -the great crater-ring, with such effect that bulky volcanic cones were -built up in the midst of the crater-ring which surrounds them like a -vast wall; examples of this are exhibited in the extinct volcanoes of -Rocca Monfina and Monte Albano. Some of the grandest volcanoes of the -globe, such as Teneriffe (fig. 73), the volcanoes of Mauritius and -Bourbon (figs. 74 and 75), and many others that might be cited, are -thus found to be surrounded by vast crater-rings. Vesuvius itself is -surrounded by the crater-ring of Somma (fig. 76). - -[Illustration: Fig. 74.--The volcano of Bourbon, rising in the midst of -a crater-ring four miles in diameter.] - -[Illustration: Fig. 75.--The volcano of Bourbon, as seen from another -point of view, with three concentric crater-rings encircling its base.] - -[Sidenote: BASALTIC CONES IN TRACHYTIC CRATER-RINGS.] - -This formation of cone within crater, often many times repeated, is -very characteristic of volcanoes. The craters mark sudden and violent -paroxysmal outbursts, the cones are the result of more moderate but -long-continued ejection. Sometimes, as at Vesuvius in 1767 (fig. 15, -p. 85), we find a nest of craters and cones which very strikingly -exemplifies this kind of action. - -[Illustration: Fig. 76.--Vesuvius, as seen from Sorrento, half -encircled by the crater-ring of Somma.] - -We shall point out, hereafter, that at most volcanic centres the -ejection of trachytic lavas precedes that of the basaltic lavas. Now it -is these trachytic lavas which principally give rise to the formation -of the light lapilli of which tuff-cones are formed. Hence it is that -we so frequently find, as in the case of Vesuvius, Rocca-Monfina, and -many other volcanoes, that a great crater-ring, largely composed of -tuffs, encloses a cone built up of more basic lavas. - -In fig. 77 we have shown by a series of outline sections the various -forms assumed by volcanoes in consequence of the different kinds of -eruptive action going on in them:-- - -1. Is an outline of Fusiyama, an almost perfect cone, with a small -crater at its summit. The sides of this volcano admirably illustrate -the beautiful double curves characteristic of volcanic cones. - -2. Hverfjall in Iceland, a volcanic cone with a large crater, reaching -almost to its base. - -3. The crater-lake of Bracciano, in which the area of the crater is out -of all proportion to the height of the crater-walls. - -4. Rocca-Monfina, in Southern Italy, a tuff-cone of large dimensions, -in the midst of which an andesitic lava-cone has been built up. - -5. Teneriffe, in the Canary Islands, in which a perfect volcanic cone -has been built up in the centre of an encircling crater-ring. - -6. Vulcano, in the Lipari Islands, in which, by the shifting of the -centre of volcanic activity along a line of fissure, a series of -overlapping volcanic cones has been produced. - -[Illustration: Fig. 77.--Outlines of various Volcanoes, illustrating -the different relations of the craters to cones.] - -[Sidenote: SUBMARINE VOLCANOES.] - -While speaking of the varieties of form assumed by volcanic cones and -craters, we must not forget to notice the effects which are produced -by denuding forces upon them. In the case of submarine volcanoes, like -the celebrated island called by the English Graham Isle, by the French -Isle Julie, and by the Germans the Insel Ferdinandez (fig. 78), which -was thrown up off the coast of Sicily in 1831, it was evident that -volcanic outbursts taking place at some depth below the level of the -sea gradually piled up a cone of scoriæ with a crater in its midst. -By constant accessions to its mass, this scoria-cone was eventually -raised above the sea-level, but the action of the waves upon the loose -materials soon destroyed the crater-walls and eventually reduced the -island to a shoal. It is evident that in all cases in which eruptions -take place beneath the sea-level, and the loose materials are exposed -during their accumulation to the beating of the sea-waves, the form -of the volcanic cone so produced will be greatly modified by the -interaction of the two sets of opposed causes, the eruptive forces from -below and the distributive action of the sea-waves. - -[Illustration: Fig. 78.--Island thrown up in the Mediterranean Sea in -July and August 1881. - -(The view was taken in the month of September, after the sides of the -crater had been washed away by the waves.)] - -Craters when once formed are often rent across, along the line of the -fissure above which they are thrown up. Thus the crater of Vesuvius was -in 1872 rent completely asunder on one side, so that it was possible -to climb through the fissure thus produced and reach the bottom of the -crater. Streams flowing down the sides of the crater, and escaping -through such a rent, may in the end greatly modify the form and -disguise the characters of a volcanic crater. Of this kind of action we -have a striking example in the Val del Bove of Etna. - -Volcanoes, as we shall point out in the sequel, are after their -extinction frequently submerged beneath the waters of the ocean. The -sea entering the craters, eats back their cliff-like sides and enlarges -their areas. Such denuded waters are called 'calderas,' the channels -into them 'barrancos.' - -Sometimes the action of the waves upon a partially submerged volcano -has led to the cutting back of its slopes into steep cliffs, at the -same time that the crater-ring is enlarged. In such cases we have left -a more or less complete rocky ring, composed of alternating lavas and -fragmentary materials. Of such a ruined crater-ring, the Island of St. -Paul in the South Atlantic affords an admirable example. - -When the action of denudation has gone still further, all the lavas and -tuffs composing the cone may be completely removed and nothing left but -masses of the hard and highly-crystalline rocks which have cooled down -slowly in the heart of the volcano. An example of this kind is afforded -to us by St. Kilda, the remotest member of the British Archipelago. - -But although the majority of volcanic craters are clearly formed by -explosive action, there are some craters, like those of Kilauea in -Hawaii, which probably owe their origin to quite a different set of -causes. In this case the explosive action at the vent is but slight, -and the crater, which is of very irregular form, appears to have -originated in a fissure, which has been slowly enlarged by the liquid -lavas encroaching upon and eating away its sides. Such craters as -these, however, appear to be comparatively rare. - -Besides the great volcanic mountains composed of lava, scoriæ, tuff and -ash, there are other structures which are formed around volcanic vents -even when these do not eject molten rock-masses. The water which issues -in these cases either as steam or in a more or less highly heated -condition frequently carries materials in suspension or solution, and -these sometimes accumulate in considerable quantities around the vent. - -[Sidenote: FORMATION OF MUD-VOLCANOES.] - -When fissures are formed in the midst of loose argillaceous materials, -such as are frequently produced by the decomposition of volcanic rocks, -the waters which issue through them are sometimes so charged with muddy -matter that this accumulates to form cones having all the general -characters of volcanic mountains, and which occasionally rise to the -height of 250 feet. The gases and vapours which issue from these -'mud-volcanoes' are those which are known to be emitted from volcanic -vents at which the action going on is not very intense. Daubeny and -others have suggested that these mud-volcanoes may be the result of -actions which have little or no analogy with those which take place -at ordinary volcanic vents, and that the combustion of subterranean -beds of sulphur and similar causes would be quite competent to their -production. But inasmuch as these mud-volcanoes are almost always -situated in regions in which the more powerful volcanic action has -only recently died away, and the gases and vapours emitted by them are -very similar in character to those which issue from volcanoes, there -does not appear to be any good reason for doubting that they should be -classed as truly volcanic phenomena. - -Mud-volcanoes are found in Northern Italy near Modena, in Sicily near -Girgenti, on the shores of the Sea of Azof and the Caspian, in Central -America, and in other parts of the globe. The gas frequently escapes -from them with such violence that mud is thrown into the air to the -height of several hundreds of feet. Sometimes this gas is inflammable, -consisting of sulphuretted hydrogen, hydrogen, or some hydrocarbons, -and these gases occasionally take fire, so that true flames issue from -these mud-volcanoes. In other cases the mud-volcanoes appear to be -formed by either hot or cold springs containing large quantities of -suspended materials, and the liquid mud issues from the vent without -any violent eruptive action. - -[Illustration: Fig. 79.--Sinter-cones surrounding the orifices of -Geysers. - -1. Basin of the Great Geyser, Iceland. 2. Hot spring cone. 3. Old -Faithful. 4. The Great Geyser. 5. Liberty Cap. (2, 3, 4 and 5 are in -the Yellowstone Park district of the Rocky Mountains.)] - -[Sidenote: FORMATION OF SINTER-CONES.] - -The soluble materials which waters issuing from volcanic vents deposit -on their sides are chiefly silica and carbonate of lime. - -Hot springs, whether intermittent or constant, often contain large -quantities of silica in solution. The solution of this silica is -effected, at the moment of its separation from combination with the -alkali or alkaline earths, during the decomposition of volcanic rocks, -and is favoured by the presence of alkaline carbonates in the water, -and the high temperature and the pressure under which it exists in the -subterranean regions. When the water reaches the surface and, being -relieved from pressure, begins to cool down the silica is deposited. -By this deposited silica the basins around the geysers of Iceland are -formed. Sometimes conical structures are built up around the vents of -hot springs by the deposition of silica from their waters. Examples -of this kind abound in the National Park of Colorado, where they have -received fanciful names, such as the Beehive, Liberty Gap, &c. This -deposited silica is known to geologists as sinter. The forms of some of -the structures which surround the orifices of geysers is shown in fig. -79. The 'Liberty Cap' is an extinct geyser-cone fifty feet high and -twenty feet in diameter. - -Hot and cold springs rising in volcanic regions are often highly -charged with carbonic acid, and in passing through calcareous rocks -dissolve large quantities of carbonate of lime. Upon exposure to the -atmosphere, the free carbonic acid escapes and the carbonate of lime -is deposited in the form known as 'travertine.' Such springs occur -in great numbers in many volcanic regions. In the Auvergne great -rock-masses occur formed of carbonate of lime deposited from a state -of solution and taking the form of natural aqueducts and bridges. In -Carlsbad the numerous hot springs have deposited masses of pisolitic -rock (Strudelstein) which have filled up the whole bottom of the -valley, and upon these deposits the town itself is mainly built. -In Central Italy the deposits of travertine formed by calcareous -springs are of enormous extent and thickness: St. Peter's and all the -principal buildings of Rome being constructed of this travertine or -'Tibur-stone.' - -[Sidenote: FORMATION OF SINTER-TERRACES.] - -When springs charged with silica or carbonate of lime rise upon the -slope of a hill composed of loose volcanic materials, they give rise -to the remarkable structures known as sinter- and travertine-terraces -(see fig. 80). The water flowing downwards from the vent forms a -hard deposit upon the lower slope of the hill, while the continual -deposition of solid materials within the vent tends to choke it up. -As a new vent cannot be forced by the waters through the hard rock -formed below, it is originated a little higher up. Thus the site of the -spring is gradually shifted farther and farther back into the hill. As -deposition takes place along the surfaces over which this water flows, -terraces are built up enclosing basins. Of structures of this kind we -have remarkable examples in the sinter-terraces of Rotomahana in New -Zealand and the travertine-terraces of the Gardiner's River in the -Yellowstone Park district of the Rocky Mountains. - -[Illustration: Fig. 80.--Diagram illustrating the mode of formation of -Travertine and Sinter Terraces on the sides of a hill of tuff.] - - - - -CHAPTER VII. - -THE SUCCESSION OF OPERATIONS TAKING PLACE AT VOLCANIC CENTRES. - - -That a volcanic vent, when once established, may display intense -activity during enormous periods of time, there cannot be the smallest -reason for doubting; for the accumulation of materials around some -existing volcanic centres must certainly have been going on during -many thousands, perhaps millions, of years. To us, whose periods of -observation are so circumscribed, it may therefore at first sight -appear a hopeless task to trace the 'life-history of a volcano,' to -discover the stages of its development, and to indicate the various -episodes which have occurred during the long periods it has been in -existence. But when it is remembered that we have the opportunity of -studying and comparing hundreds of such volcanoes, exhibiting every -varying phase of their development, we shall see that such an attempt -is by no means so unpromising as it at first sight appears to be. In -the present chapter, we shall give an account of the results which have -already been obtained by inquiries directed to this object. - -[Sidenote: CYCLES OF VOLCANIC PHENOMENA.] - -There is not the smallest room for doubt that during the past history -of our globe, exhibitions of subterranean energy have occurred at many -different parts of its surface. There is further evidence that at the -several sites where these displays of the volcanic forces have taken -place, the succession of the outbursts has run through a regular cycle, -gradually increasing in intensity to a maximum, and then as gradually -dying away. - -A little consideration will show that the first portion of this cycle -of events is the one which it is most difficult to examine and study. -The products of the earlier and feeble displays of volcanic activity, -at any particular centre, are liable to be destroyed, or masked, during -the ejection of overwhelming masses of materials in the later stages -of its more matured energy. That the feeble displays of volcanic -force now exhibited in some localities will gradually increase in -intensity in the future, and eventually reach the grandest stage of -development, there can be no reason for doubting. But, unfortunately, -we are quite unable to discriminate these feeble manifestations, which -are the embryonic stages in the development of grand exhibitions of -the volcanic forces, from slight outbursts which die away and make no -farther sign. - -From what has been proved concerning the true nature of volcanic -action, however, it is certain that the first step towards the -exhibition of such action, at any particular locality, must be the -production of an aperture in the earth's crust. Only by means of -such an aperture can the vapours, gases, and rocky materials reach -the surface, and give rise to the phenomena there displayed. There is -reason to believe that all such apertures are really of the nature of -fissures, or cracks, which have been opened through the superjacent -strata by the efforts of the repressed subterranean forces. - -Some recent writers have, it is true, endeavoured to draw a distinction -between what they call 'fissure-eruptions,' and eruptions taking -place from volcanic cones. But all volcanic outbursts are truly -'fissure-eruptions'--the subterranean materials finding their way to -the surface through great cracks, which, in a more or less vertical -position, traverse the overlying rock-masses. It is true that in many -cases portions of these cracks soon get choked up, while other portions -become widened, and the volcanic energy is concentrated at such spots. -Thus the materials ejected from these fissures are usually emitted in -greatest quantities at one or more points along the fissure, and a -single great volcanic vent, or a row of smaller vents, is established -upon the line at which the fissure reaches the surface. - -We have seen that the amount of explosive action taking place at -different volcanic vents varies according to the proportion of -imprisoned water contained in the lava. In the cases where there is -much explosive action, vast accumulations of scoriæ, lapilli, and -dust take place, and cones of great size are built up; but in those -cases where the explosive action is small the lavas flow quietly from -the vent, and only small scoriæ-cones are thrown up, these being -probably soon swept away by the lava-currents themselves or by denuding -agencies. But both kinds of eruption have equal claims to be called -'fissure-eruptions.' - -[Sidenote: FORMATION OF VOLCANIC FISSURES.] - -In the expansive force of great masses of imprisoned vapour, we have -a competent cause for the production of the fissures through which -volcanic outbursts take place. Such fissures are found traversing the -rocks lying above volcanic foci, and often extending to distances of -many miles, or even hundreds of miles, from the centres of activity. -Some of these cracks are found to be injected with fused materials from -below, others have been more or less completely filled with various -minerals that have been volatilized, or carried by superheated waters -from the deeper regions of the earth's crust. That many of the cracks -thus produced in the superjacent rocks, by the heaving forces of -imprisoned vapour seeking to escape, never reached the surface, we have -sufficient proof in many mining regions. - -If we now transfer our attention from the deeper portions of the -earth's crust to the surface, we can well understand how the attempts -of the imprisoned vapours to force a passage for themselves through the -solid rock-masses would lead to shocks and jars among the latter. Each -of these shocks or jars would give rise, in the surrounding portions -of the earth's crust, to those vibrations which we know as earthquakes. -The close connection between most earthquakes and volcanic phenomena is -a fact that does not admit of the smallest doubt; and though it would -be rash to define all earthquakes as 'uncompleted efforts to establish -a volcano,' yet, in the efforts of the repressed subterranean forces to -find a vent by the production of fissures in the overlying rock-masses, -we have a cause competent to the production of those shocks which are -transmitted to such enormous distances as waves of elastic compression. - -We have seen that the production of the fissure upon which the small -volcano of Monte Nuovo was thrown up was preceded by a succession -of earthquakes, which for a period of over two years terrified the -inhabitants of the district, and might have warned them of the -coming event. In the same manner, doubtless, the period before the -appearance of volcanic phenomena in a new area would be marked by -powerful subterranean disturbances within it, due to the efforts of the -imprisoned vapours to force for themselves a channel to the surface. - -[Sidenote: NATURE OF FIRST EJECTIONS FROM FISSURES.] - -In the case of Monte Nuovo, we have seen that the fissure, when -produced, emitted water--at first in a cold, then in a boiling -condition--and, eventually, steam and scoriæ. It is probable that -through the first cracks which reached the surface, during the heaving -of the subterranean forces, water, charged with carbonic acid, flowed -abundantly, and that these cold springs, charged with carbonic acid -and carbonate of lime, would be succeeded by others which were hot -and contained silica in solution. In Hungary, the Western Isles of -Scotland, and many other volcanic districts, we find abundant evidence -that, before the eruption of lavas in the area, great masses of -travertine and siliceous sinter were formed by the action of cold and -hot springs. - -As the volcanic action became more intense by the more perfect opening -of the fissures, the evolution of carbonic add gas would be succeeded -by the appearance of sulphurous acid, sulphuretted hydrogen, boracic -acid, and hydrochloric acid, which recent studies have shown to be -successively emitted from volcanic vents as the temperature within them -rises. At last lava or molten rock becomes visible within the fissures, -and the ejection of the frothy masses--scoriæ, pumice, lapilli and -dust--commences, and this is sometimes succeeded by the outflow of -currents of lava. - -That volcanoes originate upon lines of fissure in the earth's crust -we have the most convincing proofs. Not only have such fissures been -seen in actual course of formation at Vesuvius, Etna, and other active -volcanoes, but a study of the volcanoes dissected by denudation affords -the most convincing evidence of the same fact. The remarkable linear -arrangement seen in groups of volcanoes, which is conspicuous to the -most superficial observer, confirms this conclusion. - -[Illustration: Fig. 81.--Map of the volcanic group of the Lipari -Islands, illustrating the position of the lines of fissure on which the -volcanoes have been built up.] - -[Sidenote: SHIFTING OF VENTS ALONG FISSURES.] - -We have described the action going on at Stromboli as typical of that -which occurs at all volcanic vents. Stromboli is, however, one among -a group of islands all of which are entirely of volcanic origin. The -volcanoes of this group of islands, the Æolian or Lipari Islands, are -arranged along a series of lines which doubtless mark fissures in the -earth's crust. These fissures, as will be seen by the accompanying map -(fig. 81), radiate from a centre at which we have proofs of the former -existence of a volcano of enormous dimensions. It is a very interesting -fact, which the studies of Prof. Suess have established, that the -earthquakes which have so often desolated Calabria appear to have -originated immediately beneath this great centre of volcanic activity. - -[Illustration: Fig. 82.--The Puy de Pariou in the Auvergne, -illustrating the shifting of the centre of eruption along a line of -fissures.] - -When two volcanic cones are thrown up on the same line of fissure, -their full development is interfered with, and irregularities in their -form and characters are the consequence. In the plan (fig. 82) and the -section (fig. 83) an example is given of the results of such a shifting -of the centre of eruption along a line of fissure. By the second -outburst, one-half of the first-formed cone has been removed, and the -second-formed overlaps the first. - -[Illustration: Fig. 83.--Ideal section of the Puy de Pariou.] - -Sometimes a number of scoria- or tuff-cones are thrown up in such close -proximity to one another along a line of fissure, that they merge into -a long irregular heap on the summit of which a number of distinct -craters can be traced. An example of this kind was furnished by the -line of scoria-cones formed above the fissure which opened on the -flanks of Etna in 1865 (see fig. 84). - -[Illustration: Fig. 84.--Fissure formed on the flanks of Etna during -the eruption of 1865. - -_a._ Monte Frumento, an old parasitic cone. _b._ Line of fissure. _c, -c, c._ New scoria-cones thrown up on line of fissure. _d._ Lava from -same.] - -[Illustration: Fig. 85.--Plan of the Island of Vulcano, based on the -map of the Italian Government.] - -[Sidenote: SHIFTING OF ERUPTIONS ALONG FISSURES.] - -Even in the case of great composite cones, however, we sometimes find -proofs of the centre of eruption having shifted its place along the -line of fissure. No better example of this kind could possibly be -adduced than that of the Island of Vulcano, with the peninsula of -Vulcanello, which is joined to it by a narrow isthmus (see the map, -fig. 81, p 192). In fig. 85 we have given an enlarged plan of this -island which will make its peculiar structure more intelligible (see -also the section given in fig. 77, No. 6, facing p. 178). - -The south-eastern part of the island consists of four crater-rings, -one half of each of Which has been successively destroyed, through -the shifting of the centre of eruption towards the north-west, along -the great line of fissure shown in the general map (fig. 81). The -last formed of these four crater-rings is the one which is now most -complete, and culminates in Monte Saraceno (1581 ft.), _a_ in the -plan, the highest point in the island. The older crater-rings have -been in part removed by the inroads of the waters of the Mediterranean -on the shores of the island. In the centre of the great crater, -_b_, which we have just described, rises the present active cone of -Vulcano, 1,266 feet high, and having a crater, _c_, about 600 yards -in diameter and more than 500 feet in depth. From this cone, a great -stream of obsidian, _e_, flowed in the year 1775, and a small crater, -_d_, the Fossa Anticha, has been opened in the side of the cone. The -continuation of the same line of fissure is indicated by a ruined -tuff-cone, _f_, known as the Faraglione, and the three scoria-cones of -Vulcanello, _g, h_, which have been thrown up so close to one another -as to have their lower portions merged in one common mass, as shown in -fig. 86. - -[Sidenote: SYSTEMS OF VOLCANIC FISSURES.] - -Even in volcanoes of the largest dimensions we sometimes find proofs of -the centre of eruption having shifted along the line of fissure. Lyell -showed that such a change in the position of the central axis of the -volcano had taken place in Etna, and the same phenomenon is exhibited -in the clearest manner' by some of the ancient volcanoes of the Inner -Hebrides, which have been dissected by the denuding forces. - -[Illustration: Fig. 86.--Vulcanello, with its three craters. - -_a._ The most recently-formed and perfect crater, _b_ and _c_. Older -craters, the walls of which have been partly removed by denudation, -_e._ Lava-currents proceeding from _b_. The section exposed in the -cliff at _d_ is represented in fig. 35, p. 116.] - -In the case of the Lipari Islands, the fissures along which the -volcanic mountains have been thrown up radiate from a common centre, -and a similar arrangement can be traced in many volcanic regions, -especially those in which a great central volcano has existed. In other -cases, however, as in the Campi Phlegræi, the volcanic vents appear -to be formed along lines which assume a parallel arrangement, and -this doubtless marks the relative position of the original fissures -produced in the earth's crust when these volcanoes were formed. In some -other cases we find evidences of the existence of a principal fissure -from the sides of which smaller cracks originated. These three kinds of -arrangements of volcano-producing fissures are equally well illustrated -when we study those denuded districts, in which, as we have seen, the -ground-plans of volcanic structures are revealed to our view. - -There is now good ground for believing that in volcanic vents, at which -long-continued eruptive action takes place, the lavas of different -chemical composition make their appearance in something like a definite -order. It had been remarked by Scrope and other geologists at the -beginning of the present century, that in many volcanic areas the acid -or trachytic lavas were erupted before the basic or basaltic. - -Von Richthofen, by his studies in Hungary and the volcanic districts -of the Rocky Mountains, has been able to enunciate a law governing the -natural order of succession of volcanic products; and although some -exception to this law may be mentioned, it is found to hold good for -many other districts than those in which it was first determined. - -In a great number of cases it has been found that the first erupted -rocks in a volcanic district are those of intermediate composition -which are known as andesites. These andesites, which are especially -characterised by the nature of their felspar, sometimes contain free -quartz and are then known as quartz-andesites or dacites, from their -abundance in Transylvania, the old Roman province of Dacia. - -[Sidenote: ORDER OF ERUPTION OF VOLCANIC PRODUCTS.] - -Von Richthofen suggests that another class of volcanic rocks, to which -he gives the name of 'propylites,' were in every case erupted before -the andesites, and in support of his views adduces the fact that in -many instances propylites are found underlying andesites. But the -propylites are, in chemical composition, identical with the andesites, -and like them present some varieties in which quartz occurs, and others -in which that mineral is absent. In their microscopic characters the -propylites differ from the andesites and dacites only in the fact that -the former are more perfectly crystalline in structure, being indeed in -many cases quite undistinguishable from the diorites or the plutonic -representatives of the andesites. The propylites also contain liquid -cavities, which the andesites and dacites as a rule do not, and the -former class of rocks, as Prof. Szabo well points out, are usually much -altered by the passage of sulphurous and other vapours, in consequence -of which they frequently contain valuable metallic ores. - -The extrusion of these andesitic lavas is sometimes accompanied, and -sometimes preceded or followed, by eruptions of trachytic lavas--that -is, of lavas of intermediate composition which have a different kind of -felspar from that prevailing in the andesites. - -In the final stages of the eruptive action in most volcanic districts -the lavas poured forth belong to the classes of the rhyolitic or acid, -and the basaltic or basic lavas. - -These facts are admirably illustrated in the case of the volcanic -district of the Lipari Islands, to which we have had such frequent -occasion to refer. The great central volcano of this district, which -now in a ruined condition constitutes a number of small islets (see -the map, fig. 81, p. 192), is composed of andesitic lavas. The other -great volcanoes thrown up along the three radiating lines of fissure -are composed of andesitic and trachytic rocks. But all the more recent -ejections of the volcanoes of the district have consisted either of -rhyolites, as in Lipari and Vulcano, or of basalts, as in Stromboli and -Vulcanello. - -Von Richthofen and the geologists who most strongly maintain the -generalisations which he has made concerning the order of appearance -of volcanic products, go much farther than we have ventured to do, -and insist that in all volcanic districts a constant and unvarying -succession of different kinds of lavas can be made out. It appears to -us, however, that the exceptions to the law, as thus precisely stated, -are so numerous as to entirely destroy its value. - -The generalisation that in most volcanic districts the first ejected -lavas belong to the intermediate group of the andesites and trachytes, -and that subsequently the acid rhyolites and the basic basalts made -their appearance, is one that appears to admit of no doubt, and is -found to hold good in nearly all the volcanic regions of the globe -which have been attentively studied. - -The Tertiary volcanic rocks of our own country, those of North -Germany, Hungary, the Euganean Hills, the Lipari Islands, and many -other districts in the Old World, together with the widespread volcanic -rocks of the Rocky Mountains in the New World, all seem to conform to -this general rule. - -[Sidenote: THEORY OF VOLCANIC MAGMAS.] - -In connection with this subject, it may be well to refer to the -ideas on the composition of volcanic rocks which were enunciated by -Bunsen, and the theoretic views based on them by Durocher. Bunsen -justly pointed out that all volcanic rocks might be regarded as -mixtures in varying proportions of two typical kinds of materials, -which he named the 'normal trachytic' and the 'normal pyroxenic' -elements respectively. The first of these corresponds very closely in -composition with the acid volcanic rocks or rhyolites, and the second -with the basic volcanic rocks or basalts. Durocher pointed out that -if quantities of these different materials existed in admixture, the -higher specific gravity of the basic element would cause it gradually -to sink to the bottom, while the acid element would rise to the top. -Carrying out this idea still further, he propounded the theory that -beneath the earth's solid crust there exist two magmas, the upper -consisting of light acid materials, the lower of heavy basic ones; and -he supposed that by the varying intensity of the volcanic forces we may -have sometimes one or the other magma erupted and sometimes varying -mixtures of the two. - -The study of volcanic rocks in recent years has not lent much support -to the theoretic views of Durocher concerning the existence of two -universal magmas beneath the earth's crust; and there are not a few -facts which seem quite irreconcilable with such a theory. Thus we -find evidence that in the adjacent volcanic districts of Hungary and -Bohemia, volcanic action was going on during the whole of the latter -part of the Tertiary period. But the products of the contemporaneous -volcanic outbursts in adjacent areas were as different in character -as can well be imagined. The volcanic rocks all over Hungary present -a strong family likeness; the first erupted were trachytes, then -followed andesites and dacites in great abundance, and lastly rhyolites -and basalts containing felspar. But in Bohemia, the lavas poured out -from the volcanoes during the same period were firstly phonolites -and then basalts containing nepheline and leucite. It is scarcely -possible to imagine that such very different classes of lavas could -have been poured out from vents which were in communication with the -same reservoirs of igneous rock, and we are driven to conclude that the -Hungarian and Bohemian volcanoes were supplied from different sources. - -[Sidenote: SEPARATION OF LAVAS IN RESERVOIRS.] - -But the undoubted fact that in so many volcanic regions the eruption of -andesitic and trachytic rocks, which are of intermediate composition, -is followed by the appearance of the differentiated products, rhyolite -and basalt, which are of acid and basic composition respectively, lends -not a little support to the view that under each volcanic district a -reservoir of more or less completely molten rock exists, and that in -these reservoirs various changes take place during the long periods -of igneous activity. During the earlier period of eruption the heavier -and lighter elements of the contents of these subterranean reservoirs -appear to be mingled together; but in the later stages of the volcanic -history of the district, the lighter or acid elements rise to the top, -and the heavier or basic sink to the bottom, and we have separate -eruptions of rhyolite and basalt. We even find some traces of this -action being carried still further. Among the basalts ejected from the -volcanoes of Northern Germany, Bohemia, Styria, Auvergne, and many -other regions, we not unfrequently find rounded masses consisting of -olivine, enstatite, augite, and other heavier constituents of the rock. -These often form the centre of volcanic bombs, and are not improbably -portions of a dense mass which may have sunk to the bottom of the -reservoirs of basaltic materials. - -In consequence of the circumstance that the eruption of lavas of -intermediate composition usually precedes that of other varieties, we -usually find the central and older portions of great volcanoes to be -formed of andesites, trachytes, or phonolites, while the outer and -newer portions of the mass are made up of acid or basic lavas. This is -strikingly exemplified in the great volcanoes of the Auvergne and the -Western Isles of Scotland, in all of which we find that great mountain -masses have, in the first instance, been built up by extrusions of lava -of the intermediate types, and that through this central core fissures -have been opened conveying basic lavas to the surface. From these -fissures great numbers of basaltic lava-streams have issued, greatly -increasing the height and bulk of the volcanic cones and deluging the -country all around. - -The lavas of intermediate composition--the andesites, trachytes, and -phonolites--possess, as we have already seen, but very imperfect -liquidity as they flow from the volcanic vents. Hence we find them -either accumulating in great dome-shaped masses above the vent or -forming lava-streams which are of great bulk and thickness, but do not -flow far from the orifices whence they issue. The more fusible basaltic -lavas, on the other hand, spread out evenly on issuing from a vent, and -sometimes flow to the distance of many miles from it. This difference -in the behaviour of the intermediate and basic lavas is admirably -illustrated in the volcanic districts of the Auvergne and the Western -Isles of Scotland. - -In other cases, like Vesuvius, we find that great volcanic cones of -trachytic tuff have been built up, and that these masses of fragmentary -trachytic materials have been surrounded and enclosed by the ejection, -at a later date, of great outbursts of basaltic lavas. In still -other cases, of which Rocca Monfina in Southern Italy constitutes an -excellent example, we find that a great crater-ring of trachytic tuffs -has been formed in the first instance, and in the midst of this a cone, -composed of more basic materials, has been thrown up. - -[Sidenote: EXCEPTIONS TO THE GENERAL LAW.] - -In all these volcanoes we see the tendency towards the eruption -of intermediate lavas in the first instance, and of basaltic and -acid lavas at a later date. Valuable, however, as are the early -generalisations of Scrope, and the more precise law enunciated by Von -Richthofen concerning the 'natural order of succession of volcanic -products,' we must not forget that there are to be found a considerable -number of exceptions to them. There are some volcanic centres from -which only one kind of lava has been emitted, and this may be either -acid, basic, or intermediate in composition; and on the other hand, -there are districts in which various kinds of lava have been ejected -from the same vents within a short period of time, in such a way as to -defy every attempt to make out anything like a law as to the order of -their appearance. Nevertheless the rules which we have indicated appear -to hold good in so great a number of cases that they are well worthy -of being remembered, and may serve as a basis on which we may reason -concerning the nature of the action going on beneath volcanic vents. - -From the study of the external appearances of volcanic mountains, -combined with investigations of those which have been dissected by -denudation, we are able to picture to our minds the series of actions -by which the great volcanic mountains of the globe have been slowly and -gradually built up. - -In the first instance the eruptions appear to have taken place at -several points along a line of fissure, but gradually all of these -would become choked up except one which became the centre of -habitual eruption. From this opening, ejections, firstly of lavas of -intermediate composition, and afterwards of basic materials, would take -place, until a volcano of considerable dimensions was built up around -it. But at last a point would be reached in the piling up of this -cone, when the volcanic forces below would be inadequate to the work -of raising the liquid lava through the whole length of the continually -upward-growing tube of the volcano. Under these circumstances the -expansive force of the imprisoned steam would find it easier to rend -asunder the sides of the volcanic cone than to force the liquid -material to the summit of the mountain. If these fissures reached the -surface explosive action would take place, in consequence of the escape -of steam from the glowing mass, and scoria-, tuff-, and lava-cones -would be formed above the fissure. In this way, as we have already -pointed out, the numerous 'parasitic cones' which usually abound on -the flanks of the greater volcanic mountains have been formed. The -extrusion of these masses of scoriæ and lava on the flanks of the -mountain tends, not only to increase the bulk of the mass, but to -strengthen and fortify the sides. For by the powerful expansive force -at work below, every weak place in the cone is discovered and a fissure -produced there; but by the extrusion of material at this fissure, and -still more by the consolidation of the lava in the fissure, the weak -place is converted into one of exceptional strength. - -[Sidenote: INTRUSIVE MASSES BENEATH VOLCANOES.] - -As the sides of the cone are thus continually repaired and strengthened -they are rendered more capable of withstanding the heaving forces -acting from below, and these forces can then only find vent for -themselves by again raising the liquefied lava to the central orifice -of the mountain. Many volcanoes, like Etna, exhibit this alternation of -eruptive action from the crater at the summit of the mountain, and from -fissures opened upon its flanks, the former tending to raise the height -of the volcanic pile, the latter to increase its bulk. - -But at last a stage will be reached when the volcanic forces are -no longer able either to raise the lava up the long column of the -central vent on the one hand, or to rend asunder the strongly-built -and well-compacted flanks of the mountain on the other. It is probably -under these conditions, for the most part, that the lavas find their -way between the masses of surrounding strata and force them asunder in -the way that we have already described. - -In the case of the more fluid basaltic lavas, as was pointed out so -long ago by Macculloch, the liquefied materials may find their way -between the strata to enormous distances from the volcanic centre. Such -extended flat sheets of igneous rock retain their parallelism with the -strata among which they are intruded over large areas, and did not -probably produce any marked phenomena at the surface. - -But in the case of less fluid lavas, such as those of intermediate or -acid composition, for example, the effect would be far otherwise. Such -lavas, not flowing readily from the centre of eruption, would tend to -form great bulky lenticular masses between the strata which they forced -asunder, and, in so doing, could not fail to upheave and fissure the -great mountain-mass above. Vast lenticular masses of trachytic rock, -thus evidently forced between strata, have been described by Mr. G. -K. Gilbert, as occurring in the Henry Mountains of Southern Utah, and -by him have been denominated 'laccolites,' or stone-cisterns. Whether -the great basaltic sheets, like those described by Macculloch, and -those more bulky lenticular reservoirs of rock of which Mr. Gilbert -has given us such an admirable account, were in all cases connected -with the surface, may well be a matter for doubt. It is quite possible -that, in some cases, liquefied masses of rocky materials in seeking -to force their way to the surface only succeeded in thus finding a -way for themselves between the strata, and their energy was expended -before the surface was reached and explosive action took place. But it -is an undoubted fact that beneath many of the old volcanoes, of which -the internal structure is now revealed to us by the action of denuding -forces, great intrusive sheets and laccolites abound; and we cannot -doubt that beneath volcanoes now in a state of eruption, or in those -which have but recently become extinct, similar structures must be in -course of formation. - -[Sidenote: EFFECTS OF INTRUSION BENEATH CONES.] - -That great upheaving forces have operated on volcanoes, subsequently -to the accumulation of their materials, we have sufficient evidence in -the Val del Bove of Etna, the Caldera of Palma, the Corral of Madeira, -&c. In all of these cases we find a radial fissure ('barranco') leading -into a great crateral hollow; and these radial fissures are of such -width and depth that their origin can only be referred to a disruptive -force like that which would be exercised by the intrusion of masses of -more or less imperfectly fluid material between the subjacent strata. -These facts, of course, lend no countenance to the views formerly held -by many geologists, both in Germany and France, that the materials -of which volcanoes are built up were deposited in an approximately -horizontal position, and were subsequently blown up like a gigantic -bubble. In Etna, Palma, and Madeira we find abundant proofs that the -mass existed as a great volcanic cone before the production of the -fissures (barrancos), which we have referred to the force exercised -during the intrusion of great igneous masses beneath them. - -But besides the horizontally-disposed intrusive sheets and laccolites, -great, radiating, vertical fissures are produced by the heaving forces -acting beneath those volcanic centres which have been closed up and -'cicatrised' by the exudation from them of subterranean materials. -These vertical intrusions, which we call dykes, like the horizontal -ones, differ in character, according to the nature of the materials -of which they are composed. Dykes of acid and intermediate lava are -usually of considerable width, and do not extend to great distances -from the centres of eruption. Dykes composed of the more-liquid, basic -lavas, on the other hand, may extend to the distance of hundreds of -miles from the central vent. The way in which comparatively narrow, -basaltic dykes are found running in approximately straight lines -for such enormous distances is a very striking fact, and bears the -strongest evidence to the heaving and expanding forces at work at -volcanic centres, during and subsequently to the extrusion of the -igneous products at the surface. - -These basaltic dykes occur in such prodigious numbers around some -volcanic vents, that the whole of the stratified rocks in the immediate -vicinity are broken up by a complete network of them, crossing and -interlacing in the most complicated fashion. Farther away from the -vents, similar dykes are found in smaller numbers, evidently radiating -from the same centre, and sometimes extending to a distance of more -than a hundred miles from it. Nowhere can we find more beautiful -illustrations of such dykes than in the Western Isles of Scotland. When -composed of materials which do not so easily undergo decomposition as -the surrounding rocks, they stand up like vast walls; but when, on the -other hand, they are more readily acted on by atmospheric moisture than -are the rocks which enclose them, they give rise to deep trenches with -vertical sides, which render the country almost impassable. - -[Sidenote: STRUCTURE OF INTRUSIVE MASSES.] - -The lava consolidating in these horizontal intrusions (sheets and -laccolites), and the vertical intrusions (dykes), is usually more -crystalline in structure than the similar materials poured out at the -surface. In the same dyke or sheet, when it is of great width, we -often find every variation--from a glassy material formed by the rapid -cooling of the mass where it is in contact with other rocks, to the -perfectly crystalline or granitic varieties which form the centre of -the intrusion. It is in these dykes and other intrusions that we find -the most convincing evidence of the truth of the conclusions, which -we have enunciated in a former chapter, concerning the dependence of -the structure of an igneous rock upon the conditions under which it -has consolidated. One material is found, under varying conditions, -assuming the characters of obsidian, rhyolite, quartz-felsite, or -granite; another, under the same set of conditions, taking the form of -tachylyte, basalt, dolerite, and gabbro. - -That these great intrusive masses, sheets and dykes, in their passage -between the sedimentary rocks sometimes find places where the overlying -strata are of such thinness or incoherence that the liquefied rocks are -able to force a way for themselves to the surface, we have the clearest -proof. In some dykes we find the rock in their upper portions losing -its compact character and becoming open and scoriaceous, showing that -the pressure had been so far diminished as to allow of the imprisoned -water flashing into steam. - -All round great volcanoes which have become extinct we frequently find -series of small volcanic cones, which have evidently been thrown up -along the lines where the great lava-filled fissures, which we have -been describing, have reached the surface and given rise to explosive -action there. The linear arrangement of these small cones, which are -thrown up in the plains surrounding vast volcanic mountains that have -become extinct, is very striking. The numerous 'puys' of the Auvergne -and adjoining volcanic regions of Central France are for the most part -small scoria- and lava-cones which were thrown up along great lines -of fissure radiating from the immense, central, volcanic mountains of -the district, after they had become extinct. These scoria-cones and -the small lava-streams which flow from them, as was so well shown by -Mr. Scrope, mark the latest efforts of the volcanic forces beneath the -district before they finally sank into complete extinction. In the -Western Isles of Scotland, as I have elsewhere shown, we can study the -formation of these later-formed cones in the plains around extinct -volcanic mountains, with the additional advantage of having revealed -to us, by the action of the denuding forces, their connection with the -great radiating fissures. - -It has been shown that the several stages in the decline of each -volcanic outburst is marked by the appearance at the vent of certain -acid gases. In the same way, after the ejection of solid materials -from a volcanic vent has come to an end, certain gaseous substances -continue to be evolved; and as the temperature at the vents declines, -the nature of the volatile substances emitted from them undergoes a -regular series of changes. - -[Sidenote: ORDER OF EMISSION OF VOLCANIC GASES.] - - -M. Fouqué, by a careful series of analyses of the gases which he -collected at different gaseous vents, or fumaroles as they are called, -in the crater of Vulcano, has been able to define the general relations -which appear to exist between the temperature at a volcanic orifice -and the volatile substances which issue from it. He found that in -fumaroles, in which the temperature exceeded 360° centigrade, and -in which in consequence strips of zinc were fused by the stream of -issuing gas, the analysis of the products showed sulphurous acid and -hydrochloric add to be present in large quantities, and sulphuretted -hydrogen and carbonic acid in much smaller proportions. Around these -excessively heated fumaroles, the lips of which often appear at night -to be red-hot, considerable deposits of sulphide of arsenic, chloride -of iron, chloride of ammonium, boracic acid, and sulphur were taking -place. - -It was found, however, that as the temperature of the vent declined, -the emission of the sulphurous acid and hydrochloric acid diminished, -and the quantity of sulphuretted hydrogen and carbonic acid mingled -with them was proportionately increased. - -In the same way it appears to be a universal rule that when a volcanic -vent sinks into a condition of temporary quiescence or complete -extinction the powerfully acid gases, hydrochloric acid and sulphurous -acid, make their appearance in the first instance, and at a later stage -these are gradually replaced by sulphuretted hydrogen and carbonic acid. - -Of these facts we find a very beautiful illustration in the Campi -Phlegræi near Naples. With the exception of Monte Nuovo, the volcano -which has most recently been in a state of activity in that district -is the Solfatara. From certain apertures in the floor of the crater of -the Solfatara there issue continually watery vapours, sulphurous acid, -sulphuretted hydrogen, hydrochloric acid, and chloride of ammonium. -The action of these substances upon one another, and upon the volcanic -rocks through which they pass, gives rise to the formation of certain -chemical products which, from a very early period, have been collected -on account of their commercial value. The action of these add gases -upon the surrounding rocks is very marked; efflorescent deposits of -various sulphates and chlorides take place in all the crevices and -vesicles of the rock; sulphur and sulphide of arsenic are also formed -in considerable quantities; and the trachytic tuffs, deprived of their -iron-oxide, alkaline earths and alkalies, which are converted into -soluble sulphates and chlorides, are reduced to a white, powdery, -siliceous mass. Many volcanoes, which have sunk into a state of -quiescence or extinction like the Solfatara of Naples, exhibit the same -tendency to give off great quantities of the powerfully-acid gases -which act upon the surrounding rocks, and deprive them of their colour -and consistency. Such volcanoes are said by geologists to have sunk -into the 'solfatara stage.' - -[Sidenote: SOLFATARA-STAGE OF VOLCANOES.] - -At the Lake of Agnano and some other points in the Campi Phlegræi, -however, we find fissures from which the less-powerfully acid gases, -sulphuretted hydrogen and carbonic acid, issue. These gases as they, -are poured forth from the vents are found to be little, if at all, -above the temperature of the atmosphere. Sulphuretted hydrogen is an -inflammable gas, and in the so-called salses and mud-volcanoes, at -which it is ejected in considerable quantities, it not unfrequently -takes fire and bums with a conspicuous flame. Carbonic acid on account -of its great density tends to accumulate in volcanic fissures and -craters rather than to mingle with the surrounding atmosphere. At the -so-called Grotto del Cane, beside the Lago Agnano, it is the custom to -show the presence of this heavy and suffocating gas by thrusting a dog -into it, the poor animal being revived, before life is quite extinct, -by pouring cold water over it. At the Büdos Hegy or 'stinking hill' of -Transylvania, carbonic acid and sulphuretted hydrogen are emitted in -considerable quantities, and it is possible to take a bath of the heavy -gas, the head being kept carefully above the constant level of the -exhalations. - -Although the stories of the ancient Avernian lake, across which no bird -could fly without suffocation, and of the Guevo Upas, or Poison Valley -of Java, which it has been said no living being can cross, may not -improbably be exaggerations of the actual facts, yet there is a basis -of truth in them in the existence of old volcanic fissures and craters -which evolve the poisonous sulphuretted hydrogen and carbonic acid -gases. - -Besides the gases which we have already named, and which are the most -common at and characteristic of volcanic vents, there are some others -which are not unfrequently emitted. First among these we must mention -boracic acid, which, though not a remarkably volatile substance, is -easily carried along in a fine state of division in a current of steam. -At Monte Cerboli and Monte Rotondo in Tuscany, great quantities of -steam jets accompanied by sulphuretted hydrogen and boracic acid issue -from the rocks, and these jets being directed into artificial basins of -water, the boracic acid is condensed and is recovered by evaporation. -We have already noticed that boracic add is evolved with the gases at -Vulcano and other craters; and the part which this substance plays in -volcanic districts is shown by the fact that many of the rocks, filling -old subterranean volcanic reservoirs, are found to be greatly altered -and to have new minerals developed in their midst through the action -upon them of boracic acid. - -Ammonia and various compounds of carbon, nitrogen, and hydrogen are -among the gases evolved from volcanic vents. In some cases these gases -may be produced by the destructive distillation of organic materials -in the sedimentary rocks through which volcanic outbursts take place. -But it is far from impossible that under the conditions of temperature -and pressure which exist at the volcanic foci, direct chemical union -may take place between substances, which at the surface appear to be -perfectly inert in each other's presence. - -When the temperature at volcanic fissures is no longer sufficiently -high to cause water to issue in the condition of vapour or steam, as is -the case at the 'stufas' which we have described, it comes forth in the -liquid state. Water so issuing from old volcanic fissures may vary in -its temperature, from the boiling point downwards. - -[Sidenote: GEYSERS AND HOT-SPRINGS.] - -When the water issues at a temperature little removed from the boiling -point, it is apt to give rise to intermittent springs or geysers, the -eruptions of which exhibit a remarkable analogy with those of ordinary -volcanoes. Geysers may indeed be described as volcanoes in which heated -water, instead of molten rock, is forced out from the vent by the -escaping steam. They occur in great abundance in districts in which the -subterranean action is becoming dormant or extinct, such as Iceland, -the North Island of New Zealand, and the district of the National Park -in the Rocky Mountains. - -Many attempts have been made to explain the exact mechanism by which -the intermittent action of geysers is produced, but it is not at all -probable that any one such explanation will cover all the varied -phenomena exhibited by them. Like volcanic outbursts, geyser eruptions -doubtless originate in the escape of bubbles of steam through a liquid -mass, and this liberation of steam follows any relief of pressure. -In districts where vast masses of lava are slowly cooling down from -a state of incandescence, and surface waters are finding their way -downwards while subterranean waters are finding their way upwards, -there can be no lack of the necessary conditions for such outbursts. -Sometimes the eruptions of geysers take place at short and regular -intervals, at other times they occur at wide and irregular intervals -of time. In some cases the outbursts take place spontaneously, and at -others the action can be hastened by choking up the vent with stones or -earth. - -Other hot springs, like the Strudel of Carlsbad, rise above the surface -in a constant jet, while most of them issue quietly and flow like -ordinary springs. - -Although the violent and paroxysmal outbursts of volcanic mountains -arrest the attention, and powerfully impress us with a sense of the -volcanic activity going on beneath the earth's surface, yet it may -well be doubted whether the quantity of heat, which the earth gets rid -of by their means, at all approaches in amount that which is quietly -dissipated by means of the numerous 'stufas,' gaseous exhalations, and -thermal springs which occur in such abundance all over its surface. -For while the former are intermittent in their action, and powerful -outbursts are interrupted by long periods of rest, the action of the -latter, though feeble, is usually continuous. - -[Sidenote: EFFECTS OF HOT-SPRINGS.] - -Most people may regard the hot spring of Bath as a very slight -manifestation of volcanic activity. This spring issues at a constant -temperature of 49° C, or 120° Fahr. As, however, no less than 180,000 -gallons of water issue daily from this source, we may well understand -how great is the amount of heat of which the earth's crust is relieved -by its agency. It may indeed be doubted whether its action in this way -is not at least equal to that of a considerable volcano which, though -so much more violent, is intermittent in its action. - -Nor are thermal springs by any means ineffective agents in bringing -materials from the interior of the earth's crust and depositing it -at the surface. The Bath spring contains various saline substances, -principally sulphates and chlorides, in solution in its waters. These -are quietly carried by rivers to the sea, and are lost to our view. The -spring has certainly maintained its present condition since the time -of the Romans, and I find that if the solid materials brought from the -interior of the earth during the last 2,000 years had been collected, -they would form a solid cone equal in height to Monte Nuovo. Yet we -usually regard the Campi Phlegræi as a powerfully-active volcanic -district, and the subterranean action in our own country as quite -unworthy of notice. - -When we remember the fact that on the continent of Europe the hot and -saline springs may be numbered by thousands, and that they especially -abound in districts like Hungary, the Auvergne, the Rhine provinces, -and Central Italy, where volcanic action has recently become extinct, -we shall be able to form some slight idea of the work performed -by these agents, not only in relieving the earth's crust of its -superfluous heat, but in transporting materials in a state of solution -from the interior of that crust and depositing them at the surface. The -vast deposits of siliceous sinter and of travertine also bear witness -to the effects produced by hot and mineral springs. - -Nor is the work of these springs confined to the surface. Mr. John -Arthur Phillips has shown that metallic gold and the sulphide of -quicksilver (cinnabar) have been deposited with the silica and other -minerals formed on the sides of a fissure from which hot springs issue -at the surface. There cannot be any doubt that the metallic veins or -lodes, which are the repositories of most of the metals employed in the -arts, have been formed in cracks connected with great volcanic foci, -the transfer of the various sulphides, oxides, and salts which fill -the vein having been effected either by solution, sublimation, or the -action of powerful currents of steam. - -As the igneous activity of the district declines, the temperature -of the issuing gases and waters diminishes with it, until at last -the volcanic forces appear to wholly abandon that region and to be -transferred to another. - -Yet even after all or nearly all indications of the volcanic agencies -cease to make themselves visible at the surface, occasional tremblings -of the earth's crust show that perfect equilibrium has not been -restored below, but that movements are taking place which result in -shocks that are transmitted through the overlying and surrounding -rock-masses as earthquake vibrations. - -[Sidenote: NATURE OF VOLCANIC CYCLES.] - -Such is the cycle of changes which appears to take place at each -district of the earth's surface, as it successively becomes the scene -of volcanic activity. - -The invasion of any particular area of the earth's surface by the -volcanic forces appears to be heralded by subterranean shocks causing -earthquake vibrations. Presently the origination of fissures is -indicated by the rise of saline and thermal springs, and the issuing -of carbonic acid and other gases at the surface. As the subterranean -activity becomes more pronounced, the temperature of the springs and -emitted gases is found to increase, and at last a visible rent is -formed at the surface, exposing the incandescent materials below. - -From this open fissure which has thus been formed, the gas and vapours -imprisoned in the incandescent rock-materials escape with such violence -as to disperse the latter in scoriæ and dust, or to cause them to -well out in great streams as lava-flows. Usually the action becomes -concentrated at one or several points at which the ejected materials -accumulate to form volcanic cones. - -Sometimes the volcanic activity dies away entirely after these cones -are thrown up along the line of fissure, but at others some such -centre becomes for a longer or shorter time the habitual vent for the -volcanic forces in the district, and by repeated ejections of lavas and -fragmentary materials at longer or shorter intervals the cone increases -both in height and bulk. - -When the height of the cone has grown to a certain extent, it becomes -more easy for the volcanic energies below to rend the sides of the -cone than to raise the molten materials to its summit. In this way -lateral or parasitic cones are thrown up on the flanks of the volcanic -mountain, the mass being alternately elevated and strengthened by the -ejections from the summit and sides respectively. - -When the volcanic energies no longer suffice to raise the fluid -materials to the summit, nor to rend the sides of the volcano, fissures -with small cones may be formed in the plains around the great central -volcano. - -At last, however, this energy diminishes so far that rock materials -can no longer be forced to the surface, the fissures become sealed up -by consolidating lava, and the volcanic cones fall into a condition of -extinction and decay. - -The existence of heated materials at no great depth from the surface -is indicated by the outburst of gases and vapours, the formation of -geysers, mud-volcanoes, and ordinary thermal springs. But as the -underlying rocks cool down, the issuing jets of gas and vapour lose -their high temperature and diminish in quantity, the geysers and -mud-volcanoes become extinct, and the thermal springs lose their -peculiar character or disappear, and thus all manifestations of the -igneous energies in the district gradually die away. - -[Sidenote: DURATION OF VOLCANIC CYCLES.] - -Such a cycle of changes probably requires many hundreds of thousands, -or even many millions, of years for its accomplishment; but by the -study of volcanoes in every stage of their growth and decline we are -able to reconstruct even the minutest details of their history. - - - - -CHAPTER VIII. - -THE DISTRIBUTION OF VOLCANOES UPON THE SURFACE OF THE GLOBE. - - -It is not by any means an easy task to frame an estimate of the number -of volcanoes in the world. Volcanoes, as we have seen, vary greatly -in their dimensions--from vast mountain masses, rising to a height -of nearly 25,000 feet above the sea-level, to mere molehills; the -smaller ones being in many cases subsidiary to larger, and constituting -either parasitic cones on their flanks, or 'puys' around their bases. -Volcanoes likewise exhibit every possible stage of development and -decay: while some are in a state of chronic active eruption, others -are reduced to the condition of solfataras, and others again have -fallen into a more or less complete state of ruin through the action of -denuding forces. - -Even if we confine our attention to the larger volcanoes, which merit -the name of 'mountains,' and such of these as we have reason to -believe to be in a still active condition, our difficulties will be -diminished, but not by any means removed. Volcanoes, as we have seen, -may sink into a dormant condition that may endure for hundreds or -even thousands of years, and then burst forth into a state of renewed -activity; and it is quite impossible, in many cases, to distinguish -between the conditions of dormancy and extinction. Concerning certain -small areas in Southern Europe, Western Asia, and Northern Africa, -historical records, more or less reliable, extend back over periods of -several thousands of years; but with regard to the greater part of the -rest of the world we have no information beyond a few hundred years, -and there are considerable areas which have been known only for far -shorter periods, while some are as yet quite unexplored. In districts -almost wholly uninhabited, or roamed over by nomadic tribes, legend -and tradition constitute our only guides--and very unsafe ones they -are--in the attempt to determine what volcanoes have recently been in a -condition of activity. - -[Sidenote: NUMBER OF ACTIVE VOLCANOES.] - -We shall, however, probably be within the limits of truth in stating -that the number of great habitual volcanic vents upon the globe, -which we have reason to believe are still in an active condition, is -somewhere between 300 and 350. Most of these active volcanic vents -are marked by more or less considerable mountains, composed of the -materials ejected from them. If we include the mountains which exhibit -the external conical form, the crateral hollows, and other features -of volcanoes, but concerning the activity of which we have no record -or tradition, the number will fall little, if anything, short of -1,000. The mountains composed of volcanic materials, but which have -lost through denudation the external form of volcanoes, are still more -numerous. The smaller temporary openings which are usually subordinate -to the habitual vents, that have been active during the periods -covered by history and tradition, must be numbered by thousands and -tens of thousands. The still feebler manifestations of the volcanic -forces--such as are exhibited in 'stufas,' or steam-jets, geysers, -or intermittent hot springs, thermal and mineral waters, fumaroles, -emitting various gases, salses or spouting saline and muddy springs, -and mud volcanoes--may be reckoned by millions. It is not improbable -that these less powerful manifestations of the volcanic forces, to a -great extent make up in number what they want in individual energy; and -the relief which they afford to the imprisoned activities within the -earth's crust may be scarcely less than that which results from the -occasional outbursts at the 300 or 350 great habitual volcanic vents. - -In taking a general survey of the volcanic phenomena of the globe, -no fact comes out more strikingly than that of the very unequal -distribution, in different districts, both of the great habitual -volcanic vents, and of the minor exhibitions of subterranean energy. - -[Sidenote: VOLCANOES OF THE CONTINENTS.] - -Thus, on the whole of the continent of Europe, there is but one -habitual volcanic vent--that of Vesuvius--and this is situated upon -the shores of the Mediterranean. In the islands of the Mediterranean, -however, there are no less than six volcanoes; namely, Stromboli and -Vulcano, in the Lipari Islands; Etna, in Sicily; Graham's Isle, a -submarine volcano, off the Sicilian coast; and Santorin and Nisyros, in -the Ægean Sea. - -The African continent is at present known to contain about ten active -volcanoes--four on the west coast, and six on the east coast; about ten -other active volcanoes occur on islands close to the African coasts. In -Asia, twenty-four active volcanoes are known, but no less than twelve -of these are situated in the peninsula of Kamtschatka. No volcanoes are -known to exist in the Australian continent. - -The American continent contains a greater number of volcanoes than -the divisions of the Old World. There are twenty in North America, -twenty-five in Central America, and thirty-seven in South America. - -Thus, taken altogether, there are about one hundred and seventeen -volcanoes situated on the great continental lands of the globe, while -nearly twice as many occur upon the islands scattered over the various -oceans. - -Upon examining further into the distribution of the continental -volcanoes, another very interesting fact presents itself. The volcanoes -are in almost every case situated either close to the coasts of the -continent, or at no great distance from them. There are, indeed, only -two exceptions to this rule. In the great and almost wholly unexplored -table-land lying between Siberia and Tibet four volcanoes are said to -exist, and in the Chinese province of Mantchouria several others. More -reliable information is, however, needed concerning these volcanoes, -situated, unlike all others, at a great distance from the sea. - -It is a remarkable circumstance that all the oceanic islands which -are not coral-reefs are composed of volcanic rocks; and many of these -oceanic islands, as well as others lying near the shores of the -continents, contain active volcanoes. - -Through the midst of the Atlantic Ocean runs a ridge, which, by the -soundings of the various exploring vessels sent out in recent years, -has been shown to divide the ocean longitudinally into two basins. -Upon this great ridge, and the spurs proceeding from it, rise numerous -mountainous masses, which constitute the well-known Atlantic islands -and groups of islands. All of these are of volcanic origin, and among -them are numerous active volcanoes. The Island of Jan Mayen contains -an active volcano, while Iceland contains thirteen, and not improbably -more; the Azores have six active volcanoes, the Canaries three; while -about eight volcanoes lie off the west coast of Africa. In the West -Indies there are six active volcanoes; and three submarine volcanoes -have been recorded within the limits of the Atlantic Ocean. Altogether, -no less than forty active volcanoes are situated upon the great -submarine ridges which traverse the Atlantic longitudinally. - -[Sidenote: VOLCANOES ON THE OCEANIC ISLANDS.] - -But along the same line the number of extinct volcanoes is far greater, -and there are not wanting proofs that the volcanoes which are still -active are approaching the condition of extinction. At a somewhat -earlier period of the earth's history the whole line of the present -Atlantic Ocean was in all probability traversed by a chain of volcanoes -on the very grandest scale; but submergence has taken place, and only a -few portions of this great mountain range now rise above the sea-level, -forming the isolated islands and island-groups of the Atlantic. Here -and there among these a still active volcano exists. - -But if the great medial chain of the Atlantic presents us with an -example of a chain of volcanic mountains verging on extinction, we have -in the line of islands separating the Pacific and Indian Oceans an -example of a similar range of volcanic vents which are in a condition -of the greatest activity. In the peninsula of Kamtschatka there are -twelve active volcanoes, in the Aleutian Islands thirty-one, and in -the peninsula of Alaska three. The chain of the Kuriles contains at -least ten active volcanoes; the Japanese Islands and the islands lying -to the south of Japan twenty-five. The great group of islands lying -to the south-east of the Asiatic continent is at the present time the -grandest focus of volcanic activity upon the globe. No less than fifty -active volcanoes occur here. Farther south, the same chain is probably -continued by the four active volcanoes of New Guinea, one or more -submarine volcanoes, and several vents in New Britain, the Solomon -Isles, and the New Hebrides, the three active volcanoes of New Zealand, -and possibly by Mount Erebus and Mount Terror in the Antarctic region. -Altogether, no less than 150 active volcanoes exist in the chain of -islands which stretch from Behring's Straits down to the Antarctic -circle; and if we include the volcanoes on Indian and Pacific islands -which appear to be situated on lines branching from this particular -band, we shall not be wrong in the assertion that this great system of -volcanic mountains includes at least one half of the habitually active -vents of the globe. - -A third series of volcanoes starts from near the last in the -neighbourhood of Behring's Straits, and stretches along the whole -western coast of the American continent. In this great range there are -about eighty active volcanoes. - -[Sidenote: LINEAR ARRANGEMENT OF VOLCANOES.] - -In considering the facts connected with the distribution of volcanoes -upon the globe, the one which, by its striking character, seems to -demand our attention in the first instance is that of the remarkable -linear arrangement of volcanic vents. We have already seen that small -scoria-cones are often thrown up on the flanks, or at the base, of a -great volcanic mountain, along lines which are manifestly lines of -fissure. In the eruption of Etna, in 1865, and again in that of 1874, -Professor Silvestri, of Catania, witnessed the actual opening of great -fissures on the north-east and north sides of the mountain: and -along the bottom of these cracks the glowing lava was clearly visible -(fig. 84, page 194). In the course of a few days, there were thrown -up a number of small scoria-cones along these lines of fissure--those -formed on the fissure of 1865 being seven in number, and those on the -fissure of 1874 being no less than thirty-six in number. Precisely -familiar phenomena were witnessed upon the slopes of Vesuvius, in 1760, -when a fissure opened on the south side of the mountain, and fifteen -scoria-cones, which are still visible, were thrown up along it. - -We have already considered the evidence pointing to the conclusion that -systems of volcanoes, like that of the Lipari Islands, are similarly -ranged along lines of fissures, and there is equally good ground for -believing that the great linear bands of volcanoes, which, as we -have seen, stretch for thousands of miles, have had their positions -determined by great lines of fissure in the earth's crust. While, -however, the smaller fissures, upon which rows of scoria-cones are -thrown up, seem to have been in many cases opened by a single effort -of the volcanic forces, the enormous fissures, which traverse so large -a portion of the surface of the globe, are doubtless the result of -numerous manifestations of energy extending over vast periods of time. - -The greatest of these bands along which the volcanic forces are so -powerfully exhibited at the present day, is the one which stretches -from near the Arctic circle at Behring's Straits to the Antarctic -circle at South Victoria. The line followed by this volcanic band, -which, as we have seen, includes more than one half of the active -volcanoes of the globe, is a very sinuous one, and it gives off -numerous offshoots upon either side of it. The great focus of this -intense volcanic action may be regarded as lying in the district -between the islands of Borneo and New Guinea. From this centre there -radiate a number of great lines, along which the volcanic forces are -exhibited in the most powerful manner. The first of these extends -northwards through the Philippine Isles, Japan, the Kurile Islands, and -Kamtschatka, giving off a branch to the east, which passes through the -Aleutian Islands and the peninsula of Alaska. This band, along which -the volcanic forces are very powerfully active, is continued towards -the south-east in the New Britain, the Solomon Islands, Santa Cruz, the -New Hebrides, New Zealand, and South Victoria. East and west from the -great central focus there proceed two principal branches. The former -of these extends through the Navigator Islands and Friendly Islands -as far as Elizabeth Islands. The latter passes through Java, and then -turns north-westward through Sumatra, the Nicobar Islands, the Andaman -Islands up to the coast of Burmah. - -The great band which we have been describing exhibits the most -striking examples of volcanic activity to be found upon the globe. -Besides the 150 or more volcanoes which are known to have been in a -state of activity during the historical period, there are several -hundred very perfect volcanic cones, many of which appear to have but -recently become extinct, if indeed, they are not simply in a dormant -condition. For long distances these chains of volcanic cones are almost -continuous, and the only very considerable breaks in the series are -those between New Zealand and the New Hebrides on the one hand, and -between the former islands and South Victoria on the other. - -[Sidenote: GREAT VOLCANIC BANDS OF THE GLOBE.] - -Much less continuous, but nevertheless very important, is the great -band of volcanoes which extends along the western side of the great -American continent, and contains, with its branches, nearly a hundred -active volcanoes. On the north this great band is almost united with -the one we have already described by the chain of the Aleutian and -Alaska volcanoes. In British Columbia about the parallel of 60° N. -there exist a number of volcanic mountains, one of which, Mount St. -Elias, is believed to be 18,000 feet in height, and several of these -have certainly been seen in a state of eruption. Farther south in the -part of the United States, territories drained by the Columbia River, -a number of grand volcanic mountains exist, some of which are probably -still active, for geysers and other manifestations of volcanic activity -abound. From the southern extremity of the peninsula of California -an almost continuous chain of volcanoes stretches through Mexico and -Guatemala, and from this part of the volcanic band a branch is given -off which passes through the West Indies, and forms a connection with -the great volcanic band of the Atlantic Ocean. In South America the -line is continued by the active volcanoes of Ecuador, Bolivia and -Chili, but at many intermediate points in the chain of the Andes -extinct volcanoes occur, which to a great extent fill up the gaps -in the series. A small offshoot to the westward passes through the -Galapagos Islands. The great band of volcanoes which stretches through -the American continent is second only in importance, and in the -activity of its vents, to the band which divides the Pacific from the -Indian Ocean. - -The third volcanic band of the globe is that which traverses the -Atlantic Ocean from north to south. This series of volcanic mountains -is much more broken and interrupted than the other two, and a greater -proportion of its vents are extinct. This chain, as we shall show in a -future chapter, attained its condition of maximum activity during the -distant period of the Miocene, and now appears to be passing into a -state of gradual extinction. Beginning in the north with the volcanic -rocks of Greenland and Bear Island, we pass southwards, by way of Jan -Mayen, Iceland, and the Faroe Islands, to the Hebrides and the north -of Ireland. Thence by way of the Azores, the Canaries and the Cape de -Verde Islands, with some active vents, we pass to the ruined volcanoes -of St. Paul, Fernando de Noronha, Ascension, St. Helena, Trinidad and -Tristan d'Acunha. From this great Atlantic band two branches proceed -to the eastward, one through Central Europe, where all the vents are -now extinct, and the other through the Mediterranean to Asia Minor, -the great majority of the volcanoes along the latter line being now -extinct, though a few are still active. The vol canoes on the eastern -coast of Africa may be regarded as situated on another branch from this -Atlantic volcanic band. The number of active volcanoes on this Atlantic -band and its branches, exclusive of those in the West Indies, does not -exceed fifty. - -[Sidenote: LENGTH OF THE VOLCANIC BANDS.] - -From what has been said, it will be seen that, not only do the -volcanoes of the globe usually assume a linear arrangement, but nearly -the whole of them can be shown to be thrown up along three well-marked -bands and the branches proceeding from them. The first and most -important of these bands is nearly 10,000 miles in length, and with its -branches contains more than 150 active volcanoes; the second is 8,000 -miles in length, and includes about 100 active volcanoes; the third is -much more broken and interrupted, extends to a length of nearly 1,000 -miles, and contains about 50 active vents. The volcanoes of the eastern -coast of Africa, with Mauritius, Bourbon, Rodriguez, and the vents -along the line of the Red Sea, may be regarded as forming a fourth and -subordinate band. - -Thus we see that the surface of the globe is covered by a network -of volcanic bands, all of which traverse it in sinuous lines with a -general north-and-south direction, giving off branches which often -run for hundreds of miles, and sometimes appear to form a connection -between the great bands. - -These four bands of volcanic vents, running in a general -north-and-south direction, separate four unequal areas within which the -exhibitions of volcanic activity are feeble or quite unknown. The two -grandest of the bands of volcanic activity, with their branches, form -an almost complete series encircling the largest of the oceans. - -To this rule of the linear arrangement of the volcanic vents of the -globe and their accumulation along certain well-marked bands, there are -two very striking exceptions, which we must now proceed to notice. - -In the very centre of the continent formed by Europe and Asia, the -largest unbroken land-mass of the globe, there rises from the great -central plateau the remarkable volcanoes of the Thian Shan Range. -The existence of these volcanoes, of which only obscure traditional -accounts had reached Europe before the year 1858, appears to be -completely established by the researches of the Russian traveller -Semenof. Three volcanic vents appear to exist in this region: the -active volcanoes of Boschan and Turfan or Hot-schen, and the solfatara -of Urumtsi. At a point situated about half-way between these three -volcanoes and the sea, another active vent, that of Ujung-Holdongi, is -said to exist. Other volcanic phenomena have been stated to occur in -the great plateau of Central Asia, but the existence of some at least -of these appears to rest on very doubtful evidence. The only accounts -which we have of the eruptions of these Thian Shan volcanoes are -contained in Chinese histories and treatises on geography; and a great -service would be rendered to science could they be visited by some -competent explorer. - -[Sidenote: EXCEPTIONALLY-SITUATED VOLCANOES.] - -The second exceptionally-situated volcanic group is that of the -Sandwich Islands. While the Thian Shan volcanoes rise in the centre of -the largest unbroken land-mass, and stand on the edge of the loftiest -and greatest plateau in the world, the volcanoes of the Sandwich -Islands rise almost in the centre of the largest ocean and from almost -the greatest depths in that ocean. All round the Sandwich Islands the -sea has a depth of from 2,000 to 3,000 fathoms, and the island-group -culminates in several volcanic cones which rise to the height of nearly -14,000 feet above the sea-level. The volcanoes of the Sandwich Islands -are unsurpassed in height and bulk by those of any other part of the -globe. - -With the exception of the two isolated groups of the Thian Shan and -the Sandwich Islands, nearly all the active volcanoes of the globe -are situated near the limits which separate the great land- and -water-masses of the globe--that is to say, they occur either on the -parts of continents not far removed from their coast-lines, or on -islands in the ocean not very distant from the shores. - -The fact of the general proximity of volcanoes to the sea, is one which -has frequently been pointed out by geographers, and may now be regarded -as being thoroughly established. Even the apparently anomalous case of -the Thian Shan volcanoes is susceptible of explanation if we remember -the fact, now well ascertained by geological researches, that as -late certainly as Pliocene times, a great inland sea spread over the -districts where the Caspian, the Sea of Aral, and many other isolated -lakes are now found. Upon the southern shore of this sea rose the -volcanoes of the Thian Shan, some of which have not yet fallen into a -state of complete extinction. - -But although the facts concerning the general proximity of volcanoes to -the ocean may be admitted to be thoroughly established, yet inferences -are sometimes hastily drawn from these facts which the latter, if -fairly considered, will not be found to warrant. It is frequently -assumed that we may refer all the remarkable phenomena of volcanic -action to the penetration of sea-water to a mass of incandescent lava -in the earth's crust, and to the chemical or mechanical action which -would result from this meeting of sea-water and molten rock. And this -conclusion is supposed to find support in the circumstance that many -of the gases and volatile substances emitted from volcanic vents are -such as would be produced by the decomposition of the various salts -contained in sea-water. - -This argument in favour of the production of volcanic outbursts by -the irruption of sea-water into subterranean reservoirs, involves, as -Mr. Scrope long ago pointed out, a curious example of reasoning in a -circle. It is assumed, on the one hand, that the heaving subterranean -movements, which give rise to the fissures by which steam and other -gases escape to the surface, are the result of the passage of water -to heated masses in the earth's crust. But, on the other hand, it is -supposed that it is the production of these fissures which leads to -the influx of water to the heated materials. If it is the passage of -water through these fissures which produces the eruptions, it may be -fairly asked, what is it that gives rise to the fissures? And if, on -the other hand, there exist subterranean forces competent to produce -the fissures, may they not also give rise to the eruptions through the -openings which they have originated? Nor does the chemical argument -appear to rest upon any surer ground. It is true that many of the -volatile substances emitted from volcanic vents are such as might be -produced by the decomposition of sea-water, but, upon the other hand, -there are not a few substances which cannot possibly be regarded as so -produced, and, all the materials may equally well be supposed to have -been originally imprisoned in the masses of subterranean lava. - -[Sidenote: CAUSE OF PROXIMITY OF VOLCANOES TO SEA.] - -The problem before us is this. Granting that it is proved that active -volcanoes are always in close proximity to the ocean, are we to explain -the fact by supposing that the agency of sea-water is necessary to -volcanic outbursts, or by regarding the position of the coast-lines -as to some extent determined by the distribution of volcanic action -upon the surface of the globe? The first supposition is the one which -perhaps most readily suggests itself, but the latter, as we shall -hereafter show, is one in favour of which not a few weighty arguments -may be advanced. - -Another problem which suggests itself in connection with the -distribution of volcanoes is the following. Are the great depressed -tracts which form the bottom of the oceans, like the elevated tracts -which constitute the continents, equally free from exhibitions of -volcanic energy? - -When we remember the fact that the area of the ocean beds is two and -three-quarter times as great as that of the continents, it will be seen -how important this question of the existence of volcanoes at the bottom -of the ocean really is. - -The fact that recent deep-sea soundings have shown the deepest parts -of the ocean to be everywhere covered with volcanic _débris_ is by -no means conclusive upon this question; for, as we have seen, the -ejections of sub-aerial volcanoes are by the wind and waves distributed -over every part of the earth's surface. - -[Sidenote: SUBMARINE ERUPTIONS.] - -Submarine volcanic outbursts have occurred in many parts of the -globe, but it may well be doubted whether any such outburst has ever -commenced at the bottom of a deep ocean, and has succeeded in building -up a volcanic cone reaching to the surface. Most, if not all, of the -recorded submarine outbursts have occurred in the midst of volcanic -districts, and the volcanic cones have been built up in water of no -great depth. Indeed, when it is remembered that the pressure of each -1,000 fathoms of water is equivalent to a weight of more than one ton -on every square inch of the ocean-bottom, it is difficult to imagine -the ordinary explosive action of volcanic vents taking place at abysmal -depths. If, however, fissures were opened in the beds of the ocean, -quiet outwellings of lava might possibly occur. - -The solution of this problem of the probable existence of volcanic -outbursts on the floor of the ocean can only be hoped for from the -researches of the geologist. The small specimens of the ocean-beds -brought up by deep-sea sounding-lines, taken at wide distances apart, -and including but a few inches from the surface, can certainly afford -but little information upon the question. But the geologist has the -opportunity of studying the sea-bottoms of various geological periods -which have been upheaved and are now exposed to his view. It was at -one time supposed by geologists that in the so-called 'trap-rocks' we -have great lava-sheets which must have been piled upon one another, -without explosive action. But the more accurate researches of recent -years have shown that between the layers of 'trap-rock,' in every part -of the globe, traces of terrestrial surfaces and freshwater deposits -are found; and the supposed proofs of the absence of explosive action -break down no less signally upon re-examination; for the loose, -scoriaceous materials would either be removed by denudation, or -converted into hard and solid rocks by the infilling of their vesicles -and air-cavities with crystalline minerals. It is not possible, among -the representatives of former geological periods, to point to any rocks -that can be fairly regarded as having issued from great submarine -fissures, and it is therefore fair to conclude that no such great -outbursts of the volcanic forces take plane at the present day on the -deep ocean-floors. - -In connection with the question of the relation between the position of -the volcanic bands of the globe and the areas covered by the ocean, we -may mention a fact which deep-sea soundings appear to indicate, namely, -that the deepest holes in the ocean-floor are situated in volcanic -areas. Near Japan, the soundings of the U.S. ship 'Tuscarora' showed -that at two points the depth exceeded 4,000 fathoms; and the deepest -sounding obtained by H.M.S. 'Challenger,' amounting to 4,575 fathoms, -was taken in the voyage from New Gruinea to Japan, in the neighbourhood -of the Ladrone Islands. Depths nearly as great were found in the -soundings carried on in the neighbourhood of the volcanic group of the -West Indian Islands. It must be remembered, however, that at present -our knowledge of the depths of the abysmal portions of the ocean is -very limited. A few lines of soundings, often taken at great distances -apart, are all we have to guide us to any conclusions concerning the -floors of the great oceans, and between these lines are enormous areas -which still remain altogether unexplored. It may be wise, therefore, to -suspend our judgment upon such questions till more numerous facts have -been obtained. - -[Sidenote: RELATIONS TO MOUNTAIN-CHAINS.] - -Another fact concerning the distribution of volcanoes which is worthy -of remark is their relation to the great mountain-ranges of the globe. - -Many of the grandest mountain-chains have bands of volcanoes -lying parallel to them. This is stinkingly exhibited by the great -mountain-masses which lie on the western side of the American -continent. The Rocky Mountains and the Andes consist of folded and -crumpled masses of altered strata which, by the action of denuding -forces, have been carved into series of ridges and summits. At many -points, however, along the sides of these great chains, we find that -fissures have been opened and lines of volcanoes formed, from which -enormous quantities of lava have flowed and covered great tracts of -country. At some parts of the chain, however, the volcanoes are of such -height and dimensions as to overlook and dwarf the mountain-ranges by -the side of which they lie. Some of the volcanoes lying parallel to the -great American axis appear to be quite extinct, while others are in -full activity. - -In the Eastern continent we find still more striking examples of -the parallelism between great mountain-chains and the lands along -which volcanic activity is exhibited. Stretching in a more or less -continuous chain from east to west, through Europe and Asia, we find -the mountain-masses known in different parts of their course as the -Pyrenees, the Alps, the Balkan, the Caucasus, which form the axis -of the Eastern continent. These chains consist of numerous parallel -ridges, and give off branches on either side of them. They are -continued to the eastward by the Hindoo Koosh and the Himalaya, with -the four parallel ranges that cross the great Central-Asian plateau. -Now, on either side of this grand axial system of mountains, we find -a great parallel band of volcanoes. The northern volcanic band is -constituted by the eruptive rocks of the Auvergne, the Eifel, the -Siebengebirge, Central Germany, Bohemia, Hungary, and Transylvania, -few, if any, of the vents along this northern band being still active. -The remarkable volcanoes of the Thian Shan range and of Mantchouria may -not improbably be regarded as a continuation of the same great series. - -The southern band of volcanoes, lying parallel to the great mountain -axis of the Old World, also consists for the most part of extinct -volcanoes, but includes not a few vents which are still active. In -this band we include the extinct volcanoes of Spain and Sardinia, the -numerous extinct and active vents of the Italian peninsula and islands, -and those of the Ægean Sea and Asia Minor. We may, perhaps, consider -the scattered volcanoes of Arabia and the northern part of the Indian -Ocean as a continuation of the same series. Both of these bands may -be regarded as offshoots from the great mid-Atlantic volcanic chain, -and the condition of the vents, both in the principal band and its -offshoots, is such as to indicate that they form parts of a system -which is gradually sinking into a state of complete extinction. - -There are some other volcanic bands which exhibit a similar parallelism -with mountain chains; but, on the other hand, there are some volcanoes -between which and the nearest mountain axes no such connection can be -traced. - -[Sidenote: RELATION TO AREAS OF UPHEAVAL.] - -There is yet one other fact concerning the mode of distribution of -volcanoes upon the surface of the globe, to which we must allude. It -was first established by Mr. Darwin as one of the conclusions derived -from the valuable series of observations made by him during the voyage -of H.M.S. 'Beagle,' and relates to the position of active volcanoes -with respect to the portions of the earth's crust which are undergoing -upheaval or subsidence. - -From the relative position of the different kinds of coral-reefs, and -the fact that reef-forming corals cannot live at a depth of more than -twenty fathoms beneath the sea-level, or above tide-mark, we are led to -the conclusion that certain areas of the earth's surface are undergoing -slow elevation, while other parts are as gradually subsiding. This -conclusion is confirmed by the occurrence of raised beaches, which -are sometimes found at heights of hundreds, or even thousands, of -feet above the sea-level, and of submerged forests, which are not -unfrequently found beneath the waters of the ocean. - -By a study of the evidences presented by coral-reefs, raised beaches, -submerged forests, and other phenomena of a similar kind, it can be -shown that certain wide areas of the land and of the ocean-floor are -at the present time in a state of subsidence, while other equally -large areas are being upheaved. And the observations of the geologist -prove that similar upward and downward movements of portions of the -earth's crust have been going on through all geological times. Now, -as Mr. Darwin has so well shown in his work on 'Coral-Reefs,' if we -trace upon a map the areas of the earth's surface which are undergoing -upheaval and subsidence respectively, we shall find that nearly all the -active volcanoes of the globe are situated upon rising areas, and that -volcanic phenomena are conspicuously absent from those parts of the -earth's crust which can be proved at the present day to be undergoing -depression. - - - - -CHAPTER IX. - -VOLCANIC ACTION AT DIFFERENT PERIODS OF THE EARTH'S HISTORY. - - -It is only in comparatively recent times that the important doctrine of -geological continuity has come to be generally accepted, as furnishing -us with a complete and satisfactory explanation of the mode of origin -of the features of our globe. The great forces, which are ever at -work producing modifications in those features, operate so silently -and slowly, though withal so surely, that without the closest and -most attentive observation their effects may be easily overlooked; -while, on the other hand, there are so many phenomena upon our globe -which seem at first sight to bear testimony to the action of sudden -and catastrophic forces, very different to any which appear to be at -present at work, that the tendency to account for all past changes by -these violent actions is a very strong one. In spite of this tendency, -however, the real potency of the forces now at work upon the earth's -crust has gradually made its way to recognition, and the capability of -these forces, when their effects are accumulated through sufficiently -long periods of time, to bring about the grandest changes, is now -almost universally admitted. The modern science of geology is based -upon the principle that the history of the formation and development -of the earth's surface-features, and of the organisms upon it, has -been continuous during enormous periods of time, and that in the study -of the operations taking place upon the earth at the present day, we -may find the true key to the changes which have occurred during former -periods. - -In no branch of geological science has the doctrine of continuity -had to encounter so much opposition and misconception as in that -which relates to the volcanic phenomena of the globe. For a long time -students of rocks utterly failed to recognise any relation between the -materials which have been ejected from active volcanic vents and those -which have been formed by similar agencies at earlier periods of the -earth's history. And what was far worse, the subject became removed -from the sphere of practical scientific inquiry to that of theological -controversy, those who maintained the volcanic origin of some of the -older rocks being branded as the worst of heretics. - -[Sidenote: CONTROVERSY CONCERNING ORIGIN OF BASALT.] - -With the theological aspects of the great controversy concerning the -origin of basalt and similar rocks--a controversy which was carried -on with such violence and acrimony during the latter half of the -eighteenth century--we have here nothing to do. But it may not be -uninstructive to notice the causes of the strange misconceptions which -for so long a period stood in the way of the acceptance of rational -views upon the subject. - -At this period but little had been done in studying the chemical -characters of aqueous and igneous rock-masses respectively; and while, -on the one hand, the close similarity in chemical composition between -the ancient basalts and many modern lavas was not recognised, the -marked distinction between the composition of such materials and most -aqueous sediments remained, on the other hand, equally unknown. Nor -had anything been yet accomplished in the direction of the study of -rock-masses by the aid of the microscope. Hence there could be no -appeal to those numerous structural peculiarities that at once enable -us to distinguish the most crystalline aqueous rocks from the materials -of igneous origin. - -On the other hand, there undoubtedly exist rocks of a black colour -and crystalline structure, sometimes presenting a striking similarity -in general appearance to the basalts, which contain fossils and are -undoubtedly of aqueous origin. Thus on the shore near Portrush, in the -North of Ireland, and in the skerries which lie off that coast, there -occur great rock-masses, some of which undoubtedly agree with basalt in -all their characters, while others are dark-coloured and crystalline, -and are frequently crowded with _Ammonites_ and other fossils. We now -know that the explanation of these facts is as follows. Near where -the town of Portrush is now situated, a volcanic vent was opened in -Miocene times through rocks of Lias shale. From this igneous centre, -sheets and dykes of basaltic lava were given off, and in consequence -of their contact with these masses of lava, the Lias shales were baked -and altered, and assumed a crystalline character, though the traces of -the fossils contained in them were not altogether obliterated. In the -last century the methods which had been devised for the discrimination -of rocks were so imperfect that no distinction was recognised between -the true basalt and the altered shale, and specimens of the latter -containing _Ammonites_ found their way to almost every museum in -Europe, and were used as illustrations of the 'origin of basalt by -aqueous precipitation.' - -Another source of the widely-spread error which prevailed concerning -the origin of basalt, was the failure to recognise the nature of -the alterations which take place in the character of rock-masses in -consequence of the passage through them, during enormous periods of -time, of water containing carbonic acid and other active chemical -agents. The casual observer does not recognise the resemblance which -exists between certain ornamental marbles and the loose accumulations -of shells and corals which form many sea-beaches; but close examination -shows that the former consist of the same materials as the latter, -bound together by a crystalline infilling of carbonate of lime, which -has been deposited in all the cavities and interstices of the mass. In -the same way, as we have already seen, the vesicles and interstices -of heaps of scoriæ may, by the percolation of water through the mass, -become so filled with various crystalline substances, that its original -characters are entirely masked. - -But the progress of chemical and microscopic research has effectually -removed these sources of error. Many rocks of aqueous origin, formerly -confounded with the basalts, have now been relegated to their proper -places among the different classes of rocks; while, on the other -hand, it has been shown that the chemical and physical differences -between the ancient basalts and the modern basic lavas are slight -and accidental, and their resemblances are of the closest and most -fundamental character. - -[Sidenote: VOLCANIC ORIGIN OF 'TRAP ROCKS.'] - -The notion of the aqueous origin of basalt, which was so long -maintained by the school of Werner, has now been entirely abandoned, -and the so-called 'trap-rocks' are at the present day recognised as -being as truly volcanic in their origin as the lavas of Etna and -Vesuvius. - -There is, however, a vestige of this doctrine of Werner, which still -maintains its ground with obstinate persistence. Many geologists in -Germany who admit that volcanic phenomena, similar to those which are -going on at the present day, must have occurred during the Tertiary and -the later Secondary periods, nevertheless insist that among the earlier -records of the world's history we find no evidence whatever of such -volcanic action having taken place. By the geologists who hold these -views it is asserted that while the granites and other plutonic rocks -were formed during the earlier periods of the world's history, true -volcanic products are only known in connection with the sediment of the -later geological periods. - -Some geologists have gone farther even than this, and asserted that -each of the great geological periods is characterised by the nature -of the igneous ejections which have taken place in it. They declare -that granite was formed only during the earliest geological periods, -and that at later dates the gabbros, diabases, porphyries, dolerites -and basalts, successively made their appearance, and finally that the -modern lavas were poured out. - -A little consideration will suffice to convince us that these -conclusions are not based upon any good evidence. The plutonic rocks, -as we have already seen, exhibit sufficient proofs in their highly -crystalline character, and in their cavities containing water, -liquefied carbonic acid, and other volatile substances, that they must -have been formed by the very slow consolidation of igneous materials -under enormous pressure. Such pressures, it is evident, could only -exist at great depths beneath the earth's surface. Mr. Sorby and -others have endeavoured to calculate what was the actual thickness of -rock under which certain granites must have been formed, by measuring -the amount of contraction in the liquids which have been imprisoned -in the crystals of these rocks. The conclusions arrived at are of a -sufficiently startling character. It is inferred that the granites -which have been thus examined must have consolidated at depths varying -from 30,000 to 80,000 feet beneath the earth's surface. It is true -that in arriving at these results certain assumptions have to be made, -and to these exception may be taken, but the general conclusion that -granitic rocks could only have been formed under such high pressures as -exist at great depths beneath the surface, appears to be one which is -not open to reasonable doubt. - -If, then, granites and similar rocks were formed at the depth of some -miles, it is evident that they can only have made their appearance at -the surface by the removal of the vast thickness of overlying rocks; -and the sole agency which we know of that is capable of effecting the -removal of such enormous quantities of rock-materials, is denudation. -But the agents of denudation--rain and frost, rivers and glaciers, and -sea-waves--though producing grand results, yet work exceeding slowly; -and almost inconceivably long periods of time must have elapsed before -masses of rock several miles in thickness could have been removed, and -the subjacent granites and other highly crystalline rocks have been -exposed at the surface. - -[Sidenote: ANCIENT AND MODERN VOLCANIC ROCKS.] - -It is an admitted fact that among the older geological formations, we -much more frequently find intrusions of granitic rocks than in the case -of younger ones. It is equally true that among the sediments formed -during the most recent geological periods, no true granitic rocks have -been detected. But if, as we insist is the case, granitic rocks can -only be formed at a great depth from the surface, the £acts we have -described are only just what we might expect to present themselves -under the circumstances. The older a mass of granitic rock, the greater -chance there is that the denuding forces operating upon the overlying -masses, will have had an opportunity of so far removing the latter -as to expose the underlying crystalline rocks at the surface. And, -on the other hand, the younger crystalline rocks are still, for the -most part, buried under such enormous thicknesses of superincumbent -materials that it is hopeless for us to search for them. Nevertheless, -it does occasionally happen that, where the work of denudation has been -exceptionally rapid in its action, such crystalline rocks formed during -a comparatively recent geological period, are exposed at the surface. -This is the case in the Western Isles of Scotland and in the Pyrenees, -where masses of granite and other highly crystalline rocks are found -which were evidently formed during the Tertiary period. - -The granites which were formed in Tertiary times present no essential -points of difference from those which had their origin during the -earlier periods of the earth's history. The former, like the latter, -consist of a mass of crystals with no imperfectly crystalline base or -groundmass between them; and these crystals include numerous cavities -containing liquids. - -Between the granites and the quartz-felsites every possible -gradation may be found, so that it is impossible to say where the one -group ends and the other begins; indeed, many of the rocks called -'granite-porphyries' have about equal claims to be placed in either -class. Nor is the distinction between the quartz-felsites and rhyolites -any more strongly marked than that between the former class of rocks -and the granites; some of the more crystalline rhyolites of Hungary -being quite undistinguishable, in their chemical composition, their -mineralogical constitution, and their microscopic characters, from -the quartz-felsites. The more crystalline rhyolites are in turn found -passing by insensible gradations into the glassy varieties and finally -into obsidian. - -[Sidenote: RELATIONS BETWEEN GRANITE AND PUMICE.] - -A piece of granite and a piece of pumice may at first sight appear to -present so many points of difference, that it would seem quite futile -to attempt to discover any connection between them. Yet, if we analyse -the two substances, we may find that in ultimate chemical composition -they are absolutely identical. There is nothing irrational, therefore, -in the conclusion that the same materials under different conditions -may assume either the characters of granite on the one hand, or of -pumice on the other; the former being consolidated under circumstances -in which the chemical and crystalline forces have had the freest play -and have used up the whole of the materials to form crystallised -minerals, while the latter has cooled down and solidified rapidly at -the surface, in such a way that only incipient crystallisation has -occurred, and the glassy mass has been reduced to a frothy condition -by the escape of steam-bubbles from its midst This conclusion receives -the strongest support from the fact that examples of every stage of -the change, between the glassy condition of pumice and the crystalline -condition of granite, may be detected among the materials of which the -globe is built up. - -There is still another class of facts which may be adduced in -support of the same conclusion. Many lavas, as we have seen, contain -crystals of much larger dimensions than those constituting the mass -of the rock, which is then said to be 'porphyritic' in structure. -The porphyritically embedded crystals, when carefully examined, are -often seen to be broken and injured, and to exhibit rounded edges, -with other indications of having undergone transport. When examined -microscopically, too, they often present the cavities containing -liquids which distinguish the crystals of plutonic rocks. All the -facts connected with these porphyritic lavas point to the conclusion -that while the crystals in their groundmass have separated from the -liquefied materials near the surface, the large embedded crystal, have -been floated up from great depths within the earth's crust, where they -had been originally formed. - -[Sidenote: GRANITIC REPRESENTATIVES OF OTHER LAVAS.] - -The careful consideration of all the facts of the case leads to the -conclusion that where pumice, obsidian, and rhyolite are now being -ejected at the surface, the materials which form these substances -are, at various depths in the earth's interior, slowly consolidating -in the form of quartz-felsite, granite-porphyry and granite. It may -be that we can nowhere point to the example of a mass of rock which -can be traced from subterranean regions to the surface, and is, under -such conditions, actually seen to pass from the dense and crystalline -condition of granite to the vesicular and glassy form of pumice; -but great granitic masses often exhibit a more coarsely crystalline -condition in their interior, and the offshoots and dykes which they -give off not infrequently assume the form of quartz-felsite; while, -on the other hand, the more slowly consolidated rocks found in the -interior of some rhyolite masses are not distinguishable in any way -from some of the true quartz-felsites. - -That which is true of the lavas of acid composition is equally true -of the lavas of intermediate and basic character. The andesites, -the trachytes, the phonolites, and the basalts have all their exact -representatives among the plutonic rocks, and these have a perfectly -crystalline or granitic structure. The plutonic and the volcanic -representatives of each of these groups are identical in their chemical -composition, and numerous intermediate gradations can be found between -the most completely granitic and the most perfectly vitreous or glassy -types. In illustration of this fact, we may again refer to the series -of microscopic sections of rocks given in the frontispiece. - -Another objection to the conclusion that the volcanic products of -earlier periods of the earth's history were identical in character with -those which are being ejected at the present day is based on the fact -of the supposed non-existence of the scoriaceous and glassy materials -which abound in the neighbourhood of the active volcanic vents. Where, -it is asked, do we find among the older rocks of the globe the heaps of -lapilli, dust, and scoriæ, with the glassy and pumiceous rocks that now -occur so abundantly in all volcanic districts? - -In reply to this objection, we may point out that these accumulations -of loose materials are of such a nature as to be capable of easy -removal by denuding agents, and that as they are formed upon the land -they will, if not already washed away by the action of rain, floods, -rivers, &c., run great risk of having their materials distributed, -when the land sinks beneath the waters of the ocean and the surface -is covered by new deposits. With respect to the glassy rocks it must -be remembered that the action of water, containing carbonic acid and -other substances, in percolating through such masses has a tendency -to set up crystalline action, and these glassy rocks easily undergo -'devitrification'; it would therefore be illogical for us to expect -glassy rock-masses to retain their vitreous character through long -geological periods, during which they have been subjected to the action -of water and acid gases. - -But careful observation has shown that the scoriaceous and vitreous -rocks are by no means absent among the igneous materials ejected during -earlier periods of the earth's history. Their comparative infrequency -is easily accounted for when we remember, in the first place, the ease -with which such materials would be removed by denuding forces, and in -the second place, the tendency of the action of percolating water to -destroy their characteristic features, by filling up their vesicles -with crystalline products and by effecting devitrification in their -mass. - -[Sidenote: SIMILARITY OF ANCIENT AND RECENT LAVAS.] - -If we go back to the very oldest known rock-masses of the globe, those -which are found underlying the fossiliferous Cambrian strata, we find -abundant evidence that volcanic action took place during the period -in which these materials were being accumulated. Thus, in the Wrekin, -as Mr. Allport has so well shown, we find clear proofs that before -the long-distant period of the Cambrian, there existed volcanoes -which ejected scoriæ, lapilli, and volcanic dust, and also gave rise -to streams of lava exhibiting the characteristic structures found in -glassy rocks. In these rocks, which have undergone a curious alteration -or devitrification, we still find all those peculiar structures--the -sphærulitic, the perlitic, and the banded--so common in the rhyolites -of Hungary, with which rocks the Wrekin lavas, in their chemical -composition, precisely agree. Prof. Bonney, too, has shown that the -rocks of Charnwood Forest, which are also probably of pre-Cambrian age, -contain great quantities of altered volcanic agglomerates, tuffs, and -ashes. I have found the sphærulitic, perlitic, and banded structures -exhibited by British lavas of the Cambrian, Silurian, Devonian and -Carboniferous periods, as well as in those of Tertiary age; and in -connection with these different lavas we find vast accumulations, -sometimes thousands of feet in thickness, of volcanic agglomerates and -tuffs which have undergone great alteration. - -All these facts point to one conclusion--namely, that during all past -geological periods, materials similar to those which are now being -extruded from volcanic vents were poured out on the earth's surface -by analogous agencies. If we could trace the lava-streams of the -present day down to the great subterranean reservoirs from which their -materials have been derived, we should doubtless find that at gradually -increasing depths, where the pressure would be greater and the escape -of heat from the mass slower, the rocky materials would by degrees -assume more and more crystalline characters. We should thus find -obsidian or rhyolite insensibly passing into quartz-felsite and finally -into granite; trachyte passing into orthoclase-porphyry and syenite; -and basalt passing into dolerite, augite-porphyry, and gabbro. - -On the other hand, if we could replace the great masses of stratified -rocks which must once have overlain the granites, syenites, diorites, -and gabbros, we should find that, as we approached the original -surface, these igneous materials would gradually lose their crystalline -characters, and when they were poured out at the surface would take the -forms of rhyolite, trachyte, andesite, and basalt--all of which might -occasionally assume the glassy forms known as obsidian or tachylyte. - -[Sidenote: ALTERED FORMS OF ANCIENT LAVAS.] - -But while we insist on the essential points of similarity between -the lavas poured out upon the surface of the earth during earlier -geological periods and those which are being extruded at the present -day, we must not forget that by the action of percolating water and -acid gases, the mineral constitution, the structure, and sometimes -even the chemical composition of these ancient lavas may undergo -a vast amount of change. In not a few cases these changes in the -characters of a lava may be carried so far that the altered rock bears -but little resemblance to the lava from which it was formed, and it may -be found desirable to give it a new name. Among the rocks of aqueous -origin we find similar differences in the materials deposited at -different geological periods. Clay, shale and clay-slate have the same -composition, and the two latter are evidently only altered forms of the -first mentioned, yet so great is the difference in their characters -that it is not only allowable, but desirable, to give them distinctive -names. - -In the same way, among the deposits of the earlier geological periods -we find rocks which were doubtless originally basalts, but in which -great alterations have been produced by the percolation of water -through the mass. The original rock has consisted of crystals of -felspar, augite, olivine, and magnetite distributed through a glassy -base. But the chemical action of water and carbonic acid may have -affected all the ingredients of the rock. The outward form of the -felspar crystals may be retained while their substance is changed to -kaolinite, various zeolites, and other minerals; the olivine maybe -altered to serpentine and other analogous minerals; the magnetite -changed to hydrous peroxide of iron; the augite may be changed to -uralite or hornblende; and the surrounding glassy mass more or -less devitrified and decomposed. The hard, dense, and black rock -known as basalt has under these circumstances become a much softer, -earthy-looking mass of a reddish-brown tint, and its difference from -basalt is so marked that geologists have agreed to call it by another -name, that of 'melaphyre.' Even in their ultimate chemical compositions -the 'melaphyres' differ to some extent from the basalts, for some of -the materials of the latter may have been removed in solution, and -water, oxygen, and carbonic acid have been introduced to combine with -the remaining ingredients. - -But if we carefully study, by the aid of the microscope, a large series -of basalts and melaphyres, we shall find that many rocks of the former -class show the first incipient traces of those changes which would -reduce them to the latter class. Indeed, it is quite easy to form a -perfect series from quite unaltered basalts to the most completely -changed melaphyres. Hence we are justified in concluding that all the -melaphyres were originally basalts, just as we infer that all oaks were -once acorns. - -Now changes, similar to those which we have seen to take place in -the case of basaltic lavas, are exhibited by the lavas of every -other class, which have been exposed to the influence of the same -agencies,--namely, the passage of water and acid gases. But inasmuch as -the minerals composing the basic lavas are for the most part much more -easily affected by such agencies than are the minerals of acid lavas, -the ancient basic rocks are usually found in a much more highly altered -condition than are the acid rocks of equivalent age. - -[Sidenote: NAMES GIVEN TO ALTERED LAVAS.] - -We thus see that each of the classes of modern lavas has its -representative in earlier geological periods, in the form of rocks -which have evidently been derived from these lavas, through alterations -effected by the agency of water and acid-gases that have permeated -their mass. Thus, while the basalts are represented among the ancient -geological formation by the melaphyres, the andesites are represented -by the porphyrites, and the trachytes and rhyolites by different -varieties of felstones. And, as we can form perfect series illustrating -the gradual change from basalt to melaphyre, so we can arrange other -series demonstrating the passage of andesites into porphyrites, and of -trachytes and rhyolites into felsites. - -It must be remembered, however, that these changes do not take place -in anything like determinate periods of time. Occasionally we may -find lavas of ancient date which have undergone surprisingly little -alteration, and in other cases there occur lavas belonging to a -comparatively recent period which exhibit very marked signs of change. - -The alteration of the lavas and other igneous rocks does not, however, -stop with the production of the melaphyres, porphyrites, and felstones. -By the further action of the water and carbonic acid of the atmosphere, -the basic lavas are reduced to the soft earthy mass known as 'wacke,' -and the intermediate and acid lavas to the similar material known as -'claystone.' As the passage of water and carbonic acid gas through -these rock-masses goes on, they are eventually resolved into two -portions, one of which is insoluble in water and the other is soluble. -The insoluble portion consists principally of quartz, the crystals -of which are almost unattacked by water and carbonic acid, and the -hydrated silicate of alumina. All the sands and clays, which together -make up more than nine-tenths of the stratified rocks of the globe, -are doubtless derived, either directly or indirectly, from these -insoluble materials separated during the decomposition of volcanic and -plutonic rocks. The soluble materials, which consist of the carbonates, -sulphates and chlorides of lime, magnesia, soda, potash, and iron, -give rise to the formation of the limestones, gypsum, rock-salt, -ironstones, and other stratified masses of the earth's crust. We thus -see how the igneous materials of the globe, by their decomposition, -famish the materials for the stratified rock-masses. The relations of -the different plutonic and volcanic rocks to one another and to the -materials which are derived from them are illustrated in the following -table. - -[Sidenote: RELATIONS OF ALTERED TO UNALTERED LAVAS.] - - Unaltered Altered Decomposed - Plutonic Rocks lavas lavas Rocks - - Granite {Quartz-felsite } Rhyolite and } - { ('quartz-porphyry')} Obsidian } Felstone } - } } - Syenite {Orthoclase-porphyry } Trachyte } } Claystones - } - Diorite {Hornblende-porphyry } Andesite Porphyrite } - - Miascite {Liebnerite porphyry } Phonolite ? -- - - Gabbro {Augite-porphyry and } Basalt Melaphyre Wacke - {Dolerite } - -Some petrographers, indeed, have maintained the principle that rocks -belonging to widely separated geological periods, even when they -exhibit no essential points of difference, should nevertheless be -called by distinct names. But such a system of classification is -calculated rather to hinder than to advance the cause of science. If -the palæontologist were to adopt the same principle and give distinct -names to the same fossil, when it was found to occur in two different -geological formations, we can easily understand what confusion would -be occasioned, and how the comparison of the fauna and flora of the -different formations would be thereby rendered impossible. But the -naturalist, in his diagnosis of a species, wisely confines himself -to the structure and affinities of the organism before him; and in -the same way the petrographer, in giving a name to a rock, ought -to be guided only by his studies of its chemical composition, its -mineralogical constitution, and its structure, putting altogether -out of view its geographical distribution and geological age. Only -by strict attention to this principle can we hope to arrive at such -comparisons of the rocks of different areas and different periods, as -may serve as the basis for safe inductions. - -Before leaving this question of the relation which exists between the -igneous rocks of different ages, it may be well to notice several facts -that have been relied upon, as proving that the several geological -periods are distinguished by characteristic igneous products. - -It has frequently been asserted that the acid igneous rocks are present -in much greater quantities in connection with the older geological -formations than axe the basic; while, on the other hand, the basic -igneous rocks are said to have been extruded in greater abundance in -the more recent geological periods. But in considering this question it -must not be forgotten that, as a general rule, the basic rocks undergo -decomposition and disintegration far more rapidly than do the acid -rocks. In consequence of this circumstance the chance of our finding -their recognisable representatives among the older formations, is much -less in the case of the former class of rocks than in the latter. As a -matter of fact, however, we do find great masses of gabbro, diabase, -and melaphyre associated even with the oldest geological formations, -while trachytes and rhyolites abound in many volcanic districts where -active vents exist at the present day. Upon a general review of the -subject, it may well be doubted whether the supposed preponderance of -acid igneous materials in the earlier periods of the earth's history, -and of basic igneous materials during the later periods, rests on any -substantial basis of observation. - -[Sidenote: AUGITIC AND HORNBLENDIC ROCKS.] - -Another difference which has frequently been relied upon, as -distinguishing the older igneous rocks from those of more recent date, -is the supposed fact that the former are characterised by the presence -of hornblende, the latter by the presence of augite. It may be admitted -that this distinction is a real one, but its significance and value are -greatly diminished when we remember the relations which exist between -the two minerals in question. Hornblende and augite are interesting -examples of a dimorphous substance; in chemical composition they are -identical, or rather they are liable to variation between the same -limits, but in their crystalline forms and optical characters they -differ from one another. It has been proved that hornblende is the -stable, and augite the unstable condition of the substance in question. -If hornblende be fused and allowed to cool, it crystallises in the form -of augite. On the other hand, augite-crystals in rocks of ancient date -are found undergoing gradual change and passing into hornblende. The -mineral uralite has the outward form of augite, but the cleavage and -optical properties of hornblende; and there are not wanting many facts -pointing to the conclusion that rocks which now contain hornblende were -originally augitic masses, in which the unstable mineral in their -midst has been gradually converted into the stable one. - -There are, however, two minerals which up to the present time have -been found in association only with the older and newer rock-masses -respectively. These are _muscovite_, or the white form of mica, which -occurs in so many granites, but has not yet been discovered in any -modern representative of that rock; and leucite, which is not yet known -in rocks of older date than the Tertiary. - -When we remember that muscovite would appear to be a product of -deep-seated igneous action, and is only found in rock-masses that have -been formed under such conditions, we shall be the less surprised at -its non-occurrence in rocks of recent date, especially if we bear in -mind the fact that very few of the younger granitic rocks have as yet -been exposed at the surface by denudation. - -With respect to leucite, on the other hand, it must be remembered that -it is a very unstable mineral which appears to be easily changed into -felspar. It is by no means improbable, therefore, that some ancient -igneous rocks which now contain felspar were originally leucitic rocks. - -To the view that the action of volcanic forces upon the globe during -past geological times was similar in kind to that which we now observe -going on around us, still another objection has been raised. It has -been asserted that some of the deposits of igneous rock associated -with the older geological formations are of such a nature that they -could not possibly have been accumulated around volcanic vents of the -kind which we see in operation around us. - -[Sidenote: VOLCANIC ORIGIN OF ANCIENT IGNEOUS ROCKS.] - -Mr. Mallet has declared that the igneous products of the Palæozoic -period differ fundamentally in character from those materials formed -by volcanic action during the later Secondary and the Tertiary -periods. Upon what observations these generalisations are based he -has given us no information, and the enormous mass of facts which -have been collected in recent years concerning the structure of the -lavas and fragmental volcanic deposits of the pre-Cambrian, Cambrian, -Silurian, Devonian and Carboniferous periods, all point to a directly -opposite conclusion. The more carefully we carry on our investigations -concerning these ancient lavas, by the aid of chemical analysis and -microscopic study, the more are we convinced of the essential identity -of the ancient and modern volcanic rocks, both in their composition -and their minute structure. Of great masses of dust produced by -crushing, such as Mr. Mallet has supposed to have been formed during -the earlier geological periods, there is not the smallest evidence; -but we everywhere find proofs, when the rocks are minutely examined, -of the vesicular structure so characteristic of materials produced by -explosive volcanic action. - -It has frequently been asserted that in the great districts covered by -basaltic lavas which we find in the Rocky Mountains of North America, -in the Deccan of India, in Abyssinia, and even in the Western Isles of -Scotland, we have proofs of the occurrence, during earlier geological -periods, of volcanic action very different in character from that -which at present takes place on our globe. It has been asserted that -the phenomena observed in these districts can only be accounted for by -supposing that great fissures have opened in their midst, from which -lavas have issued in enormous floods unaccompanied by the ordinary -explosive phenomena of volcanoes. - -It must be remembered, however, that none of the districts in question -have been subjected to careful and systematic examination with a view -to the discovery of the vents from which these masses of lava have -issued, with the exception of that which occurs in our own islands. In -this case, in which superficial observers have spoken of the district -as being covered with horizontal lava-sheets piled upon one another to -the depth of 3,000 feet, careful study of the rock-masses has shown -that the accumulations of basalt really consist of a great number of -lava-currents which have issued at successive epochs covering enormous -periods of time. During the intervals between the emission of these -successive lava-currents the surfaces of the older ones have been -decomposed, and formed soils upon which forests have grown up; they -have been eroded by streams, the valleys so formed being filled with -gravels; and lakes have been originated on their surfaces in which -various accumulations have taken place. - -[Sidenote: TRULY-VOLCANIC ORIGIN OF LAVA PLATEAUX.] - -It has been demonstrated, moreover, that the basal-wrecks of no less -than five volcanic mountains, each of which must have rivalled Etna in -its proportions, existed within this area, and the connection of the -lava-currents, which have deluged the surrounding tracts, with these -great volcanoes has been clearly proved. It is probable that when -more careful and systematic researches are carried on in the other -districts, in which widely-spread sheets of basaltic rocks exist, -similar volcanic vents will be discovered. It must also be remembered -that if such a country as Iceland were subjected to long-continued -denudation, the mountain peaks and cones of loose materials would be -worn away, the whole island being thus reduced to a series of plateaux -composed of lava-sheets, the connection of which with the crystalline -materials filling the great volcanic vents, a superficial observer -might altogether fail to recognise. - -But even where we cannot trace the former existence of great volcanic -mountains, like those which once rose in the Hebrides, it would -nevertheless be very rash to conclude that the vast plateaux of -lava-rock must have been formed as gigantic floods unaccompanied by -ordinary volcanic action. Mr. Darwin has pointed out that in crossing -districts covered by lava, he was frequently only able to determine -the limits of the different currents of which it was made up, by an -examination of the age of the trees and the nature of the vegetation -which had sprung up on them. And everyone who has travelled much in -volcanic districts can confirm this observation; what appears at first -sight to be a great continuous sheet of lava proves upon more careful -observation to be composed of a great number of distinctly different -lava-currents, which have succeeded one another at longer or shorter -intervals. - -We must remember, too, how various in kind are the volcanic -manifestations which present themselves under different circumstances. -Sometimes the amount of explosive action at a volcanic vent is very -great, and only fragmental ejections take place, composed of the frothy -scum of the lava produced by the escape of gases and vapours from its -midst. But in other cases the amount of explosive action may be small, -and great volumes of igneous materials may issue as lava-streams. In -such cases, only small scoriæ-cones would be formed around the vents, -and one half of such cones is commonly swept away by the efflux of the -lava-currents, while the remainder may be easily removed by denuding -action or be buried under the lava-currents issuing from other vents -in the neighbourhood. Thus it may easily come to pass that what a -superficial observer takes for an enormous mass of basaltic lava poured -out from a great fissure at a single effort, may prove upon careful -observation to be made up of innumerable lava-currents, each of which -is of moderate dimensions; and it may further be found that these -lava-currents, instead of being the product of a single paroxysmal -effort from one great fissure, have been accumulated by numerous small -outbursts taking place at wide intervals, from a great number of minor -orifices. - -[Sidenote: SHIFTING OF VOLCANIC ACTION IN DIFFERENT AREAS.] - -Having then considered the arguments which have been adduced in support -of the view that the volcanic phenomena of former geological periods -differ from those which are still occurring upon the globe, we may -proceed to state the general conclusions which have been drawn from the -study of the volcanic rocks of the different geological periods. - -From a survey of the volcanic rocks of different ages, we are led to -the interesting and important conclusion that the scene of volcanic -action has been continually shifting to fresh areas at different -periods of the earth's history. We find repeated proofs that the -volcanic energy has made its appearance at a certain part of the -earth's crust, has gradually increased in intensity to a maximum, and -then as slowly declined. But as these manifestations have died away -at one part of the earth's surface, they have gradually made their -appearance at another. In every district which has been examined, we -find abundant proofs that volcanic energy has been developed at certain -periods, has disappeared during longer or shorter periods, and then -reappeared in the same area. And on the other hand, we find that there -is no past geological period in which we have not abundant evidence -that volcanic outbursts took place at some portion of the earth's -surface. - -To take the case of our own islands for example. We know that during -the pre-Cambrian periods volcanic outbursts occurred, traces of which -are found both in North and South Wales, in the Wrekin Chain in -Shropshire, in Charnwood Forest, and in parts of Scotland and Ireland. - -In Cambro-Silurian times we have abundant proofs, both in North Wales -and the Lake district, that volcanic action on the very grandest -scale was taking place during the Arenig and the older portion of -the Llandeilo periods, and again during the deposition of the Bala -or Caradoc beds. The lavas, tuffs, and volcanic agglomerates ejected -during these two periods have built up masses of rock many thousands of -feet in thickness. Snowdon and Cader Idris among the Welsh mountains, -and some of the higher summits of the Lake district, have been carved -by denudation from the vast piles of volcanic materials ejected during -these periods. - -In Devonian or Old-Red-Sandstone times, volcanic activity was renewed -with fresh violence upon that part of the earth's surface now occupied -by the British Islands. Along the line which now forms the Grampians -there rose a series of volcanoes of the very grandest dimensions. Ben -Nevis, and many others among the higher Scotch mountains, have been -carved by denudation from the hard masses of granite, quartz-felsite, -and other plutonic rocks which formed the central cores of these -ancient volcanic piles. The remains of the great lava-sheets, and of -the masses of volcanic agglomerate ejected from these grand Devonian -volcanoes, make up hill-ranges of no mean altitude, like the Sidlaws, -the Ochils, and the Pentlands. - -[Sidenote: ANCIENT BRITISH VOLCANOES.] - -The volcanic action of the Devonian period was prolonged into -Carboniferous times, but was then evidently diminishing gradually in -violence. Instead of great central volcanoes, such as existed in the -earlier period, we find innumerable small vents which threw out tuffs, -agglomerates and lavas, and were scattered over the districts lying -around the bases of the now extinct Devonian volcanoes. In the central -valley of Scotland and in many parts of England, we find abundant -proofs of the existence of these small and scattered volcanic vents -during Carboniferous times. The well-known hill of Arthur's Seat, which -overlooks the city of Edinburgh, and many castle-crowned crags of the -Forth and Clyde valleys, are the worn and denuded relics of these small -volcanoes. There are some indications which point to the conclusion -that the volcanic action of the Newer Palæozoic epoch had not entirely -died out in Permian times, but the evidence upon this point is not -altogether clear and satisfactory. - -During nearly the whole of the Secondary or Mesozoic periods the -volcanic forces remained dormant in the area of the British Isles. -Some small volcanic outbursts, however, appear to have occurred in -Triassic times in Devonshire. But in other areas, such as the Tyrol, -South-eastern Europe and Western America, the Triassic, Jurassic, and -Cretaceous periods were marked by grand manifestations of volcanic -activity. - -The volcanic forces which had during the long Mesozoic periods deserted -our part of the earth's surface, appear to have returned to it in -full rigour in the Tertiary epoch. In the Newer-Palæozoic periods -the direction of the great volcanic band which traversed our islands -appears to have been from north-east to south-west; but in Tertiary -times a new set of fissures were opened running from north to south. -There is evidence that during the Eocene or Nummulitic period, the -first indications of the subterranean forces having gathered strength -below the district were afforded by the issue of calcareous and -siliceous springs, and soon fissures were opened which emitted scoriæ, -tuffs, and lavas. The intensity of the volcanic action gradually -increased till it attained its maximum in the Miocene period, when -a great chain of volcanic mountains stretched north and south along -the line of the Inner Hebrides, the north-east of Ireland, and the -sea which separates Great Britain from Ireland. The basal-wrecks of a -number of these volcanoes can be traced in the islands of Skye, Mull, -Rum, and parts of the adjoining mainland. We have already seen that -along this great band of volcanic action, which traverses the Atlantic -Ocean from north to south, a number of active vents still exist, though -their energy is now far less intense than was the case in former times. -The only vestiges of the action of these now declining volcanic forces, -at present found in our islands, are the hot springs of Bath and a few -other warm and mineral springs; but in connection with this subject it -must be remembered that our country occasionally participates in great -earthquake-vibrations, like that which destroyed Lisbon in the year -1759. - -[Sidenote: ANCIENT VOLCANOES IN OTHER DISTRICTS.] - -If we were to study any other part of the earth's surface, we should -arrive at precisely the same conclusion as those to which we have -been conducted by our examination of the British Islands--namely, -that during past geological times the subterranean forces had made -themselves felt in the area, had gradually attained a maximum, and then -as gradually declined, passing through all those varied cycles which we -have described in a former chapter. And we should also find that these -periods of volcanic activity alternated with other periods of complete -quiescence which were of longer or shorter duration. But on comparing -two different districts, we should discover that what was a period of -volcanic activity in the one was a period of repose in the other, and -_vice versâ_. - -From these facts geologists have been led to the conclusion which we -have already enunciated--namely, that the subterranean forces are in a -state of continual flux over the surface of the globe. At one point of -the earth's crust these forces gradually gather such energy as to rend -asunder the superincumbent rock-masses and make themselves manifest -at the surface in the series of phenomena characteristic of volcanic -action. But after a longer or shorter interval of time--an interval -which must probably be measured by millions of years--the volcanic -forces die out in that area to make their appearance in another. - -Hence, although we may not be able to prove the fact by any -mathematical demonstration, a strong presumption is raised in favour -of the view that the subterranean energy in the earth's crust is a -constant quantity, and that the only variations which take place are in -the locality of its manifestation. - -Upon this question whether the amount of this subterranean energy -within the earth's crust is at the present time increasing, stationary, -or declining, we are not altogether destitute of evidence. There are -some considerations connected with certain astronomical hypotheses, to -which we shall hereafter have to refer, that might lead us to entertain -the view that the subterranean activity was once far greater than it -is at present, and that during the long periods of the earth's past -history it has been slowly and gradually declining. And those who -examine the vast masses of igneous materials which have been poured -out from volcanic vents during the earlier periods of the earth's -history may be inclined, at first sight, to point to them as affording -conclusive proof of this gradual decline. - -[Sidenote: SUPPOSED DECLINE OF VOLCANIC ACTION.] - -But a more careful study of the rocks in question will probably cause -a geologist to pause before jumping to such a conclusion. If we look -at the vast masses of volcanic materials erupted in Miocene times in -our own island and in Ireland, for example, we might be led to imagine -that we have the indications of a veritable 'Reign of Fire,' and that -the evidence points to a condition of things very different indeed -from that which prevails at the present day. But in arriving at such -a conclusion we should be neglecting a most important consideration, -the disregard of which has been the fertile parent of many geological -errors. Many independent lines of evidence all point to the inference -that these volcanic ejections are not the result of one violent effort, -but are the product of numerous small outbreaks which have been -scattered over enormous periods of time. - -When we examine with due care the lavas, tuffs, and other volcanic -ejections which constitute such mountain-masses as those of the -Hebrides, of the Auvergne, and of Hungary, we find clear proofs -that the ancient Miocene volcanoes of these districts were clothed -with luxuriant forests, through which wild animals roamed in the -greatest abundance. The intervals between the ejections of successive -lava-streams were often so great, that soils were formed on the -mountain-slope, and streams cut deep ravines and valleys in them. - -The island of Java is situated near the very heart of what is at the -present day the most active volcanic centre on the face of the globe, -yet vegetable and animal life flourish luxuriantly there, and the -island is one of the richest and most fertile spots upon the face of -the globe. Not all the terrors of occasional volcanic outbreaks will -ever drive the Neapolitan vine-dressers from the fertile slopes of -Vesuvius, for its periods of repose are long, and its eruptions are of -short duration. - -These considerations lead the geologist to conclude that the evidence -afforded by the ancient volcanic rocks is clear and positive in support -of the view that the manifestations of the subterranean forces in the -past agree precisely in their nature and in their products with those -taking place around us at the present time. On the question of great -secular changes having occurred in the amount of volcanic energy in -past geological periods, the evidence must be pronounced negative, or -at the best doubtful. - -But even if the geologist confesses himself unable to establish the -fact of any decline in the subterranean energies during the vast -periods of which he takes cognisance, it must be remembered that such -decline may really be going on; for vast as was the duration of the -geological epochs, they probably constitute but a fraction of those far -grander periods which are required by the speculations of the physical -astronomer. - - - - -CHAPTER X. - -THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF NATURE. - - -The first impression which is produced upon the mind, when the -phenomena of volcanic action are studied, is that here we have -exhibitions of destructive violence the effects of which must be -entirely mischievous and disastrous to the living beings occupying the -earth's surface. A little consideration will convince us, however, that -the grand and terrible character of the displays of volcanic energy -have given rise to exaggerated notions concerning their destructive -effects. The fact that districts situated over the most powerful -volcanic foci, like Java and Japan, are luxuriant in their productions, -and thickly inhabited, may well lead us to pause ere we condemn -volcanic action as productive only of mischief to the living beings -on the earth's surface. The actual slopes of Vesuvius and Etna, and -many other active volcanoes, are abundantly clothed with vineyards and -forests and are thickly studded with populous villages. - -As a matter of fact, the actual amount of damage to life and property -which is effected by volcanic eruptions is small. Usually, the -inhabitants of the district have sufficient warning to enable them -to escape with their lives and to carry away their most valuable -possessions. And though fertile tracts are covered by loose dust and -ashes, or by lava- and mud-currents, yet the sterility thus produced -is generally of short duration, for by their decomposition volcanic -materials give rise to the formation of the richest and most productive -soils. - -Earthquakes, as we have already seen, are far more destructive in -their effects than are volcanoes. Houses and villages, nay even entire -cities, are, by vibrations of portions of the earth's crust, reduced to -heaps of ruins, and famines and pestilences too frequently follow, as -the consequence of the disorganisation of our social systems by these -terrible catastrophes. - -It may well be doubted, however, whether the annual average of -destruction to life and property caused by all kinds of subterranean -action, exceeds that produced either by floods or by hurricanes. Yet -we know that the circulation of water and air over our globe are -beneficial and necessary operations, and that the mischief occasionally -wrought by the moving bodies of water and air is quite insignificant -compared with the good which they effect. - -In the same way, we shall be able to show that the subterranean -energies are necessary to the continued existence of our globe as -a place fitted for the habitation of living beings, and that the -mischievous and destructive effects of these energies bear but a small -and insignificant proportion to the beneficial results with which they -must be credited. - -[Sidenote: LEVELLING ACTION OF DENUDING FORCES.] - -We have had frequent occasion in the preceding pages to refer to the -work--slow but sure, silent but effective--wrought by the action of -the denuding forces ever operating upon the surface of our globe. The -waters condensing from the atmosphere and falling upon the land in -the form of rain, snow, or hail, are charged with small quantities of -dissolved gases, and these waters penetrating among the rock-masses -of which the earth's crust is composed, give rise to various -chemical actions of which we have already noticed such remarkable -illustrations in studying the ancient volcanic products of our globe. -By this action the hardest and most solid rock-masses are reduced -to a state of complete disintegration, certain of their ingredients -undergoing decomposition, and the cementing materials which hold -their particles together being removed in a state of solution. In the -higher regions of the atmosphere this work of rock-disintegration -proceeds with the greatest rapidity; for there the chemical action is -reinforced by the powerful mechanical action of freezing water. On -high mountain-peaks the work of breaking up rock-masses goes on at the -most rapid rate, and every craggy pinnacle is swathed by the heaps -of fragments which have fallen from it. The Alpine traveller justly -dreads the continual fusillade of falling rock-fragments which is -kept up by the ever-active power of the frost in these higher regions -of the atmosphere; and fears lest the vibrations of his footsteps -should loosen, from their position of precarious rest, the rapidly -accumulating piles of detritus. No mountain-peak attains to any very -great elevation above the earth's surface, for the higher we rise in -the atmosphere the greater is the range of temperature and the more -destructive are the effects of the atmospheric water. The moon, which -is a much smaller planet than our earth, has mountains of far greater -elevation; but the moon possesses neither an atmosphere nor moisture on -its surface, to produce those levelling effects which we see everywhere -going on around us upon the earth. - -The disintegrated materials, produced by chemical and mechanical -actions of the atmospheric waters upon rock-masses, are by floods, -rivers, and glaciers, gradually transported from higher to lower -levels; and sooner or later every fragment, when it has once been -separated from a mountain-top, must reach the ocean, where these -materials are accumulated and arranged to form new rocks. - -Over every part of the earth's surface these three grand operations of -the disintegration of old rock-masses, the transport of the materials -so produced to lower levels, and the accumulation of these materials -to form new rocks, is continually going on. It is by the varied action -of these denuding agents upon rocks of unequal hardness, occupying -different positions in relation to one another, that all the external -features of hills, and plains, and mountains owe their origin. - -It is a fact, which is capable of mathematical demonstration, that by -the action of these denuding forces the surface of all the lands of the -globe is being gradually but surely lowered; and this takes place at -such a rate that in a few millions of years the whole of the existing -continents must be washed away and their materials distributed over the -beds of the oceans. - -[Sidenote: NECESSITY FOR COMPENSATING AGENCIES.] - -It is evident that there exists some agency by which this levelling -action of the denuding forces of the globe is compensated; and a little -consideration will show that such compensating agency is found in the -subterranean forces ever at work within the earth's crust. The effects -of these subterranean forces which most powerfully arrest our attention -are volcanic outbursts and earthquake shocks, but a careful study of -the subject proves that these are by no means the most important of the -results of the action of such forces. Exact observation has proved that -almost every part of the earth's surface is either rising or falling, -and the striking and destructive phenomena of volcanoes and earthquakes -probably bear only the same relation to those grand and useful -actions of the subterranean forces, which floods do to the system of -circulating waters, and hurricanes to the system of moving air-currents. - -If we ride in a well-appointed carriage with good springs, upon -a railway which is in excellent order, the movement is almost -imperceptible to us; and the rate of speed may be increased -indefinitely, without making itself apparent to our senses. The -smallest impediment to the evenness of the movement--such as that -produced by a small object placed upon the rails--at once makes itself -felt by a violent jar and vibration. How perfectly insensible we may -be of the grandest and most rapid movements is taught us by the facts -demonstrated by the astronomer. By the earth's daily rotation, we are -borne along at a rate which in some places amounts to over 1,000 miles -an hour; and by its annual revolution we are every hour transported -through a distance of 70,000 miles; yet concerning the fact and -direction of these movements we are wholly unconscious. - -In the case both of the railway train and of our planet, we can only -establish the reality of the movement, and its direction and rate, by -means of observations upon external objects, which appear to us to -have a movement in the opposite direction. In the same way we can only -establish the fact of the movement of portions of the earth's crust -by noticing the changing positions of parts of the earth's surface in -relation to the constant level of the ocean. When this is done we find -abundant proof that while some parts of the earth's crust are rising, -others are as undoubtedly undergoing depression. - -[Sidenote: POTENCY OF THE SUBTERRANEAN FORCES.] - -We shall be able to form some idea of the vastness of the effects -produced by the subterranean forces, by a very simple consideration. -It is certain that during the enormous periods of time of which the -records have been discovered by the geologist, there have always been -continents and oceans upon the earth's surface, just as at present, -and it is almost equally certain that the proportions of the earth's -surface occupied by land and water respectively, have not varied very -widely from those which now prevail. But, at the same time, it is an -equally well-established bet that the denuding forces ever at work -upon the earth's surface would have been competent to the removal -of existing continents many times over, in the vast periods covered -by geological records. Hence we are driven to conclude that the -subterranean movements have in past times entirely compensated for the -waste produced by the denuding forces ever at work upon our globe. But -this is not all. The subterranean forces not only produce upheaval; -in a great many cases the evidences of subsidence are as clear and -conclusive as are those of upheaval in others. Hence we are driven to -conclude that the forces producing upheaval of portions of the earth's -crust are sufficient, not only to balance those producing subsidence, -but also to compensate for the destructive action of denuding agents -upon the land-masses of the globe. - -It is only by a careful and attentive study and calculation of the -effects produced by the denuding agents at work all around us, aided -by an examination of the enormous thicknesses of strata formed by the -action of such causes during past geological times, that we are able -to form any idea of the reality and vastness of the agents of change -which are ever operating to modify the earth's external features. When -we have clearly realised the grand effects produced on the surface -of the globe by these external forces, through the action of its -investing atmosphere and circulating waters, then, and only then, -shall we be in a position to estimate the far greater effects resulting -from the internal forces, of which the most striking, but not the most -important, results are seen in the production of volcanic eruptions and -earthquake-shocks. - -Another series of facts which serve to convince the geologist of the -reality and potency of the forces ever at work within the earth's -crust, and the way in which these have operated during past geological -periods, is found in the disturbed condition of many of the stratified -rock-masses of which it is composed. Such stratified rock-masses, it is -clear, must have been originally deposited in a position of approximate -horizontality; but they are now often found in inclined and even -vertical positions; they are seen to be bent, crumpled, puckered, and -folded in the most remarkable manner, and have not unfrequently been -broken across by dislocations--'faults'--which have sometimes displaced -masses, originally in contact, to the extent of thousands of feet. -The slate-rocks of the globe, moreover, bear witness to the fact that -strata have been subjected to the action of lateral compression of -enormous violence and vast duration; while in the metamorphic rocks -we see the effects of still more extreme mechanical strains, which -have been in part transformed into chemical action. No one who has -not studied the crushed, crumpled, fractured, and altered condition -of many of the sedimentary rocks of the globe, can form the faintest -idea of the enormous effects of the internal forces which have been in -operation within the earth's crust during earlier geological periods. -And it is only by such studies as these that we at last learn to regard -the earthquake and volcanic phenomena of our globe, not as the grandest -and most important effects of these forces, but as their secondary -and accidental accompaniments. 'Volcanoes,' it has been said, 'are -the safety-valves of the globe;' and when we come to realise the real -extent and nature of the internal forces ceaselessly working in the -earth's crust we shall scarcely be disposed to regard the simile as an -overstrained one. - -[Sidenote: RELATION TO CONTINENTAL MOVEMENTS.] - -The first geologist who attempted to show the exact relations existing -between those subterranean forces which cause the movements of -continental masses of land, and those more startling displays of energy -which are witnessed in volcanic outbursts, was the late Mr. Poulett -Scrope. At a somewhat later date Mr. Darwin, in his remarkable paper -'On the Connexion of certain Volcanic Phenomena in South America, and -on the Formation of Mountain-chains and Volcanoes as the effect of -Continental Elevations,' threw much new and important light upon the -question. - -While, on the one hand, we are led by recent geological investigations -to reject the notions which were formerly accepted, by which -mountain-ranges were supposed to be suddenly and violently upheaved by -volcanic forces, we are, on the other hand, driven to conclude that -without the action of these subterranean forces, the irregularities -which are exhibited on the earth's surface could not have had any -existence. - -It is true that the actual forms of the mountain-ranges are due -directly to the action of denuding forces, which have sculptured -out from the rude rocky masses all the varied outlines of peaks and -crags, of ravines and valleys. But it is none the less true that -the determining causes which have directed and controlled all this -earth-sculpture, are found in the relative positions of hard and soft -masses of rock; but these rock-masses have acquired their hardness and -consistency, and have assumed their present positions, in obedience -to the action of subterranean forces. Hence we see that though the -formation of mountain-ranges is proximately due to the denuding -forces, which have sculptured the earth's surface, the primary cause -for the existence of such mountain-chains must be sought for in the -fact that subterranean forces have been at work, folding, crumpling, -and hardening the soft sediments, and placing them in such positions -that, by the action of denudation, the more indurated portions are left -standing as mountain-masses above the general surface. - -The old notion that mountain-chains are due to a vertical upthrust -from below, finds but little support when we come to study with due -care the positions of the rock-masses composing the earth's crust. On -the contrary, we find that mountain-ranges are usually carved out of -the crushed and crumpled edges of strata which have along certain lines -been influenced by great mechanical strains, and subjected to more or -less induration and chemical alteration. When we compare these folded -and contorted portions of the strata with those parts of the same beds -which are not so affected, we find the effects produced in the former -are not such as would result from an upthrust from below, but from -movements by which a tangential strain would be brought about. If we -imagine certain lines of weakness to exist in the solid crust of the -earth, then any movements in the portions of the crust between these -lines of weakness would cause crushing and crumpling of the strata -along the latter. - -[Sidenote: FORMATION OF MOUNTAIN-CHAINS.] - -Recent investigations of Dana and other authors have thrown much new -light upon the question of the mode of formation of mountain-chains, -and the relation between the movements by which they are produced and -the sudden and violent manifestations of force witnessed in volcanic -outbursts. We cannot, perhaps, better illustrate this subject than by -giving a sketch of the series of operations to which the great Alpine -chains owe their origin. - -There are good grounds for believing that the great mountain-axis of -Southern Europe, with its continuation in Asia, had no existence -during the earlier geological periods. Indeed, it has been proved that -all the higher among the existing mountain-chains of the globe have -been almost entirely formed in Tertiary times. The reason of this -remarkable fact is not far to seek. So rapid is the work of denudation -in the higher regions of the atmosphere, that the elevated crags and -pinnacles are being broken up by the action of moisture and frost at -an exceedingly rapid rate. This fact is attested by the existence -of those enormous masses of angular rock-fragments which are found -lodged on every vantage-ground among the mountain-summits, as well as -by the continually descending materials which are borne by glaciers -and mountain-torrents to the valleys below. Where such a rate of -disintegration as this is maintained, no elevated mountain-crests -could exist through long geological periods. It is true we find in all -parts of the globe relics of many mountain-chains which were formed -before the Tertiary period; but these have by long-continued denudation -been worn down to 'mere stumps.' Of such worn-down and degraded -mountain-ranges we have examples in the Scandinavian chains, and some -of the low mountain-regions of Central Europe and North America. - -Let us now proceed to illustrate this subject by briefly sketching the -history of that series of operations by which the great mountain-chains -of the Alpine system have been formed. - -The first stage of that grand series of operations appears from recent -geological researches to have consisted in the opening of a number -of fissures running along a line near to that at which, in a long -subsequent period, the elevation of the mountain-masses took place. -This betrayal of the existence of a line of weakness in this part of -the earth's crust occurred in the Permian period, and from that time -onward a series of wonderful movements and changes have been going -forward, which have resulted in the production of the Alpine chains as -we now see them. - -[Sidenote: VOLCANIC FISSURES OF PERMIAN PERIOD.] - -From the great fissures opened in Permian times along this line of -weakness, great quantities of lava, scoriæ, and tuff were poured out, -and these accumulated to form great volcanic mountains, which we can -now only study at a few isolated spots, as in the Tyrol, Carinthia, -and about Lake Lugano. Everywhere else, these Permian volcanic rocks -appear to be deeply buried under the later-formed sediments, from which -the Alpine chains have been carved. Few and imperfect, however, as are -the exposures of these ancient rhyolite and quartz-andesite lavas and -agglomerates formed at the close of the Palæozoic epoch, their greatly -denuded relics form masses which are in places more than 9,000 feet in -thickness. From this fact we are able to form some slight idea of the -scale upon which the volcanic outbursts in question must have taken -place during Permian times. - -The second stage in the series of operations by which the Alpine chains -have been formed, consisted in a general sinking of the surface along -that line of weakness in the earth's crust, the existence of which had -been betrayed by the formation of fissures and the eruption of volcanic -rocks. We have already had occasion to remark how frequently such -subsidences follow upon the extrusion of volcanic masses at any part -of the earth's surface; and we have referred these downward movements -in part to the removal of support from below the portion of the crust -affected, and in part to the weight of the materials piled upon its -surface by the volcanic forces. - -The volcanic energy which had been manifested with such violence -during the Permian period, does not appear to have died out altogether -during the succeeding Triassic period. A number of smaller volcanic -vents were opened from time to time, and from these, lavas, tuffs, and -agglomerates, chiefly of basic composition, were poured out. The relics -of these old Triassic volcanoes are found at many points along the -Alpine chain, but it is evident that the igneous forces were gradually -becoming exhausted during this period, and before the close of it they -had fallen into a state of complete extinction. - -But the great subsidence which had commenced in the Triassic period, -along what was to become the future line of the Alpine chain, was -continued almost without interruption during the Rhætic, the Jurassic, -the Tithonian, the Neocomian, the Cretaceous and the Nummulitic -periods. With respect to the strata formed during all these periods, -it is found that their thiknesses, which away from the Alpine axis -may be measured by hundreds of feet, is along that axis increased -to thousands of feet. The united thickness of sediments accumulated -along this great line of subsidence between the Permian and Nummulitic -periods probably exceeds 60,000 feet, or ten miles. The subsidence -appears to have been very slow and gradual, but almost uninterrupted, -and the deposition of sediments seems to have kept pace with the -sinking of the sea-bottom, a fact which is proved by the circumstance -that nearly the whole of these sediments were such as must have been -accumulated in comparatively shallow water. - -[Sidenote: FORMATION OF ALPINE GEOSYNCLINAL.] - -By the means we have described there was thus formed a 'geosynclinal,' -as geologists have called it, that is, a trough-like hollow filled -with masses of abnormally thickened sediments, which had been piled -one upon another during the long periods of time in which almost -uninterrupted subsidence was going on along the Alpine line of -weakness in the earth's crust. In this way was brought together that -enormous accumulation of materials from which the hard masses of the -Alpine chains were subsequently elaborated, and out of which the -mountain-peaks were eventually carved by denudation. - -The third stage in this grand work of mountain-making commenced in the -Oligocene period. It consisted of a series of movements affecting the -parts of the earth's crust on either side of the line of weakness -which had first exhibited itself in Permian times. By these movements -a series of tangential strains were produced, which resulted in the -violent crushing, folding, and crumpling of the sedimentary materials -composing the geosynclinal. - -One effect of this action was the violent flexure and frequent fracture -of these stratified masses, which are now found in the Alpine regions -assuming the most abnormal and unexpected positions and relations to -one another. Sometimes the strata are found tortured and twisted into -the most complicated folds and puckerings; at others they are seen to -be completely inverted, so that the older beds are found lying upon the -newer; and in others, again, great masses of strata have been traversed -by numerous fractures or faults, the rocks on either side of which are -displaced to the extent of thousands of feet. - -Another effect of the great lateral thrusts by which the thick -sedimentary masses of the geosynclinal were being so violently -disturbed, was the production of a great amount of induration and -chemical change in these rocks. Masses of soft clay, of the age of -that upon which London is built, were by violent pressure reduced to -the condition of roofing-slate, similar to that of North Wales. One -of the most important discoveries of modern times is that which has -resulted in the recognition of the fact of the mutual convertibility -of different kinds of energy. We now know that mechanical force -may be transformed into heat-force or chemical force; and of such -transformations we find abundant illustrations in the crushed and -crumpled rock-masses of the Alpine chains. - -Under the influence of these several kinds of force, not only was -extreme consolidation and induration produced among the rock-masses, -but chemical affinity and crystalline action had the fullest play -among the materials of which they were composed. In many cases we find -the originally soft muds, sands, and shell-banks converted into the -most highly crystalline rocks, which retain their primary chemical -composition, but have entirely lost all their other original features. - -[Sidenote: FORMATION OF ALPINE GEANTICLINAL.] - -To the mass of folded, crumpled, and altered strata, formed from a -geosynclinal by lateral pressure, geologists have given the name -of a 'geanticlinal.' The formation of the Alpine geanticlinal was -due to movements which commenced in the Oligocene period, attained -their maximum in the Miocene, and appear to have declined and almost -altogether died out in the Pliocene period. - -The movements which resulted in the crushing and crumpling of the -thickened mass of sediments along the Alpine line of weakness, also -gave rise to the formation of a series of fissures from which volcanic -action took place. These fissures were not, however, formed along the -original line of weakness, for this had been strengthened and repaired -by the deposition of ten-miles' thickness of sediments upon it, but -along new fissures opened in directions parallel to the original lines -of weakness, and in areas where a much less considerable amount of -deposition had taken place since Permian times. - -We have abundant evidence that, just at the period when those great -movements were commencing which resulted in the formation of the great -Alpine and Himalayan geanticlinal, earth-fissures were being opened -upon either side of the latter from which volcanic outbursts took -place. At the period when the most violent mountain-forming movements -occurred, these fissures were in their most active condition, and at -this time two great volcanic belts stretched east and west, on either -side of, and parallel to, the great Alpine chain. The Northern volcanic -band was formed by the numerous vents, now all extinct, in Auvergne, -Central Germany, Bohemia, and Hungary, and was probably continued in -the volcanoes of the Thian Shan and Mantchouria. The Southern volcanic -band was formed by the numerous vents of the Iberian and Italian -peninsulas, and the islands of the Mediterranean, and were continued -to the eastward by those of Asia Minor, Arabia, and the North Indian -Ocean. As the earth-movements which produced the geanticlinal died -away, the volcanic energy along these parallel volcanic bands died -away at the same time. In studying the geology of Central and Southern -Europe, no fact comes out more strikingly than that of the synchronism -between the earth-movements by which the geanticlinal of the Alps was -formed, and the volcanic manifestations which were exhibited along -lines of fissure parallel to that geanticlinal. The earth-movements -and the volcanic outbursts both commenced in the Oligocene period, -gradually attained their maximum in the Miocene, and as slowly declined -in the Pliocene. - -[Sidenote: SCULPTURING OF ALPS BY DENUDATION.] - -The fourth stage in the great work of mountain-building in the case -of the Alps consisted in the operation of the denuding forces, the -disintegrating action of rain and frost, the transporting action -of rivers and glaciers, by which the Alpine peaks were gradually -sculptured out of the indurated and altered masses constituting the -geanticlinal. The action of this fourth stage went on to a great -extent side by side with that of the third stage. So soon as the -earth-movements had brought the submerged sedimentary masses of the -geosynclinal under the action of the surface tides and currents of the -ocean, marine denudation would commence; and, as the work of elevation -went on, the rock-masses would gradually be brought within the reach -of those more silently-working but far more effective agents which -are ever operating in the higher regions of the atmosphere. It is -impossible to say what would have been the height of the Alpine chain -if the work of denudation had not to a great extent kept pace with -that of elevation. Only the harder and more crystalline masses have -for the most part escaped destruction, and stand up in high craggy -summits; while flanking hills, like the well-known Rigi, are Been to -be composed of conglomerates thousands of feet in thickness, composed -of their disintegrated materials. It is a remarkable fact, as showing -how enormous was the work of elevation daring the formation of the -geanticlinal, that some of the youngest and least consolidated rocks of -the Nummulitic period are still found at a height of 11,000 feet in the -Alps, and of 16,000 feet in the Himalaya. - -From what has been said, it will be seen that mountain-chains may be -regarded as cicatrised wounds in the earth's solid crust. A line of -weakness first betrays itself at a certain part of the earth's surface -by fissures, from which volcanic outbursts take place; and thus the -position of the future mountain-chain is determined. Next, subsidence -during many millions of years permits of the accumulation of the raw -materials out of which the mountain-range is to be formed; subsequent -earth-movements cause these raw materials to be elaborated into the -hardest and most crystalline rock-masses, and place them in elevated -and favourable positions; and lastly, denudation sculptures from these -hardened rock-masses all the varied mountain forms. Thus the work of -mountain-making is not, as was formerly supposed by geologists, the -result of a simple upheaving force, but is the outcome of a long and -complicated series of operations. - -[Sidenote: ORIGIN OF OTHER MOUNTAIN-CHAINS.] - -The careful study of other mountain-chains, especially those of the -American continent, has shown that the series of actions which we -have described as occurring in the Alps, took place in the same order -in the formation of all mountain-masses. It is doubtful whether the -line of weakness is always betrayed in the first instance by the -formation along its course of volcanic fissures. But in all cases we -have evidence of the production of a geosynclinal, which is afterwards, -by lateral pressure, converted into a geanticlinal, and from this the -mountain-chains have been carved by denudation. Professor Dana has -shown that the geosynclinal of the Appalachian chain was made up of -sediments attaining a thickness of 40,000 feet, or eight miles; while -Mr. Clarence King has shown that a part of the geosynclinal of the -Rocky Mountains was built up of no less than 60,000 feet, or twelve -miles of strata. - -It has thus been established that a very remarkable relation exists -between the forces by which continental masses of land are raised -and depressed, and mountain-ranges have been developed along lines -of weakness separating such moving continental masses, and those -more sudden and striking manifestations of energy which give rise to -volcanic phenomena. It is in this relation between the widespread -subterranean energies and the local development of the same forces -at volcanic vents, that we must in all probability seek for the -explanation of those interesting peculiarities of the distribution -of volcanoes upon the face of the globe which we have described -in a former chapter. The parallelism of volcanic bands to great -mountain-chains is thus easily accounted for; and in the same way -we may probably explain the position of most volcanoes with regard -to coast-lines. We have already pointed out the objections to the -commonly-received view that volcanoes depend for their supplies of -water on the proximity of the ocean. This proximity of the ocean to -volcanic vents we are thus inclined to regard, not as the cause, but as -the effect of the subterranean action. The positions of both volcanoes -and coast-lines are determined by the limits of those great areas of -the earth's crust which are subjected to slow vertical movements, often -in opposite directions. - -Terrible and striking, then, as are the phenomena connected with -volcanic action, such sudden and violent manifestations of the -subterranean energy must not be regarded as the only, or indeed the -chief, effects which they produce. The internal forces continually -at work within the earth's crust perform a series of most important -functions in connection with the economy of the globe, and were the -action of these forces to die out, our planet would soon cease to be -fit for the habitation of living beings. - -There is no fact which the geological student is more constantly -called upon to bear in mind than that of the potency of seemingly -insignificant causes which continue in constant operation through long -periods of time. Indeed these small and almost unnoticed agencies at -work upon the earth's crust are often found, in the long ran, to -produce far grander effects than those of which the action is much -more striking and obvious. It is to the silent and imperceptible -action of atmospheric moisture and frost that the disintegration of -the solid rock-masses must be mainly ascribed; and the noisy cataract -and ocean-billow produce effects which are quite insignificant -compared with those which must be ascribed to the slight and almost -unnoticed forces. Great masses of limestone are built up of the remains -of microscopic organisms, while the larger and higher life-forms -contribute but little to the great work of rock-building. - -[Sidenote: EFFECTS OF SLOW CONTINENTAL MOVEMENTS.] - -In the same way it is to the almost unnoticed action of the -subterranean forces in raising some vast areas of the earth's crust, -in depressing others, and in bringing about the development of -mountain-chains between them, that we must ascribe a far more important -part in the economy of our globe than to the more conspicuous but less -constant action of volcanoes. - -A few simple considerations will serve to convince us, not only of the -beneficial effects of the action of the subterranean energies within -the earth's crust, but of the absolute necessity of the continued -operation of those energies to the perpetuation of that set of -conditions by which our planet is fitted to be the habitation of living -beings. - -We have already referred to the prodigious effects which are constantly -being produced around us by the action of the external forces at work -upon the globe. The source of these external forces is found in the -movements and changes which are ever going on within the aqueous and -atmospheric media in which the globe is enveloped. The circulation of -the air, influencing the circulation of the waters in the shape of -clouds, rain, snow, rivers, glaciers, and oceans, causes the breaking -up of even the hardest rock-masses, and the continual removal of their -disintegrated fragments from higher to lower levels. This work goes on -with more or less regularity over every part of the land raised above -the level of the ocean, but the rate of destruction in the higher -regions of the atmosphere is far more rapid than at lower levels. Hence -the circulating air and water of the globe are found to be continually -acting as levellers of the land-masses of the earth. - -It is by no means a difficult task to calculate the approximate rate -at which the various continents and islands are being levelled down, -and such calculations prove that in a very few millions of years the -existing forces operating upon the earth's surface would reduce the -whole of the land-masses to the level of the ocean. - -But a little consideration will convince us that the circulation of -the air and waters of the globe are themselves dependent upon the -existence of those irregularities of the land-surfaces which they are -constantly tending to destroy. Without elevated mountain ridges the -regular condensation of moisture, and its collection and distribution -in streams and rivers over every part of the land surfaces, could -not take place. Under these circumstances the unchecked evaporation -of the oceanic waters would probably go on, till the proportion -of water-vapour increased to such an extent in the atmosphere as -effectually to destroy those nicely-balanced conditions upon which the -continued existence of both vegetable and animal life depend. - -But the repeated upward and downward movements which have been shown -to be going on in the great land-masses of the globe, giving rise -in turns to those lateral thrusts and tangential strains to which -mountain-chains owe their formation, afford a perfect compensation -to the action of the external forces ever operating upon the earth's -surface. - -If, however, the uncompensated effect of the external forces acting on -the earth's crust is calculated to bring about the destruction of those -conditions upon which the existence of life depends, the uncompensated -effect of the internal forces acting on the earth's crust are fraught -with at least equal dangers to those necessary conditions. - -[Sidenote: CONTRAST BETWEEN THE EARTH AND MOON.] - -In our nearest neighbour among the planets--the moon--the telescope has -revealed to us the existence of a globe, in which the internal forces -have not been checked and controlled by the operation of any external -agencies--for the moon appears to be destitute of both atmosphere and -water. - -Under these circumstances we find its surface, as we might expect, to -be composed of rocks which appear to be entirely of igneous origin; -the mountain-masses, unworn by rain or frost, river or glacier, being -of most prodigious dimensions as compared with those of our own globe, -while no features at all resembling valleys, or plains, or alluvial -flats are anywhere to be discerned upon the lunar surface. - -But by the admirable balancing of the external and internal forces -on our own globe, the conditions necessary to animal and vegetable -existence are almost constantly maintained, and those interruptions -of such conditions, produced by hurricanes and floods, by volcanic -outbursts and earthquakes, may safely be regarded as the insignificant -accidents of what is, on the whole, a very perfectly working piece of -machinery. - -The ancients loved to liken the earth to a living being--the macrocosm -of which man was the puny representative or microcosm; and when we -study the well-adapted interplay of the forces at work upon the -earth's crust, both from within and without, the analogy seems a -scarcely strained one. In the macrocosm and the microcosm alike, slight -interferences with the regular functions occasionally take place, and -both of them exhibit the traces of a past evolution and the germs of an -eventual decay. - - - - -CHAPTER XI. - -WHAT VOLCANOES TEACH US CONCERNING THE NATURE OF THE EARTH'S INTERIOR. - - -In entering upon any speculations or enquiries concerning the nature of -the interior of our globe, it is necessary before all things that we -should clearly realise in our minds how small and almost infinitesimal -is that part of the earth's mass which can be subjected to direct -examination. The distance from the surface to the centre of our globe -is nearly 4,000 miles, but the deepest mines do not penetrate to much -more than half a mile from the surface, and the deepest borings fall -far short of a mile in depth. Sometimes, it is true, the geologist -finds means for drawing inferences as to the nature of the rocks at -depths of ten or fifteen miles below the surface; but the last-named -depth must be regarded as the utmost limit of that portion of our -globe which can be made the object of direct observation and study. -This thin exterior film of the earth's mass, which the geologist is -able to investigate, we call the 'crust of the globe'; but it must be -remembered that in using this term, it is not intended to imply that -the outer part of our globe differs in any essential respect from the -interior. The term 'crust of the globe' is employed by geologists as -a convenient way of referring to that portion of the earth which is -accessible to their observation. - -But if we are unable to make direct investigations concerning the -nature of the internal portions of the globe, there are nevertheless a -number of facts from which we may draw important inferences upon the -subject. These facts and the inferences based upon them we shall now -proceed to consider. - -First in importance among these we may mention the results which have -been obtained by weighing our globe. Various methods have been devised -for accomplishing this important object, and the conclusions arrived -at by different methods agree so closely with one another, that there -is no room for doubt as to the substantial accuracy of those results. -It may be taken as proved beyond the possibility of controversy that -our globe is equal in weight to five and a half globes of the same size -composed of water, or, in other words, that the average density of the -materials composing the globe is five and a half times as great as that -of water. - -Now the density of the materials which compose the crust of the globe -is very much less than this, varying from about two-and-one-third to -three times that of water. Hence we are compelled to conclude that the -interior portions of the globe are of far greater density than the -exterior portions; that, as a matter of fact, the mass of the globe is -composed of materials having twice the density of the rocks exposed at -the surface. - -[Sidenote: DENSITY OF EARTH'S INTERIOR.] - -It has been sometimes argued that as all materials under intense -pressure appear to yield to an appreciable extent, and to allow their -particles to be packed into a smaller compass, we may find in this fact -an explanation of the great density of the internal parts of the globe. -It has in fact been suggested that under the enormous pressure which -must be exerted by masses of rock several thousand feet in thickness, -the materials of which our earth is composed may be compelled to pack -themselves into less than one-half the compass which they occupy at the -surface. But the ascription of such almost unlimited compressibility to -solid substances can be supported neither by experiment nor analogy. -Various considerations point to the probability that solid bodies yield -to pressure up to a certain limit and no farther, and that when this -limit is reached an increase in pressure is no longer attended with a -reduction in bulk. - -If then we are compelled to reject the idea of the unlimited -compressibility of solid substances, we must conclude that the interior -portions of our globe are composed of _materials of a different kind_ -from those which occur in its crust. And this conclusion, as we shall -presently see, is borne out by a number of independent facts. - -The study of the materials ejected from volcanic vents proves that even -at very moderate depths there exist substances differing greatly in -density, as well as in chemical composition. The lightest lavas have a -specific gravity of 2·3, the heaviest of over 3. And that materials of -even greater density are sometimes brought by volcanic action from the -earth's interior, we have now the clearest proofs. - -[Sidenote: RELATION BETWEEN EARTH AND OTHER PLANETS.] - -But in considering a question of this kind, it will be well to remember -that analogy may furnish us with hints upon the subject which may prove -to be by no means unimportant. There is no question upon which modern -science has wrought out a more complete revolution in our ideas, than -that of the relation of our earth to the other bodies of the universe. -We know, as the result of recent research, that our globe is one of -a great family of bodies, moving through space in similar paths and -in obedience to the same laws. A hundred years ago the primary and -secondary planets of the solar system could be almost numbered upon -the fingers; now we recognise the fact that they exist in countless -millions, presenting every variety of bulk from masses 1,400 times -as large as our earth down to the merest planetary dust. Between the -orbits of Mars and Jupiter, more than 200 small planets have been -recognised as occurring, and every year additions are made to the -number of these asteroids. Comets have now been identified with streams -of such planetary bodies, of minute size, moving in regular orbits -through our system. The magnificent showers of 'shooting-stars' have -been proved to be caused by the passage of the earth through such -bands of travelling bodies, and 'the zodiacal light' finds its most -probable explanation in the supposition that the sun is surrounded by -a great mass of such minute planets. Every increase in the power of -the telescope reveals to us the existence of new secondary planets -or moons, revolving about the primaries; and the wonderful system -of the Saturnian rings is now explained by the proved existence of -great streams of such secondary planets circling around it. The solar -system was formerly conceived of as a vast solitude through which a -few gigantic bodies moved at awful distances from one another. Now we -know that the supposed empty void is traversed by countless myriads of -bodies of the most varied dimensions, all moving in certain definite -paths, in obedience to the same laws, ever acting and reacting upon -each other, and occasionally coming into collision. - -There are not wanting further facts to prove that the other planets are -like our own in many of their phenomena and surroundings. In some of -them atmospheric phenomena have been detected, such as the formation of -clouds and the deposition of snow, so that the external forces at work -on our globe act upon them also. And that internal forces, like those -we have been considering in the case of our earth, are at work in our -neighbours, is proved by the great solar storms and the condition of -the moon's surface. - -But the results of spectrum-analysis in recent years have furnished new -facts in proof of the close relationship of our earth to the numerous -similar bodies by which it is surrounded. So far as observation has yet -gone we have reason for believing that not only the members of the -solar system, but the more distant bodies of the universe, are all -composed of the same elementary substances as those which enter into -the composition of our globe. - -The most satisfactory information concerning the composition and nature -of other planetary bodies is derived from the study of those small -planets which occasionally come into collision with our globe, and -which have their own proper motion in space thereby arrested. These -meteorites, as such falling planetary bodies are called, have justly -attracted great attention, and their fragments are treasured as the -most valuable objects in our museums. - -[Sidenote: COMPOSITION OF METEORITES.] - -The first fact concerning these meteorites, which it is necessary -to notice, is that they are composed of the same chemical elements -as occur in the earth's crust. No element has yet been found in any -meteorite which was not previously known as existing in the earth, and -of the sixty-five or seventy known terrestrial elements no less than -twenty-two have already been detected in meteorites. - -There are, however, a dozen elements which occur in overwhelming -proportions in the earth's crust. We shall probably not be going too -far in saying that these twelve elements--namely, oxygen, silicon, -aluminium, calcium, magnesium, sodium, potassium, iron, carbon, -hydrogen, sulphur, and chlorine--make up amongst them not less than 999 -out of 1,000 parts of the earth's crust, and that all the other fifty -or sixty elements are 80 comparatively rare that they do not constitute -when taken altogether more than one part in 1,000 of the rocks of the -globe. Now all of these twelve common terrestrial elements occur in -meteorites, and the fact that the rarer terrestrial elements have not -as yet been found in them will not surprise anyone, who remembers how -small is the bulk of all the specimens of these meteorites existing in -our museums. - -We have hitherto insisted on the points of resemblance in the chemical -composition of meteorites and that of the rocks of the globe, but we -shall now have to indicate some very important points in which they -differ. - -While in the rocks composing the earth's crust oxygen forms one-half of -their mass, and silicon another quarter, we find that in the meteorites -these elements, though present, play a much less important part. The -most abundant element in the meteorites is iron; and nickel, chromium, -cobalt, manganese, sulphur, and phosphorus, are much more abundant in -these extra-terrestrial bodies than they are in the earth's crust. - -We have already referred to the remarkable fact that in our earth's -crust nearly all the other elementary substances are found combined -in the first instance with oxygen, and that most rocks consist of the -oxide of silicon combined with the oxides of various metals. But this -is by no means the case with the meteorites. In them we find metals -like iron, nickel, cobalt, &c., in their uncombined condition, and -forming alloys with one another. The same and other metals also occur -in combination with carbon, phosphorus, chlorine, and sulphur, and -some of the substances thus formed are quite unknown among terrestrial -rocks. Compounds of the oxide of silicon with the oxides of the metals -such as form the mass of the crust of the globe do occur in meteorites, -but they play a much less important part than in the case of the -terrestrial rocks. - -Among the substances found in meteorites are several which do not exist -among the terrestrial rocks--some, indeed, which it seems impossible -to conceive of as being formed and preserved under terrestrial -conditions. Among these we may mention the phosphide of iron and nickel -(Schreibersite), the sulphide of chromium and iron (Daubréelite), the -protosulphide of iron (Troilite), the sulphide of calcium (Oldhamite), -the protochloride of iron (Lawrencite), and a peculiar form of -crystallised silica, called by Professor Maskelyne 'Asmanite.' - -[Sidenote: DIFFERENT KINDS OF METEORITES.] - -There are other phenomena exhibited by meteorites which indicate that -they must have been formed under conditions very different to those -which prevail upon the earth's surface. Thus we find that fused iron -and molten slag-like materials have remained entangled with each other, -and have not separated as they would do if a great body like the earth -were near to exercise the varying force of gravity upon the two -classes of substances. Again, meteorites are found to have absorbed -many times their bulk of hydrogen gas, and to exhibit peculiarities in -their microscopic structure which can probably be only accounted for -when we remember that they were formed in the interplanetary spaces, -far away from any great attracting body. - -But in recent years a number of very important facts have been -discovered which may well lead us to devote a closer attention to the -composition and structure of meteorites. It has been shown, on the -one hand, that some meteorites contain substances precisely similar -to those which are sometimes brought from the earth's interior during -volcanic outbursts; and, on the other hand, there have been detected, -among some of the ejections of volcanoes, bodies which so closely -resemble meteorites that they were long mistaken for them. Both kinds -of observation seem to point to the conclusion that the earth's -interior is composed of similar materials to those which we find in the -small planets called meteorites. - -M. Daubrée has proposed a very convenient classification for -meteorites, dividing them into the following four groups:-- - -I. _Holosiderites_; consisting almost entirely of metallic iron, or of -iron alloyed with nickel, stony matter being absent; but sulphides, -phosphides, and carbides of several metals are often diffused through -the mass. The polished surfaces of these meteoric irons, when etched -with acid, often exhibit a remarkable crystalline structure. - -II. _Syssiderites_; in which a network of metallic iron encloses a -number of granular masses of stony materials. - -III. _Sporadosiderites_; which consist of a mass of stony materials, -through which particles of metallic iron are disseminated. - -IV. _Asiderites_; containing no metallic iron, but consisting entirely -of stony materials. - -There are, besides the meteorites belonging to these principal groups, -a few of peculiar and exceptional composition, which we need not notice -further for our present purpose. - -From the above classification it will be seen that most meteorites -consist of a mixture in varying proportions of metallic and stony -materials. Sometimes the metallic constituents are present in greater -proportions than the stony, at other times the stony materials -predominate, while occasionally one or other of these elements may be -wholly wanting. - -The stony portions of meteorites, upon careful examination, prove to be -built up of certain minerals, agreeing in their chemical composition -and their crystalline forms with those which occur in the rocks of the -earth's crust. Among the ordinary terrestrial minerals occurring in -the stony portions of meteorites, we may especially mention olivine, -enstatite, augite, anorthite, chromite, magnetite, and pyrrhotite. - -[Sidenote: METEORITES AND ULTRA-BASIC ROCKS.] - -The minerals which occur in meteorites are in every case such as are -found in the more basic volcanic rocks--quartz, and the acid felspars, -with the other minerals which occur in acid rocks, being entirely -absent in the 'extra-terrestrial' rocks. - -Now, besides the three great classes of lavas which we have described -as being ejected from volcanic vents, there are some rarer materials -occasionally brought from the earth's interior by the same agency, -that present a most wonderful resemblance to the stony portions of -meteorites. These materials we may call 'ultra-basic rocks.' Their -specific gravity is very high, usually exceeding 3, and they contain -a very low percentage of silica; on the other hand, the proportion of -iron and magnesia is often much greater than in ordinary terrestrial -rocks. But the most remarkable fact about these ultra-basic rocks is, -that they are almost entirely composed of the minerals which occur in -meteorites; namely, olivine, enstatite, augite, anorthite, magnetite, -and chromite. - -The ultra-basic rocks often occur under very peculiar conditions. -Sometimes they are found forming ordinary volcanic protrusions -through the sedimentary rocks. The rocks named pikrites, lherzolites, -dunites, &c., are examples of such igneous protrusions composed of -these ultra-basic materials, and probably all the true serpentines are -rocks of the same class which have absorbed water and undergone great -alteration. The ultra-basic rocks sometimes contain platinum and other -metals in the free or uncombined state. But not unfrequently we find -among the ordinary ejections of volcanoes, nodules and fragments of -such ultra-basic materials, which have clearly been carried up with the -other lavas from great depths in the earth's crust. Thus in Auvergne, -the Eifel, Bohemia, Styria, and many other volcanic districts, the -basaltic lavas and tuffs are found to contain nodules composed of -the minerals which are so highly characteristic of meteorites. Such -nodules, too, often form the centres of the volcanic bombs which are -thrown out of craters during eruptions. - -We thus see that materials identical in composition and character with -the stony portions of meteorites, exist within the earth's interior, -and are thrown out on its surface by volcanic action. A still more -interesting discovery has been made in recent years; namely, that -materials similar to the metallic portion of meteorites, and consisting -of nickeliferous iron, also occur in deep-seated portions of the -earth's crust, and are brought to the surface during periods of igneous -activity. - -In the year 1870, Professor Nordenskiöld made a most important -discovery at Ovifak, on the south side of the Island of Disko, off -the Greenland coast. On the shore of the island a number of blocks of -iron were seen, and the chemical examination of these proved that, -like ordinary metallic meteorites, they consisted of iron alloyed with -nickel and cobalt. - -[Sidenote: IRON-MASSES OF OVIFAK.] - -Now, when the facts concerning the masses of native iron of Ovifak were -made known, the first and most natural explanation which presented -itself to every mind was, that these were a number of meteorites which -at some past period had fallen upon the earth's surface. - - Metallic iron. - - Opaque crystals of magnetite (black oxide of iron). - - Transparent crystals of felspar, augite, and olivine. - -[Illustration: Fig. 87.--Section of basalt from Ovifak, Greenland, with -particles of metallic iron diffused through its mass.] - -But a further examination of the locality revealed a number of facts -which, as Professor Steenstrup pointed out, it is very difficult -to reconcile with the theory that the Ovifak masses of iron are of -meteoric origin. The district of Western Greenland, where these masses -were discovered, has been the scene of volcanic outbursts on the -grandest scale during the Miocene period. In close proximity to the -great iron masses, there are seen a number of basaltic dykes; and, when -these dykes are carefully examined, the basaltic rock of which they are -composed is seen to be full of particles of metallic iron. In fig. 87, -we have a drawing made from a section of the Ovifak basalts magnified -four or five diameters. The rock-mass is seen to be composed of black, -opaque magnetite, and transparent crystals of augite, labradorite, -olivine, &c.; while, through the whole, particles of metallic iron are -found entangled among the different crystals in the most remarkable -manner. - -It has been suggested that this singular rock might have been formed by -a meteorite falling, in Miocene times, into a lava-stream in a state -of incandescence. But the relation of the metallic particles to the -stony materials is such as to lend no support whatever to this rather -strained hypothesis. - -A careful study of all the facts of the case by Lawrence Smith, -Daubrée, and others well acquainted with the phenomena exhibited by -meteorites, has led to the conclusion that the large iron-masses of -Ovifak, as well as the particles of metallic iron diffused through -the surrounding basalts, are all of terrestrial origin, and have been -brought by volcanic action from the earth's interior. It is probable -that, just as we find in many basaltic lavas nodules of ultra-basic -materials similar to the stony parts of meteorites, so in these basalts -of Ovifak we have masses of iron alloyed with nickel, similar to the -metallic portions of meteorites. Both the stony and metallic enclosures -in the basalt are in all probability derived from deeper portions of -the earth's crust. By the weathering away of the basalt of Ovifak, the -larger masses of metallic iron have been left exposed upon the shore -where they were found. - -There are a number of other facts which seem to support this startling -conclusion. Thus it has been shown by Professor Andrews that certain -basalts in our own islands contain particles of metallic iron of -microscopic dimensions, and it is not improbable that some of the -masses of nickeliferous iron found in various parts of the earth's -surface, which have hitherto been regarded as meteorites, are, like -those of Ovifak, of terrestrial origin. - -[Sidenote: MATERIALS FILLING METALLIC-VEINS.] - -Another piece of evidence pointing in the same direction, is derived -from those great fissures communicating with the interior of our globe -which become filled with metallic minerals, and are known to us as -mineral-veins. In these mineral-veins the native metals, their alloys, -and combinations of these with sulphur, chlorine, phosphorus, &c., are -frequently present. But oxides of the metals, except as products of -subsequent alteration, occur far less frequently than in the earth's -crust generally. Hence we are led to conclude that the substances which -in the outer part of the earth's crust always exist in combination -with oxygen, are at greater depths in a free and uncombined condition. - -Nor is it a circumstance altogether unworthy of attention that the -researches of Mr. Norman Lockyer and other astronomers, based on the -known facts of the relative densities of the several members of the -solar system, and the ascertained relations of the different solar -envelopes, have led to conclusions closely in accord with those arrived -at by geologists. These researches appear to warrant the hypothesis -that the interior of our globe consists of metallic substances -uncombined with oxygen, and that among these metallic substances iron -plays an important part. Our globe, as we know, is a great magnet, -and the remarkable phenomena of terrestrial magnetism may also not -improbably find their explanation in the fact that metallic iron forms -80 large a portion of the earth's interior. - -The interesting facts which we have been considering may be made -clearer by the accompanying diagram (fig. 88). The materials ejected -from volcanic vents (lavas) are in almost all cases compounds of -silicon and the various metals with oxygen. In the lighter or acid -lavas oxygen constitutes one-half of their weight, and the proportion -of metals of the iron-group is very small. As we pass to the heavier -intermediate and basic lavas, we find the proportion of oxygen -diminishing, and the metals of the alkaline earths (magnesium and -calcium) with the metals of the iron-group increasing, in quantity. In -the small and interesting group of the ultra-basic lavas the proportion -of oxygen is comparatively small, and the proportion of magnesium and -iron very high. So much for the terrestrial rocks. - -[Illustration: Fig. 88.--Diagram illustrating the relation between the -Terrestrial and the Extra-Terrestrial Rock.] - -[Sidenote: TERRESTRIAL AND EXTRA-TERRESTRIAL ROCKS.] - -Now let us turn our attention to the extra-terrestrial rocks or those -found in meteorites. The Asiderites are quite identical in composition -with the ultra-basic lavas of our globe, but in the Sporadosiderites -and the Syssiderites we find the proportion of oxygen rapidly -diminishing, and that of metallic iron increasing. Finally, in the -Holosiderites the oxygen entirely disappears, and the whole mass -becomes metallic. - -From the Holosiderites at one end of the chain to the add lavas -at the other, we find there is a complete and continuous series; -the rocks of terrestrial origin overlapping, in their least -oxydized representatives, the most highly oxydized representatives -of the extra-terrestrial rocks. But the discovery at Ovifak of -the iron-masses, and the basalts with iron disseminated, has -afforded another very important link, placing the terrestrial and -extra-terrestrial rocks in closer relations with one another. - -All these facts appear to point to the conclusion that the earth's -interior consists of metallic substances either quite uncombined -or simply alloyed with one another, and among these iron is very -conspicuous by its abundance. The outer crust, which is probably of no -great thickness, contains an enormous proportion of oxygen and silicon -combined with the materials which constitute the interior portions of -our globe. It may be, as has been suggested by astronomers, that our -earth consisted at one time of a solid metallic mass surrounded by a -vaporous envelope of metalloids, and that the whole of the latter, with -the exception of the constituents of the atmosphere and ocean, have -gradually entered into combination with the metals of the nucleus to -form the existing crust of the globe. But of this period the geologist -can take no cognisance. The records which he studies evidently -commenced at a long subsequent period, when the conditions prevailing -at the earth's surface differed but little, if at all, from those which -exist at the present day. Equally little has the geologist to do with -speculations concerning a far distant future when, as some philosophers -have suggested, the work of combination of the waters and atmosphere of -the earth's surface with the metallic substances of its interior shall -be completed, and our globe, entirely deprived of its fluid envelopes, -reduced to the condition in which we find our satellite, the moon. - - * * * * * - -[Sidenote: PHYSICAL CONDITION OF EARTH'S INTERIOR.] - -There is another class of enquiries concerning the earth's interior to -which the attention of both geologists and astronomers has long been -directed--that, namely, which deals with the problem of the _physical -condition_ of the interior of our globe. - -The fact that masses of molten materials are seen at many points of the -earth's surface to issue from figures in the crust of our globe, seems -at first sight to find a simple explanation if we suppose our planet -to consist of a fluid central mass surrounded by a solid crust. Hence -we find that among those who first thought upon this subject, this -hypothesis of a liquid centre and a solid crust was almost universally -accepted. This hypothesis was supposed to find further support in the -fact that, as we penetrate into the earth's crust by mines or boring -operations, the temperature is found to continually increase. It was -imagined, too, that this condition of our planet would best agree with -the requirements of the nebular hypothesis of Laplace, which explains -the formations and movements of the bodies of the solar system by the -cooling down of a nebulous mass. - -But a more careful and critical examination of the question has led -many geologists and astronomers to reject the hypothesis that the earth -consists of a great fluid mass surrounded by a comparatively thin shell -of solid materials. - -Volcanic outbursts and earthquake tremors, though so terrible and -destructive to man and his works, are but slight and inconsiderable -disturbances in a globe of such vast dimensions as that on which we -live. The condition of the crust of the globe is, in spite of volcanic -and earthquake manifestations, one of general stability; and this -general stability has certainly been maintained during the vast periods -covered by the geological record. Such a state of things seems quite -irreconcilable with the supposition that, at no great depth from the -surface, the whole mass of the globe is in a liquid condition. If, on -the other hand, it be supposed that the solid crust of the globe is -several hundreds of miles in thickness, it is difficult to understand -how the local centres of volcanic activity could be supplied from such -deep-seated sources. - -There are other facts which seem equally irreconcilable with the -hypothesis of a fluid centre and a thin solid crust in our globe. If -all igneous products were derived from one central reservoir, we might -fairly expect to find a much greater uniformity of character among -those products than really exists. But in some cases, materials of -totally different composition are ejected at the same time from closely -adjoining volcanic districts. Thus in Hungary and Bohemia, as we have -seen, lavas of totally different character were being extruded during -the Miocene period. In the island of Hawaii, as Professor Dana has -pointed out, igneous ejections have taken place at a crater 14,000 feet -above the sea-level, while a closely adjoining open vent at a level -10,000 feet lower exhibited no kind of sympathy with the disturbance. -Whatever may be the cause of volcanic action, it seems clear that it -does not originate in a universal mass of liquefied material situated -at no great depth from the earth's surface. - -The conclusions arrived at by astronomers and physicists is one quite -in accord with those which geologists have reached by totally different -methods. It is now very generally admitted that if the earth were not -a rigid mass, its behaviour under the attract live influences of the -surrounding members of the solar system would be very different to what -is found to be the case. - -[Sidenote: ARGUMENTS AGAINST LIQUID INTERIOR.] - -That the earth is in a solid condition to a great depth from the -surface, and possibly quite to the centre, is a conclusion concerning -which there can be little doubt; and in the next chapter we shall -endeavour to show that such a condition of thirds is by no means -incompatible with those manifestations of internal energy, the -phenomena of which we are considering in this work. The question, -therefore, of the complete solidity of our globe, or of its consisting -of a solid and a liquid portion, is one of speculative interest only, -and is in no way involved in our investigations concerning the nature -and origin of volcanic activity. We may conclude this chapter by -enumerating the several hypotheses which have at different times been -maintained concerning the nature of the interior of our globe. - -_First._ It has been suggested that the earth consists of a fluid or -semi-fluid nucleus surrounded and enclosed in a solid shell. Some -have maintained this shell to be of such insignificant thickness, as -compared with the bulk of the interior liquid mass, that portions of -the latter are able to reach the earth's surface through movements -and fractures of the outer shell, and that in this manner volcanic -manifestations originate. Others, impressed with the general stability -and rigidity of the globe as a whole, have maintained that the outer -solid shell must have a very considerable thickness, amounting -probably to not less than several hundreds of miles. But through a -shell of such thickness it is difficult to conceive of the liquid -masses of the interior finding their way to the surface, and those who -have held this view are driven to suggest some other means by which -local developments of volcanic action might be brought about. - -_Secondly._ Some physicists have asserted that a globe of liquid -matter radiating its heat into space, would tend to solidify both at -the surface and the centre, at the same time. The consequence of this -action would be the production of a sphere with a solid external shell -and a solid central nucleus, but with an interposed layer in a fluid -or semi-fluid condition. It has been pointed out that if we suppose -the solidification to have gone so far, as to have caused the partial -union of the interior nucleus and the external shell, we may conceive a -condition of things in which the stability and rigidity is sufficient -to satisfy both geologists and astronomers, but that in still -unsolidified pockets or reservoirs, filled with liquefied rock, between -the nucleus and the shell, we should have a competent cause for the -production of the volcanic phenomena of the globe. In this hypothesis, -however, it is assumed that the cooling at the centre and the surface -of the globe would go on at such rates that the reservoirs of liquid -material would be left at a moderate depth from the surface, so that -easy communication could be opened between them and volcanic vents. - -[Sidenote: REVIEW OF THE SEVERAL HYPOTHESES.] - -_Thirdly._ It has been maintained that the earth may have become -perfectly solid from the centre to the surface. Those who hold this -view endeavour to account for the phenomena of volcanoes in one of two -ways. It may be, they say, that the deep-seated rock-masses, though -actually solid, are in a state of _potential_ liquidity; that though -reduced to a solid state by the intense pressure of the superincumbent -masses, yet such is the condition of unstable equilibrium in the whole -mass, that the comparatively slight movements and changes taking place -at the earth's surface suffice to bring about the liquefaction of -portions of its crust and consequent manifestations of volcanic energy. -But It may be, as other supporters of the doctrine of the earth's -complete solidity have maintained, that the phenomena of volcanoes -have no direct connection with a supposed incandescent condition of -our planet at all, and that there are chemical and mechanical forces -at work within our globe which are quite competent to produce at the -surface all those remarkable phenomena which we identify with volcanic -action. - -From this summary of the speculative views which have been entertained -upon the subject of the physical condition of the earth's interior, -it will be clear that at present we have not sufficient evidence for -arriving at anything like a definite solution of the problem. The -conditions of temperature and pressure which exist in the interior of -a globe of such vast dimensions as our earth, are so far removed from -those which we can imitate in our experimental enquiries, and it is so -unsafe to push the application of laws arrived at by the latter to the -extreme limits required by the former, that we shall do well to pause -before attempting to dogmatise on such a difficult question. - -In the next chapter we shall endeavour to grapple with a somewhat more -hopeful task, to point out how far observation and experiment have -enabled us to offer a reasonable explanation of the wonderful series of -phenomena which are displayed during outbursts of volcanic activity. - - - - -CHAPTER XII. - -THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN THE CAUSES OF VOLCANIC -ACTION. - - -Every completed scientific investigation must consist of four series -of operations. In the first of these an attempt is made to collect the -whole of the facts bearing on the question, by means of observation -and experiment; the latter being only observation under conditions -determined by ourselves. In the second stage of the enquiry, the -attention is directed to classifying and grouping the isolated facts, -so as to determine their bearings upon one another, and the general -conclusions to which they appear to point. In the third stage, it is -sought to frame an hypothesis which shall embrace all the observed -facts, and shall be in harmony with the general conclusions derived -from them. In the fourth stage, this hypothesis is put to the most -rigid test; comparing the results which must follow, if it be true, -with the phenomena actually observed, and rejecting or amending our -hypothesis accordingly. Every great scientific theory has thus been -established by these four processes--observation, generalisation, -hypothesis, and verification. - -The enquiry concerning the nature and causes of volcanic action is far -from being a completed one. It is true that many hypotheses upon the -subject have been framed, but in too many instances these have not been -based on accurate observations and careful generalisations, and can be -regarded as little better than mere guesses. Indeed, the state of the -enquiry at the present time would seem to be as follows. Although much -remains to be done in the direction both of observation and experiment, -the main facts of the case have been established upon irrefragable -evidence. The classification and comparison of these facts have led to -the recognition of certain laws, which seem to embrace all the known -facts. To account for these facts and their demonstrated relations to -one another, certain tentative hypotheses have been suggested; but in -no case can it be truly said that these latter have so far stood the -test of exact enquiry as to deserve to rank as demonstrated truths. A -complete and consistent theory of volcanic action still remains to be -discovered. - -[Sidenote: VALUE AND LIMITS OF HYPOTHESES.] - -In accordance with the plan which we have sketched out for ourselves -at the commencement of this work, we shall aim at following what has -been the order of investigation and discovery in our study of volcanic -action; and in this concluding chapter we shall indicate the different -hypotheses by which it has been proposed to account for the varied -phenomena, which we have discussed in the preceding pages, and their -remarkable relations to one another. We shall endeavour, in passing, -to indicate how far these several hypotheses appear to be probable, -as satisfying a larger or smaller number of those conditions of the -problem which have been established by observation, experiment, and -careful reasoning; but we shall at the same time carefully avoid such -advocacy of any particular views as would tend to a prejudgment of -the question. Hypothesis is, as we have seen, one of the legitimate -and necessary operations in scientific investigation. It only becomes -a dangerous and treacherous weapon when it is made to precede rather -than to follow observation and experiment, or when being regarded -with paternal indulgence, an attempt is made to shield it from the -relentless logic of facts. Good and bad hypotheses must be allowed to -'grow together till the harvest;' such as are unable to accommodate -themselves to the surrounding conditions imposed by newly-discovered -facts and freshly-established laws will assuredly perish; and in this -'struggle for existence' the true hypothesis will in the end survive, -while the false ones perish. - -It may well happen, however, that among the hypotheses which have up to -the present time been framed, none will be found to entirely satisfy -all the conditions of the problem. New discoveries in physics and -chemistry have suggested fresh explanations of volcanic phenomena in -the past, and may continue to do so in the future; and the true theory -of volcanic action, when it is at last discovered, may combine many of -the principles which now seem to be peculiar to different hypotheses. - -Let us, in the first place, enquire what are the facts which must be -accounted for in any theory of volcanic action. We have already been -led to the conclusion that the phenomena exhibited by volcanoes were -entirely produced by the escape of imprisoned water and other gases -from masses of incandescent and fluid rock. Our subsequent examination -of the problem confirmed the conclusion that in all cases of volcanic -outburst we have molten rock-materials from which water and other -gases issue with greater or less violence. The two great facts to -be accounted for, then, in any attempted explanation of volcanic -phenomena, are the existence of this high temperature at certain points -within the earth's crust, and the presence of great quantities of water -and gas, imprisoned in the rocks. We shall perhaps simplify the enquiry -if we examine these two questions separately, and, in the first place, -review those hypotheses which have been suggested to account for high -temperatures in the subterranean regions, and, in the second place, -examine those which seek to explain the presence of large quantities of -imprisoned water and gases. - -[Sidenote: INCREASE OF TEMPERATURE WITH DEPTH.] - -That a high temperature exists in the earth's crust at some depth from -the surface is a £act which does not admit of any doubt. Every shaft -sunk for mining operations, and every deep boring made for the purpose -of obtaining water, proves that a more or less regular increase of -temperature takes place as we penetrate downwards. The average rate -of this increase of temperature has been estimated to be about 1° -Fahrenheit for every 50 or 60 feet of depth. - -Now if it be assumed that this regular increase of temperature -continues to great depths, a simple calculation proves that at a depth -of 9,000 feet a temperature of 212° Fahrenheit will be found--one -sufficient to boil water at the earth's surface--while at a depth of 28 -miles the temperature will be high enough to melt cast-iron, and at 34 -miles to fuse platinum. - -So marked is this steady increase of temperature as we go downwards, -that it has been seriously proposed to make very deep borings in order -to obtain supplies of warm water for heating our towns. Arago and -Walferdin suggested this method for warming the Jardin des Plantes at -Paris; and now that such important improvements have been devised in -carrying borings to enormous depths, the time may not be far distant -when we shall draw extensively upon these supplies of subterranean -heat. At the present time the city of Buda-Pesth is extensively -supplied with hot-water from an underground source. Should our -coal-supply ever fail it may be well to remember that we have these -inexhaustible supplies of heat everywhere beneath our feet. - -But although we may conclude that at the moderate depths we have -indicated such high temperatures exist, it would not be safe to -infer, as some have done, that at a distance of only 40 or 50 miles -from the surface the materials composing our globe are in a state -of actual fusion. Both theory and experiment indicate that under -increased pressure the fusing point of solid bodies is raised; and -just as in a Papin's digester we may have water retained by high -pressure in a liquid condition at a temperature far above 212° F., -so in the interior of the earth, masses of rock may exist in a solid -state, at a temperature far above that at which they would fuse at -the earth's surface. We may speak of such rock-masses, retained in a -solid condition by intense pressure, at a temperature far above their -fusing point at the earth's surface, as being in a 'potentially liquid -condition.' Upon any relief of pressure such masses would at once -assume the liquid state, just as the superheated water in a Papin's -digester immediately flashes into steam upon the fracture of the strong -vessel by which it is confined. We have already seen how the action -at volcanic vents often appears to indicate just such a manifestation -of elastic forces, as would be exhibited by the relief of superheated -masses from a state of confinement by pressure. - -In reasoning upon questions of this kind, however, we must always be -upon our guard against giving undue extension to principles and laws -which seem to be clearly established by experiment at the earth's -surface. It is well to remember how exceedingly limited is our command -of extreme pressures and high temperatures, when compared with those -which may exist within a body of the dimensions of our globe. - -[Sidenote: EFFECT OF PRESSURE ON FUSION-POINT.] - -If we were to imagine a set of intelligent creatures, who were able to -command only a range of temperatures from 50° to 200° F., engaged upon -an investigation of the properties of water, we shall easily understand -how unsafe it may be to extend generalisations far beyond the limits -covered by actual experiment. Such beings, from their observation of -the regular changes of volume of water at all the temperatures they -could command, might infer that at still higher and lower temperatures -the same rates of expansion and contraction would be maintained. Yet, -as we well know, such an inference would be quite wide of the truth; -for a little above 200° F. water suddenly expands to 1,700 times its -volume, and not far below 50° F. the contraction is suddenly changed -for expansion. - -It has been argued by the late Mr. David Forbes and others that, -inasmuch as experiment has shown that--though the fusing points of -solids are raised by pressure, yet that this rise of the fusing -points goes on in a diminishing ratio as compared with the pressures -applied--a limit will probably be reached at which the most intense -pressure will not be sufficient to retain substances at a high -temperature in their solid state. The fact that gases cannot be -retained in a liquid condition by the most intense pressure at a -temperature above their critical point, may seem by analogy to favour -the same conclusion. Hence, David Forbes, Dana, and other authors, have -argued in favour of the existence of a great liquid nucleus in our -globe covered by a comparatively thin, solid crust. And if we accept -the supposed proofs of a constant increase of temperature from the -surface to the centre of the globe, such a conclusion appears to be at -least as well founded as that which regards the central masses of the -earth as maintained in a solid condition by intense pressure. - -A little consideration will, however, convince us that the facts which -have been relied upon as proving the intensely heated condition of the -central masses of our globe, are by no means so conclusive as has been -supposed. - -The earth's form, which mathematicians have shown to be exactly that -which would be acquired by a globe composed of yielding materials -rotating on its axis at the rate which our planet does, has often -been adduced as proving that the latter was not always in a rigid and -unyielding condition. In the same way, all the remarkable facts and -relations of the bodies of the solar system, which have been shown by -astronomers to lend such support to the nebular hypothesis, have been -thought, at the same time, to favour the view that our earth is still -in a condition of uncompleted solidification. - -But it is quite admissible to accept the nebular hypothesis and the -view that our globe attained its present form while still in a state -of fluidity, and at the same time to maintain that our earth has long -since reached its condition of complete solidification. And there are -not a few facts which appear to lend support to such a conclusion. - -[Sidenote: SUPPOSED PROOFS OF LIQUID NUCLEUS.] - -If the rapid rate of increase in temperature which has been -demonstrated to occur at so many parts of the earth's surface be -maintained to the centre, then, as argued by David Forbes and Dana, it -is difficult to conceive of our earth as being in any other condition -than that of a liquid mass covered by a comparatively thin crust. The -objection to this view, both upon geological and astronomical grounds, -we have pointed out in the previous chapter. - -Before accepting as a demonstrated conclusion this notion of a constant -increase of temperature from the surface to the centre of our globe, it -may be well to re-examine the facts which are relied upon as proving it. - -That there is a general increase of temperature so far as we are able -to go downwards in the earth's crust, there can, as we have seen, -be no doubt whatever. Yet it may be well to bear in mind how very -limited is the range of our observation on the subject. The deepest -mines extend to little more than half-a-mile from the surface, and the -deepest borings to little more than three-quarters of a mile, while the -distance from the earth's surface to its centre is nearly 4,000 miles. -We may well pause before we extend conclusions, derived from such very -limited observations, to such enormous depths. - -But when we examine critically these observations themselves, we -shall find equal grounds for caution in generalising from them. There -is the greatest and most startling divergence in the results of the -observations which have been made at different points at the earth's -surface. Even when every allowance is made for errors of observation, -these discrepancies still remain. In some places the increase of -temperature as we go downwards is so rapid that it amounts to 1° -Fahrenheit for every 20 feet in depth, while in other cases, in order -to obtain the same increase in temperature of 1° Fahrenheit, we have to -descend as much as 100 feet. - -Now if, as is so often assumed, this increase of temperature as we -go downwards be due to our approach to incandescent masses forming -the interior portions of the globe, it is difficult to understand -why greater uniformity is not exhibited in the rate of increase in -different areas. No difference in the conducting powers of the various -rock-materials is sufficient to account for the fact that in some -places the rate of increase in temperature in going downwards is no -less than five times as great as it is in others. - -[Sidenote: VARIATIONS IN UNDERGROUND TEMPERATURES.] - -Again, there are some remarkable facts concerning the variation in -the rate of increase in temperature with depth which seem equally -irreconcilable with the theory that the heat in question is directly -derived from a great, central, incandescent mass. M. Walferdin, by a -series of careful observations in two shafts at Creuzot, proved that -down to the depth of 1,800 feet the increase of temperature amounted to -1° Fahrenheit for every 55 feet of descent, but below the depth named, -the rate of increase was as much as 1° Fahrenheit for every 44 feet. On -the other hand, in the great boring of Grenelle at Paris, the increase -in temperature down to the depth of 740 feet amounted to 1° Fahrenheit -for every 50 feet of descent, but from 740 feet down to 1,600 feet, -the rate of increase diminished to 1° for 75 feet of descent. The -same remarkable fact was strikingly shown in the case of the deepest -boring in the world--that of Sperenberg, near Berlin, which attained -the great depth of 4,052 feet. In this case, the rate of increase in -temperature for the first 1,900 feet, was 1° Fahrenheit for every 55 -feet of descent, and for the next 2,000, it diminished to 1° Fahrenheit -for every 62 feet of descent. In the deep well of Buda-Pesth there was -actually found a decline in temperature below the depth of 3,000 feet. - -Perhaps the most interesting fact in connection with this question -which has been discovered of late years, is that in districts which -have recently been the seat of volcanic agencies, the rate of increase -in temperature, as we go downwards in the earth's crust, is abnormally -high. Thus at Monte Massi in Tuscany, the temperature was found to -increase at the rate of 1° Fahrenheit for every 24 feet of descent. -In Hungary several deep wells and borings have been made, which prove -that a very rapid increase of temperature occurs. The deep boring at -Buda-Pesth penetrates to a depth of 3,160 feet, and a temperature of -178° Fahrenheit has been observed near the bottom. The rate of increase -of temperature in this boring was about 1° for every 23 feet of -descent. In the mines opened in the great Comstock lode, in the western -territories of the United States, an abnormally high temperature has -been met with amounting in some cases to 157° Fahrenheit. Although -this is the richest mineral-vein in the world, having yielded since -1859, when it was first discovered, 60,000,000_l._ worth of gold and -silver, this rapid increase in temperature in going downwards threatens -in the end to entirely baffle the enterprise of the miner. The rate -of increase in temperature in the case of the Comstock mines has been -estimated at 1° Fahrenheit for every 46 feet of descent, between 1,000 -and 2,000 feet from the surface, but as much as 1° Fahrenheit for every -25 feet, at depths below 2,000 feet. - -The facts which we have stated, with others of a similar kind, have -led geologists to look with grave feelings of doubt upon the old -hypothesis which regarded the increase of temperature found in making -excavations into the earth's crust as a proof that we are approaching a -great incandescent nucleus. They have thus been led to enquire whether -there are any conceivable sources of high temperatures at moderate -depths--temperatures which would be quite competent to produce locally -all the phenomena of volcanic action. - -There are not wanting other facts which seem to point to the same -conclusion: namely, that volcanic action is not due to the existence -of a universal reservoir of incandescent material occupying the central -portion of our globe, but to the local development of high temperatures -at moderate depths from the surface. - -[Sidenote: DEPTHS AT WHICH EARTHQUAKES ORIGINATE.] - -The close connection between the phenomena of volcanoes and earthquakes -cannot be doubted. It is true that some of those vibrations or tremors -of the earth's crust, to which we apply the name of earthquakes, -occur in areas which are not now the seat of volcanic action; and it -is equally true that the stratified rock-masses of our globe, far -away from any volcanic centres, exhibit proofs of violent movement -and fracture, in the production of which, concussions giving rise -to earthquake vibrations, could scarcely fail to have occurred. But -it is none the less certain that earthquakes as a rule take place -in those areas which are the seats of volcanic action, and that -great earthquake-shocks precede and accompany volcanic outbursts. -Sometimes, too, it has been noticed that the manifestation of -activity at a volcanic centre is marked by the sudden decline of the -earthquake-tremors of the district around, as though a safety-valve had -been opened at that part of the earth's surface. - -Mr. Mallet has shown that by the careful study of the effects -produced at the surface by earthquake-vibrations, we may determine -with considerable accuracy the point at which the shock or concussion -occurred which gave rise to the vibration. Now it is a most remarkable -fact that such calculations have led to the conclusion that, so far -as is at present known, earthquake shocks never originate at greater -depths than thirty miles from the surface, and that in some cases -the focus from which the waves of elastic compression producing an -earthquake proceed is only at the depth of seven or eight miles. -As we have already seen, there can be no doubt that in the great -majority of instances the forces originating earthquake-vibrations and -volcanic outbursts are the same, and independent lines of reasoning -have conducted us to the conclusion that these forces operate at very -moderate distances from the earth's surface. - -Under these circumstances, geologists have been led to enquire -whether there are any means by which we can conceive of such an -amount of heat, as would be competent to produce volcanic outbursts, -being locally developed at certain points within the earth's crust. -Recent discoveries in physical science which have shown the close -relation to one another of different kinds of force, and their mutual -convertibility, have at least suggested the possibility of the -existence of causes by which such high temperatures within certain -portions of the earth's crust may be originated. - -[Sidenote: DAVY'S CHEMICAL THEORY.] - -When, at the commencement of the present century, Sir Humphry Davy -discovered the remarkable metals of the alkalies and alkaline earths, -and at the same time demonstrated the striking phenomena which are -exhibited if these metals be permitted to unite with oxygen, he at -once perceived that if such metals existed in an uncombined condition -within the earth's crust, the access of water and air to the mass might -give rise to the development of such an amount of heat, as would be -competent to produce volcanic phenomena at the surface. It is true that -at a later date Davy recognised the chemical theory of volcanoes as -being beset with considerable difficulties, and was disposed to abandon -it altogether. It was argued, with considerable show of reason, that if -the heat at volcanic centres were produced by the access of water to -metallic substances, great quantities of hydrogen would necessarily be -evolved, and this gas ought to be found in prodigious quantities among -the emanations of volcanoes. The fact that such enormous quantities of -hydrogen gas are not emitted from volcanic vents has been held by many -authors to be fatal to the chemical theory of volcanoes. - -But the later researches of Graham and others have made known facts -which go far towards supplying an answer to the objections raised -against the chemical theory of volcanoes. Various solids and liquids -have been shown to possess the power of absorbing many times their -volume of certain gases. Among the gases thus absorbed in large -quantities by solids and liquids, hydrogen is very conspicuous. In -some cases gases are absorbed by metals or other solids in a state of -fusion, and yielded up again by them as they cool. - -It is a very remarkable circumstance that some meteorites are found -to have absorbed large quantities of hydrogen gas, and this is given -off when they are heated in vacuo. Thus it has been demonstrated that -certain meteorites have contained as much as forty seven times their -own volume of hydrogen gas. - -We have already pointed out that there are reasons for believing the -internal portions of our globe to be composed of materials similar to -those found in meteorites. If such be the case, the access of water -to these metallic substances may result in the formation of oxides, -attended with a great local development of heat, the hydrogen which -is liberated being at once absorbed by the surrounding metallic -substances. That this oxidation of the metallic substances in the -interior of our globe by the access of water and air from the surface -is continually going on, can scarcely be doubted. We may even look -forward to a far-distant period when the whole of the liquid and -gaseous envelopes of the globe shall have been absorbed into its -substance, and our earth thereby reduced to the condition in which we -now find the moon to be. - -There is a second method by which high temperatures might be locally -developed within the earth's crust, which has been suggested by Vose, -Mallet, and other authors. - -We have good grounds for believing that the temperature of our globe -is continually diminishing by its radiation of heat into space. This -cooling of our globe is attended by contraction, which results in -movements of portions of its crust. It may at first sight appear that -such movements would be so small and insignificant as to be quite -unworthy of notice. But if we take into account the vast size of our -earth it will be seen that the movements of such enormous masses may be -attended with the most wonderful results. - -It has been shown that if a part of the earth's crust fifty miles in -thickness were to have its temperature raised 200° Fahrenheit, its -surface would be raised to the extent of 1,000 or 1,500 feet Le Conte -has pointed out that if we conceive the conduction of heat to take -place at slightly different rates along different radii of our globe, -we should at once be able to account for the existing inequalities of -the earth's surface, and for all those continental movements which can -be shown to have taken place in past geological periods. - -[Sidenote: DYNAMICAL THEORIES.] - -But if we admit, as we have good grounds for doing, that the loss of -heat from the external portions of our globe goes on more rapidly than -in the case of the central masses, we have thereby introduced another -powerful agent for the production of high temperatures within the -earth's crust. The external shell of the globe will tend to contract -upon the central mass, and in so doing a series of tangential strains -will result which will be capable of folding and crumpling the rocks -along any lines of weakness. That such crushing and crumpling has -during all geological periods taken place along lines of weakness -in the earth's crust, is proved, as we have seen, by the phenomena -presented by mountain-ranges. Now these crushings, crumplings, and -other violent movements of great rock-masses must result in the -development of a vast amount of heat, just as the forcing down of a -break upon a moving wheel produces heat. This conclusion is strikingly -confirmed by the well-known geological fact that nearly all rocks which -have undergone great movement and contortion are found to present -evidence of having been subjected to such chemical and crystalline -actions, as would result from the development of a high temperature -within their mass. - -[Sidenote: RECAPITULATION OF SEVERAL THEORIES.] - -Let us sum up briefly the various methods which have been suggested to -account for the high temperatures within certain parts of the earth's -crust by which volcanic phenomena are produced. - -Our globe may be conceived of as an incandescent liquid mass surrounded -by a cooler, solid shell. If we regard this liquid interior mass as -supplying directly the various volcanic vents of the earth, it must be -conceded that the outer shell is of comparatively slight thickness. -But astronomers are almost universally agreed that such a thin outer -shell and inner liquid mass are quite incompatible with that rigidity -which our planet exhibits under the attractions of its neighbours. -Geologists are almost equally unanimous in regarding this hypothesis -of a liquid nucleus and thin, solid shell as contradicted by the -stability of the conditions which have been maintained during such -long past periods, and which exist at the present day. The extent and -character of volcanic action do not indicate a condition of general -instability in our earth, but one of stability subject to small and -local interferences The grandest volcanic disturbances appear small and -insignificant, if we take into account the vast dimensions of the globe -upon which they are displayed. - -If, on the other hand, we consider the outer solid shell to be of -great thickness, we are met by the difficulty of accounting for the -upheaval of liquid matter through such vast thicknesses of a solid -shell. The differences in character of lavas extruded from closely -adjoining volcanic districts seem equally difficult of explanation on -any theory of a central, fluid nucleus and a solid, outer shell. Nor is -the distribution of heat within the earth's crust so uniform as might -be anticipated, if the source of that heat be a great central mass of -highly heated materials. - -Under these circumstances, geologists and physicists have enquired -whether any other conditions can be imagined as existing in the earth's -interior, which would better account for the observed phenomena than -does the hypothesis of a liquid nucleus and a solid outer shell. Two -such alternative hypotheses have been suggested. - -Mr. Hopkins, adopting the theory that the earth has solidified both at -the centre and its outer surface, endeavoured to explain the occurrence -of volcanoes and earthquakes by supposing that cavities of liquid -material have been left between the solid nucleus and the solid shell, -and these cavities full of liquid material constitute the sources from -which the existing volcanoes of the globe draw their supplies. But -this hypothesis is found to be beset with many difficulties when we -attempt to apply it to the explanation of the phenomena of volcanic -action. It entirely fails, among other things, to account for the -remarkable fact that during past geological periods the scene of -volcanic action has been continually shifting over the surface of the -earth, so that there is probably no considerable area of our globe -which has not at one time or other been invaded by the volcanic forces. - -By some other theorists, who have felt the full force of this last -objection, an attempt has been made to explain the phenomena of -volcanoes by supposing that the globe is solid from its surface to its -centre, but that the internal portions of the globe are at such a high -temperature that they are only retained in a solid condition by the -enormous pressure to which they are subjected. The central masses of -the globe are thus regarded as being in an _actually_ solid, but in a -_potentially_ liquid condition, and any local relief of pressure is at -once followed by the conversion of solid to liquefied materials, in the -district where the relief takes place, resulting in the manifestation -of volcanic phenomena at the spot. It may be granted that this -hypothesis better accords with the known facts of Vulcanology than any -of those which we have previously described, but it is impossible to -shut our eyes to the fact that not a few serious difficulties still -remain. Thus it is based upon the assumption that the law of the -elevation of the point of fusion by pressure is true at temperatures -and pressures almost infinitely above those at which we are able to -conduct observations; but neither experiment nor analogy warrant this -conclusion, for the former shows that the elevation of the point of -fusion by pressure goes on in a continually diminishing ratio, and the -latter famishes us with the example of volatile liquids which, above -their critical points, obstinately remain in a gaseous condition under -the highest pressures. Nor is it easy upon this hypothesis to account -for the very irregular distribution of temperatures within the earth's -crust, as demonstrated by observations in mines, wells, and borings. -The hypothesis further requires the assumption that, at such very -moderate depths as are required for the reservoirs of volcanoes, the -effects of pressure and temperature on the condition of rock-materials -are so nicely balanced that the smallest changes at the surface lead to -a disturbance of the equilibrium. - -[Sidenote: DIFFICULTIES NOT YET EXPLAINED.] - -It is the weight of these several objections that has led geologists -in recent years to regard with greater favour those hypotheses which -seek to account for the production of high temperatures within parts of -the earth's crust, without having recourse to a supposed incandescent -nucleus. If it can be shown that there are any chemical or mechanical -forces at work within the crust of the globe which are capable of -producing local elevations of temperature, then we may conceive of a -condition of things existing in the earth's interior which is free -from the objections raised by the astronomer on the score of the -earth's proved rigidity, and by the geologist on the ground of its -general stability, and which at the same time seems to harmonise better -with the observed facts of the distribution of temperature within the -earth's crust. How far the existence of such chemical and mechanical -agencies capable of producing high temperatures within the crust of the -globe have been substantiated, we have already endeavoured to point out. - -It must be admitted, then, that the questions of the nature of the -earth's interior and the cause of the high temperatures which produce -volcanic phenomena, are still open ones. We have not yet got beyond the -stage of endeavouring to account for the facts observed by means of -tentative hypotheses. Some of these, as we have seen, agree with the -facts, so far as they are at present known, much better than others; -but the decision between them or the rejection of the whole of them in -favour of some new hypothesis, must depend on the results of future -observation and enquiry. - -It may be well, before leaving this subject, to remark that they are -all equally reconcilable with the nebular theory of Kant and Laplace. -Granting that the matter composing our globe has passed successively -through the gaseous and liquid conditions, it is open to us to imagine -the earth as now composed of a liquid nucleus with either a thick or -a thin solid shell; of a solid nucleus and a solid shell with more or -less liquid matter between them; or, lastly, to conceive of it as -having become perfectly solid from the centre to the surface. - -[Sidenote: CAUSE OF THE PRESENCE OF WATER IN LAVAS.] - -But it is not upon the existence of a high temperature within certain -parts of the earth's crust that the production of volcanic activity -alone depends. The presence of water and other liquid and gaseous -substances in a state of the most intimate admixture with the fused -rock-masses, is, as we have seen, the main cause of the violent -displays of energy exhibited at volcanic centres. And We shall now -proceed to notice the hypotheses which have been suggested to account -for the presence of these liquid and gaseous bodies in the midst of the -masses of incandescent materials poured out from volcanic vents. - -There is an explanation of this presence of water and various gases in -the masses of molten rock-materials within the earth's crust which at -once suggests itself, and which was formerly very generally accepted. -Volcanoes, as we have seen, are usually situated near coast-lines, and -if we imagine fissures to be produced by which sea-water finds access -to masses of incandescent rock-materials, then we can regard volcanic -outbursts as resulting from this meeting of water with rock-masses in a -highly healed condition. This supposition has been thought to receive -much support from the fact that many of the gases evolved from volcanic -vents are such as would be produced by the decomposition of substances -present in sea-water. - -But it frequently happens that an explanation which at first sight -appears to be very simple and obvious, turns out on more critical -examination to be quite the reverse, and this is the case with the -supposed origination of volcanic outbursts by the access of sea-water -to incandescent rock-material by means of earth-fissures. It is -difficult to understand how, by such means, that wonderfully intimate -union between the liquefied rock and the water, evolved in such -quantities during volcanic outbursts, could be brought about; and -moreover, we can scarcely regard the production of fissures in the -earth's crust as being at the same time both the cause and the effect -of this influx of water to the deep-seated rock-masses at a high -temperature. - -[Sidenote: ABSORPTION OF GASES BY LIQUIDS AND SOLIDS.] - -During recent years the attention of both geologists and physicists -has been directed to a remarkable property exhibited by many liquids -and solids, as supplying a possible explanation of the phenomena of -volcanic action. The property to which we refer is that whereby some -liquid and solid substances are able to absorb many times their volume -of certain gases--which gases under different conditions may be given -off again from the liquids or solids. This power of absorption is a -very remarkable one; it is not attended with chemical combination, but -the amount of condensation which gases must undergo within the solid or -liquid substances is sometimes enormous. Water may be made to absorb -more than 1,000 times its volume of ammonia, and more than 500 times -its volume of hydrochloric acid. Alcohol may absorb more than 300 -times its volume of sulphurous acid. Charcoal may absorb 100 times its -volume of ammonia, 85 times its volume of hydrochloric acid, 65 times -its volume of sulphuretted hydrogen, 55 times its volume of sulphurous -acid, and 35 times its volume of carbonic acid. Platinum-black absorbs -many times its volume of oxygen and other gases. - -This power of absorption of gases varies in different solids and -liquids according to the conditions to which they are subjected. Dr. -Henry showed it to be a general law in liquids that, as the pressure is -augmented, the weight of the gas absorbed is proportionately increased. - -Sometimes this absorption of gases takes place only at high -temperatures. Thus silver in a state of fusion is able to absorb 22 -times its volume of oxygen gas. When the metal is allowed to cool this -gas is given off, and if the cooling takes place suddenly a crust is -formed on the surface, and the phenomenon known as the 'spitting of -silver' is exhibited. Sometimes during this operation miniature cones -and lava-streams are formed on the surface of the cooling mass, which -present a striking resemblance to those formed on a grand scale upon -the surface of the globe. Similar phenomena are exhibited by several -other metals and by the oxide of lead. - -The researches of Troost and others have shown that molten iron and -steel possess the property of absorbing considerable quantities of -oxygen, hydrogen, carbonic acid, and carbonic oxide, and that these -gases are given off in the operation known as 'seething,' when either -the pressure or the temperature is diminished. - -Hochstetter has shown that in the process of extracting sulphur from -the residues obtained during the manufacture of soda, some very -interesting phenomena are manifested. The molten sulphur is exposed -to a temperature of 262° Fahrenheit, and a pressure of two or three -atmospheres, in the presence of steam; under these circumstances it -is found that the sulphur absorbs a considerable quantity of water, -which is given off again with great violence from the mass as it -undergoes solidification. The hardened crust which forms on the surface -of the molten sulphur is agitated and fissured, miniature cones and -lava-streams being formed upon it, which have a striking resemblance to -the grander phenomena of the same kind exhibited upon the crust of the -globe. - -The observations which we have described prove conclusively that many -liquids and solids in a molten condition have the power of absorbing -many times their volume of certain gases, and that this action is aided -by heat and pressure. - -That the molten materials which issue from volcanic vents have -absorbed enormous quantities of steam and other gases, we have the -most undisputable evidence. The volume of such gases given off -during volcanic outbursts, and while the lava-streams are flowing -and consolidating, is enormous, and can only be accounted for by -supposing that the masses of fluid rock have absorbed many times their -volume of the gases. But we have another not less convincing proof -of the same fact in the circumstance that volcanic materials which -have consolidated under great pressure--such as granites, gabbros, -porphyries, &c.--exhibit in their crystals innumerable cavities -containing similar gases in a liquefied state. - -It is to the violent escape of these gases from the molten rock-masses, -as the pressure upon them is relieved, that nearly all the active -phenomena of volcanoes must be referred; and it was the recognition of -this bet by Spallanzani, while he was watching the phenomena displayed -in the crater of Stromboli, which laid the foundations of the science -of Vulcanology. - -[Sidenote: SOURCE OF THE ABSORBED GASES.] - -But here another question presents itself to the investigator of the -phenomena of volcanoes: it is this. At what period did the molten -rock-masses issuing from vents absorb those gaseous materials which -are given off so violently from their midst during eruptions? Two -different answers to this question have been suggested. It may be that -the original materials of which our globe was composed consisted of -metallic substances in a state of fusion which had absorbed many gases, -and that, in the fluid masses below the solid crust, vast quantities of -vapour and gas are stored up, which are being gradually added to the -atmosphere during volcanic outbursts. The fact that meteorites, which, -as we have seen, in all probability closely resemble the materials -forming the earth's interior, sometimes yield many times their volume -of hydrogen and other gases, may be thought to lend some support to -this idea. If it be the correct one, we must regard our globe as -gradually parting with its pent-up stores of energy, in those absorbed -gases and vapours held in bondage by the solid and fluid materials of -its interior. - -But there is another hypothesis which is, to say the least, equally -probable. Water containing various gases in solution is continually -finding its way downwards by infiltration into the earth's crust. -Much of this water, after passing through pervious beds, reaches -some impervious stratum and is returned to the surface in the form -of springs. But that some of this percolating water penetrates to -enormous depths is shown by the fact that the deepest mines and borings -encounter vast underground supplies of water. When we remember that -nearly three-fourths of the earth's surface is covered by the waters -of the ocean, and that the average depth of these oceanic waters is -more than 10,000 feet, we may easily understand how great a portion of -the earth's crust must be penetrated by infiltrating waters which can -find no outlet in springs. The penetration of the waters of the ocean -into the earth's crust will be aided, too, by the enormous pressure -amounting to not less than several tons to the square-inch upon the -greater part of the ocean-floor. It might be thought that this downward -penetration of water would be counteracted by the upward current of -steam that would be produced as these subterranean waters reach the -hotter portions of the earth's crust. But the experiments of Daubrée -have conclusively shown that the penetration of water through rocks -takes place in opposition to the powerful pressure of steam in the -contrary direction. Hence, we may assume that certain quantities of -water, containing various gases and solids in solution, are continually -finding their way by capillary infiltration from the surface to -the deeply seated portions of the earth's crust, there to undergo -absorption by the incandescent rock-masses and to produce oxidation of -some of their materials. - -[Sidenote: POSITION OF THE ISOGEOTHERMS.] - -The deep-sea soundings of the 'Challenger' have shown that the floor of -the ocean is constantly maintained at a temperature but little above -that of the freezing point of water. This low temperature is probably -produced by the absorption of heat from the earth's crust by the waters -of the ocean, which distribute it by means of convection currents on -the grandest scale. Hence, the isogeotherms, or lines indicating the -depths at which the same mean temperature is found within the earth's -crust, are probably depressed beneath the great ocean-floors, and rise -towards the land-masses. It is to this circumstance, combined with -that of the enormous pressure of water on the ocean-beds, that we must -probably ascribe the general absence of volcanoes in the deep seas and -their distribution near coast-lines. - -We have thus briefly reviewed the chief hypotheses which have been -suggested in order to account for the two great factors in all volcanic -phenomena--namely, the presence of highly heated rock-masses within -the earth's crust, and the existence of various vapours and gases in a -state of most intimate mechanical, but not chemical, union with these -incandescent materials. It must be admitted that we do not at present -appear to have the means for framing a complete and consistent theory -of volcanic action, but we may hopefully look forward to the time when -further observation and experiment shall have removed many of the -existing difficulties which beset the question, and when by the light -of such future researches untenable hypotheses shall be eliminated and -the just ones improved and established. - -But if we are constrained to admit that a study of the observed -phenomena and established laws of volcanic action have not as yet -enabled us to frame any complete and satisfactory theory on the -subject, we cannot lose sight of the fact that all modern speculation -upon this question appears to be tending in one definite direction. It -is every day becoming more and more clear that our earth is bound by -ties of the closest resemblance to the other members of that family of -worlds to which it belongs, and that the materials entering into their -constitution, and the forces operating in all are the same. - -We have had occasion in a previous chapter to point out that there -are the strongest grounds for believing the interior of our globe to -consist of similar materials to those found in the small planetary -bodies known as meteorites. That the comets are merely aggregations -of such meteorites, and that the planets differ from them only in -their greater dimensions, may be regarded as among the demonstrated -conclusions of the astronomer. The materials found most abundantly in -meteorites and in the interior of our globe are precisely the same as -those which are proved to exist in an incandescent state in our sun. -Hence we are led to conclude that the whole of the bodies of the solar -system are composed of the same chemical elements. - -[Sidenote: ERUPTIVE ACTION IN THE SUN.] - -That the forces operating in each of these distant bodies present -striking points of analogy is equally clear. The sun is of far greater -dimensions than our earth, and is still in great part, if not entirely, -in a gaseous condition. The great movements in the outer envelopes of -the sun exhibited in the 'sun-spots' and 'solar prominences,' recall -to the mind the phenomena of volcanic activity upon our globe. But -the vast energy still existing in the intensely heated mass of the -sun, and the wonderful mobility of its gaseous materials, give rise to -appearances beside which all terrestrial outbursts seem to sink into -utter insignificance. Vast cavities of such dimensions that many globes -of the size of our earth might be swallowed up in them are formed -in the solar envelopes in the course of a few days or hours. Within -these cavities or sun-spots incandescent vapours are observed, rushing -upwards and downwards with almost inconceivable velocity. - -The drawings made by Secchi, and reproduced in figs. 89 and 90, will -give some idea of the appearances presented by these great holes in the -solar envelopes. - -[Illustration: Fig. 89.--A group of Sun-spots. (After Secchi.)] - -In fig. 89 a group of sun-spots is represented and, in their circular -outlines and tendency to a linear arrangement, they can scarcely fail -to remind anyone familiar with volcanic phenomena of terrestrial -craters, though their dimensions are so much greater. - -In fig. 90 the sun-spot represented shows the presence of large -floating masses of incandescent materials rushing upwards and downwards -within the yawning gulf. - -[Sidenote: PHENOMENA OF SUN-SPOTS.] - -[Illustration: Fig. 90.--A Sun-spot, showing the great masses of -incandescent vapour rising or falling within it. (After Secchi.)] - -[Illustration: Fig. 91.--The edge of a Sun-spot, showing a portion of -the prominent masses of incandescent gas (A), which detached itself at -E and floated into the midst of the cavity.] - -From fig. 91, taken from a drawing by Mr. Norman Lockyer, we may -understand the movements of these great protuberances of incandescent -gas which are seen on the sides of the sun-spots. - -The so-called solar prominences present even more striking resemblances -to the volcanic outbursts of our globe. - -Two drawings made by Mr. Norman Lockyer will serve to give some idea -of the vast dimensions of these solar prominences, and of the rapid -changes which take place in their form. - -[Illustration: Fig. 92.--Drawing of a Solar prominence, made by Mr. -Norman Lockyer on March 14, 1869, at 11 H. 5 M. A.M.] - -The masses of incandescent gas were estimated as being no less than -27,000 feet in height, yet in ten minutes they had totally changed -their form and appearance, as shown in fig. 93. - -Even still more striking are the changes recorded by Professor Young, -of New-Haven, in a solar prominence, which he observed on September 7, -1871. - -[Illustration: Fig. 93.--The same object, as seen at 11 H. 15 M. on the -same day.] - -[Sidenote: SOLAR PROMINENCES.] - -That astronomer described a mass of incandescent gas rising from -the surface of the sun to the height of 54,000 miles. In less than -twenty-five minutes he saw the whole mass torn to shreds and blown -upwards, some of the fragments being in ten minutes hurled to the -height of 200,000 miles above the sun's surface. The masses of -incandescent gas thus hurled upwards were of enormous dimensions, the -smallest being estimated as having a greater area than the whole of the -British Islands, and the force with which they were urged upwards was -so great that they acquired a velocity of 166 miles per second. The -accompanying woodcut shows the successive appearances presented by this -grand eruptive outburst on the surface of the sun. - -[Illustration: Fig. 94.--Drawings of a Solar prominence at four -different periods on Sept. 7, 1871. (After Young.)] - -[Sidenote: EXTINCT VOLCANOES OF THE MOON.] - -The moon, which is of far smaller size than our earth, exhibits on -its surface sufficiently striking evidences of the action of volcanic -forces. Indeed the dimensions of the craters and fissures which cover -the whole visible lunar surface are such that we cannot but infer -volcanic activity to have been far more violent on the moon than it -is at the present day upon the earth. This greater violence of the -volcanic forces on the moon is perhaps accounted for by the fact that -the force of gravity on the surface of the moon is only one-sixth of -that at the surface of the earth; and thus the eruptive energy will -have a much less smaller resistance to overcome in bursting asunder the -solid crust and accumulated heaps of ejected materials on its surface. -But the volcanic action on the moon appears now to have wholly ceased, -and the absence of both water and atmosphere in our satellite suggests -that this extinction of volcanic energy may have been caused by the -complete absorption of its gaseous envelope. The appearance presented -by a portion of the moon's surface is shown in fig. 95. - -The sun and the moon appear to exhibit two widely separated extremes -in the condition assumed during the cooling down from a state -of incandescence of great globes of vaporised materials. The -several planets, our own among the number, probably exhibit various -intermediate stages of consolidation. - -[Illustration: Fig. 95.--A group of Lunar craters (Maurolycus, -Barocius, etc.), the largest being more than 60 miles in diameter.] - -[Sidenote: ERUPTIVE ACTION IN THE SUN, EARTH AND MOON.] - -Our earth is, as we have seen, closely allied to the other bodies of -the solar system in its movements, its relations, and its composition; -and a true theory of terrestrial vulcanicity, when it is discovered, -may be expected not only to afford an explanation of the phenomena -displayed on our own globe, but to account for those displays of -internal energy which have been manifested in other members of the same -great family of worlds. - - - - -INDEX. - - -[The subjects illustrated in the engravings are indicated by _italics_, -the names of authors are in Capitals.] - - ABICH, cited, 122 - -- researches of, 4 - Absorption of gases by liquids and solids, 354, 355 - Acid lavas, 48 - Æolian Islands. _See_ Lipari Islands - Æolus, origin of myth, 35 - Africa, volcanoes of, 227 - -- South, diamonds of, 147 - Agates, formation of, 150 - Allport, Mr., cited, 259 - Alps, formation of, 292 - Altered lavas, names given to, 261 - America, volcanoes of, 227 - Amygdaloids, formation of, 140, 141 - Andesites, 50, 59 - Andesite-volcanoes, 126 - Andrews, Professor, cited, 321 - Anne Boleyn and Etna, 3 - Armstrong, Sir W., hydro-electric machine, 29 - Arthur's Seat, 275 - Artificial stone, 55 - Asia, volcanoes of, 227 - Asiderites, 316 - Asmanite, 314 - Astroni, crater-ring of, 170 - Atlantic, volcanoes in, 223 - Auvergne, _breached cones of_, 123, fig. 40 - -- _denuded cones in_, 124, fig. 42 - -- incrusting springs of, 184 - -- puys of, 152, 212 - -- volcanic cones in, 79 - - BALL-AND-SOCKET structure in basaltic columns, 107 - Barrancos, formation of, 209 - Basalt, controversy concerning origin of, 249 - Basalts, 49, 50, 59 - Basaltic columns of Bohemia, 107 - -- -- of Central Germany, 107 - -- -- of Monte Albano, 107 - -- -- _from the Giant's Causeway_, 107, fig. 29 - Basic lavas, 48 - Bath, hot spring of, 219 - Ben Nevis, 274 - Bohemia, volcanoes of, 126 - -- lavas of, 103 - Boiling. _See_ Ebullition - Bonney, Professor, cited, 69, 109, 259 - Boracic acid at volcanic vents, 216 - _Bourbon, volcano of_, 176, figs. 74, 75 - _Bracciano, crater-lake of_, 178, fig. 77 - _Breached cones_, 123, fig. 40 - Babbles of steam, escape from lava, 21 - _Bubbles, spontaneous movement of, in liquid cavities_, 62, fig. 8 - -- -- cause of, 65 - Buch, Von, researches of, 4 - Buda-Pesth, deep well of, 335, 341 - Büdos Hegy, Transylvania, 215 - Bunsen, cited, 201 - Burning, does not take place at volcanoes, 2 - - CADER IDRIS, 274 - 'Calderas,' formation of, 180 - Caldera of Palma, 209 - Cambro-Silurian volcanoes of British Islands, 274 - _Campi-Phlegræi, map of_, fig. 11 - -- -- volcanoes of, 79 - -- -- tuff-cones of, 118 - -- -- fissures in, 197 - Carbonic acid in cavities of crystals, 63 - Carboniferous volcanoes of British Islands, 275 - Carlsbad, Strudel of, 218 - -- Strudelstein of, 184 - Caspian Sea, mud-volcanoes of, 182 - Catacecaumene, volcano cones in, 79 - Cause of proximity of volcanoes to sea, 239 - Central Asia, volcanoes of, 236 - -- America, mud-volcanoes, 182 - -- Pacific, volcanoes of, 236 - 'Challenger,' H.M.S., voyage of, 73 - -- -- soundings of, 359 - Chance, Messrs., of Birmingham, 55 - Charnwood Forest, ancient volcanic rocks of, 259 - Chemical deposits at Vulcano, 44 - -- -- on surfaces of lavas, 110 - -- elements present in lavas, 46 - -- theory of volcanoes, 344, 346 - Chiaja di Luna, 108 - Chimborazo, size of, 44 - -- 151 - Chodi-Berg, Hungary, 161 - _Citlaltepetl, view of_, 169, fig. 69 - Coast-lines, proximity of volcanoes to, 228 - Cole, Mr. Grenville, 110 - Colours of lavas, 49 - Columns in lava, 105 - -- -- dimensions of, 105 - -- radiating in intrusive masses, 136 - Columnar structure in lavas, 104 - -- -- origin of, 105 - _Columnar lava-stream on the Ardèche_, 107, fig. 28 - Combustion, does not take place at volcanoes, 2 - Composite cones, 128, 161 - Comstock mines, temperature of, 342 - _Concentric jointing in lava_, 108, fig. 30 - _Cones composed of viscid lava_, 129, fig. 43 - -- miniature on lava-streams, 100, 101, figs. 25, 26 - -- natural sections of, 129 - -- shifting of axis in, 167 - Coolin Hills, Skye, 144 - Cotopaxi, volcanic dust of, 69 - -- _view of_, 168, fig. 68 - Craters, formation of, 82 - -- origin of, 167 - -- position of, 167 - -- fissuring of sides, 180 - Crater of Stromboli, aperture at bottom of, 15 - Crater-lakes, formation of, 171 - -- of Agnano, 171 - -- of Albano, 171 - -- of Avernus, 171 - -- _of Bagno_, 171, fig. 71 - -- of Bolsena, 171 - -- of Bracciano, 171 - -- of Frascati, 173, 175 - -- _of Gustavila_, 171, fig. 72 - -- of Laach, 171 - -- of Nemi, 171 - Crater-rings, formation of, 170 - _Crater-ring of Somma_, 177, fig. 76 - Crater-ring of Pianura, 174 - -- -- of Piano di Quarto, 174 - -- -- of Vallariccia, 174 - Creuzot, shafts at, 340 - 'Critical point' of liquids, 63 - Crust of globe, definition of, 308 - Crystals in lavas, 51 - -- -- formed of crystallites, 54-57 - -- -- formed in subterranean reservoirs, 60 - -- -- interruption in growth of, 60 - -- pressure under which formed, 65 - -- deposited on surface of lava, 110 - -- porphyritic, origin of, 256 - Crystalline minerals formed beneath volcanoes, 146, 147 - -- -- ejected from volcanoes, 147 - Crystallised minerals of volcanoes, 46 - Crystallites, aggregates of, 54, _Frontispiece_ - Crystallites in lavas, 53, _Frontispiece_ - Crypto-crystalline base, 57 - 'Cupolas,' 135 - Corral of Madeira, 209 - - DACITES, 198 - Dana, Professor, J. D., cited, 100, 159, 291, 301, 327, 338, 339 - Darwin, Mr., cited, 245, 246, 271, 289 - Daubeny, cited, 182 - Daubrée, M., cited, 147, 315, 320, 358 - Daubréelite, 314 - Davy, Sir Humphry, chemical theory of volcanoes, 344, 345 - Deccan of India, 103 - Density of the earth, 306 - _Denuded cones and craters_, 158, fig. 59 - Denudation, effects of, on volcanoes, 114 - Deposits about volcanic fissures, 42 - Detonations at Vesuvius, 26 - Devonian volcanoes of British Islands, 274 - Diorite, 59 - Dolomieu, cited, 4, 39 - Durocher, cited, 201 - Dykes, formation of, 116, 117, 209, 210 - -- structure of rock in, 211 - -- pseudo-, 119 - Dynamical theory of volcanoes, 347, 348 - - EARTH'S interior, nature of, 309 - -- -- physical condition of, 325 - -- -- hypothesis concerning, 328-330 - -- relation to other planets, 310, 311 - Earthquakes, depth of origin of, 343, 344 - -- connection with volcanoes, 343 - -- accompanying Vesuvian eruption of 1872, 27 - Ebullition, compared to volcanic eruptions, 19, 20 - Eifel, volcanic cones of, 45 - Ejected blocks, 45 - -- materials, height to which thrown, 72 - -- -- stratification of, 117-119 - Elements, pyroxenic and trachytic, theory of, 201 - Elevation-craters, theory of, 135, 200 - Erroneous opinions, sources of, in regard to volcanoes, 2 - Eruptions, feeble and violent compared, 31 - -- prediction of, not possible, 32 - -- intervals between, 33 - -- of varying intensity, 33 - -- and barometric pressure, 36 - -- effects of repetition of from same fissure, 80 - Eruptive action in sun and moon, 360-369 - Etna, ideas of ancients concerning, 3 - -- and Anne Boleyn, 3 - -- observatory on, 37 - -- size of, 44 - --, 151 - -- eruptions at summit and on flanks, 207 - _Etna, dyke and lava-stream in_, 133, fig. 54 - _Etna, views of_, 162, 163, figs. 62, 63 - Euganean Hills, 139 - -- -- volcanoes of, 201 - Europe, volcanoes of, 227 - Extra-terrestrial rocks, 316 - _Extra-terrestrial rocks, relation to ultra-basic rocks_, 322, fig. 88 - - FELSTONES, 263 - Ferric-chloride, mistaken for sulphur, 41 - _Fissure on flanks of Etna_, 194, fig. 84 - Fissure-eruptions, 188 - Fissures, volcanic cones on, 194 - -- systems of, 198 - Fingal's Cave, 106 - Flames, phenomena mistaken for, 2 - -- at volcanoes, feebly luminous, 17 - -- false appearance of, in volcanoes, 17 - Flames at volcanic vents, 41 - Flashing lighthouse, compared to Stromboli, 10 - Floods, accompanying volcanic outbursts, 30 - Forbes, Mr. David, cited, 337, 339 - Fossils, from beneath Vesuvius, 45 - -- supposed in basalt, 250 - Fouqué, M., cited, 110, 213 - Fumaroles, gases emitted from, 213 - Fusiyama, form of, 90, 166 - _Fusiyama_, 178, fig. 77 - - GABBRO, 59 - Gardiner's river, travertine terraces of, 185 - Gases emitted from volcanoes, 40 - -- -- volcanic vents, 212-216 - Geanticlinals, formation of, 297 - Gems, formation of, 147 - -- mode of occurrence, 148 - Geological continuity, doctrine of, 247 - Geosynclinals, formation of, 294 - Geysers, formation of, 217 - -- intermittent action of, 218 - -- of Colorado, 184, 217 - -- of Iceland, 184, 217 - Giant's Causeway, 108 - Gilbert, Mr. G. K., cited, 208 - Girgenti, mud-volcanoes of, 182 - Glass, formed by fusion of lavas, 52 - Glasses, composed of certain silicates, 58 - Glassy base, 57 - Goethe, cited, 112 - _Graham Isle_, 178, 179, fig. 78 - Graham, cited, 345 - Grand Sarcoui, Auvergne, 161 - Granite, 59 - Granite of Secondary and Tertiary ages, 254 - Granitic rocks, position beneath volcanoes, 145 - Great earth movements, nature of, 286 - Great volcanic bands of the globe, 232-234 - Grenelle, boring of, 341 - Greystones, 49 - Groundmass of lavas, 52 - Grotto del Cane, 215 - Guevo Upas, Java, 215 - Guiscardi, Professor, referred to, 45 - _Gustavila, crater-lake of_, 172, fig. 72 - - HAMILTON, Sir W., researches of, 4, 75, 84 - -- -- observations on Vesuvius, 80 - Hannay, Mr., referred to, 147 - Hartley, Mr. Noel, referred to, 65 - Hawaii, volcanoes of, 100, 125 - -- -- lava-masses of, 159 - -- -- volcanic eruptions at different levels, 327 - Hebrides, volcanoes of, 271 - Henry, Dr., cited, 355 - Henry Mountains, Southern Utah, 208 - Hephæstus, forge of, 3 - Hochstetter, cited, 135, 356 - Holosiderites, 315 - Hopkins, Mr., cited, 349 - Hot springs, numbers of, 219 - Humboldt, researches of, 4 - Hungary, lavas of, 96, 103 - -- volcanoes of, 126, 201 - -- deep wells of, 341 - _Hverfjall, Iceland_, 178, fig. 77 - Hydro-electric machine of Sir W. Armstrong, 29 - Hypothesis, value of, 331-333 - - ICE under lava of Vesuvius in 1872, and of Etna, 110 - Iceland, volcanic dust of, carried to Norway, 72 - Indian Ocean, volcanoes in, 229 - _Insel Ferdinandez_, 178, 179, fig. 78 - Intermediate lavas, 48 - Intervals between Eruptions, 33 - Ireland, north-east of, 103 - _Iron in Ovifak-basalts_, 319, fig. 87 - Iron, seething of, 356 - -- of Ovifak, terrestrial origin of, 320 - Ischia, eruption in 1301, 164 - -- _crater-lake of Bagno in_, 172, fig. 71 - -- _plan of_, 163, fig. 64 - -- _parasitic cones in_, 164, fig. 65 - Island of Bourbon, 93 - _Isle Julie_, 178, 179, fig. 78 - Isogeotherms, 359 - - JANSSEN, referred to, 42 - Joint-structures in lava, 104-110 - - _KAMMERBÜHL_, 112-114, fig. 33 - -- _section of_, 114, fig. 34 - -- _section in side of_, 118, fig. 36 - Kant, nebular hypothesis of, 352 - Kilauea, volcano of, 71, 138 - -- crater of, 181 - King, Mr. Clarence, cited, 301 - - LAACHER SEE, minerals ejected at, 149 - _Lac Paven, Auvergne_, 171, fig. 70 - 'Laccolites,' formation of, 208 - Lago di Bolsena, 173, 175 - Lago di Bracciano, dimensions of, 172, 173 - Lake Avernus, 215 - Lapilli, 70 - Laplace, nebular hypothesis of, 325, 352 - Lavas, action of acid gases on, 41 - -- resemblance to slags, 46 - -- chemical elements in, 46 - -- oxygen in, 47 - -- silicon in, 47 - -- proportion of silica and other oxides in, 47 - -- silicates in, 47 - -- acid, intermediate, basic, 48 - -- specific gravities of, 49 - -- colours of, 49 - -- microscopic study of, 50 - -- fusibility of, 51 - -- minerals in, 51 - -- artificially fused, 51 - -- crystals in, 51, 93 - -- ground mass of, 52 - -- crystalline forms of, 59 - -- of Bohemia, 103 - -- of Hungary, 96, 103 - -- of Kilauea, 95 - -- of Lipari, 96 - -- of Niedermendig, 103 - -- of Vesuvius, 104 - -- of Volvic, 95 - -- of Volcano, 95 - -- presence of water In, 102 - -- chemical deposits on, 110 - -- different fluidity of, 204 - -- augite and hornblende in, 267 - _Lava, cascade of_, 93, fig. 18 - Lava-cones, composed of liquid lava, 125 - -- -- of viscid lava, 126, 127 - -- characters of, of liquid lava, 159 - -- -- of viscid lava, 160 - _Lava-cones, outlines of_, 160, fig. 60 - Lava, in deep-seated reservoirs, 138 - -- consolidation of, at great depths, 139 - Lava-fountains, 94 - _Lava-sheets, intrusive_, 136, 137, fig. 56 - 'Lava' ornaments of Naples, 45 - 'Lava,' slow-cooling of, 110 - -- a bad conductor of heat, 110 - -- ice under, 110 - Lava-streams, nature of movements, 92 - -- difference in liquidity of, 92 - -- miniature cones on, 100, 101 - -- vast dimensions of, 102 - -- structure of, 103 - -- position of columns in, 106 - -- sinking of surface of, 111 - 'Lave di fango,' 30 - 'Lave di fuoco,' 30 - Lawrencite, 314 - Laws of volcanic action, 38 - Le Conte, cited, 347 - Leucite, absence from ancient lavas, 268 - Lightning, accompanying volcanic outbursts, 28 - Linear arrangement of volcanic vents, 191 - -- -- of volcanoes, 231 - Lipari Islands, 3, 39 - -- -- fissures in, 197 - -- -- pumice-cones in, 154 - -- -- order of appearance of lavas in, 200 - -- -- _breached pumice-cones in_, 124, fig. 41 - -- -- _map of_, 192, fig. 81 - -- -- _lavas of_, 96, figs. 20, 21 - Liquids in cavities of crystals, 63 - _Liquid cavities in lavas_, 60, fig. 7 - -- -- _spontaneous movement of bubbles in_, 62, fig. 8 - -- -- spontaneous movement of bubbles in, cause of, 65 - Lockyer, Mr. Norman, cited, 322, 363, 364 - _Lunar craters_, 368, fig. 95 - Lyell, Sir Charles, cited, 135, 167, 197 - - MACCULLOCH, cited, 207, 208 - _Madeira, cliff-section in_, 128, fig. 47 - Magmas, theory of, 201 - -- objections to, 202, 203 - Mallet, Mr., cited, 269, 343, 346 - _Mamelons of Bourbon_, 126, 127, figs. 45, 46 - Maskelyne, Professor, cited, 314 - Massa di Somma, destruction of, 26 - Mauna Loa, 138 - Metamorphism around volcanic vents, 145 - Meteorites, nature of, 312 - -- composition of, 313 - -- minerals of, 314 - -- classification of, 315 - Melaphyres, 262 - Miascite, 59 - Michel Lévy, M., 110 - Micro-crystalline base, 58 - Microliths. _See_ Crystallites - Microscopic study of lavas, 50 - Minerals in lavas, 51 - -- of Vesuvius, 46 - Mineral-veins, formation of, 149 - -- connection with volcanoes, 220 - -- nature of materials in, 321 - _Misenum, Cape of, section of tuff-cone of_, 121, fig. 38 - Modena, mud-volcanoes of, 182 - _Mont Dore, section at_, 130, fig. 48 - Monte Cerboli, Tuscany, 216 - Monte Massi, Tuscany, well at, 341 - Monte Nuovo, history of formation of, 76 - -- -- _description of_, 77, 78, fig. 10 - -- -- 152 - -- -- crater of, 168 - -- -- production of fissure at, 190 - Monte Rotondo, Tuscany, 216 - Moon, effect of internal forces on, 305 - Mountains, all volcanoes not, 2 - Mountain-chains, formation of, 291 - -- -- all of recent date, 292 - Mud-streams at volcanoes, 30 - Mud-volcanoes, formation of, 181, 182 - _Mull, dissected volcano of_, 142-4, figs. 57, 58 - Muscovite, absence of, from modern lavas, 268 - - NEBULAR hypothesis of Laplace, 325, 352 - -- -- of Kant, 352 - New Zealand, geysers of, 217 - -- -- volcanoes of, 135 - -- -- volcanic cones in, 79 - Niedermendig, lava of, 103 - Nordenskiöld, Professor, cited, 318 - - OBSERVATORY on Vesuvius, 24, 37 - -- on Etna, 37 - Obsidian, 59 - Oceans, depth of, in volcanic areas, 242 - Oceanic islands, volcanoes in, 228 - Oliver, Capt. S. P., 92 - Oldhamite, 314 - _Outlines of Vesuvius_, 87, fig. 17 - Ovifak, iron-masses of, 319 - Oxidation of materials of globe, 324 - Oxygen, proportion in lavas, 47 - - PACIFIC, volcanoes in, 229 - Palmieri, Professor, cited, 25, 37 - Papandayang, eruption of, 169 - Papin's digester, nature of action in, 22 - _Parasitic cones, formation of_, 161, 162, fig. 61 - Pele's Hair, 71 - Perlitic structure, 109 - Phillips, Mr. J. A., cited, 220 - Phonolites, 50, 59 - Phonolite-volcanoes, 126 - _Photograph of Vesuvius eruption_, 24, fig. 5 - 'Pine-tree, appendage of Vesuvius, 29 - Pitchstones, porphyritic, 60 - Plateaux formed of lava-sheets, 270 - Pliny, Elder, death of, 7 - Plombières, hot springs of, 147 - Plutonic rocks, 61 - Pompeii, nature of materials covering, 117 - Ponza Islands, 39 - _Ponza, sections in_, 131, 132, figs. 51, 52 - Porphyrites, 263 - Porphyritic pitchstones, 60 - Potentially liquid rock, 250 - Pre-Cambrian volcanoes of British Islands, 274 - Presence of water in lavas, 353 - Pressure under which crystals were formed, 65 - _Predazzo, ancient volcano of_, 165, fig. 67 - Propylites, 199 - Pseudo-dykes, 119 - Pumice, how formed, 68 - -- cause of white colour of, 71 - -- floating on ocean, 73 - -- on ocean-beds, 73 - Pumice-cones, 154 - _Puy de Pariou, Auvergne_, 193, 194, figs. 82, 83 - Puzzolana, 89 - - RAIN, accompanying volcanic outbursts, 30 - Rate of movement of lava-streams, 97 - Rath, Professor Vom, 72 - Red clay of ocean-beds, 74 - Red Mountains, Skye, 144 - Reservoirs beneath volcanoes, 145 - Reyer, Dr. Ed., experiments of, 125, 160 - Reykjanes, eruption of, in 1783, 102 - Rhyolites, 50, 59 - Richthofen, Von, cited, 196, 199, 200, 205 - _Rocca-Monfina_, 178, fig. 77 - -- --, 204 - Rock-masses, movements of, 288 - Rocky Mountains, 103 - -- -- volcanoes of, 201 - Rotomahana, sinter-terraces of, 185 - _Ropy-lavas_, 98, fig. 24 - - _SALINA, section in_, 132, fig. 53 - Sandwich Islands, lavas of, 125 - San Sebastiano, destruction of, 26 - _San Stephano, section in_, 131, fig. 50 - Santorin, 42 - _Sarcoui, Grand Puy of_, 126, fig. 44 - Sciarra del fuoco, 13 - Scoria, how formed, 68, 70 - Scoria-cones, altered by acid gas, 155 - -- breached, 156 - -- characters of, 153 - -- preservation of, 155 - -- red colour of, 154 - _Scoria-cone in Vesuvius_, 122, fig. 39 - _Scoria-cone near Auckland, N. Z._, 1656, fig. 66 - Schmidt, referred to, 153 - Schreibersite, 314 - Scrope, Mr. Poulett, cited, 5, 69, 106, 135, 198, 205, 212, 238, 289 - Sea of Azof, mud-volcanoes of, 182 - Secchi, Father, cited, 362 - Shiant Isles, 105 - Silica, presence in lavas, 47 - Silicates in lavas, 47 - Silicon, proportion in lavas, 47 - Siliceous sinter, deposits of, 220 - Silver, spitting of, 355 - Silvestri, Professor, cited, 230 - Similarity of lavas of different ages, 260 - _Sinter-cones, forms of_, 183, fig. 79 - Skye, dissected volcano of, 144 - Slags, compared with lavas, 46 - Smith, Lawrence, cited, 320 - Smoke, appearance of, due to steam, 2 - Snowdon, 274 - _Solar prominences_, 364-366, figs. 92, 93, 94 - Solfatara of Naples, 214 - Solfatara-stage of volcanoes, 215 - Somma, 133 - -- crater-ring of, 83 - Sorby, Mr. H. C., referred to, 59, 252 - Spallanzani, early researches of, 4 - -- observations on Stromboli, 8 - -- cited, 39, 367 - Specific gravities of lavas, 49 - -- -- of glassy and crystalline rocks, 59 - Spectroscope in vulcanology, 41 - Spectrum-analysis, results of, 311 - Specular-iron, deposited on lava-streams, 110 - Sperenberg, boring of, 341 - _Sphærulites_, 54, _Frontispiece_ - Sporadosiderites, 316 - Stability of crust of globe, 326 - Staffa, Isle of, 106 - Steenstrup, cited, 319 - Steam-engine compared to volcano, 8 - Steam, emitted by lava of Vesuvius, 27 - Sternberg, referred to, 113 - St. Kilda, 181 - Stokes, Professor, 65 - St. Paul, Island of, 180 - Stromboli, 42, 158 - -- apertures at bottom of crater, 15 - -- appearances in crater of, 16 - -- -- at night, 10 - -- compared with Vesuvius, 23 - -- crater of, 13 - -- dependence of eruptions on atmospheric conditions, 34 - -- eruption of, 14, fig. 4 - -- general features of, 11 - -- map of, 11, fig. 2 - -- observations by Spallanzani, 8 - -- resemblance to flashing light, 10 - -- _section of_, 13, fig. 8 - -- soundings around, 12 - -- vapour-cloud above, 9 - -- violent eruptions of, 23 - Strombolian stage, 23 - Stufas, nature of, 217 - Submarine volcanoes, 179 - Subterranean forces, beneficial effects of, 303 - Subsidence in centre of volcanoes, 165 - Sulphur, absorption of water by molten, 356 - -- deposited on lava-streams, 110 - -- how formed at volcanoes, 18 - -- not the cause of volcanic outbursts, 18 - _Surfaces of lava-streams_, 97-99, figs. 22, 23 - _Sun-spots_, 361-363, figs. 89, 90, 91 - Syenite, 59 - Syssiderites, 316 - Szabo, Professor, cited, 199 - - TACHYLYTE, 59 - Tertiary volcanoes of British Islands, 276 - _Terraces, sinter- and travertine-formation of_, 185, fig. 80 - Temperature, increase in deeper parts of earth's crust, 335 - -- rate of increase in different areas, 340 - Teneriffe, 44, 151 - _Teneriffe_, 178, fig. 77 - -- _peak of_, 175, fig. 73 - Tenon-and-mortise structure in basaltic columns, 107 - Theodosius and Vulcano, 3 - Thunder, accompanying volcanic outbursts, 28 - Trachytes, 49, 50, 69 - Trap-rocks, origin of, 241 - Trass, 90 - Travertine or Tibur-stone, 184 - -- deposits of, 220 - Triassic volcanoes of British Islands, 275 - Tridymite deposited on lava-streams, 110 - Troilite, 314 - Troost, cited, 355 - Tufa, or tuff, 90 - Tuff-cones, character of, 157 - -- denudation of, 157, 158 - Typhon, fable of, 3 - - ULTRA-BASIC lavas, 50, 66 - -- rocks, 317 - - VAL DEL BOVE, Etna, 133, 180, 209 - -- -- _dykes in_, 134, fig. 55 - Vapour-cloud over Vesuvius, 26, 29 - -- -- Stromboli, 9 - Ventotienne, Island of, section at, 130, fig. 49 - Vesuvius, 37 - -- changes in form of, 81 - -- compared with Stromboli, 23 - -- _crater of in 1756_, 84, fig. 14 - -- -- _of in 1767_, 85, fig. 16 - -- -- _of in 1822_, 82, fig. 13 - -- -- _of in 1843_, 86, fig. 16 - -- detonations at, 26 - -- early history of, 83 - -- eruption of year 79, 84 - -- -- of 1822, 69 - -- -- of April 1872, 24 - -- -- of October 1822, 24 - -- ejected blocks of, 45 - -- first eruption of, 7 - -- form of, 166 - -- fossils of, 45 - -- growth of cone of, 80 - -- history of, 204 - -- last eruption of, 7 - -- lava-stream of 1855, 101 - -- lava-streams of 1858, 1872, 97 - -- lavas of, 104 - -- minerals of, 46 - -- -- ejected at, 149 - -- observatory on, 24, 37 - -- outlines of, 87 - -- pine-tree appendage of, 29 - -- scoria-cones in lava, 122 - -- -- on lava of 1855, 153 - -- steam emitted by lava of, 27 - -- vapour-cloud over, 26, 29 - Vesuvian stage, 23 - -- _eruption, photograph of_, 24, fig. 5 - Viscid lavas of Lipari Islands, 94-96 - Vitreous lavas, devitrification of, 259 - Volcanic action, laws of, 32 - -- bombs, 70, 71 - -- cycles, nature of, 221, 222 - -- -- duration of, 223 - -- cones, internal structure of, 115-122 - -- -- _experimental illustration of formation of_, 120, fig. 37 - -- -- limits to height of, 166 - -- -- form of, 152 - -- -- dimensions of, 152 - -- -- irregular development of, 90 - -- -- slopes of sides of, 91 - -- -- composed of ejected rock-fragments, 156 - -- -- curved slopes of, 156 - -- _débris_ on sea-bottom, 240 - -- dust, fineness of, 69 - -- districts, areas of upheaval, 245 - -- ejections, alteration of, 258 - -- eruptions, compared to ebullition, 19, 20 - -- forces, compensate for denudation, 283 - -- -- intensity at former periods, 278 - -- -- necessity for action of, 285 - -- -- shifting of from one area to another, 277 - -- mountains, origin of conical forms, 89 - -- -- mode of growth, 89 - -- phenomena of the past similar to those at present, 273 - -- products, order of appearance of, 198, 199 - Volcanic rocks, 61 - -- -- similarity of ancient and modern, 253 - Volcano, origin of name, 3 - -- craters of, 167 - -- Island of. _See_ Vulcano. - -- compared to steam-engine, 8 - Volcanoes, blocks, ejected from, 45 - -- built up of ejected fragments, 74 - -- destruction caused by, 281 - -- dissected by denudation, 115, 139 - -- erroneous ideas concerning, 1 - -- ejection of different materials from, 205 - -- known to ancients, 3 - -- life-history of, 186 - -- number of, 224, 225 - -- of Africa, 227 - -- of America, 236 - -- of Asia, 236 - -- of Bohemia, 126 - -- of Central Asia, 236 - -- of Central Pacific, 236 - -- of Europe, 227 - -- of Hungary, 126 - -- position in relation to mountain chains, 243 - -- popular ideas concerning, 1 - -- reservoirs beneath, 145 - Volvic lava of, 103 - Vose, cited, 346 - Vulcan, forge of, 3 - Vulcano, island of, 3, 158 - Vulcano and Theodosius, 3 - _Vulcano_, 178, fig. 77 - -- _and Vulcanello, view of_, 43, fig. 6 - -- chemical deposits at, 44 - -- eruption in 1786, 43 - -- -- in 1873, 43 - -- -- _lava-stream in_, 95, fig. 19 - -- --, 103, fig. 27 - -- _plan of_, 195, fig. 85 - -- _section of volcanic cone in_, 116, fig. 35 - -- section in, 129 - -- shifting of centre of eruption in, 196 - _Vulcanello, craters of_, 197, fig. 86 - Vulcanology, origin of the science, 4 - -- earliest treatise on, 5 - - Walferdin, M., cited, 340 - Water in lavas, 353 - -- penetration through rocks, 358 - -- presence of in lavas, 102 - -- and saline solutions in cavities of crystals, 63 - Werner, cited, 201 - Western Isles of Scotland, 103, 139, 142 - -- -- volcanoes of, 212 - Whymper, Mr., 69 - Woodward, Mr., experiments of, 119 - Wrekin, ancient volcanic rocks of, 259 - - YOUNG, Professor, cited, 365 - - ZEOLITES, formation of, 150 - - - - PRINTED BY - - SPOTTISWOODE AND CO. 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You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online -at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you -are not located in the United States, you will have to check the laws of the -country where you are located before using this eBook. -</div> - -<p style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: Volcanoes: What They are and What They Teach</p> -<p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Author: John Wesley Judd</p> -<p style='display:block; text-indent:0; margin:1em 0'>Release Date: April 18, 2022 [eBook #67873]</p> -<p style='display:block; text-indent:0; margin:1em 0'>Language: English</p> - <p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em; text-align:left'>Produced by: Tom Cosmas compiled from materials made availbe at The Internet Archive and placed in the Public Domain.</p> -<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK VOLCANOES: WHAT THEY ARE AND WHAT THEY TEACH ***</div> - - - - -<div class="figcenter" id="cover" style="width: 336px;"> - <img src="images/cover.png" width="336" height="530" alt="Volcanoes: What They Are and What They Teach, by John W. Judd" /> -</div> - -<p><span class="pagenum" id="Page_i">- i -</span></p> - - -<hr class="chap x-ebookmaker-drop" /> - -<p class="pmt4 tdc">THE</p> - -<p class="tdc">International Scientific Series</p> - -<p class="pmb4 tdc">VOL. XXXV.</p> - -<p><span class="pagenum" id="Page_iv">- iv -</span></p> - -<div class="figcenter" id="frontispiece" style="width: 414px;"> -<p class="tdr"><i>Frontispiece.</i></p> - - <img src="images/frontispiece.png" width="414" height="620" alt="" /> - -<p class="figcaption"><span class="smcap">Sections of Igneous Rocks, illustrating the passage from the -glassy to the crystalline structure.</span></p> - -<div class="blockquot"> - -<p class="smaller">1. Vitreous Rock. 2. Semi-Vitreous Rock. 3. Vitreous Rock with -Sphærulites. 4. Rock with Crypto-crystalline Base. 5. Rock with -Micro-crystalline Base. 6. Rock of Granite Structure built up -entirely of Crystals.</p> -</div> - -<p class="tdr smaller">[<i>See pp.</i> <a href="#Page_63">63-68</a>.</p> -</div> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_v">- v -</span></p> - -<h1 class="nobreak">VOLCANOES<br /> - -<span class="smaller"><span class="smcap">WHAT THEY ARE and WHAT THEY TEACH</span></span></h1> -</div> - - -<p class="pmt2 tdc">BY</p> - -<h2>JOHN W. JUDD, F.R.S.</h2> - -<p class="tdc">PROFESSOR OF GEOLOGY IN THE ROYAL SCHOOL OF MINES</p> - - - -<p class="pmt4 pmb4 tdc"><i>WITH 96 ILLUSTRATIONS</i></p> - - - -<p class="tdc">SIXTH EDITION</p> - - - -<p class="pmt2 pmb4 tdc">LONDON<br /> - -<span class="smcap">KEGAN PAUL, TRENCH, TRÜBNER & CO. Ltd.</span><br /> - -PATERNOSTER HOUSE, CHARING CROSS ROAD<br /> - -1903</p> - -<p><span class="pagenum" id="Page_vi">- vi -</span></p> - - -<p class="tdc">(<i>The rights of translation and of reproduction are reserved.</i>)</p> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_vii">- vii -</span></p> - -<h2 class="nobreak" id="PREFACE">PREFACE.</h2> -</div> - - -<p><span class="smcap">In preparing this work</span>, I have aimed at carrying out -a design suggested to me by the late Mr. Poulett -Scrope, the accomplishment of which has been unfortunately -delayed, longer than I could have wished, -by many pressing duties.</p> - -<p>Mr. Scrope's well-known works, 'Volcanoes' and -'The Geology and Extinct Volcanoes of Central France'—which -passed through several editions in this country, -and have been translated into the principal -European languages—embody the results of much -careful observation and acute reasoning upon the -questions which the author made the study of his life. -In the first of these works the phenomena of volcanic -activity are described, and its causes discussed; in the -second it is shown that much insight concerning these -problems may be obtained by a study of the ruined and -denuded relics of the volcanoes of former geological -periods. The appearance of these works, in the years -<span class="pagenum" id="Page_viii">- viii -</span> -1825 and 1827 respectively, did much to prepare the -minds of the earlier cultivators of science for the -reception of those doctrines of geological uniformity -and continuity, which were shortly afterwards so ably -advocated by Lyell in his 'Principles of Geology.'</p> - -<p>Since the date of the appearance of the last editions -of Scrope's works, inquiry and speculation concerning -the nature and origin of volcanoes have been alike -active, and many of the problems which were discussed -by him, now present themselves under aspects entirely -new and different from those in which he was accustomed -to regard them. No one was ever more ready -to welcome original views or to submit to having long-cherished -principles exposed to the ordeal of free -criticism than was Scrope; and few men retained to -so advanced an age the power of subjecting novel -theories to the test of a rigorous comparison with -ascertained facts.</p> - -<p>But this eminent geologist was not content with -the devotion of his own time and energies to the -advancement of his favourite science, for as increasing -age and growing infirmities rendered travel and -personal research impossible, he found a new source of -pleasure in seeking out the younger workers in those -fields of inquiry which he had so long and successfully -cultivated, and in furthering their efforts by his judicious -<span class="pagenum" id="Page_ix">- ix -</span> -advice and kindly aid. Among the chosen disciples of -this distinguished man, who will ever be regarded as -one of the chief pioneers of geological thought, I had -the good fortune to be numbered, and when he committed -to me the task of preparing a popular exposition -of the present condition of our knowledge on -volcanoes, I felt that I had been greatly honoured.</p> - -<p>In order to keep the work within the prescribed -limits, and to avoid unnecessary repetitions, I have -confined myself to the examination of such selected -examples of volcanoes as could be shown to be really -typical of all the various classes which exist upon the -globe; and I have endeavoured from the study of these -to deduce those general laws which appear to govern -volcanic action. But it has, at the same time, been -my aim to approach the question from a somewhat -new standpoint, and to give an account of those investigations -which have in recent times thrown so -much fresh light upon the whole problem. In this -way I have been led to dwell at some length upon -subjects which might not at first sight appear to be -germane to the question under discussion;—such as -the characters of lavas revealed to us by microscopic -examination; the nature and movements of the liquids -enclosed in the crystals of igneous rocks; the relations -of minerals occurring in some volcanic products to -those found in meteorites; the nature and origin of -<span class="pagenum" id="Page_x">- x -</span> -the remarkable iron-masses found at Ovifak in Greenland; -and the indications which have been discovered -of analogies between the composition and dynamics of -our earth and those of other members of the family of -worlds to which it belongs. While not evading the -discussion of theoretical questions, I have endeavoured -to keep such discussions in strict subordination to that -presentation of the results attained by observation and -experiment, which constitutes the principal object of -the work.</p> - -<p>The woodcuts which illustrate the volume are in -some cases prepared from photographs, and I am indebted -to Mr. Cooper for the skill with which he has -carried out my wishes concerning their reproduction. -Others among the engravings are copies of sketches -which I made in Italy, Hungary, Bohemia, and other -volcanic districts. The whole of the wood-blocks employed -by Mr. Poulett Scrope in his work on Volcanoes -were placed at my disposal before his death, and such -of them as were useful for my purpose I have freely -employed. To Captain S. P. Oliver, R.A., I am obliged -for a beautiful drawing made in the Island of Bourbon, -and to Mr. Norman Lockyer and his publishers, Messrs. -Macmillan & Co., for the use of several wood-blocks -illustrating sun-spots and solar prominences.</p> - -<p class="tdr"> -J. W. J.<br /> -</p> - -<p><span class="smcap">London</span>: <i>May 1881</i>.</p> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_xi">- xi -</span></p> - -<h2 class="nobreak" id="CONTENTS">CONTENTS.</h2> -</div> - - -<table class="tblcont" summary="TOC"> -<tr> - <td class="tdc">CHAPTER I.</td> -</tr> -<tr> - <td class="tdr smaller">PAGE</td> -</tr> -<tr> - <td class="tdc">INTRODUCTORY: NATURE OF THE ENQUIRY</td> -</tr> -<tr> - <td class="tdr"><a href="#CHAPTER_I">1</a></td> -</tr> -<tr> - <td class="tdc">CHAPTER II.</td> -</tr> -<tr> - <td class="tdc">THE NATURE OF VOLCANIC ACTION</td> -</tr> -<tr> - <td class="tdr"><a href="#CHAPTER_II">7</a></td> -</tr> -<tr> - <td class="tdc">CHAPTER III.</td> -</tr> -<tr> - <td class="tdc">THE PRODUCTS OF VOLCANIC ACTION</td> -</tr> -<tr> - <td class="tdr"><a href="#CHAPTER_III">39</a></td> -</tr> -<tr> - <td class="tdc">CHAPTER IV.</td> -</tr> -<tr> - <td class="tdc">THE DISTRIBUTION OF THE MATERIALS EJECTED FROM VOLCANIC VENTS</td> -</tr> -<tr> - <td class="tdr"><a href="#CHAPTER_IV">67</a></td> -</tr> -<tr> - <td class="tdc">CHAPTER V.</td> -</tr> -<tr> - <td class="tdc">THE INTERNAL STRUCTURE OF VOLCANIC MOUNTAINS</td> -</tr> -<tr> - <td class="tdr"><a href="#CHAPTER_V">112</a></td> -</tr> -<tr> - <td class="tdc">CHAPTER VI.</td> -</tr> -<tr> - <td class="tdc">THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC VENTS</td> -</tr> -<tr> - <td class="tdr"><a href="#CHAPTER_VI">161</a></td> -</tr> -<tr> - <td class="tdc">CHAPTER VII.</td> -</tr> -<tr> - <td class="tdc">THE SUCCESSION OF OPERATIONS TAKING PLACE AT VOLCANIC CENTRES</td> -</tr> -<tr> - <td class="tdr"><a href="#CHAPTER_VII">186</a></td> -</tr> -<tr> - <td class="tdc">CHAPTER VIII. - <span class="pagenum" id="Page_xii">- xii -</span></td> -</tr> -<tr> - <td class="tdc">THE DISTRIBUTION OF VOLCANOES UPON THE SURFACE OF THE GLOBE</td> -</tr> -<tr> - <td class="tdr"><a href="#CHAPTER_VIII">224</a></td> -</tr> -<tr> - <td class="tdc">CHAPTER IX.</td> -</tr> -<tr> - <td class="tdc">VOLCANIC ACTION AT DIFFERENT PERIODS OF THE EARTH'S HISTORY</td> -</tr> -<tr> - <td class="tdr"><a href="#CHAPTER_IX">247</a></td> -</tr> -<tr> - <td class="tdc">CHAPTER X.</td> -</tr> -<tr> - <td class="tdc">THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF NATURE</td> -</tr> -<tr> - <td class="tdr"><a href="#CHAPTER_X">281</a></td> -</tr> -<tr> - <td class="tdc">CHAPTER XI.</td> -</tr> -<tr> - <td class="tdc">WHAT VOLCANOES TEACH US CONCERNING THE NATURE OF THE EARTH'S INTERIOR</td> -</tr> -<tr> - <td class="tdr"><a href="#CHAPTER_XI">307</a></td> -</tr> -<tr> - <td class="tdc">CHAPTER XII.</td> -</tr> -<tr> - <td class="tdc">THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN THE CAUSES OF VOLCANIC ACTION</td> -</tr> -<tr> - <td class="tdr"><a href="#CHAPTER_XII">331</a></td> -</tr> -<tr> - <td class="tdc">INDEX</td> -</tr> -<tr> - <td class="tdr"><a href="#INDEX">371</a></td> -</tr> -</table> - -<p><span class="pagenum" id="Page_xiii">- xiii -</span></p> - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h2 class="nobreak" id="ILLUSTRATIONS">ILLUSTRATIONS.</h2> -</div> -<table class="tblcont" summary="Illustrations"> -<tr> - <td class="tdl vtop" colspan="2">Sections of igneous rocks illustrating the passage from the - glassy to the crystalline structure</td> - <td class="tdr smaller"><i>Frontispiece</i></td> -</tr> -<tr> - <td class="tdl smaller" colspan="2"><span class="smcap">Fig.</span></td> - <td class="tdr smaller"><span class="smcap">Page</span></td> -</tr> -<tr> - <td class="tdr vtop">1.</td> - <td class="tdl">Stromboli, viewed from the north-west, April 1874</td> - <td class="tdr"><span class="smaller"><i>to face p.</i> </span><a href="#fig01">10</a></td> -</tr> -<tr> - <td class="tdr">2.</td> - <td class="tdl">Map of the Island of Stromboli</td> - <td class="tdr"><a href="#fig02">11</a></td> -</tr> -<tr> - <td class="tdr vtop">3.</td> - <td class="tdl">Section through the Island of Stromboli from north-west to - south-east</td> - <td class="tdr vbot"><a href="#fig03">13</a></td> -</tr> -<tr> - <td class="tdr vtop">4.</td> - <td class="tdl">The crater of Stromboli as viewed from the side of the - Sciarra during an eruption on the morning of April 24, - 1874.</td> - <td class="tdr vbot"><a href="#fig04">14</a></td> -</tr> -<tr> - <td class="tdr vtop">5.</td> - <td class="tdl">Vesuvius in eruption, as seen from Naples, April 26, 1872. - (<i>From a photograph</i>)</td> - <td class="tdr vbot"><span class="smaller"><i>to face p.</i> </span><a href="#fig05">24</a></td> -</tr> -<tr> - <td class="tdr vtop">6.</td> - <td class="tdl">View of Vulcano, with Vulcanello in the foreground—taken - from the south end of the Island of Lipari</td> - <td class="tdr vbot"><a href="#fig06">43</a></td> -</tr> -<tr> - <td class="tdr vtop">7.</td> - <td class="tdl">Minute cavities, containing liquids, in the crystals of rocks. - (<i>After Zirkel</i>)</td> - <td class="tdr vbot"><span class="smaller"><i>to face p.</i> </span><a href="#fig07">60</a></td> -</tr> -<tr> - <td class="tdr vtop">8.</td> - <td class="tdl">Minute liquid-cavity in a crystal, with a moving bubble. - (<i>After Hartley</i>)</td> - <td class="tdr vbot"><a href="#fig08">63</a></td> -</tr> -<tr> - <td class="tdr vtop">9.</td> - <td class="tdl">Cavity in crystal, containing carbonic-acid gas at a temperature - of 86° F., and passing from the liquid to the gaseous condition. - (<i>After Hartley</i>)</td> - <td class="tdr vbot"><a href="#fig09">64</a></td> -</tr> -<tr> - <td class="tdr vtop">10.</td> - <td class="tdl">Monte Nuovo (440 ft high) on the shores of the Bay of Naples. - (<i>After Scrope</i>)</td> - <td class="tdr"><a href="#fig10">76</a></td> -</tr> -<tr> - <td class="tdr vtop">11.</td> - <td class="tdl">Map of the district around Naples, showing Monte Nuovo and the - surrounding volcanoes of older date</td> - <td class="tdr vbot"><a href="#fig11">78</a></td> -</tr> -<tr> - <td class="tdr vtop">12.</td> - <td class="tdl">Outlines of the summit of Vesuvius during the eruption of - 1767. (<i>After Sir W. Hamilton</i>) - <span class="pagenum" id="Page_xiv">- xiv -</span></td> - <td class="tdr vbot"><span class="smaller"><i>to face p.</i> </span><a href="#fig12_neg">80</a></td> -</tr> -<tr> - <td class="tdr vtop">13.</td> - <td class="tdl">Crater of Vesuvius formed during the eruption of 1822 - (<i>After Scrope</i>)</td> - <td class="tdr vbot"><a href="#fig13">82</a></td> -</tr> -<tr> - <td class="tdr vtop">14.</td> - <td class="tdl">Crater of Vesuvius in 1756, from a drawing made on the spot. - (<i>After Sir W. Hamilton</i>)</td> - <td class="tdr vbot"><a href="#fig14">84</a></td> -</tr> -<tr> - <td class="tdr vtop">15.</td> - <td class="tdl">The summit of Vesuvius in 1767, from an original drawing. - (<i>After Sir W, Hamilton</i>)</td> - <td class="tdr vbot"><a href="#fig15">85</a></td> -</tr> -<tr> - <td class="tdr">16.</td> - <td class="tdl">Summit of Vesuvius in 1843</td> - <td class="tdr"><a href="#fig16">86</a></td> -</tr> -<tr> - <td class="tdr vtop">17.</td> - <td class="tdl">Outlines of Vesuvius, showing its form at different periods - of its history</td> - <td class="tdr vbot"><a href="#fig17">87</a></td> -</tr> -<tr> - <td class="tdr vtop">18.</td> - <td class="tdl">Cascade of lava tumbling over a cliff in the Island of - Bourbon. (<i>After Capt. S. P. Oliver, R.A.</i>)</td> - <td class="tdr vbot"><a href="#fig18">93</a></td> -</tr> -<tr> - <td class="tdr vtop">19.</td> - <td class="tdl">Lava-stream (obsidian) in the Island of Vulcano, showing - the imperfect liquidity of the mass</td> - <td class="tdr vbot"><a href="#fig19">95</a></td> -</tr> -<tr> - <td class="tdr vtop">20.</td> - <td class="tdl">Interior of a rhyolitic lava-stream in the Island of Lipari, - showing broad, sigmoidal folds, produced by the slow - movements of the mass</td> - <td class="tdr vbot"><a href="#fig20">96</a></td> -</tr> -<tr> - <td class="tdr vtop">21.</td> - <td class="tdl">Interior of a rhyolitic lava-stream in the Island of Lipari, - showing the complicated crumplings and puckerings, - produced by the slow movements of the mass</td> - <td class="tdr vbot"><a href="#fig21">96</a></td> -</tr> -<tr> - <td class="tdr vtop">22.</td> - <td class="tdl">Vesuvian lava-stream of 1858, exhibiting the peculiar - 'ropy' surfaces of slowly-moving currents. - (<i>From a photograph</i>)</td> - <td class="tdr vbot"><span class="smaller"><i>to face p.</i> </span><a href="#fig22">98</a></td> -</tr> -<tr> - <td class="tdr vtop">23.</td> - <td class="tdl">Vesuvian lava-stream of 1872, exhibiting the rough cindery - surfaces characteristic of rapidly flowing currents. - (<i>From a photograph</i>)</td> - <td class="tdr vbot"><span class="smaller"><i>to face p.</i> </span><a href="#fig23">96</a></td> -</tr> -<tr> - <td class="tdr vtop">24.</td> - <td class="tdl">Concentric folds on mass of cooled lava. (<i>After Heaphy</i>)</td> - <td class="tdr vbot"><a href="#fig24">100</a></td> -</tr> -<tr> - <td class="tdr vtop">25.</td> - <td class="tdl">Mass of cooled lava formed over a spiracle on the slopes - of Hawaii. (<i>After Dana</i>)</td> - <td class="tdr vbot"><a href="#fig25">100</a></td> -</tr> -<tr> - <td class="tdr vtop">26.</td> - <td class="tdl">Group of small cones thrown up on the Vesuvian lava-current - of 1855. (<i>After Schmidt</i>)</td> - <td class="tdr vbot"><a href="#fig26">101</a></td> -</tr> -<tr> - <td class="vtop tdr">27.</td> - <td class="tdl">Natural section of a lava-stream in the Island of Vulcano, - showing the compact central portion and the scoriaceous - upper and under surfaces</td> - <td class="tdr vbot"><a href="#fig27">104</a></td> -</tr> -<tr> - <td class="tdr vtop">28.</td> - <td class="tdl">Section of a lava-stream exposed on the side of the river - Ardèche, in the south-west of France. (<i>After Scrope</i>)</td> - <td class="tdr vbot"><a href="#fig28">106</a></td> -</tr> -<tr> - <td class="tdr vtop">29.</td> - <td class="tdl">Portion of a basaltic column from the Giant's Causeway, - exhibiting both the ball-and-socket and the - tenon-and-mortise structure - <span class="pagenum" id="Page_xv">- xv -</span></td> - <td class="tdr vbot"><a href="#fig29">107</a></td> -</tr> -<tr> - <td class="tdr vtop">30.</td> - <td class="tdl">Vein of green pitchstone at Chiaja di Luna, in the Island - of Ponza, breaking up into regular columns and into - spherical masses with a concentric series of joints. - (<i>After Scrope</i>)</td> - <td class="tdr vbot"><a href="#fig30">108</a></td> -</tr> -<tr> - <td class="tdr vtop">31.</td> - <td class="tdl">Illustration of the 'perlitic structure' in glassy rocks</td> - <td class="tdr vbot"><a href="#fig31">109</a></td> -</tr> -<tr> - <td class="tdr vtop">32.</td> - <td class="tdl">Transverse section of a lava-stream</td> - <td class="tdr vbot"><a href="#fig32">111</a></td> -</tr> -<tr> - <td class="tdr vtop">33.</td> - <td class="tdl">The Kammerbühl, or Kammerberg, Bohemia (as seen from - the south-west)</td> - <td class="tdr vbot"><a href="#fig33">113</a></td> -</tr> -<tr> - <td class="tdr vtop">34.</td> - <td class="tdl">Section of the Kammerbühl in Bohemia</td> - <td class="tdr vbot"><a href="#fig34">114</a></td> -</tr> -<tr> - <td class="tdr vtop">35.</td> - <td class="tdl">Natural section of a volcanic cone in the Island of Vulcano</td> - <td class="tdr vbot"><a href="#fig35">116</a></td> -</tr> -<tr> - <td class="tdr vtop">36.</td> - <td class="tdl">Section in the side of the Kammerbühl, Bohemia</td> - <td class="tdr vbot"><a href="#fig36">118</a></td> -</tr> -<tr> - <td class="tdr vtop">37.</td> - <td class="tdl">Experimental illustration of the mode of formation of - volcanic cones, composed of fragmental materials</td> - <td class="tdr vbot"><a href="#fig37">120</a></td> -</tr> -<tr> - <td class="tdr vtop">38.</td> - <td class="tdl">Natural section of a tuff-cone, forming the Cape of Misenum, - and exhibiting the peculiar internal arrangement, - characteristic of volcanoes composed of fragmentary - materials. (<i>After Scrope</i>)</td> - <td class="tdr vbot"><a href="#fig38">121</a></td> -</tr> -<tr> - <td class="tdr vtop">39.</td> - <td class="tdl">Section of a small scoria-cone formed within the crater of - Vesuvius in the year 1835, illustrating the filling up of - the central vent of the cone by subsequent ejections. - (<i>After Abich</i>)</td> - <td class="tdr vbot"><a href="#fig39">122</a></td> -</tr> -<tr> - <td class="tdr vtop">40.</td> - <td class="tdl">Volcanic cones composed of scoriæ, and breached on one - side by the outflow of lava-currents. (<i>After Scrope</i>)</td> - <td class="tdr vbot"><a href="#fig40">128</a></td> -</tr> -<tr> - <td class="tdr vtop">41.</td> - <td class="tdl">Campo Bianco, in the Island of Lipari. A pumice-cone - breached by the outflow of an obsidian lava-stream</td> - <td class="tdr vbot"><span class="smaller"><i>to face p.</i> </span><a href="#fig41">124</a></td> -</tr> -<tr> - <td class="tdr vtop">42.</td> - <td class="tdl">Volcanic cones in Auvergne, which have suffered to some - extent from atmospheric denudation. (<i>After Scrope</i>)</td> - <td class="tdr vbot"><a href="#fig42">124</a></td> -</tr> -<tr> - <td class="tdr vtop">43.</td> - <td class="tdl">Experimental illustration of the mode of formation of - volcanic cones composed of viscid lavas. (<i>After Reyer</i>)</td> - <td class="tdr vbot"><a href="#fig43">126</a></td> -</tr> -<tr> - <td class="tdr vtop">44.</td> - <td class="tdl">The Grand Puy of Sarcoui, composed of trachyte, rising - between two breached scoria-cones (Auvergne). (<i>After - Scrope</i>)</td> - <td class="tdr vbot"><a href="#fig44">126</a></td> -</tr> -<tr> - <td class="tdr vtop">45.</td> - <td class="tdl">Volcanic cone (Mamelon) composed of very viscid lava - (Island of Bourbon). (<i>After Bory de St. Vincent</i>) - <span class="pagenum" id="Page_xvi">- xvi -</span></td> - <td class="tdr vbot"><a href="#fig45">127</a></td> -</tr> -<tr> - <td class="tdr vtop">46.</td> - <td class="tdl">Another Mamelon in the Island of Bourbon, with a crater - at its summit. (<i>After Bory de St. Vincent</i>)</td> - <td class="tdr vbot"><a href="#fig46">127</a></td> -</tr> -<tr> - <td class="tdr vtop">47.</td> - <td class="tdl">Cliff-section in the Island of Madeira, showing how a - composite volcano is built up of lava-streams, beds of - scoriæ, and dykes. (<i>After Lyell</i>)</td> - <td class="tdr vbot"><a href="#fig47">125</a></td> -</tr> -<tr> - <td class="tdr vtop">48.</td> - <td class="tdl">Section seen at the cascade, Bains du Mont Dore. (<i>After - Scrope</i>)</td> - <td class="tdr vbot"><a href="#fig48">130</a></td> -</tr> -<tr> - <td class="tdr vtop">49.</td> - <td class="tdl">Section in the Island of Ventotienne, showing a great - stream of andesitic lava overlying stratified tuffs. - (<i>After Scrope</i>)</td> - <td class="tdr vbot"><a href="#fig49">130</a></td> -</tr> -<tr> - <td class="tdr vtop">50.</td> - <td class="tdl">Cliff on the south side of the Island of San Stephano</td> - <td class="tdr vbot"><a href="#fig50">131</a></td> -</tr> -<tr> - <td class="tdr vtop">51.</td> - <td class="tdl">The headland of Monte della Guardia, in the Island of Ponza</td> - <td class="tdr vbot"><a href="#fig51">131</a></td> -</tr> -<tr> - <td class="tdr vtop">52.</td> - <td class="tdl">Western side of the same headland, as seen from the north - side of Luna Bay</td> - <td class="tdr vbot"><a href="#fig52">132</a></td> -</tr> -<tr> - <td class="tdr vtop">53.</td> - <td class="tdl">Sea-cliff at Il Capo, the north-east point of Salina, - showing stratified agglomerates traversed by numerous - dykes, the whole being unconformably overlaid by - stratified, aqueous deposits</td> - <td class="tdr vbot"><a href="#fig53">137</a></td> -</tr> -<tr> - <td class="tdr vtop">54.</td> - <td class="tdl">Section observed in the Val del Bove, Etna, showing a - basaltic dyke, from the upper part of which a - lava-current has flowed</td> - <td class="tdr vbot"><a href="#fig54">138</a></td> -</tr> -<tr> - <td class="tdr vtop">55.</td> - <td class="tdl">Basaltic dykes projecting from masses of stratified scoriæ - in the sides of the Val del Bove, Etna</td> - <td class="tdr vbot"><a href="#fig55">134</a></td> -</tr> -<tr> - <td class="tdr vtop">56.</td> - <td class="tdl">Sheets of igneous rock (basalt) intruded between beds of - sandstone, clay, and limestone (Island of Skye)</td> - <td class="tdr vbot"><a href="#fig56">137</a></td> -</tr> -<tr> - <td class="tdr vtop">57.</td> - <td class="tdl">Plan of the dissected volcano of Mull in the Inner - Hebrides</td> - <td class="tdr vbot"><span class="smaller"><i>to face p.</i> </span><a href="#fig57">142</a></td> -</tr> -<tr> - <td class="tdr vtop">58.</td> - <td class="tdl">Section of the volcano of Mull along the line A B</td> - <td class="tdr vbot"> " <a href="#fig58">142</a></td> -</tr> -<tr> - <td class="tdr vtop">59.</td> - <td class="tdl">Summit of the volcano of Monte Sant' Angelo, in Lipari, - exhibiting a crater with walls worn down by denudation</td> - <td class="tdr vbot"><a href="#fig59">158</a></td> -</tr> -<tr> - <td class="tdr vtop">60.</td> - <td class="tdl">Outlines of lava-cones</td> - <td class="tdr vbot"><a href="#fig60_neg">160</a></td> -</tr> -<tr> - <td class="tdr vtop">61.</td> - <td class="tdl">Diagram illustrating the formation of parasitic cones along - lines of fissure formed on the flanks of a great volcanic - mountain</td> - <td class="tdr vbot"><a href="#fig61">162</a></td> -</tr> -<tr> - <td class="tdr vtop">62.</td> - <td class="tdl">Outline of Etna, as seen from Catania - <span class="pagenum" id="Page_xvii">- xvii -</span></td> - <td class="tdr vbot"><a href="#fig62">162</a></td> -</tr> -<tr> - <td class="tdr vtop">63.</td> - <td class="tdl">Outline of Etna, as seen from the Val del Bronte</td> - <td class="tdr vbot"><a href="#fig63">163</a></td> -</tr> -<tr> - <td class="tdr vtop">64.</td> - <td class="tdl">Plan of the volcano forming the Island of Ischia</td> - <td class="tdr vbot"><a href="#fig64">163</a></td> -</tr> -<tr> - <td class="tdr vtop">65.</td> - <td class="tdl">A primary parasitic cone, with a secondary one at its - base—Ischia</td> - <td class="tdr vbot"><a href="#fig65">164</a></td> -</tr> -<tr> - <td class="tdr vtop">66.</td> - <td class="tdl">Scoria-cone near Auckland, New Zealand, with a lava-current - flowing from it. (<i>After Heaphy</i>)</td> - <td class="tdr vbot"><a href="#fig66">165</a></td> -</tr> -<tr> - <td class="tdr vtop">67.</td> - <td class="tdl">Section of rocks below the ancient triassic volcano of - Predazzo in the Tyrol</td> - <td class="tdr vbot"><a href="#fig67">165</a></td> -</tr> -<tr> - <td class="tdr vtop">68.</td> - <td class="tdl">Cotopaxi, as seen from a distance of ninety miles. (<i>After - Humboldt</i>)</td> - <td class="tdr vbot"><a href="#fig68">168</a></td> -</tr> -<tr> - <td class="tdr vtop">69.</td> - <td class="tdl">Citlaltepetl, or the Pic d'Orizaba, in Mexico, as seen from - the Forest of Xalapa. (<i>After Humboldt</i>)</td> - <td class="tdr vbot"><a href="#fig69">169</a></td> -</tr> -<tr> - <td class="tdr vtop">70.</td> - <td class="tdl">Lac Paven, in the Auvergne. (<i>After Scrope</i>)</td> - <td class="tdr vbot"><a href="#fig70">171</a></td> -</tr> -<tr> - <td class="tdr vtop">71.</td> - <td class="tdl">The crater-lake called Lago del Bagno, in Ischia, converted - into a harbour</td> - <td class="tdr vbot"><a href="#fig71">172</a></td> -</tr> -<tr> - <td class="tdr vtop">72.</td> - <td class="tdl">Lake of Gustavila, in Mexico. (<i>After Humboldt</i>)</td> - <td class="tdr vbot"><a href="#fig72">172</a></td> -</tr> -<tr> - <td class="tdr vtop">73.</td> - <td class="tdl">Peak of Teneriffe, surrounded by great crater-rings. (<i>After - Piazzi-Smyth</i>)</td> - <td class="tdr vbot"><a href="#fig73">175</a></td> -</tr> -<tr> - <td class="tdr vtop">74.</td> - <td class="tdl">The volcano of Bourbon, rising in the midst of a crater-ring - four miles in diameter. (<i>After Bory de St. Vincent</i>)</td> - <td class="tdr vbot"><a href="#fig74">176</a></td> -</tr> -<tr> - <td class="tdr vtop">75.</td> - <td class="tdl">The volcano of Bourbon, as seen from another point of - view, with three concentric crater-rings encircling its - base. (<i>After Bory de St. Vincent</i>)</td> - <td class="tdr vbot"><a href="#fig75">176</a></td> -</tr> -<tr> - <td class="tdr vtop">76.</td> - <td class="tdl">Vesuvius as seen from Sorrento, half encircled by the - crater-ring of Somma</td> - <td class="tdr vbot"><a href="#fig76">177</a></td> -</tr> -<tr> - <td class="tdr vtop">77.</td> - <td class="tdl">Outlines of various volcanoes illustrating the different - relations of the craters to cones</td> - <td class="tdr vbot"><span class="smaller"><i>to face p.</i> </span><a href="#fig77_neg">178</a></td> -</tr> -<tr> - <td class="tdr vtop">78.</td> - <td class="tdl">Island thrown up In the Mediterranean Sea in July and - August, 1831. (<i>After the Prince de Joinville</i>)</td> - <td class="tdr vbot"><a href="#fig78">179</a></td> -</tr> -<tr> - <td class="tdr vtop">79.</td> - <td class="tdl">Sinter-cones surrounding the orifices of geysers</td> - <td class="tdr vbot"><a href="#fig79_neg">183</a></td> -</tr> -<tr> - <td class="tdr vtop">80.</td> - <td class="tdl">Diagram illustrating the mode of formation of travertine- - and sinter-terraces on the sides of a hill of tuff</td> - <td class="tdr vbot"><a href="#fig80_neg">185</a></td> -</tr> -<tr> - <td class="tdr vtop">81.</td> - <td class="tdl">Map of the volcanic group of the Lipari Islands, illustrating - the position of the lines of fissure upon which - the volcanoes have been built up - <span class="pagenum" id="Page_xviii">- xviii -</span></td> - <td class="tdr vbot"><a href="#fig81">192</a></td> -</tr> -<tr> - <td class="tdr vtop">82.</td> - <td class="tdl">The Puy de Pariou, in the Auvergne, illustrating the shifting - of eruption along a line of fissures</td> - <td class="tdr vbot"><a href="#fig82">193</a></td> -</tr> -<tr> - <td class="tdr vtop">83.</td> - <td class="tdl">Ideal section of the Puy de Pariou</td> - <td class="tdr vbot"><a href="#fig83">194</a></td> -</tr> -<tr> - <td class="tdr vtop">84.</td> - <td class="tdl">Fissure formed on the flanks of Etna during the emotion - of 1865. (<i>After Silvestri</i>)</td> - <td class="tdr vbot"><a href="#fig84">194</a></td> -</tr> -<tr> - <td class="tdr vtop">85.</td> - <td class="tdl">Plan of the Island of Vulcano, based on the map of the - Italian Government</td> - <td class="tdr vbot"><a href="#fig85">196</a></td> -</tr> -<tr> - <td class="tdr vtop">86.</td> - <td class="tdl">Vulcanello, with its three craters</td> - <td class="tdr vbot"><a href="#fig86">197</a></td> -</tr> -<tr> - <td class="tdr vtop">87.</td> - <td class="tdl">Section of basalt from Ovifak, Greenland, with particles of - metallic iron diffused through its mass</td> - <td class="tdr vbot"><a href="#fig87">319</a></td> -</tr> -<tr> - <td class="tdr vtop">88.</td> - <td class="tdl">Diagram illustrating the relations between the terrestrial - and the extra-terrestrial rocks</td> - <td class="tdr vbot"><span class="smaller"><i>to face p.</i> </span><a href="#fig88">322</a></td> -</tr> -<tr> - <td class="tdr vtop">89.</td> - <td class="tdl">A group of sun-spots. (<i>After Secchi</i>)</td> - <td class="tdr vbot"><a href="#fig89">362</a></td> -</tr> -<tr> - <td class="tdr vtop">90.</td> - <td class="tdl">A sun-spot, showing the great masses of incandescent - vapour rising or falling within it. (<i>After Secchi</i>)</td> - <td class="tdr vbot"><a href="#fig90">363</a></td> -</tr> -<tr> - <td class="tdr vtop">91.</td> - <td class="tdl">The edge of a sun-spot, showing a portion of the prominent - masses of incandescent gas (<span class="allsmcap">A</span>) which detached itself - at <span class="allsmcap">B</span> and floated into the midst of the cavity. - (<i>After Norman Lockyer</i>)</td> - <td class="tdr vbot"><a href="#fig91">363</a></td> -</tr> -<tr> - <td class="tdr vtop">92.</td> - <td class="tdl">Drawing of a solar prominence made by Mr. Norman - Lockyer, March 14, 1869, at 11 h. 5 m. <span class="allsmcap">A.M.</span></td> - <td class="tdr vbot"><a href="#fig91">364</a></td> -</tr> -<tr> - <td class="tdr vtop">93.</td> - <td class="tdl">The same object, as seen at 11 h. 15 m. on the same day. - (<i>After Norman Lockyer</i>)</td> - <td class="tdr vbot"><a href="#fig93">365</a></td> -</tr> -<tr> - <td class="tdr vtop">94.</td> - <td class="tdl">Drawings of a solar prominence at four different periods - on September 7, 1871. (<i>After Young</i>)</td> - <td class="tdr vbot"><a href="#fig94">366</a></td> -</tr> -<tr> - <td class="tdr vtop">95.</td> - <td class="tdl">A group of Lunar craters (Maurolycus, Barocius, &c.), the - largest being more than sixty miles in diameter</td> - <td class="tdr vbot"><a href="#fig95">368</a></td> -</tr> -</table> - -<p><span class="pagenum" id="Page_1">- 1 -</span></p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h1 class="nobreak">VOLCANOES.</h1> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h2 class="nobreak" id="CHAPTER_I">CHAPTER I.<br /> - -<span class="smaller">INTRODUCTORY: NATURE OF THE INQUIRY.</span></h2> -</div> - - -<p class="p0">'<span class="smcap">What is a volcano?</span>' This is a familiar question, -often addressed to us in our youth, which 'Catechisms -of Universal Knowledge,' and similar school manuals, -have taught us to reply to in some such terms as -the following: 'A volcano is a burning mountain, from -the summit of which issue smoke and flames.' Such -a statement as this, it is probable, does not unfairly -represent the ideas which are, even at the present day, -popularly entertained upon the subject.</p> - -<p>But in this, as in so many other cases, our first -step towards the acquirement of scientific or exact -knowledge, must be the unlearning of what we have -before been led to regard as true. The description -which we have quoted is not merely incomplete and -inadequate as a whole, but each individual proposition -of which it is made up is grossly inaccurate, and, what -<span class="pagenum" id="Page_2">- 2 -</span> -is worse, perversely misleading. In the first place, the -action which takes place at volcanoes is not 'burning,' -or combustion, and bears, indeed, no relation whatever -to that well-known process. Nor are volcanoes necessarily -'mountains' at all; essentially, they are just -the reverse—namely, holes in the earth's crust, or outer -portion, by means of which a communication is kept -up between the surface and the interior of our globe. -When mountains do exist at centres of volcanic activity, -they are simply the heaps of materials thrown -out of these holes, and must therefore be regarded not -as the causes but as the consequences of the volcanic -action. Neither does this action always take place at -the 'summits' of volcanic mountains, when such exist, -for eruptions occur quite as frequently on their sides -or at their base. That, too, which popular fancy regards -as 'smoke' is really condensing steam or watery vapour, -and the supposed raging 'flames' are nothing more -than the glowing light of a mass of molten material -reflected from these vapour clouds.</p> - -<p>It is not difficult to understand how these false -notions on the subject of volcanic action have come to -be so generally prevalent. In the earlier stages of its -development, the human mind is much more congenially -employed in drinking in that which is marvellous -than in searching for that which is true. It must -be admitted, too, that the grand and striking phenomena -displayed by volcanoes are especially calculated -to inspire terror and to excite superstition, and such -<span class="pagenum" id="Page_3">- 3 -</span> -feelings most operate in preventing those close and -accurate observations which alone can form the basis -of scientific reasoning.</p> - -<div class="sidenote">IDEAS OF THE ANCIENTS.</div> - -<p>The ancients were acquainted only with the four or -five active volcanoes in the Mediterranean area; the -term 'volcano' being the name of one of these (Vulcano, -or Volcano, in the Lipari Islands), which has -come to be applied to all similar phenomena. It is -only in comparatively modern times that it has become -a known £act that many hundreds of volcanoes exist -upon the globe, and are scattered over almost every -part of its surface. Classical mythology appropriated -Vulcano as the forge of Hephæstus, and his Roman -representative Vulcan, while Etna was regarded as -formed by the mountains under which a vengeful -deity had buried the rebellious Typhon; it may be -imagined, therefore, that any endeavour to more -closely investigate the phenomena displayed at these -localities would be regarded, not simply as an act of -temerity, but as one of actual impiety. In mediæval -times similar feelings would operate with not less -force in the same direction, for the popular belief -identified the subterranean fires with a place of everlasting -torment; Vulcano was regarded as the place of -punishment of the Arian Emperor Theodosius, while -Etna was assigned to poor Anne Boleyn, the perverter -of faith in the person of its stoutest defender. That -such feelings of superstitious terror in connection with -volcanoes are, even at the present day, far from being -<span class="pagenum" id="Page_4">- 4 -</span> -extinct, will be attested by every traveller who, in -carrying on investigations about volcanic centres, has -had to avail himself of the assistance of guides and -attendants from among the common people.</p> - -<p>Among the great writers of antiquity we find -several who had so far emancipated their minds from -the popular superstitions as to be able to enunciate -just and rational views upon the subject of volcanoes. -Until quite recent times, however, their teaching was -quite forgotten or neglected, and the modern science -of Vulcanology may be said to have entirely grown up -within the last one hundred years.</p> - -<p>The great pioneer in this important branch of research -was the illustrious Italian naturalist Spallanzani, -who, in the year 1788, visited the several volcanoes of -his native land, and published an account of the numerous -valuable and original observations which he had -made upon them. About the same time the French -geologist Dolomieu showed how much light might be -thrown on the nature of volcanic action by a study of -the various materials which are ejected from volcanic -vents; while our own countryman. Sir William Hamilton, -was engaged in a systematic study of the changes -in form of volcanic mountains, and of the causes -which determine their growth. At a somewhat later -date the three German naturalists. Von Buch, Humboldt, -and Abich, greatly extended our knowledge of -volcanoes by their travels in different portions of the -globe.</p> - -<p><span class="pagenum" id="Page_5">- 5 -</span></p> - -<div class="sidenote">CHARACTER OF MODERN RESEARCHES.</div> - -<p>The first attempt, however, to frame a satisfactory -theory of volcanic action, and to show the part which -volcanoes have played in the past history of our globe, -together with their place in its present economy, was -made in 1825, by Poulett Scrope, whose great work, -'Considerations on Volcanoes,' may be regarded as the -earliest systematic treatise on Vulcanology. Since the -publication of this work, many new lines of inquiry -have been opened up in connection with the subject, -and fresh methods of research have been devised and -applied to it. More exact observations of travellers -over wider areas have greatly multiplied the facts -upon which we may reason and speculate, and many -erroneous hypotheses which had grown up in connection -with the subject have been removed by patient -and critical inquiry.</p> - -<p>We propose in the following pages to give an outline -of the present state of knowledge upon the subject, -and to indicate the bearings of those conclusions which -have already been arrived at, upon the great questions -of the history of our globe and the relations which -it bears to the other portions of the universe. In -attempting this task we cannot do better than take -up the several lines of inquiry in the order in which -they have been seized upon and worked out by the -original investigators; for never, perhaps, is the development -of thought in the individual mind so natural -in its methods, and so permanent in its effects, as when -it obeys those laws which determined its growth in the -<span class="pagenum" id="Page_6">- 6 -</span> -collective mind of the race. In our minds, as in our -bodies, development in the individual is an epitome, -or microcosmic reproduction, of evolution in the -species.</p> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_7">- 7 -</span></p> - -<h2 class="nobreak" id="CHAPTER_II">CHAPTER II.<br /> - -<span class="smaller">THE NATURE OF VOLCANIC ACTION.</span></h2> -</div> - - -<p class="p0"><span class="smcap">The</span> dose investigation of what goes on within a -volcanic vent may appear at first sight to be a task -beset with so many difficulties and dangers that we -may be tempted to abandon it as altogether hopeless. -At the first recorded eruption of Vesuvius the elder -Pliny lost his life in an attempt to approach the -mountain and examine the action which was taking -place there; and during the last great outburst of the -same volcano a band of Neapolitan students, whose -curiosity was greater than their prudence, shared the -same fate.</p> - -<p>But in both these cases the inquirers paid the -penalty of having adopted a wrong method. If we -wish to examine the mode of working of a complicated -steam-engine, it will be of little avail for us to -watch the machinery when the full blast of steam is -turned on, and the rapid movements of levers, pinions, -and slides baffle all attempts to follow them, and render -hopeless every effort to trace their connection with -one another. But if some friendly hand turn off the -<span class="pagenum" id="Page_8">- 8 -</span> -greater part of the steam-supply, then, as the rods -move slowly backwards and forwards, as the wheels -make their measured revolutions, and the valves axe -seen gradually opening and shutting, we may have an -opportunity of determining the relations of the several -parts of the machine to one another, and of arriving -at just conclusions concerning the plan on which it is -constructed. Nor can we doubt that the parts of the -machine bear the same relation to one another, and -that their movements take place in precisely the same -order, when the supply of steam is large as when it is -small.</p> - -<p>Now, as we shall show in the sequel, a volcano is a -kind of great natural steam-engine, and our best method -of investigating its action is to watch it when a part of -the steam-supply is cut off. It is true that we cannot -at will control the source of supply of steam to a -volcano, as we can in a steam-engine, but as some -volcanoes have usually only a small steam-supply, and -nearly all volcanoes vary greatly in the intensity of -their action at different periods, we can, by a careful -selection of the object or the time of our study, gain -all those advantages which would be obtained by regulating -its action for ourselves.</p> - -<p>Spallanzani appears to have been the first to perceive -the important fact, that the nature of volcanic -action remains the same, however its intensity may -vary. Taking advantage of the circumstance that -there exists in the Mediterranean Sea a -<span class="pagenum" id="Page_9">- 9 -</span> -volcano—Stromboli—which for at least 2,000 years has been in -a constant and regular, but not in a violent or dangerous, -state of activity, he visited the spot, and made -the series of careful observations which laid the foundation -of our knowledge of the 'physiology of volcanoes.' -Since the time of Spallanzani, many other -investigators have visited the crater of Stromboli, and -they have been able to confirm and extend the observations -of the great Italian naturalist, as to the -character of the action which is constantly taking place -within it. We cannot better illustrate the nature of -volcanic action than by describing what has been witnessed -by numerous observers within the crater of -Stromboli, where it is possible to watch the series of -operations going on by the hour together, and to do so -without having our judgment warped either by an -excited imagination or the sense of danger.</p> - -<div class="sidenote">APPEARANCE OF STROMBOLI FROM A DISTANCE.</div> - -<p>In the sketch, <a href="#fig01">fig. 1</a>, which was made on April -20, 1874, I have shown the appearance which this -interesting volcano usually presents, when viewed from -a distance. The island is of rudely circular outline, -and conical form, and rises to the height of 3,090 feet -above the level of the Mediterranean. From a point -on the side of the mountain, masses of vapour are -seen to issue, and these unite to form a cloud over the -mountain, the outline of this vapour-cloud varying -continually according to the hygrometric state of the -atmosphere, and the direction and force of the wind. -At the time when this sketch was made, the vapour-cloud -<span class="pagenum" id="Page_10">- 10 -</span> -was spread in a great horizontal stratum overshadowing -the whole island, but it was clearly seen to -be made up of a number of globular masses, each of -which, as we shall hereafter see, is the product of a -distinct outburst of the volcanic forces.</p> - -<p>Viewed at night-time, Stromboli presents a far -more striking and singular spectacle. The mountain, -with its vapour canopy, is visible over an area -having a radius of more than 100 miles. When -watched from the deck of a vessel anywhere within -this area, a glow of red light is seen to make its appearance -from time to time above the summit of the -mountain; this glow of light may be observed to -increase gradually in intensity, and then as gradually -to die away. After a short interval the same appearances -are repeated, and this goes on till the increasing -light of the dawn causes the phenomenon to be no -longer visible. The resemblance presented by Stromboli -to a 'flashing light' on a most gigantic scale is -very striking, and the mountain has long been known -as 'the lighthouse of the Mediterranean.'</p> - -<p>It must be pointed out, however, that in two very -important particulars the appearances presented by -Stromboli differ markedly from those rhythmical gleams -exhibited by the 'flashing-lights' of our coasts. In -the first place, the intervals between successive flashes -are very unequal, varying from less than one minute -to twenty minutes, or even more; and in the second -place, the duration and intensity of the red glow above -the mountain are subject to like variation, being sometimes -a momentary scarcely visible gleam, and at others -a vivid burst of light which illuminates the sky to a -considerable distance round.</p> - -<div class="figcenter" id="fig01" style="width: 626px;"> - <img src="images/fig01.png" width="626" height="425" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 1.—Stromboli, viewed from the North-west, April 1874.</span></div> -</div> - -<p><span class="pagenum" id="Page_11">- 11 -</span></p> - -<div class="figcenter" id="fig02" style="width: 450px;"> - <img src="images/fig02.png" width="383" height="334" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 2.—Map of tub Island of Stromboli.</span> -(Scale about two inches to a mile.)</div> -</div> - -<div class="sidenote">GENERAL FEATURES OF THE MOUNTAIN.</div> - -<p>Let us now draw near and examine this wonderful -phenomenon of a mountain which seemingly ever -burns with fire, and yet is not consumed. The general -form of the Island of Stromboli will be gathered from -an inspection of the plan, <a href="#fig02">fig. 2</a>, which is copied from -a map published by the Italian Government. When -we land upon the island, we find that it is entirely -built up of such materials as we know to be ejected -<span class="pagenum" id="Page_12">- 12 -</span> -from volcanoes; indeed, it resembles on a gigantic -scale the surroundings of an iron furnace, with its -heaps of cinders and masses of slag. The irregularity -in the form of the island is at once seen to be due to -the action of the wind, the rain, and the waves of the -surrounding sea, which have removed the loose, cindery -materials at some points, and left the hard, slaggy -masses standing up prominently at others.</p> - -<p>This great heap of cindery and slaggy materials -rises, as we have said, to a height of more than 3,000 -feet above the sea-level, but even this measurement -does not give a just idea of its vast bulk. Soundings -in the sea surrounding the island show that the -bottom gradually shelves around the shores to the -depth of nearly 600 fathoms, so that Stromboli is a -great conical mass of cinders and slaggy materials, -having a height of over 6,000 feet, and a base whose -diameter exceeds four miles.</p> - -<p>The general form and proportions of this mass will -be better understood by an examination of the section, -<a href="#fig03">fig. 3</a>, which is also constructed from the materials -furnished by the map of the island issued by the -Italian Government. The same section, and the map, -<a href="#fig02">fig. 2</a>, will serve to make clear the position and relations -of the point on the mountain at which the -volcanic activity takes place. At a spot on the north-west -slope of the mountain, about 1,000 feet below its -summit, and 2,000 feet above the level of the sea, -there exists a circular depression, the present active -<span class="pagenum" id="Page_13">- 13 -</span> -'crater' of the volcano; and leading down from this -to the sea there is a flat slope making an angle of -about 35° with the horizon, and known as the 'Sciarra.' -The Sciarra is bounded by steep cliffs, as shown -in the sketch <a href="#fig01">fig. 1</a>, and the plan <a href="#fig02">fig. 2</a>.</p> - -<div class="figcenter" id="fig03" style="width: 433px;"> - <img src="images/fig03.png" width="433" height="143" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 3.—Section through the Island of Stromboli from -n.w. to s.e.</span><br /> - -<div class="blockquot"> -<p><i>a.</i> Highest summit of the mountain, <i>c.</i> Cratère del Fossa, <i>b.</i> Point overlooking -the crater, <i>d.</i> Steep slope known as the Sciarra del Fuoco. <i>e.</i> -Continuation of the same slope beneath the level of the sea. <i>f.</i> Steep cliffs of the -Punta dell' Omo.</p></div> -</div> -</div> - -<div class="sidenote">FORM AND FUNCTION OF THE CRATER.</div> - -<p>If we climb up to this scene of volcanic activity, -we shall be able to watch narrowly the operations -which are going on there. On the morning of the -24th of April, 1874, I paid a visit to this interesting -spot in order to get a near view of what was taking -place. On reaching a point upon the side of the -Sciarra, from which the crater was in full view before -me, I witnessed, and made a sketch of, an outburst -which then took place, and this sketch has been reproduced -in <a href="#fig04">fig. 4</a>. Before the outburst, numerous -light curling wreaths of vapour were seen ascending -from fissures on the sides and bottom of the crater. -Suddenly, and without the slightest warning, a sound -was heard like that produced when a locomotive blows -<span class="pagenum" id="Page_14">- 14 -</span> -off its steam at a railway-station; a great volume of -watery vapour was at the same time thrown violently -into the atmosphere, and with it there were hurled -upwards a number of dark fragments, which rose to -the height of 400 or 500 feet above the crater, describing -curves in their course, and then falling -back upon the mountain. Most of these fragments -tumbled into the crater with a loud, rattling noise, but -some of them fell outside the crater, and a few rolled -down the steep slope of the Sciarra into the sea. -Some of these falling fragments were found to be -still hot and glowing, and in a semi-molten condition, -so that they readily received the impression of a coin -thrust into them.</p> - -<div class="figcenter" id="fig04" style="width: 427px;"> - <img src="images/fig04.png" width="427" height="255" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 4.—The Crates of Stromboli as viewed from the side of the -Sciarra during an eruption on the morning of April 24, 1874.</span></div> -</div> - -<p><span class="pagenum" id="Page_15">- 15 -</span></p> - -<div class="sidenote">APERTURES AT THE BOTTOM OF THE CRATER.</div> - -<p>But on the upper side of the crater, at the point -marked 6, on the section <a href="#fig03">fig. 3</a>, there exists a spot from -which we can look down upon the bottom of the crater, -and view the operations taking place there. This is -the place where Spallanzani and other later investigators -have carried on their observations, and, when the wind -is blowing from the spectator towards the crater, he -may sit for hours watching the wonderful scene displayed -before him. The black slaggy bottom of the -crater is seen to be traversed by many fissures or -cracks, from most of which curling jets of vapour issue -quietly, and gradually mingle with and disappear in -the atmosphere. But besides these smaller cracks at -the bottom of the crater, several larger openings are -seen, which vary in number and position at different -periods; sometimes only one of these apertures is -visible, at others as many as six or seven, and the -phenomena presented at these larger apertures are -especially worthy of careful investigation.</p> - -<p>These larger apertures, if we study the nature of -the action taking place at them, may be divided into -three classes. From those of the first class, steam is -emitted with loud, snorting puffs, like those produced by -a locomotive-engine, but far less regular and rhythmical -in their succession. In the second class of apertures -masses of molten material are seen welling out, and, if -the position of the aperture be favourable, flowing outside -the crater; from this liquid molten mass steam -is seen to escape, sometimes in considerable quantities. -The openings of the third class present still more -<span class="pagenum" id="Page_16">- 16 -</span> -interesting appearances. Within the walls of the -aperture a viscid or semi-liquid substance is seen slowly -heaving up and down. As we watch the seething mass -the agitation within it is observed to increase gradually, -and at last a gigantic bubble is formed which violently -bursts, when a great rush of steam takes place, carrying -fragments of the scum-like surface of the liquid -high into the atmosphere.</p> - -<p>If we visit the crater by night, the appearances -presented are found to be still more striking and -suggestive. The smaller cracks and larger openings -glow with a ruddy light. The liquid matter is seen to -be red- or even white-hot, while the scum or crust which -forms upon it is of a dull red colour. Every time a -bubble bursts and the crust is broken up by the escape -of steam, a fresh, glowing surface of the incandescent -material is exposed. If at these moments we look up -at the vapour-cloud covering the mountain, we shall at -once understand the cause of the singular appearances -presented by Stromboli when viewed from a distance -at night, for the great masses of vapour are seen to be -lit up with a vivid, ruddy glow, like that produced when -an engine-driver opens the door of the furnace and -illuminates the stream of vapour issuing from the -funnel of his locomotive.</p> - -<p>Let us now endeavour to analyse the phenomena so -admirably displayed before us in the crater of Stromboli. -The three essential conditions on which the production -of these phenomena seems to depend are the following: -<span class="pagenum" id="Page_17">- 17 -</span> -first, the existence of certain apertures or cracks communicating -between the interior and the surface of the -earth; secondly, the presence of matter in a highly -heated condition beneath the surface; and thirdly, the -existence of great quantities of water imprisoned in the -subterranean regions—which water, escaping as steam, -gives rise to all those active phenomena we have been -describing.</p> - -<div class="sidenote">CAUSE OF THE GLOWING LIGHT.</div> - -<p>We have said, at the outset, that there exists no -analogy whatever between the action which takes place -in volcanoes and the operation of burning or combustion. -Occasionally, it is true, certain inflammable substances -are formed by the action going on within the volcano, -and these inflammable substances, taking fire, produce -real flames. Such flames are, however, in almost all -cases only feebly luminous, and do not give rise to any -conspicuous appearances. What is usually taken for -flame during volcanic eruptions is simply, as we have -already pointed out, the glowing red-hot surface of a -mass of molten rock, reflected from a vapour-cloud -hanging over it. The red glow observed over a volcano -in eruption is indeed precisely similar in its nature and -origin to that which is seen above London during a -night of heavy fog, and which is produced by the reflection -of the gas-lights of the city from the innumerable -particles of water-vapour diffused through the -atmosphere. Fires, of course, occur when the molten -and incandescent materials poured out from a volcano -come in contact with inflammable substances, such as -<span class="pagenum" id="Page_18">- 18 -</span> -forests and houses, but in these cases the combustion -is quite a secondary phenomenon.</p> - -<p>There is another popular delusion concerning volcanic -action, which it may be necessary to refer to and -to combat. From the well-known fact that sulphur or -brimstone is found abundantly in volcanic regions, the -popular belief has arisen that this highly inflammable -substance has something to do with the production of -the eruptions of volcanoes. In school-books which were, -until comparatively recent years, in constant use in this -country, the statement may be found that by burying -certain quantities of sulphur, iron-pyrites, and charcoal -in a hole in the ground, we may form a miniature -volcano, and produce all the essential phenomena of a -volcanic eruption. No greater mistake could possibly -be made. The chemical reactions which take place -when sulphur and other substances are made to act -upon each other differ entirely from the phenomena -of volcanic action. The sulphur which is found in -volcanic regions is the result and not the cause of -volcanic action. Among the most common substances -emitted from volcanic vents along with the steam are -the two gases, sulphurous acid and sulphuretted hydrogen. -When these two gases come into contact -with one another, chemical action takes place, and the -elements contained in them—oxygen, hydrogen, and -sulphur—are free to group themselves together in an -entirely new fashion; the consequence of this is that -water and sulphuric acid (oil of vitriol) are formed, and -<span class="pagenum" id="Page_19">- 19 -</span> -a certain quantity of sulphur is set free. The water -escapes into the atmosphere, the sulphuric acid combines -with lime, iron, or other substances contained in -the surrounding rocks, and the sulphur builds up crystals -in any cavities which may happen to exist in these -rocks.</p> - -<div class="sidenote">VOLCANIC ACTION RESEMBLES BOILING.</div> - -<p>If, however, careful and exact observations, like -those carried on at Stromboli, compel us to reject the -popular notions concerning the supposed resemblance -between volcanic action and the combustion of sulphur -or other substances, they nevertheless suggest analogies -with certain other simple and well-known operations. -And in pursuing these analogies, we are led to -the recognition of some admirable illustrations both -of the attendant phenomena and of the true cause of -volcanic outbursts.</p> - -<p>No one can look down on the mass of seething -material in violent agitation within the fissures at the -bottom of the crater of Stromboli, without being forcibly -reminded of the appearances presented by liquids in -a state of boiling or ebullition. The glowing material -seems to be agitated by two kinds of movements, the -one whirling or rotatory, the other vertical or up-and-down -in its direction. The fluid mass in this way -appears to be gradually impelled upwards, till it approaches -the lips of the aperture, when vast bubbles -are formed upon its surface, and to the sudden bursting -of these the phenomena of the eruption are due.</p> - -<p>Now if we take a tall narrow vessel and fill it with -<span class="pagenum" id="Page_20">- 20 -</span> -porridge or some similar substance of imperfect fluidity, -we shall be able, by placing it over a fire, to imitate -very closely indeed the appearances presented in the -crater of Stromboli. As the temperature of the mass -rises, steam is generated within it, and in the efforts of -this steam to escape, the substance is set in violent -movement. These movements of the mass are partly -rotatory and partly vertical in their direction; as fresh -steam is generated in the mass its surface is gradually -raised, while an escape of the steam is immediately -followed by a fall of the surface. Thus an up-and-down -movement of the liquid is maintained, but as the -generation of steam goes on faster than it can escape -through the viscid mass, there is a constant tendency -in the latter to rise towards the mouth of the vessel. -At last, as we know, if heat continues to be applied to -the vessel, the fluid contents will be forced up to its -edge and a catastrophe will occur; the steam being -suddenly and violently liberated from the bubbles -formed on the surface of the mass, and a considerable -quantity of the material forcibly expelled from the -vessel. The suddenness and violence of this catastrophe -is easily accounted for, if we bear in mind that -the escaping steam acts after the manner of a compressed -spring which is suddenly released. Steam is -first formed at the bottom of the vessel which is in -contact with the fire; but here it is under the pressure -of the whole mass of the liquid, and moreover, the -viscidity of the substance tends to retard the union of -<span class="pagenum" id="Page_21">- 21 -</span> -the steam bubbles and their rise to the surface of the -mass. But when the pressure is relieved by the bursting -of bubbles at the surface, the whole of the generated -steam tends to escape suddenly.</p> - -<div class="sidenote">ESCAPE OF STEAM-BUBBLES FROM LAVA.</div> - - -<p>Now within the crater of Stromboli we have precisely -the necessary conditions for the display of the -same series of operations. In the apertures at the -bottom there exists a quantity of imperfectly fluid -materials at a higher temperature, containing water -entangled in its mass. As this water passes into the -state of steam it tends to escape, and in so doing puts -the whole mass into violent movement. When the steam -rises to the surface, bubbles are formed, and the formation -of these bubbles is promoted by the circumstance -that the liquid mass, where exposed to the atmosphere, -becomes chilled, and thereby rendered less perfectly -fluid. By the bursting of these bubbles the pressure -is partially relieved, and a violent escape of the pent-up -steam takes place through the whole mass. Equilibrium -being thus restored, there follows a longer or -shorter interval of quiescence, during which steam is -being generated and collected within the mass, and -the series of operations which we have described then -recommences.</p> - -<p>There is one other consideration which must be -borne in mind in connection with this subject. It is -well known that if water be subjected to sufficiently -great pressure it may be raised to a very high temperature -and still retain its liquid condition. When this -<span class="pagenum" id="Page_22">- 22 -</span> -pressure is removed, however, the whole mass passes -at once into the condition of steam or water-gas; and -the gas thus formed at high temperatures has a proportionably -high tension. In a Papin's digester water -confined in a strong vessel is raised to temperatures -far above its ordinary boiling-point, and from any -opening in such a vessel the steam escapes with prodigious -violence. Now, at considerable depths beneath -the earth's surface, and under the pressure of many -hundreds or thousands of feet of solid rock, water still -retaining its liquid condition may become intensely -heated. When the pressure is relieved by the formation -of a crack or fissure in the superincumbent mass -of rock, the escape of the superheated steam will be -of very violent character, and may be attended with -the most striking and destructive results. In the -existence of high temperatures beneath the earth's -surface, and the presence in the same regions of -imprisoned water capable of passing into the highly -elastic gas which we call steam, we have a cause fully -competent to produce all the phenomena which we -have described as occurring at Stromboli.</p> - -<p>It may at first sight appear that the grand and -terrible displays of violence witnessed during a great -volcanic eruption differ fundamentally in their character -and their origin from those feeble outbursts -which we are able to examine closely and analyse -rigorously at Stromboli. But that such is not the case -a few simple considerations will soon convince us.</p> - -<p><span class="pagenum" id="Page_23">- 23 -</span></p> - -<div class="sidenote">STROMBOLI COMPARED WITH VESUVIUS.</div> - -<p>Although Stromboli usually displays the subdued -and moderate activity which we have been describing, -yet the intensity of the action going on within it is -subject to considerable variation. Occasionally the -violence of the outbursts is greatly increased—the -roaring of the steam-jets may be heard for many miles -around, considerable streams of incandescent liquefied -rock flow down the Sciarra into the sea, and the explosions -in the crater are far more frequent and energetic, -cinders and fragments of rock being scattered -all over the island and the surrounding seas.</p> - -<p>On the other hand, volcanoes like Vesuvius, which -are sometimes the scene of eruptions on the very -grandest scale, at others subside into a temporary state -of moderate activity quite similar in character to that -which is the normal condition of Stromboli. Thus, -shortly before the great eruption of Vesuvius in April -1872, a small cone was formed near the edge of the -crater, and during some months observers could watch, -in ease and safety, a series of small explosions taking -place, quite similar in their character and attendant -phenomena to those which we have described as occurring -at Stromboli. French geologists are in the -habit of defining the condition of activity in a volcano -by speaking of the more quiet and, regular state as the -'Strombolian stage,' and the more violent and paroxysmal -as the 'Vesuvian stage'; but the two conditions -are, as we have seen, presented by the same volcano at -different periods, and pass into one another by the -most insensible gradations.</p> - -<p><span class="pagenum" id="Page_24">- 24 -</span></p> - -<p>We must now proceed to compare the grand and -terrible appearances presented during a great eruption -with those more feeble displays which we have been -describing, to show that in all their essential features -these different kinds of outbursts are identical with -one another, and must be referred to the action of -similar causes.</p> - -<p>The volcanic eruption which has been most carefully -studied in recent times is that which we have -already referred to as occurring at Vesuvius, in the -month of April 1872. With the exception, perhaps, -of that which took place in October 1822, this eruption -was the grandest which has broken out at Vesuvius -during the present century. Owing to the circumstance -of its proximity to the great city of Naples, -Vesuvius has always been the most carefully watched -of all volcanoes, and in recent years the erection of an -observatory, provided with instruments for recording -the smallest subterranean tremors affecting the mountain, -has facilitated the carrying on of those continuous -and minute observations which are so necessary for -exact scientific inquiry.</p> - -<div class="figcenter" id="fig05" style="width: 659px;"> - <img src="images/fig05.png" width="659" height="454" alt="" /> - <div class="figcaption"><span class="smcap">Fig 5. Vesuvius in Eruption, as seen from Naples, April 26, 1872.</span> - (<i>From a photograph</i>)</div> -</div> - -<p><span class="pagenum" id="Page_25">- 25 -</span></p> - -<div class="sidenote">VESUVIUS ERUPTION OF 1872.</div> - -<p>On the occasion of this outburst, the aid of instantaneous -photography was first made available for -obtaining a permanent record of the appearances displayed -at volcanic eruptions. In <a href="#fig05">fig. 5</a> we have one of -these photographs, which was taken at 5 o'clock <span class="allsmcap">P.M.</span> -on April 26, 1872, transferred to a wood-block and -engraved. In examining it we feel sure that we -are not being misled by any exaggeration or error on -the part of the artist. Vesuvius rises to the height -of nearly 4,000 feet above the level of the sea, and an -inspection of the photograph proves that the vapours -and rock-fragments were thrown to the enormous -height of 20,000 feet, or nearly four miles, into the -atmosphere.</p> - -<p>The main features of this terrifying outburst were -as follows. For more than a twelvemonth before, the -activity of the forces at work within the mountain -appeared to be gradually increasing, and the great -eruption commenced on April 24, attained its climax -on the 26th, and began to die out on the following -day. During the eruption the bottom of the crater -was entirely broken up, and the sides of the mountain -were rent by fissures in all directions. So numerous -were these fissures and cracks that liquid matter -appeared to be oozing from every part of its surface, -and, as Professor Palmieri, who witnessed the outburst -from the observatory, expressed it, 'Vesuvius sweated -fire.' One of the fissures was of enormous size, extending -from the summit to far beyond the base of the -cone; the scar left by this gigantic rent being plainly -visible at the present day.</p> - -<p>From the great opening or crater at the summit, -and from some of the fissures on the sides of the -mountain, enormous volumes of steam rushed out with -a prodigious roaring sound, the noise being so terrific -that the inhabitants of Naples, five miles off, fled from -<span class="pagenum" id="Page_26">- 26 -</span> -their houses and spent the night in the open streets. -Although this roaring sound appeared at a distance to -be continuous, yet those upon the mountain could perceive -that it was produced by detonations or explosions -rapidly following one another. Each of these explosions -was accompanied by the formation of a great -globe of white vapour, which, rising into the atmosphere, -swelled the bulk of the vast cloud overhanging -the mountain. An inspection of the photographs (see -<a href="#fig05">fig. 5</a>) shows that the great vapour-cloud over Vesuvius -was made up of the globular masses ejected at successive -explosions. Each of these explosive upward rushes -of steam carried along with it a considerable quantity -of solid fragments, and these fell in great numbers all -over the surface of the mountain, breaking the windows -of the observatory, and making it dangerous to be out -of doors.</p> - -<p>We have said that lava, or molten rock, appeared -to be issuing from the very numerous cracks formed -all over the flanks of the mountain. But at three -points this molten rock issued in such quantities as to -form great, fiery floods, which rushed down the sides -of the mountain, and flowed to a considerable distance -beyond its base. The largest of these lava-floods overwhelmed -and destroyed the two villages of Massa di -Somma and San Sebastiano, besides many country -houses in the neighbourhood.</p> - -<div class="sidenote">STEAM EMITTED FROM LAVA-CURRENT.</div> - -<p>A very marked and interesting feature exhibited -by these three lava-floods was the quantity of watery -<span class="pagenum" id="Page_27">- 27 -</span> -vapour which they gave off during their flow. All -along their course, enormous volumes of steam were -evolved from them, as will be seen by an inspection of -the photograph. Indeed, such was the abundance and -tension of the steam thus escaping from the surfaces -of the lava-currents that it forced the congealing rock -up into great bubbles and blisters, and gave rise to the -formation of innumerable miniature volcanoes, varying -in size from a beehive to a cottage, some of which -remained in a state of independent activity for a -considerable time.</p> - -<p>So far, what we have described as taking place at -Vesuvius, in April 1872, has been only the repetition -on a £Eur grander scale of the three kinds of action -which we have shown to be constantly taking place at -Stromboli; namely, the formation of cracks or fissures -in the earth's surface, the escape of steam with explosive -violence from these openings, often propelling -rock-fragments into the atmosphere, and the outwelling, -under the influence of this compressed steam, of -masses of molten materials.</p> - -<p>There were some other appearances presented at -the great outburst at Vesuvius, which do not seem at -first sight to find any analogies in the manifestations -of the more feeble action continually going on at -Stromboli.</p> - -<p>Before and during the great outbreak of April 1872, -Vesuvius itself and the whole country round were -visited with earthquake-shocks, or tremblings of the -<span class="pagenum" id="Page_28">- 28 -</span> -ground. The sensitive instruments in the Vesuvian -Observatory showed the mountain daring the eruption -to be in a constant state of tremor. These earthquakes -are not, as is commonly supposed, actual upheavings -of the earth's surface, but are vibrations propagated -through the solid materials of which the earth is built -up. We cannot stamp our feet upon the ground -without giving rise to such vibrations, though our -senses may not be sufficiently acute to perceive them. -The explosive escape of steam from a crack is a cause -sufficiently powerful to produce a shock which is propagated -and may be felt for a considerable distance -round. Even on Stromboli an observer at the edge of -the crater may notice that each explosive outburst of -steam is accompanied by a perceptible tremor of the -ground, and in the case of Vesuvius the violent shocks -produced by the escape of far larger volumes of steam -give rise to proportionately stronger vibrations. The -nature and origin of those far more terrible and destructive -shocks which sometimes accompany, and -more frequently precede, great volcanic eruptions, we -shall consider in the sequel.</p> - -<div class="sidenote">CAUSE OF LIGHTNING DURING ERUPTIONS.</div> - -<p>Another striking phenomenon which was exhibited -in the great eruption of Vesuvius in 1872 was the -vivid display of lightning accompanied by thunder. -The uprushing current of steam and rock-fragments -forms a vertical column, but as the steam condenses it -spreads out into a great horizontal cloud which is seen -to be made up of the great globes of vapour emitted at -<span class="pagenum" id="Page_29">- 29 -</span> -successive explosions. When there is little or no wind -the vertical column with a horizontal cloud above it -bears a striking resemblance to the stone-pine trees -which form so conspicuous a feature in every Neapolitan -landscape. Around this column of vapour the most -vivid lightning constantly plays and adds not a little -to the grand and awful character of the spectacle of a -volcanic eruption, especially when it is viewed by night.</p> - -<p>In the eruption of 1872 a strong wind blowing -from the north-west destroyed the usual regular appearance -of this 'pine-tree appendage' to the mountain, -which is so well known to, and dreaded by the inhabitants -of Naples; the cloud, as will be seen from -the photograph (<a href="#fig05">fig. 5</a>, <i>facing</i> p. 24), was blown on one -side, and most of the falling fragments took the same -direction.</p> - -<p>It is well known that when high-pressure steam -IS allowed to escape through an orifice, electricity is -abundantly generated by the friction, and Sir William -Armstrong's hydro-electric machine is constructed on -this principle. Every volcano in violent eruption is a -very efficient hydro-electric machine, and the uprushing -column is in a condition of intense electrical -excitation. This result is probably aided by the friction -of the solid particles as they are propelled upwards and -fall back into the crater. The restoration of the condition -of electrical stability between this column and -the surrounding atmosphere is attended with the production -of frequent lightning-flashes and thunder-claps, -<span class="pagenum" id="Page_30">- 30 -</span> -the found of the latter being usually, however, drowned -in the still louder roar of the uprushing steam-column.</p> - -<p>The discharge of Buch large quantities of steam into -the atmosphere soon causes the latter to be saturated -with watery vapour, and there follows an excessive rainfall; -long-continued rain and floods were an accompaniment -of the great Vesuvian outbreak of 1872, as they -have been of almost all great volcanic eruptions. The -Italians, indeed, dread the floods which follow an eruption -more than the fiery streams of lava which accompany -it—for they have found the mud-streams (<i>lave -di fango</i>), formed by rain-water sweeping along the loose -volcanic materials, to be more widely destructive in their -effects than the currents of molten rock (<i>lave di fuoco</i>).</p> - -<p>Besides the phenomena which we have now described -as accompanying a great volcanic outburst, -many others have undoubtedly been recorded by apparently -trustworthy authorities. But, in dealing with -the descriptions of these grand and terrible events, we -must always be on our guard against accepting as -literal facts, the statements made by witnesses, often -writing at some distance from the scene of action, and -almost always under the influence of violent excitement -and terror. The desire to administer to the universal -love of the marvellous, and the tendency to exaggeration, -will usually account for many of the wonderful -statements contained in such records; and, even where -the witness is accurately relating events which he thinks -passed before his eyes, we must remember that it is -<span class="pagenum" id="Page_31">- 31 -</span> -probable he may have had neither the opportunity nor -the capacity for exact observation.</p> - -<p>The more carefully we sift the accounts which have -been preserved of great volcanic outbursts, the more -are we struck by the fact that the appearances described -can be resolved into a few simple operations, the true -character of which has been distorted or disguised by -the want of accurate observation on the part of the -witnesses.</p> - -<div class="sidenote">SIMILARITY OF FEEBLE AND VIOLENT ERUPTIONS.</div> - -<p>We are thus led to the conclusion that the grand -and terrible appearances displayed at Vesuvius and -other volcanoes in a state of violent eruption do not -differ in any essential respect from the phenomena -which we have witnessed accompanying the miniature -outbursts of Stromboli. And we are convinced, by -the same considerations, that the forces which give -rise to the feeble displays in the latter case would produce, -if acting with greater intensity and violence, all -the magnificent spectacles presented in the former.</p> - -<p>In Vesuvius and Stromboli alike, the active cause -of all the phenomena exhibited is found to be the -escape of steam from the midst of masses of incandescent -liquefied rock. The violence, and therefore the -grandeur and destructive effects of an eruption, depend -upon the abundance and tension of this escaping steam.</p> - -<p>There is one respect in which volcanic phenomena -are especially calculated to excite the fear and wonder -of beholders—namely, in the sudden and apparently -spontaneous character of their manifestations. Eclipses -<span class="pagenum" id="Page_32">- 32 -</span> -were regarded as equally portentous with volcanic eruptions -till astronomers learned not only to explain the -causes which gave rise to them, but even to predict to -the second the times of their occurrence. If we were -able in like manner to warn the inhabitants of volcanic -regions of the approach of a grand eruption, the fear -and superstition with which these events are now regarded -would doubtless be in great part dispelled. -The power of prediction is alike the crucial test and -the crowning triumph of a scientific theory.</p> - -<p>But, although natural philosophers are able to assign -the causes to which the grand operations of volcanoes -are due, and also to explain all the varied appearances -which accompany them, they have not as yet so far -mastered the laws which govern volcanic action as to -be able to predict the periods of their manifestation.</p> - -<p>That these operations, like all others going on upon -the globe, are governed by great natural laws we cannot -for a moment doubt. And that, in all probability, -more careful and exact observation and reasoning will -at some future time lead us to the recognition of -these laws, every student of nature is sanguine. But -at the present time, it must be confessed, we are very -far indeed from being able to afford that crowning -proof of the truth of our theories of volcanic action -which is implied in the power of predicting the period -and degree of intensity of their manifestations.</p> - -<p><span class="pagenum" id="Page_33">- 33 -</span></p> - -<div class="sidenote">ERUPTIONS AND THE INTERVALS BETWEEN THEM.</div> - -<p>There are, however, some observations which lead -us to hope that the time may not be far distant when -we shall have so £Eur obtained a knowledge of the conditions -on which volcanic action depends as to be able -to form some judgment as to its manifestations in the -future at any particular locality. But we must recollect -that these conditions axe very numerous and complicated, -and that some of them may lie almost entirely -outside our sphere of observation; hence hasty attempts -in this direction, such as have recently been made, are -to be deprecated by every true lover of science.</p> - -<p>Concerning the eruptions that have taken place at -those volcanic centres which have been known from a -remote antiquity, we have records from which we can -determine the intervals separating these outbursts and -their relative violence. A critical examination of these -records leads to the following conclusions:—</p> - -<p>(1.) A long period of quiescence is generally followed -by an eruption which is either of long duration -or of great violence.</p> - -<p>(2.) A long-continued, or very violent eruption is -usually followed by a prolonged period of repose.</p> - -<p>(3.) Feeble and short eruptions usually succeed -one another at brief intervals.</p> - -<p>(4.) As a general rule, the violence of a great eruption -is inversely proportional to its duration.</p> - -<p>It will be seen that these general conclusions are -in perfect harmony with the theory that volcanic outbursts -are due to the accumulation of steam at volcanic -centres, and that the tension of this imprisoned -gas eventually overcomes the repressing forces which -<span class="pagenum" id="Page_34">- 34 -</span> -tend to prevent its manifestation. Before astronomers -had learnt to determine all the conditions on which -the production of eclipses depends, they had found -that these phenomena succeed one another at regular -intervals. The discovery of such astronomical cycles -was a great advance in our knowledge of the heavenly -bodies, and in the same way the determination of -these general relations between the intensity and -duration of volcanic outbursts and the intervals of -time which separate them may be regarded as the first -step towards the discovery of the laws which govern -volcanic activity.</p> - -<p>In the actual determination of the conditions -upon which the occurrence of volcanic eruptions -depends, it must be confessed, however, that very little -has as yet been done. This is in part due to the fact -that some at least of these conditions lie beyond the -limits of direct observation. But it must also be admitted, -on the other hand, that little has been as yet -accomplished towards the careful and systematic observation -of those phenomena which may, and probably -do, exert an influence in bringing about volcanic outbursts.</p> - -<div class="sidenote">INFLUENCE OF ATMOSPHERIC CONDITIONS.</div> - -<p>In the Lipari Islands there has prevailed a belief, -from the very earliest period of history, that the feeble -eruptions of Stromboli are in some way dependent -upon the condition of the atmosphere. These islands -were known to the ancients as the Æolian Isles, from -the fact that they were once ruled over by a king of -<span class="pagenum" id="Page_35">- 35 -</span> -the name of Æolus. It seems not improbable that -Æolus was gifted with natural powers of observation and -reasoning far in advance of those of his contemporaries. -A careful study of the vapour-cloud which covers Stromboli -would certainly afford him information concerning -the hygrometric condition of the atmosphere; the form -and position assumed by this vapour-cloud would be a -no less perfect index of the direction and force of the -wind; and, if the popular belief be well founded, the -frequence and violence of the explosions taking place -from the crater would indicate the barometric pressure. -From these data an acute observer would be able -to issue 'storm-warnings' and weather-prognostics of -considerable value. In the vulgar mind, the idea of the -prediction of natural events is closely bound up with -that of their production; and the shrewd weather-prophet -of Lipari was after his death raised to the -rank of a god, and invested with the sovereignty of the -winds.</p> - -<p>Whether the popular idea that the outbursts of -Stromboli are regulated by atmospheric conditions -has any foundation is still open to grave doubt. It -seems to be certain, however, that during autumn and -winter the more violent paroxysms of the volcano -occur, and that in summer the action which takes -place is far more regular and equable. It would be of -the greatest benefit to science if an observatory were -erected beside the crater of Stromboli, where a careful -record might be kept of all atmospheric changes, -<span class="pagenum" id="Page_36">- 36 -</span> -and of the synchronous manifestations of the volcanic -forces.</p> - -<p>A little consideration will show that it is a by no -means unreasonable supposition that variations in atmospheric -pressure may exercise a very important influence -in bringing about volcanic outbursts. Changes -in the barometer to the extent of two inches within a -very short period are not uncommon occurrences. A -very simple calculation will show that the fall of the -mercury in the barometer to the extent of two inches -indicates the removal of a weight of two millions of -tons from each square mile of the earth's surface where -this change takes place. Now, if we suppose, as we -have good ground for doing, that under volcanic areas -vast quantities of superheated water are only prevented -from flashing into steam by the superincumbent pressure, -a relief of this pressure to the extent of two -millions of tons on every square mile could scarcely -fail to produce very marked effects. The way in which -explosions in fiery coal-mines generally follow closely -upon sudden falls in the atmospheric pressure is now -well known; and coal-mine explosions and volcanic -outbursts have this in common, that both result from -the sudden and violent liberation of subterranean -gases. There are not a few apparently well-authenticated -accounts of volcanic and earthquake phenomena -following closely on peculiar atmospheric conditions, -and the whole question of the relation of the volcanic -forces to atmospheric pressure, as Spallanzani himself -<span class="pagenum" id="Page_37">- 37 -</span> -so long ago pointed out, is deserving of a most careful -and rigorous investigation.</p> - -<div class="sidenote">SUPPOSED TIDAL EFFECTS.</div> - -<p>There is one other consideration which has frequently -been urged as worthy of especial attention, in -dealing with the question of the exciting causes of -volcanic outbursts. If volcanoes were, as was at one -time almost universally supposed, in direct communication -with a great central mass of liquefied materials, -or even if any large reservoirs of such liquids existed -beneath volcanic districts, as others have imagined, -then the different mobility of the solid and liquid portions -of the earth's mass would give rise to tidal effects -similar to those occurring in the surface waters of the -globe. Under such circumstances, volcanic outbursts, -like the tides, would be determined by the relative -positions of the sun and moon to our globe. It is certain, -however, that no very direct relation has yet been -established between the lunar periods and those of -volcanic outbursts, though recent close observations -upon the crater of Vesuvius, by Professor Palmieri, do -seem to lend support to the view that such relations -may exist.</p> - -<p>At the present time, therefore, it must be admitted -that vulcanologists have only just commenced those -series of exact and continuous observations which are -necessary to determine the conditions that regulate the -appearance of volcanic phenomena. The study of the -laws of volcanic action is yet in its infancy. But the -establishment of observatories on Vesuvius and Etna -<span class="pagenum" id="Page_38">- 38 -</span> -18 fall of promise for the future, and when we consider -the advances which have been made, during the -last one hundred years, in our knowledge of the true -nature of volcanic action, we need not despair that the -extension of the same methods of inquiry will lead to -equally important results concerning the conditions -which determine and the laws which govern it.</p> - -<p>In the meanwhile, it is no small gain to have established -the fact that volcanic phenomena, divested of -all those wonderful attributes with which superstition -and the love of the marvellous have surrounded them, -are operations of nature obeying definite laws, which -laws we may hope by careful observation and accurate -reasoning to determine; and that the varied appearances, -presented alike in the grandest and feeblest -outbursts, can all be referred to one simple cause—namely, -the escape, from the midst of masses of molten -materials, of imprisoned steam or water-gas.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_39">- 39 -</span></p> - -<h2 class="nobreak" id="CHAPTER_III">CHAPTER III.<br /> - -<span class="smaller">THE PRODUCTS OF VOLCANIC ACTION.</span></h2> -</div> - - -<p class="p0"><span class="smcap">While</span> Spallanzani was engaged in investigating the -nature of the action going on at Stromboli and other -Italian volcanoes, his contemporary Dolomieu was laying -the foundation of another important branch of vulcanology -by studying the characters of the different materials -of which volcanoes are built up. Since the publication -of Dolomieu's admirable works on the rocks of -the Lipari and Ponza Islands, science has advanced with -prodigious strides. The chemist has taught us how -to split up a rock into its constituent elements and -to determine the proportions of these to one another -with mathematical precision; the mineralogist has -done much in the investigation of the characters and -mode of origin of the crystalline minerals which occur -in these rocks; and the microscopist has shown how the -minute internal structure of these rocks may be made -clearly manifest. We shall proceed to give a sketch of -the present state of knowledge obtained by these different -kinds of investigations, concerning the materials -which are ejected from volcanic vents.</p> - -<p><span class="pagenum" id="Page_40">- 40 -</span></p> - -<p>The most abundant of the substances which are -ejected from volcanoes is steam or water-gas, which, as -we have seen, issues in prodigious quantities during -every eruption. But with the steam a great number of -other volatile materials frequently make their appearance. -The chief among these are the add gases known -as hydrochloric acid, sulphurous acid, sulphuretted -hydrogen, carbonic add, and boracic acid; and with -these acid gases there issue hydrogen, nitrogen, ammonia, -the volatile metals arsenic, antimony, and mercury, -and some other substances. In considering the -nature of the products which issue from volcanic fissures, -it must be remembered that many substances which -under ordinary circumstances do not exhibit marked -volatility are nevertheless easily carried away in fine -particles when a current of steam is passed over them. -As we shall have to point out in the sequel, different -volatile substances have a tendency to make their appearance -at volcanic vents according as the intensity -of the action going on within it varies.</p> - -<p>The volatile substances issuing from volcanic fissures -at high temperatures react upon one another, -and many new compounds are thus formed. We have -already seen how, by the action of sulphurous acid and -sulphuretted hydrogen on each other, the sulphur so -common in volcanic districts has been separated and -deposited. The hydrochloric acid acts very energetically -on the rocks around the vents, uniting with the -iron in them to form the yellow ferric-chloride. The -<span class="pagenum" id="Page_41">- 41 -</span> -rocks all round a volcanic vent are not unfrequently -found coated with this yellow substance, which is almost -always mistaken by casual observers for sulphur. In -many volcanoes the constant passage through the rocks -of the various acid gases has caused nearly the whole of -the iron, lime, and alkaline materials of the rocks to be -converted into soluble compounds known as sulphates, -chlorides, carbonates, and borates; and, on the removal -of these by the rain, there remains a white, powdery -substance, resembling chalk in outward appearance, but -composed of almost pure silica. There are certain -cases in which travellers have visited volcanic islands -where chemical action of this kind has gone on to such -an extent, that they have been led to describe the -islands as composed entirely of chalk.</p> - -<div class="sidenote">GASES EMITTED FROM VOLCANOES.</div> - -<p>Some of the substances issuing from volcanic vents, -such as hydrogen and sulphuretted hydrogen, are inflammable, -and when they issue at a high temperature, -these gases burst into flame the moment that they -come into contact with the air. Hence, when volcanic -fissures axe watched at night, faint lambent flames are -frequently seen playing over them, and sometimes these -flames are brilliantly coloured, through the presence of -small quantities of certain metallic oxides. Such volcanic -flames, however, are scarcely ever strongly luminous -and, as we have already seen, the red, glowing light -which is observed over volcanic mountains in eruption -is due to quite another cause. The study by the aid of -the spectroscope of the flames which issue from volcanic -<span class="pagenum" id="Page_42">- 42 -</span> -vents promises to throw much new light on the rarer -materials ejected by volcanoes. Spectroscopic observations -of this kind have already been commenced by -Janssen, at Stromboli and Santorin.</p> - -<p>Some of the volatile substances issuing from volcanic -vents, are at once deposited when they come in contact -with the cool atmosphere, others form new compounds -with one another and the constituents of the atmosphere, -while others again attack the materials of the -surrounding rocks and form fresh chemical compounds -with some of their ingredients. Thus, there are continually -accumulating on the sides and lips of volcanic -fissures deposits of sulphates, chlorides, and borates of -the alkalies and alkaline earths, with sal-ammoniac, sulphur, -and the oxides and sulphides of certain metals. -The lips of the fissures from which steam and acid -gases issue in volcanoes are constantly seen to be coated -with yellow and reddish-brown incrustations, consisting -of mixtures, in varying proportions, of these different -materials, and these sometimes assume the form of -stalactites and pendent masses.</p> - -<div class="sidenote">DEPOSITS AROUND VOLCANIC VENTS.</div> - -<p>Some of these products of volcanic action are of -considerable commercial value. At Vulcano regular -chemical works have been established in the crater of -the volcano, by an enterprising Scotch firm, a great -number of workmen being engaged in collecting the -materials which are deposited around the fissures, and -are renewed by the volcanic action almost as soon as -they are removed. In <a href="#fig06">fig. 6</a>, I have given a sketch of -<span class="pagenum" id="Page_43">- 43 -</span> -this singular spot, taken from the high ground of the -neighbouring Island of Lipari. From the village at -the foot of the volcano, where the workmen live, a zig-zag -road has been constructed leading up the side, and -down into the crater of the volcano. On this road, -workmen and mules, laden with the various volcanic -materials, may be seen constantly passing up and down.</p> - -<div class="figcenter" id="fig06" style="width: 427px;"> - <img src="images/fig06.png" width="427" height="250" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 6.—View of Vulcano, with Vulcanello in the foreground - taken from the south end of the Island of Lipari.</span></div> -</div> - -<p>Vulcano appears to have been frequently in a state of -violent eruption during the past 2,000 years—the last -great outburst having taken place in 1786. In 1873 the -activity in the crater of Vulcano suddenly became more -pronounced in character, and the workmen hastened -to escape from the dangerous spot, but, before they -could do so, several of them were severely injured by -the explosions. After this outburst, which did not prove -<span class="pagenum" id="Page_44">- 44 -</span> -to be of very violent character, the quantity of gases -issuing from the fissures in the crater was for a time -much greater than before, and the productiveness of -these great natural chemical works was proportionately -increased: but eventually the action died out almost -entirely. The chief products of Vulcano which are of -commercial value, are sal-ammoniac, sulphur, and boracic -acid. At one time it was even contemplated that great -leaden chambers should be erected over the principal -fissures at the bottom of the crater of Vulcano, in which -chambers the volatile materials might be condensed -and collected. The change in the condition of the -volcano has unfortunately prevented the carrying out -of this bold project.</p> - -<p>Besides the volatile substances which issue from -volcanic vents, mingling with the atmosphere or condensing -upon their sides, there are also many solid -materials ejected, and these may accumulate around -the orifices, till they build up mountains of vast dimensions, -like Etna, Teneriffe, and Chimborazo. Some of -these solid materials are evidently fragments of the -rock-masses, through which the volcanic fissure has -been rent; these fragments have been carried upwards -by the force of the steam-blast and scattered over -the sides of the volcano. But the principal portion of -the solid materials ejected from volcanic orifices consists -of matter which has been extruded from sources -far beneath the surface, in a highly-heated and fluid or -semi-fluid condition.</p> - -<p><span class="pagenum" id="Page_45">- 45 -</span></p> - -<div class="sidenote">EJECTED ROCK-FRAGMENTS.</div> - -<p>The fragments torn from the sides of volcanic -fissures consist of the rocks through which the eruptive -forces may happen to have opened their way; pieces -of sandstone, limestone, slate, granite, &c., are thus -frequently found in considerable numbers among -materials which build up volcanic mountains. Thus, -some of the volcanic cones in the Eifel are very largely -made up of fragments of slate, which have been torn -from the sides of the vents by the uprushing currents -of steam. At Vesuvius masses of limestone are frequently -ejected, and may be picked up all over the -slopes of the mountains. These limestone-fragments -frequently contain fossils, and Professor Guiscardi, of -Naples, has been able to collect several hundred species -of shells, transported thus by volcanic action from the -rock-masses which form the foundation of the volcano -of Vesuvius. The action of water at a high temperature, -and under such enormous pressure as must -exist beneath volcanic mountains, has often produced -changes in the rocks of which fragments are ejected -from volcanic vents. The so-called 'lava' ornaments, -which are so extensively sold at Naples, are not made -from the materials to which geologists apply that -name, but from the fragments of altered limestone -that have been torn from the rocks beneath the -mountain, and scattered by the eruptive forces all over -its sides. The chemical action of the superheated -and highly-compressed steam on the rocks beneath -volcanoes frequently results in the formation of beautifully -<span class="pagenum" id="Page_46">- 46 -</span> -crystallised minerals. Such crystallised minerals -abound in the rock-fragments scattered over the sides -of Vesuvius and other volcanoes, both active and extinct. -They have been formed in the great chemical -laboratories which exist beneath the volcano, and have -been brought to the surface by the action of the steam-jets -issuing from its fissures.</p> - -<p>Of still greater interest are those materials which -issue from volcanic orifices in an incandescent, and -often in a molten, condition, and which are evidently -derived from sources far below the earth's surface. It -is to these materials that the name of 'lavas' is -properly applied.</p> - -<p>Lavas present a general resemblance to the slags -and clinkers which are formed in our furnaces and -brick-kilns, and consist, like them, of various stony -substances which have been more or less perfectly -fused. When we come to study the chemical composition -and the microscopical structure of lavas, however, -we shall find that there are many respects in which -they differ entirely from these artificial products.</p> - -<p>Let us first consider the facts which are taught us -concerning the nature and origin of lavas, by a chemical -analysis of them.</p> - -<div class="sidenote">CHEMICAL COMPOSITION OF LAVAS.</div> - -<p>Of the sixty-five or seventy chemical elements, only a -very small number occur at all commonly in lavas. Eight -elements, indeed, make up the great mass of all lavas—these -are oxygen, silicon, aluminium, magnesium, -calcium, iron, sodium, and potassium. But even these -<span class="pagenum" id="Page_47">- 47 -</span> -eight elements are present in very unequal proportions. -Oxygen makes up nearly one-half the weight of -all lavas. Almost all the other elements found in lavas -exist in combination with oxygen, so that lavas consist -entirely of what chemists call 'oxides.' This is a most -remarkable circumstance, which, as we shall presently -see, is of great significance. The metalloid silicon -makes up about one-fourth of the weight of most lavas, -and the metal aluminium about one-tenth. The other -five elements vary greatly in their relative proportions -in different lavas.</p> - -<p>In all lavas the substance which forms the greatest -part of the mass is the compound of oxygen and silicon, -known as silica or silicic acid. In its pure form, this -substance is familiar to us as quartz, or rock-crystal -and flint. Silica is present in all lavas in proportions -which vary from one-half to four-fifths of the whole -mass. Now, this substance, silica, has the property of -forming more complex compounds by uniting with the -other oxides present in lavas—namely, the oxides of -aluminium, magnesium, calcium, iron, potassium, and -sodium. Silica is called by chemists an <i>acid</i>, the other -oxides in lavas are termed <i>bases</i>, and the compounds -of silica with the bases are known as <i>silicates</i>. Hence -we see that lavas are composed of a number of different -silicates—the silicates of aluminium, magnesium, calcium, -iron, potassium, and sodium.</p> - -<p>The above statements will perhaps be made clearer -by the accompanying table from which it will be seen -<span class="pagenum" id="Page_48">- 48 -</span> -that lavas are compounds in varying proportions of six -kinds of salts—namely, the silicates of alumina, magnesia, -lime, iron, potash, and soda.</p> - -<p class="caption3nb"><span class="smcap">Composition of Lavas.</span></p> - -<table summary="lava"> -<tr> - <td class="tdc" colspan="2"><span class="smcap">Elements</span></td> - <td class="tdc" colspan="2"><span class="smcap">Binary Compounds</span></td> - <td class="tdc"><span class="smcap">Salts</span></td> -</tr> -<tr> - <td class="tdl" colspan="2">Oxygen</td> - <td class="tdc">Acid</td> - <td class="tdc">Bases</td> - <td></td> -</tr> -<tr> - <td class="tdl" rowspan="7"><img src="images/bracel_132.png" width="11" height="132" alt="" /></td> - <td class="tdl">Silicon</td> - <td>Silica—</td> - <td>┐</td> - <td></td> -</tr> -<tr> - <td class="tdl">Aluminum </td> - <td></td> - <td>┠—Alumina</td> - <td> " " Alumina</td> -</tr> -<tr> - <td class="tdl">Magnesium</td> - <td></td> - <td>┠—Magnesia</td> - <td> " " Magnesia</td> -</tr> -<tr> - <td class="tdl">Calcium</td> - <td></td> - <td>┠—Lime</td> - <td> " " Lime</td> -</tr> -<tr> - <td class="tdl">Iron</td> - <td></td> - <td>┠—Iron</td> - <td> " " Iron</td> -</tr> -<tr> - <td class="tdl">Potassium</td> - <td></td> - <td>┠—Potash</td> - <td> " " Potash</td> -</tr> -<tr> - <td class="tdl">Sodium</td> - <td></td> - <td>┠—Soda</td> - <td> " " Soda</td> -</tr> -</table> - - - -<p>Now, in some lavas the acid constituent, or silica, is -present in much larger proportions than in others. -Those lavas with a large proportion of silica are called -'acid lavas,' those with a lower percentage of silica, -and therefore a higher proportion of the bases, are -known as the 'basic lavas.' It is convenient to employ -the term 'intermediate lavas' for those in which the -proportion of silica is lower than in the acid lavas, and -the proportion of the bases is lower than in the basic -lavas.</p> - -<p>The acid lavas contain from 66 to 80 per cent, of -silica; they are poor in lime, magnesia, and oxide of -iron, but rich in potash and soda. The basic lavas -contain from 45 to 55 per cent, of silica; they are rich -in magnesia, lime, and oxide of iron, but poor in soda -<span class="pagenum" id="Page_49">- 49 -</span> -and potash. In the intermediate lavas the proportion -of silica varies from 55 to 66 per cent.</p> - -<p>As the basic-lavas contain a larger proportion of -oxide of iron and other heavy oxides than the acid-lavas, -the former have usually a higher specific -gravity than the latter; it is, indeed, possible in most -cases to distinguish between these different varieties -by simply weighing them in water and in air.</p> - -<div class="sidenote">DIFFERENT KINDS OF LAVA.</div> - -<p>The basic lavas are usually of much darker colour -than the add lavas—the terms acid lavas, intermediate -lavas, and basic lavas correspond indeed pretty closely -with the names trachytes, greystones and basalt, which -were given to the varieties of lavas by the older writers -on volcanoes, at a time when their chemical constitution -had not been accurately studied. Fresh lavas of acid -composition are usually nearly white in colour, intermediate -lavas are of various tints of grey, and basic -lavas nearly black. It must be remembered, however, -that colour is one of the least persistent, and therefore -one of the least valuable, characters by means of which -rocks can be discriminated, and also that by exposure -to the influence of the atmospheric moisture the iron -present in all lavas is affected, and the lavas belonging -to all classes, when weathered, assume reddish and -reddish-brown tints.</p> - -<p>Geologists have devised a great number of names -for the various kinds of lava which have been found -occurring round volcanic vents in different parts of the -world, and the study of these varieties is full of interest. -<span class="pagenum" id="Page_50">- 50 -</span> -For our present purpose, however, it will be sufficient -to state that they nearly all fall into five great groups, -known as the Rhyolites, the Trachytes, the Andesites, -the Phonolites, and the Basalts. The Rhyolites are -acid lavas, the Basalts are basic lavas, and the Trachytes, -Andesites, and Phonolites, different kinds of -intermediate lavas, distinguished by the particular -minerals which they contain.</p> - -<p>Before we part from this subject of the classification -of lavas according to their chemical composition, it will -be well to point out that there exists a small group of -lavas which stand quite by themselves, and cannot be -referred to either of the classes we have indicated. -They contain a smaller proportion of silica, and a much -larger proportion of magnesia and oxide of iron than -the other lavas, and may be made to constitute a small -sub-group, to which we may apply the term of 'ultra-basic -lavas.' Although much less widely distributed -than the other varieties, they are, in some respects, -as we shall presently have to point out, of far greater -interest to the geologist than all the other kinds of -lavas.</p> - -<div class="sidenote">MINUTE STRUCTURE OF LAVAS.</div> - -<p>We will now proceed to consider the facts which -are brought to light concerning the nature of lavas, -when they are studied by the aid of the microscope. -Although most lavas appear at first sight to be opaque -substances, yet it is easy to prepare slices of them -which are sufficiently thin to transmit light. In such -thin transparent slices we are able to make out, by the -<span class="pagenum" id="Page_51">- 51 -</span> -aid of the microscope, certain very interesting details -of structure, which afford new and important evidence -bearing on the mode of origin of these rocks.</p> - -<p>Host lavas are capable of being melted by the heat -of our furnaces; but the different kinds of lava vary -greatly in the degree of their fusibility. The basic -lavas, or those with the smallest proportion of silica, -are usually much more easily fusible than those which -contain a high percentage of silica, the add lavas.</p> - -<p>Now, it is a very noteworthy circumstance, that when -a lava is artificially fused it assumes on cooling very -different physical characters to those which were presented -by the original rock.</p> - -<p>If we examine the freshly-broken surface of a piece -of lava, we shall, in most cases, find that it contains a -great number of those regular-shaped bodies which we -call crystals; in some cases these crystals are so small -as to be scarcely visible to the naked eye, in others -they may be an inch or more in length. Most lavas -are thus seen to be largely made up of crystals of -different minerals. The minerals which are usually -contained in lavas are quartz, the various kinds of -felspar, augite, hornblende, the different kinds of mica, -olivine, and magnetite.</p> - -<p>But when a piece of lava is melted in a furnace, all -these crystalline minerals disappear, and the resulting -product is the homogeneous substance which we call -glass. If, as many suppose, lavas acquire the fluidity -which they possess when issuing from volcanic vents -<span class="pagenum" id="Page_52">- 52 -</span> -as the result of simple fusion it is strange that artificially -fused lavas do not agree more closely in character -with the natural products.</p> - -<p>A careful examination of different kinds of lavas, -however, will show that they vary very greatly in -character among themselves. Some lavas are as perfectly -glassy in structure as those which have been -artificially fused, while others contain great numbers -of crystals, which may sometimes be of very large size.</p> - -<p>If we prepare thin transparent slices of these different -kinds of lavas, and examine them by the aid -of the microscope, we shall find that lavas are made -up of two kinds of materials, a base or groundmass of -a glassy character, and distinct crystals of different -minerals, which are irregularly distributed through this -glassy base, like the raisins, currants, and pieces of -candied peel in a cake. In some cases the glassy base -makes up the whole mass of the rock; in others, -smaller or larger numbers of crystals are seen to be -scattered through a glassy base; while in others again -the crystals are so numerous that the presence of an -intervening glassy base or groundmass can only be -detected by the aid of the microscope.</p> - -<div class="sidenote">STUDY OF LAVAS WITH THE MICROSCOPE.</div> - -<p>If thin slices of the glassy materials of lavas be -examined with high magnifying powers, new and interesting -facts are revealed. Through the midst of the -clear glassy substance cloudy patches are seen to be -diffused; and, if we examine them with a still higher -power, these cloudy patches resolve themselves into -<span class="pagenum" id="Page_53">- 53 -</span> -innumerable particles, some transparent and others -opaque, having very definite outlines. At the same -time fresh cloudy patches are brought into view, which -can only be resolved by yet higher powers of the -microscope. In examining these natural glasses by -the aid of the microscope, we are forcibly reminded of -what occurs when the 'Milky Way' and some other -parts of the heavens are studied with a telescope. As -the power of the instrument is increased the nebulous -patches are resolved into distinct stars, but fresh nebulous -masses come into view, which are in turn resolved -into stars, when higher powers of the instrument are -employed.</p> - -<p>In the Frontispiece, No. 1 illustrates the appearance -presented by these volcanic glasses when examined -with a high power of a microscope. Through a glassy -base is seen a number of diffused nebulous patches, -which are in places resolved into definite particles.</p> - -<p>These minute particles of definite form, which the -microscope has revealed in the midst of the glassy -portions of lava, have received the name of microliths, -or crystallites. The study of the characters and mode -of arrangement of these microliths or crystallites has -in recent years thrown much new light on the interesting -problems presented by lavas.</p> - -<p>In some glassy lavas the microliths or crystallites, -instead of being indiscriminately diffused through the -mass of the base or groundmass, are found to be collected -together into groups of very definite form. In -<span class="pagenum" id="Page_54">- 54 -</span> -No. 2 of the Frontispiece we have a section of a glassy -rock in which the crystallites have united together, so -as to build up groups presenting the most striking -resemblance to fronds of ferns. Around these groups -spaces of dear glass have been left by the gathering -up of the crystallites, which in other parts of the mass -are seen to be equally diffused through it. In this -formation of groups of microliths we cannot but recognise -the action of those crystalline forces, which on -frosty mornings cover our windows with a mimic vegetation -composed of icy particles.</p> - -<p>In other cases, again, the crystallites scattered -through the glassy portions of lavas unite in radial -groups about certain centres, and thus build up globular -masses to which the name of 'sphærulites' has been -given. No. 3 in the Frontispiece illustrates the formation -of these sphærulites.</p> - -<p>Now, a careful study of the microliths or crystallites -has proved that they are the minute elements of -which those wonderfully beautiful objects which we -call crystals are built up. In some cases we can see -that the crystallites are becoming united together in -positions determined by mathematical laws, and the -group is gradually assuming the outward form and internal -structure of a crystal. In other cases crystals -may be found which are undergoing a disintegrating -action, and are then seen to be made up of minute -elements similar to the crystallites or microliths of -glassy rocks.</p> - -<p><span class="pagenum" id="Page_55">- 55 -</span></p> - -<div class="sidenote">CRYSTALLITES AND CRYSTALS.</div> - -<p>The conclusion is confirmed by the fact that if we -take an artificially fused lava and allow it to cool slowly, -it will be found that the glassy mass into which it has -resolved itself contains numerous crystallites. If the -cooling process be still further prolonged, these crystallites -will be found to have united themselves into -definite groups, and sometimes distinct crystals are -formed in the mass; under these circumstances the -rock frequently loses its glassy appearance and assumes -a stony character.</p> - -<p>In connection with this subject, it may be mentioned -that some years ago a very ingenious invention -was submitted to trial in the Works of the Messrs. -Chance, of Birmingham. It had been suggested that -if certain lavas of easy fusibility were melted and -poured into moulds, we might thus obtain elaborately -ornamented stone-work, composed of the hardest -material, without the labour of the mason. The molten -rock when quickly cooled was found to assume the -form of a black glass, but when very slowly cooled -passed into a stony material. Unfortunately, it was -found that this material did not withstand the weather -like ordinary building stones, and, in consequence, the -manufacture had to be abandoned.</p> - -<p>Now, the study of the products of volcanoes has led -geologists to recognise the true relations between -glassy and crystalline rocks.</p> - -<p>In the amorphous mixture of various silicates which -compose a glass, chemical affinity causes the separation -<span class="pagenum" id="Page_56">- 56 -</span> -of certain portions of definite composition, and these -form the microliths or elements of which different -crystalline minerals are built up. Under the influence -of the crystalline forces, there is a great shaking or -agitation in the mass, and the microliths of similar kind -come together and become united, like the fragments -in Ezekiel's valley of dry bones.</p> - -<p>Although we cannot see this process taking place -under our eyes, in a mass of lava, yet we may study -specimens in which the action has been arrested in -its different stages. In order to understand the -development of an acorn into an oak-tree, it is not -necessary to watch the whole series of changes in a -particular case. A visit to an oak-thicket, in which -illustrations of every stage of the transformation may -be found, will afford us equally certain information on -the subject.</p> - -<p>In the same way by the examination of such a -series of rock-sections as that represented in the Frontispiece, -we may understand how, in the midst of a -mass of mixed silicates constituting a natural glass, -the separation of microliths takes place; these unite -into groups which are the skeletons of crystals, and -finally, by the filling up of the empty spaces in these -skeletons, complete crystals are built up. The series -of operations may, however, be interrupted at any -stage, and this stage we may have the chance of -studying.</p> - -<p><span class="pagenum" id="Page_57">- 57 -</span></p> - -<div class="sidenote">GLASSY AND CRYSTALLINE LAVAS.</div> - -<p>We are able, as we shall show in a future chapter, -to examine many rock-masses that have evidently -formed the reservoirs from which volcanoes have been -supplied, and others that fill up the ducts which constituted -the means of communication between these -subterranean reservoirs, and the surface of the earth. -Now in these subterranean regions the lavas have been -placed under conditions especially favourable for the -action of the crystalline forces—they must have cooled -with extreme slowness, and they must have been under -an enormous pressure, produced in part by the weight -of the superincumbent rocks, and in part by the expansive -force of the imprisoned steam. We are not, -therefore, surprised to find that in these subterranean -regions, the lavas, while retaining the same chemical -composition, have assumed a much more perfectly -crystalline condition. In some cases, indeed, the whole -rock has become a mass of crystals without any base -or groundmass at all.</p> - -<p>An examination of the Frontispiece will illustrate -this perfect gradation from the glassy to the crystalline -condition of lavas. No. 1 represents a glass through -which microliths or crystallites of different dimensions -and character are diffused. In Nos. 2 and 3, these crystallites -have united to form regular groups. In No. 4, -which may be taken as typical of the features presented -by most lavas, we have a glassy groundmass containing -microliths (a 'crypto-crystalline base'), through which -distinct crystals are distributed. Nos. 5 and 6 illustrate -the characters presented by lavas which have consolidated -<span class="pagenum" id="Page_58">- 58 -</span> -at considerable depths beneath the surface; in -the former we have a mans of small crystals (a 'micro-crystalline -base') with larger crystals scattered through -it; while the latter is entirely made up of large -crystals without any trace of a base or groundmass.</p> - -<p>Now, as all lavas are found sometimes assuming the -glassy condition at the surface, so when seen in the -masses which have consolidated with extreme slowness, -and under great pressure, in subterranean regions, the -same materials are found in the condition of a rock -which is built up entirely of crystals. Chemists have -found that artificial mixtures of silicates in which soda -and potash are present in considerable quantities, have -a great tendency to assume the glassy condition on cooling -from a state of fusion, and glass manufacturers are -always careful to use considerable proportions of the -alkalis as ingredients, in making glass. It is found, in -like manner, that those lavas which contain the largest -portion of the silicates of soda and potash (the 'acid -lavas') most frequently assume the condition of a -natural glass.</p> - -<p>Geologists have given distinct names to the glassy -and the perfectly crystalline conditions of the different -kinds of lavas, the glassy varieties being found in -masses which have cooled rapidly near the surface, and -the crystalline varieties in masses which have cooled -slowly at great depths. The names of these two conditions -of the five great classes into which we have -divided lavas are as follows:—</p> - -<p><span class="pagenum" id="Page_59">- 59 -</span></p> - -<div class="sidenote">HIGHLY CRYSTALLINE IGNEOUS ROCKS.</div> - -<table style="width: 25em;" summary="rocks"> -<tr> - <td class="tdc smaller"><i>Crystalline Forms.</i></td> - <td class="tdc smaller" colspan="2"><i>Lavas.</i></td> - <td class="tdc smaller"><i>Glassy Forms.</i></td> -</tr> -<tr> - <td class="tdl2">Granite</td> - <td class="tdl">Rhyolite</td> - <td class="tdl" rowspan="4"><img src="images/bracer_70.png" width="10" height="70" alt="" /></td> - <td class="tdl" rowspan="4">Obsidian.</td> -</tr> -<tr> - <td class="tdl2">Syenite</td> - <td class="tdl">Trachyte</td> -</tr> -<tr> - <td class="tdl2">Diorite</td> - <td class="tdl">Andesite</td> -</tr> -<tr> - <td class="tdl2">Miascite</td> - <td class="tdl">Phonolite</td> -</tr> -<tr> - <td class="tdl2">Gabbro</td> - <td class="tdl">Basalt</td> - <td></td> - <td class="tdl">Tachylyte.</td> -</tr> -</table> - -<p>As vitreous rocks have little in their general appearance -to distinguish them from one another, the glassy -forms of the first four classes of lava have not hitherto -received distinct names, but have been confounded -together under the name of obsidian. If we determine -the specific gravities of rocks having the same -composition but different structures, we shall find that -they become heavier in proportion as the crystalline -structure is developed in them. Thus gabbro is -heavier, but tachylyte is lighter than basalt, bulk for -bulk, though all have the same chemical composition.</p> - -<p>Nor are the crystals contained in lavas less worthy -of careful study, by the aid of the microscope, than -the more or less glassy groundmass in which they are -embedded. Mr. Sorby has shown that the crystals -found in lavas, exhibit many interesting points of -difference from those which separate out in the midst -of a mass of the same rock, when it has been artificially -melted and slowly cooled. There are other facts which -also point to the conclusion that, while the glassy -groundmass of lavas may have been formed by cooling -from a state of fusion, the larger and well-formed -crystals in these lavas must have been formed under -other and very different conditions.</p> - -<p><span class="pagenum" id="Page_60">- 60 -</span></p> - -<p>The larger crystals in lavas exhibit evidence of -having been slowly built up in the midst of a glassy -mass, containing crystallites and small crystals. We -can frequently detect evidence of the interruptions -which have occurred in the growth of these crystals in -the concentric zones of different colour or texture -which they exhibit; and portions of the glassy base or -groundmass are often found to have been caught up -and enclosed in these crystals during their growth.</p> - -<p>But when we find, as in the porphyritic pitchstones, -a glassy base containing only minute crystallites, -through which large and perfectly formed crystals are -distributed, we can scarcely doubt that the minute -crystallites and the larger crystals have separated from -the base under very different conditions. This is indicated -by the bet that we detect in these cases no connecting -links between the embryo microliths and the -perfect crystals; and a confirmation of the conclusion -is seen in the circumstance that many of the crystals -are found to have suffered injury as if from transport, -their edges and angles being rounded and abraded, and -portions being occasionally broken off from them.</p> - -<p>Hence we are led to conclude that the larger crystals -in lavas were probably separated from the amorphous -mass in the subterranean reservoirs beneath the volcano, -and were carried up to the surface in the midst of the -liquefied glassy material which forms the groundmass -of lavas. When we come to examine these crystals -more closely, we find that certain very curious phenomena -are exhibited by them which lend powerful -support to this conclusion.</p> - -<div class="figcenter" id="fig07" style="width: 439px;"> - <img src="images/fig07.png" width="439" height="649" alt="" /> - <div class="figcaption"><p><span class="smcap">Fig. 7.—Minute Cavities, containing Liquids, in the - Crystals of Rocks.</span></p></div> -</div> - -<p><span class="pagenum" id="Page_61">- 61 -</span></p> - -<div class="sidenote">LIQUID CAVITIES IN CRYSTALS.</div> - -<p>It is found convenient by geologists to designate -those rocks which have consolidated in deep-seated -portions of the earth's crust as Platonic Rocks, confining -the name of Volcanic rocks to those consolidating -At the surface; but Plutonic and Volcanic Rocks shade -into one another by the most insensible gradations.</p> - -<p>When the crystals embedded in granitic rocks, and -in some lavas, are examined with the higher powers of -the microscope, they are frequently seen to contain great -numbers of excessively minute cavities. Each of these -cavities resembles a small spirit-level, having a quantity -of liquid and a bubble of gas within it. In <a href="#fig07">fig. 7</a> we -have given a series of drawings of these cavities in -crystals as seen under a high power of the microscope. -In No. 1 a group of such cavities is represented, one of -which is full of liquid, while two others are quite empty; -the remaining cavities all contain a liquid with a -moving bubble of gas. In No. 2 two larger cavities are -shown, containing a liquid and a bubble of gas; and it -will be seen from these how varied in form these -cavities sometimes are. In Nos. 3, 4 and 6 the liquid -in the cavities contains, besides the bubbles, several, -minute crystals; and in No. 6 we have a cavity containing -two liquids and a bubble.</p> - -<p>In the largest of such cavities the bubble is seen to -change its place so as always to lie at the upper side of -the cavity, when the position of the latter is altered, just -<span class="pagenum" id="Page_62">- 62 -</span> -as in a spirit-level. But in the smallest cavities the -bubbles appear to be endowed with a power of spontaneous -movement; like imprisoned creatures trying to -escape, these bubbles are seen continually oscillating -from side to side and from end to end of the cavities -which enclose them. In <a href="#fig08">fig. 8</a> a minute cavity containing -a liquid and bubble is shown, the path pursued -by the latter in its wonderful gyrations being indicated -by the dark line. These cavities are exceedingly minute, -and so numerous that in some crystals there -must be millions of them present; indeed, in certain -cases, as we increase the magnifying power of our microscopes, -new and smaller cavities continually become -visible. It has been estimated that in some instances -the number of these minute liquid-cavities in the crystals -of rocks amounts to from one thousand millions -to ten thousand millions in a cubic inch of space.</p> - -<div class="figcenter" id="fig08" style="width: 418px;"> - <img src="images/fig08.png" width="418" height="161" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 8—Minute Liquid-cavity in a Crystal, with a moving - Bubble.</span> (The path of the bubble is indicated by the dark line.)</div> -</div> - -<p><span class="pagenum" id="Page_63">- 63 -</span></p> - -<div class="sidenote">NATURE OF LIQUIDS IN CAVITIES.</div> - -<p>What is the nature of the liquids which are thus -imprisoned in these cavities contained in the crystals of -lavas and granites? Careful experiments have given -a conclusive answer to this question. In many cases -the liquid is water, usually containing considerable -quantities of saline matter dissolved in it. Sometimes -the saline matters are present in such abundance -that they cannot all pass into solution, but -crystallise out, as in <a href="#fig07">fig. 7</a>—Nos. 3, 4, 5—where cubic -crystals of the chlorides of sodium and potassium are -seen floating in the liquid; in other cases the liquid is -a hydrocarbon like the mineral oil which is present in -great abundance in deep-seated rocks in many parts of -the globe. But in some other cases the liquid contained -in the cavities of crystals is found to be one -which could scarcely be anticipated to occur under -such circumstances—the gas known as carbonic add, -which under extreme pressure can be reduced to a -liquid condition. In cavities containing liquefied carbonic -acid, if the rock be warmed up to 86° or 90° Fahrenheit -the bubble suddenly vanishes, sometimes with -an appearance like ebullition or boiling, as represented -in <a href="#fig09">fig. 9</a>. Now the temperature which we have indicated -is the 'critical point' of carbonic acid, and -above that temperature it cannot exist in a liquid condition, -however great may be the pressure to which it -is subjected. The liquid has been converted into a -gas which completely fills the cavity. The carbonic -acid in the cavities of crystals has frequently been -isolated and its nature placed beyond doubt by spectroscopic -and ordinary chemical tests.</p> - -<p><span class="pagenum" id="Page_64">- 64 -</span></p> - -<p>The presence of these liquids in the cavities of -crystals clearly proves that the latter must have been -formed under enormous pressure—a pressure sufficiently -great to reduce, not only steam, but also volatile hydrocarbons -and even gaseous carbonic acid, to the bulk of -a liquid.</p> - -<div class="figcenter" id="fig09" style="width: 413px;"> - <img src="images/fig09.png" width="413" height="301" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 9.—Cavity in Crystal containing Carbonic-Acid Gas at a - temperature of 86° F., and passing from the liquid to the - gaseous condition.</span></div> -</div> - -<p>Such conditions of enormous pressure we may infer -to exist in the deep-seated reservoirs beneath volcanoes, -where, besides the weight of the superincumbent rock-masses, -we have the compressing force of great quantities -of elastic vapour held in confinement. The crystals of -which granitic rocks are entirely built up exhibit clear -<span class="pagenum" id="Page_65">- 65 -</span> -evidence of having been all formed under these conditions -of enormous pressure. The glassy base or -groundmass of lavas, on the other hand, presents all -the characters of materials that have cooled from a -state of fusion. Most lavas consist in part of crystals, -exhibiting fluid-cavities like those present in granite, -and in part of a base, which has evidently been formed -by the cooling of a fused mass. We are therefore -justified in concluding that the crystals have been -formed in subterranean recesses, and that the groundmass -or base has consolidated at the surface. The -bearing of these conclusions upon some of the great -problems presented by volcanoes we shall have occasion -to point out in the sequel.</p> - -<div class="sidenote">CAUSE OF MOVEMENT OF BUBBLES.</div> - -<p>One of the most interesting inquiries suggested by -the study of the liquid-cavities in volcanic rocks is -that of the cause of the apparently spontaneous movement -of the bubbles which we have described as taking -place in some of the smaller of them. The ingenious -experiments of Mr. Noel Hartley have suggested to -Professor Stokes an explanation which is probably the -true one. It appears that these minute globes of -vapour are in such a state of unstable equilibrium as -to be affected by the smallest changes of temperature, -and that the variations in the heat of the atmosphere, -due to currents of air and the movement of warm -or cold bodies through it, are sufficient to cause the -oscillation of these sensitively poised bubbles.</p> - -<p>The short account which we have been able to give -<span class="pagenum" id="Page_66">- 66 -</span> -in the foregoing pages of the researches that have been -carried on concerning the nature of the materials ejected -from volcanoes will serve to show that these investigations -have already made known many facts of great -interest, and that the farther pursuit of them is full of -the highest promise. To the scientific worker no subject -is too vast for his research, no object so minute as -to be unworthy of his most patient study. In some of -our future inquiries concerning the nature of volcanic -action, we shall be led to an investigation of the phenomena -displayed in the sun, moon, comets and other -great bodies of the universe; but another road to truths -of the same grandeur and importance is found, as we -have seen, in an examination of the mode of development -of crystallites, and a study of the materials -contained in the microscopic cavities of the minutest -crystals.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_67">- 67 -</span></p> - -<h2 class="nobreak" id="CHAPTER_IV">CHAPTER IV.<br /> - -<span class="smaller">THE DISTRIBUTION OF THE MATERIALS EJECTED FROM -VOLCANIC VENTS.</span></h2> -</div> - - -<p class="p0"><span class="smcap">The</span> escape of great quantities of steam and other gases -from the midst of a mass of fluid or semi-fluid lava -gives rise to the formation of vast quantities of froth -or foam upon its surface. This froth or foam, which -is formed upon the surface of lava by the escape of -gaseous matters from within it, is made up of portions -of the lava distended into vesicles, in the same way -that bubbles are formed on the surface of water. It -bears precisely the same relation to the liquid mass of -lava that the white crest of foam upon an advancing -wave does to the sea-water, from the bubbles of which -it is formed.</p> - -<p>This froth upon the surface of lavas varies greatly -in character according to the nature of the material -from which it is formed. In the majority of cases the -lavas consist, as we have seen, of a mass of crystals -floating in a liquid magma, and the distension of such -a mass by the escape of steam from its midst gives rise -to the formation of the rough cindery-looking material -<span class="pagenum" id="Page_68">- 68 -</span> -to which the name of 'scoria' is applied. But when -the lava contains no ready-formed crystals, but consists -entirely of a glassy substance in a more or less perfect -state of fusion, the liberation of steam gives rise to the -formation of the beautiful material known as 'pumice.' -Pumice consists of a mass of minute glass bubbles; -these bubbles have not usually, however, retained their -globular form, but have been elongated in one direction -through the movement of the mass while it was still in -a plastic state.</p> - -<p>The steam frequently escapes from lava with such -violence that the froth or scum on its surface is broken -up and scattered in all directions, as the foam crests of -waves are dispersed by the wind during a storm. In -this way fragments of scoria or pumice are often thrown -to the height of many hundreds or thousands of feet -into the atmosphere, as we have seen is the case at -Stromboli and Vesuvius. Indeed, during violent eruptions, -a continuous upward discharge of these fragments -is maintained, the ragged cindery masses hurtling one -another in the atmosphere, as they are shot perpendicularly -upwards to an enormous height and fall back -into the vent; or they may rise obliquely and describe -curves so as to descend outside the orifice from which -they were ejected.</p> - -<div class="sidenote">FINENESS OF VOLCANIC DUST.</div> - -<p>During their upward discharge and downward fall, -the cindery fragments are by attrition continually -reduced to smaller dimensions. The noise made by -these fragments, as they strike against one another in -<span class="pagenum" id="Page_69">- 69 -</span> -the air during their rise and fall, is one of the most -noteworthy accompaniments of volcanic eruptions. It -has been noticed that in many cases there is a constant -diminution in the size of the fragments ejected during -a volcanic outburst, this being doubtless due to the -friction of the masses as they are ejected and re-ejected -from the vent. Thus it is related by Mr. Poulett -Scrope, who watched the Vesuvian eruption of 1822, -which lasted for nearly a month, that during the earlier -stages of the outburst fragments of enormous size were -thrown out of the crater, but by constant re-ejection -these were gradually reduced in size, till at last only -the most impalpable dust issued from the vent. This -dust filled the atmosphere, producing in the city of -Naples 'a darkness that might be felt,' and so excessively -finely divided was it, that it penetrated into all -drawers, boxes, and the most closely fastened receptacles, -filling them completely. Mr. Whymper relates that, -while standing on the summit of Chimborazo, he witnessed -an eruption of Cotopaxi, which is distant more -than fifty miles from the former mountain. The fine -volcanic dust fell in great quantities around him, and -he estimated that no less than two millions of tons -must have been ejected during this slight outburst. -Professor Bonney has examined this volcanic dust from -Cotopaxi, and calculates that it would take from 4,000 -to 25,000 particles to make up a grain in weight.</p> - -<p>Various names have been given by geologists to -the fragments ejected from volcanic vents, which, as we -<span class="pagenum" id="Page_70">- 70 -</span> -have seen, differ greatly in their dimensions and other -characters. Sometimes masses of more or less fluid -lava are flung bodily to a great height in the atmosphere. -During their rise and fall these masses are -caused to rotate, and in consequence assume a globular -or spheroidal form. The water imprisoned in these -masses, during their passage through the atmosphere, -tends to expand into steam, and they become more or -less completely distended with bubbles. Such masses, -which sometimes assume very regular and striking -forms, are known as 'volcanic bombs.' Many volcanic -bombs have a solid nucleus of refractory materials. The -large, rough, angular, cindery-looking fragments are -termed 'scoriæ.' When reduced to the dimensions of -a marble or pea they are usually called by the Italian -name of 'lapilli.' The still finer materials are known -as volcanic sand and dust.</p> - -<p>There are, however, two names which are frequently -applied to these fragmentary materials ejected from -volcanoes, which are perhaps liable to give rise to misconception. -These are the terms 'cinders' and 'ashes.' -It must be remembered that the scoriæ or cindery-looking -masses are not, like the cinders of our fires, the -product of the partial combustion of a material containing -inflammable gases, but are, like the clinkers of -furnaces and brick-kilns, portions of partially vitrified -and fused rock distended by gases. So, too, volcanic -ashes only resemble the ashes of our grates in being -very finely divided; they are not, like the latter, -<span class="pagenum" id="Page_71">- 71 -</span> -the incombustible residue of a mass which has been -burnt.</p> - -<div class="sidenote">VOLCANIC BOMBS AND PELE'S HAIR.</div> - -<p>The glassy lavas, when distended by escaping gases, -give rise to the formation of pumice, the white colour -of which, as in the case of the foam of a wave, is due -to the reflection of a portion of the light in its frequent -passage from one medium to another—in this case from -air to glass, and from glass to air. The volcanic bombs -formed from glassy lavas are often of especially beautiful -and regular forms. Sometimes the passage of steam -through a mass of molten glass produces large quantities -of a material resembling spun glass. Small particles or -shots of the glass are carried into the air and leave -behind them thin, glassy filaments like a tail. At the -volcano of Kilauea in Hawaii this filamentous volcanic -glass is abundantly produced, and is known as -'Pele's Hair'—Pele being the name of the goddess of -the mountain. Birds' nests are sometimes found composed -of this beautiful material. In recent years an -artificial substance similar to this Pele's hair has been -extensively manufactured by passing jets of steam -through the molten slag of iron-furnaces; it resembles -cotton-wool, but is made up of fine threads of glass, -and is employed for the packing of boilers and other -purposes.</p> - -<p>The very finely-divided volcanic dust is often borne -to enormous distances from the volcano out of which -it has been ejected. The force of the steam-current -carrying the fragments into the atmosphere is often so -<span class="pagenum" id="Page_72">- 72 -</span> -great that they rise to the height of several miles above -the mountain. Here they may actually pass into the -upper currents of the atmosphere and be borne away to -the distance of many hundreds or thousands of miles. -Hence it is not an unusual circumstance for vessels at -sea to encounter at great distances from land falling -showers of this finely divided, volcanic dust. We sometimes -meet with this far-travelled, volcanic dust under -very unexpected circumstances. Thus, in the spring -of 1875 I had occasion to visit Prof. Vom Rath of Bonn, -who showed me a quantity of fine volcanic dust which -had during the past winter fallen in considerable quantities -in certain parts of Norway. This dust, upon -microscopic examination, proved to be so similar to what -was known to be frequently ejected from the Icelandic -volcanoes that a strong presumption was raised that -volcanic outbursts had been going on in that island. -On returning to England I found that the first steamer -of the season had just reached Leith from Iceland, -bringing the intelligence that very violent eruptions -had taken place during the preceding months.</p> - -<div class="sidenote">DISPERSION OF PUMICE AND VOLCANIC DUST.</div> - -<p>This finely-divided volcanic dust is thus carried by -the winds and spread over every part of the ocean. -Everyone is familiar with the fact that pumice floats -upon water; this it does, not because it is a material -specifically lighter than water, but because cavities -filled with air make up a great part of its bulk. If -we pulverise pumice, we find the powder sinks readily -in water, but the rock in its natural condition floats -<span class="pagenum" id="Page_73">- 73 -</span> -for the same reason that an iron ship does—because of -the air-chambers which it encloses. When this pumice -is ejected from a volcano and falls into a river or the -ocean, it floats for a long time, till decomposition -causes the breaking down of the thin glassy partitions -between the air chambers, and causes the admission of -water into the latter, by which means the whole mass -gets water-logged. Near the Liparis and other volcanic -islands the sea is sometimes covered with fragments -of pumice to such an extent that it is difficult -for a boat to make progress through it, and the same -substance is frequently found floating in the open ocean -and is cast up on every shore.</p> - -<p>During the year 1878 masses of floating pumice -were reported as existing in the vicinity of the Solomon -Isles, and covering the surface of the sea to such extent -that it took ships three days to force their way through -them. Sometimes these masses of pumice accumulate -in such quantities along coasts that it is difficult -to determine the position of the shore within a mile or -two, as we may land and walk about on the great floating -raft of pumice. Now, recent deep-sea soundings, -carried on in the 'Challenger' and other vessels, have -shown that the bottom of the deepest portion of the -ocean, far away from the land, is covered with these -volcanic materials which have been carried through the -air or floated on the surface of the ocean. To these -deeper parts of the ocean no sediments carried down by -the rivers are borne, and the remains of calcareous -<span class="pagenum" id="Page_74">- 74 -</span> -organisms are, in these abysses, soon dissolved; under -such conditions, therefore, almost the only material -accumulating on the sea bottom is the ubiquitous wind- -and wave-borne volcanic products. These particles of -volcanic dust and fragments of pumice by their disintegration -give rise to a clayey material, and the -oxidation of the magnetite, which all lavas contain, -communicates to the mass a reddish tint. This appears -to be the true origin of those masses of 'red-clay' -which, according to recent researches, are found to -cover all the deeper parts of the ocean, but which -probably attain to no great thickness.</p> - -<p>But while some portion of the materials ejected -from volcanoes may thus be carried by winds and waves, -so as to be dispersed over every part of the land and -the ocean-bed, another, and in most cases by far the -largest, portion of these ejections falls around the volcanic -vent itself. It is by the constant accumulation -of these ejected materials that such great mountain -masses as Etna, Teneriffe, Fusiyama, and Chimborazo -have been gradually built up around centres of volcanic -action.</p> - -<p>There are cases in which the formation of volcanic -mountains on a small scale has actually been observed -by trustworthy witnesses. There are other cases in -which volcanic mountains of larger size can be shown -to have increased in height and bulk by the fall upon -their sides and summits of fragmentary materials ejected -from the volcanic vent. In all cases the examination -<span class="pagenum" id="Page_75">- 75 -</span> -of these mountain-masses leads to the conclusion that -they are entirely built up of just such materials as we -constantly see thrown out of volcanoes during eruption.</p> - -<div class="sidenote">FORMATION OF VOLCANIC MOUNTAINS.</div> - -<p>Thus we are led to the conclusion that all volcanic -mountains are nothing but heaps of materials ejected -from fissures in the earth's crust, the smaller ones -having been formed during a single volcanic outburst, -the larger ones being the result of repeated eruptions -from the same orifice which may, in some cases, have -continued in action for tens or hundreds of thousands -of years.</p> - -<p>No observer has done such useful work in connection -with the study of the mode of formation of volcanic -mountains as our countryman, Sir William Hamilton, -who was ambassador at Naples from 1764 to 1800, and -made the best possible use of his opportunities for -examining the numerous volcanoes in Southern Italy.</p> - -<p>A little to the west of the town of Puzzuoli on the -Bay of Naples there stands a conical hill rising to the -height of 440 feet above the level of the Mediterranean, -and covering an area more than half a mile in diameter. -Now we have the most conclusive evidence that in -ancient times no such hill existed on this site, which -was partly occupied by the Lucrine Lake, and the fact -is recognised in the name which the hill bears, that of -Monte Nuovo, or the 'New Mountain.' See <a href="#fig10">fig. 10</a>.</p> - -<p>Sir William Hamilton rendered admirable service -to science by collecting all the contemporary records -relating to this interesting case, and he was able to -<span class="pagenum" id="Page_76">- 76 -</span> -prove, by the testimony of several intelligent and trustworthy -witnesses, that during the week following the -29th of September, 1538, this hill had gradually been -formed of materials ejected from a volcanic vent which -had opened upon this site.</p> - -<div class="figcenter" id="fig10" style="width: 445px;"> - <img src="images/fig10.png" width="445" height="149" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 10. Monte Nuovo (440 ft. high) on the shores of the Bay of - Naples.</span></div> -</div> - -<div class="sidenote">HISTORY OF THE FORMATION OF MONTE NUOVO.</div> - -<p>The records collected by Hamilton with others which -have been discovered since his death prove most conclusively -the following facts. During more than two years, -the country round was affected by earthquakes, which -gradually increased in intensity and attained their -climax in the month of September 1538; on the 27th -and 28th of that month these earthquake shocks are said -to have been felt almost continuously day and night. -About 8 o'clock on the morning of the 29th, a depression -of the ground was noticed on the site of the future -hill, and from this depression, water, which was at first -cold and afterwards tepid, began to issue. Four hours -afterwards the ground was seen to swell up and open, -forming a gaping fissure, within which incandescent -<span class="pagenum" id="Page_77">- 77 -</span> -matter was visible. From this fissure numerous masses -of stone, some of them 'as large as an ox,' with vast -quantities of pumice and mud, were thrown: up to a -great height, and these falling upon the sides of the -vent formed a great mound. This violent ejection of -materials continued for two days and nights, and on -the third day a very considerable hill was seen to have -been built up by the falling fragments, and this hill -was climbed by some of the eye-witnesses of the eruption. -The next day the ejections were resumed, and -many persons who had ventured on the hill were injured, -and several killed by the falling stones. The -later ejections were however of less violence than the -earlier ones, and seem to have died out on the seventh -or eighth day after the beginning of the outburst. -The great mass of this considerable hill would appear, -according to the accounts which have been preserved, -to have been built up by the materials which were -ejected during two days and nights.</p> - -<p>Monte Nuovo is a hill of truncated conical form, -which rises to the height of 440 feet above the waters -of the Mediterranean, and is now covered with thickets -of stone-pine. The hill is entirely made up of volcanic -scoriæ, lapilli, and dust, and the sloping sides have -evidently been produced by these fragmentary materials -sliding over one another till they attained the angle of -rest; just as happens with the earth and stones tipped -from railway-waggons during the construction of an -embankment. In the centre of this conical hill is a -<span class="pagenum" id="Page_78">- 78 -</span> -vast circular depression, with steeply sloping sides, -which is of such depth that its bottom is but little -above the sea-level. This cup-shaped depression is -the 'crater' of the volcano, and it has evidently been -formed by the explosive action which has thrown out -the materials immediately above the vent, and caused -them to be accumulated around it.</p> - -<div class="figcenter" id="fig11" style="width: 473px;"> - <img src="images/fig11.png" width="473" height="257" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 11.—Map of the district around Naples, showing Monte - Nuovo and the surrounding volcanoes of older date.</span></div> -</div> - -<p>The district lying to the west of Naples, in which -the Monte Nuovo is situated, contains a great number -of hills, all of which present a most striking similarity -to that volcano. All these hills are truncated cones, -with larger or smaller circular depressions at their summits, -and they axe entirely composed of volcanic scoriæ, -lapilli, and dust. Some of these hills are of considerably -larger dimensions than the Monte Nuovo, while others -<span class="pagenum" id="Page_79">- 79 -</span> -are of smaller size, as shown in the annexed map, -<a href="#fig11">fig. 11</a>. No stranger visiting the district, without -previous information upon the subject, would ever -suspect the fact that, while all the other hills of the -district have existed from time immemorial, and are -constantly mentioned in the works of Greek and Roman -writers, this particular hill of Monte Nuovo came -into existence less than 350 years ago.</p> - -<div class="sidenote">OLDER VOLCANOES OF THE CAMPI PHLEGRÆI.</div> - -<p>The evidently fused condition of the materials of -which these hills are built up is a dear sign of the -volcanic action which has taken place in it; and this -feet was so fully recognised by the ancients that they -called the district the Campi Phlegræi, or 'the Burning -Fields,' and regarded one of the circular depressions -in it as the entrance to Hades.</p> - -<p>It is impossible for anyone to examine this district -without being convinced that all the numerous cones -and craters which cover it have been formed by the -same agency as that by which Monte Nuovo was produced. -We have shown that there is the most satisfactory -historical evidence as to what that agency was.</p> - -<p>Now volcanic cones with craters in their centres -occur in great numbers in many parts of the earth's -surface. In some districts, like the Auvergne, the -Catacecaumene in Asia Minor, and certain parts of -New Zealand, these volcanic cones occur by hundreds -and thousands. In some instances, these volcanic cones -have been formed in historic times, but in the great -majority of cases we can only infer their mode of origin -<span class="pagenum" id="Page_80">- 80 -</span> -from their similarity to others of which the formation -has been witnessed.</p> - -<p>Most of the smaller volcanic hills, with their -craters, have been thrown up during a single eruption -from a volcanic fissure; but, as Hamilton conclusively -proved, the grandest volcanic mountains must have -been produced by frequent repetitions of similar operations -upon the same site. For not only are these -great volcanic piles found to be entirely composed of -materials which have evidently been ejected from -volcanic vents, but, when carefully watched, such -mountains are found undergoing continual changes in -form, by the addition of materials thrown out from the -vent, and falling upon their sides.</p> - -<p>This fact will be well illustrated by a comparison -of the series of drawings of the summit of Vesuvius -which were made by Sir William Hamilton in 1767, -and which we have copied in <a href="#fig12_neg">fig. 12</a>. During the earlier -months of that year the summit of the mountain was -seen to be of truncated form, a great crater having -been originated by the violent outbursts of the preceding -year. This condition of the mountain-top is -represented in the first figure of the series. The -drawing made by Hamilton, on July 8, shows that not -only was the outer rim of the great crater being -modified in form by the fall of materials upon it, but -that in the centre of the crater a small cone was being -gradually built up by the quiet ejections which were -taking place.</p> - -<div class="figcenter" id="fig12_neg" style="width: 418px;"> -<a href="images/fig12.png"><img src="images/fig12_neg.png" width="418" height="623" alt="" /></a> - <div class="figcaption"><span class="smcap">Fig. 12.—Outlines of the Summit of Vesuvius during the - Eruption of 1767.</span><br /> - <span class="smaller">Click on image to view original negative image.</span></div> -</div> - -<p><span class="pagenum" id="Page_81">- 81 -</span></p> - -<div class="sidenote">CHANGES IN FORM OF VESUVIUS.</div> - -<p>If we compare the drawings made at -successive dates, we shall find that the constant showers -of falling materials were not only raising the edge of the -great crater but were at the same time increasing the -size of the small cone inside the crater. By the end of -October the small cone had grown to such an extent -that its sides were confluent with those of the principal -cone, which had thus entirely lost its truncated form -and been raised to a much greater height. The comparison -of these drawings will be facilitated by the -dotted lines, which represent the outline of the top of -the mountain at the preceding observation; so that the -space between the dotted and the continuous line in -each drawing shows the extent to which the bulk of -the cone had increased in the interval between two -observations.</p> - -<p>But, although the general tendency of the action -going on at volcanic mountains is to increase their -height and bulk by the materials falling upon their -summits and aides, it must be remembered that this -action does not take place by any means continuously -and regularly. Not only are there periods of rest in -the activity of the volcano, during which the rain -and winds may accomplish a great deal in the way of -crumbling down the loose materials of which volcanic -mountains are largely built up, but sudden and violent -eruptions may in a very short time undo the slow work -of years by blowing away the whole summit of the -mountain at once. Thus, before the great eruption of -1822, the cone of Vesuvius, by the almost constant -<span class="pagenum" id="Page_82">- 82 -</span> -ejection of ashes during several years, had been raised -to the height of more than 4,000 feet above the level of -the sea; but by the terrible outburst which then took -place the cone was reduced in height by 400 feet, and -a vast crater, which had a diameter of nearly a mile, -and a depth of nearly 1,000 feet (see <a href="#fig13">fig. 13</a>), was -formed at the top of the mountain. The enormous -quantity of material thus removed was either distributed -over the flanks of the mountain, or, when reduced to a -finely comminuted condition, was carried by the wind -to the distance of many miles, darkening the air, and -coating the surface of the ground with a thick covering -of dust.</p> - -<div class="figcenter" id="fig13" style="width: 448px;"> - <img src="images/fig13.png" width="448" height="236" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 13.—Crater of Vesuvius formed during the eruption in - 1822.</span> (It was nearly 1 mile in diameter and 1,000 ft. deep.)</div> -</div> - -<p><span class="pagenum" id="Page_83">- 83 -</span></p> - -<div class="sidenote">EARLY HISTORY OF VESUVIUS.</div> - -<p>The volcano of Vesuvius, although of somewhat -insignificant dimensions when compared with the -grander volcanic mountains of the globe, possesses -great interest for the student of Vulcanology, inasmuch -as being situated in the midst of a thickly -populated district and in close proximity to the city -of Naples, it has attracted much attention during past -times, and there is no other volcano concerning which -we have so complete a series of historical records. -The present cone of Vesuvius, which rises within the -great encircling crater-ring of Somma, has a height of -about 1,000 feet. But there is undoubted evidence -that this cone, to the top of which a railway has recently -been constructed for the convenience of tourists, -has been entirely built up during the last 1,800 years, -and, what is more, that during this period it has been -many times almost wholly destroyed and reconstructed.</p> - -<p>Nothing is more certain than the bet that the -Vesuvius upon which the ancient Romans and the -Greek settlers of Southern Italy looked, was a mountain -differing entirely in its form and appearance from -that with which we are familiar. The Vesuvius known -to the ancients was a great truncated cone, having a -diameter at its base of eight or nine miles, and a -height of about 4,000 feet. The summit of this mountain -was formed by a circular depressed plain, nearly -three miles in diameter, within which the gladiator -Spartacus, with his followers, were besieged by a Roman -army. There is no evidence that at this time the volcanic -character of the mountain was generally recognised, -and its slopes are described by the ancient -geographers as being clothed with fertile fields and -<span class="pagenum" id="Page_84">- 84 -</span> -vineyards, while the hollow at the top was a waste -overgrown with wild vines.</p> - -<div class="figcenter" id="fig14" style="width: 431px;"> - <img src="images/fig14.png" width="431" height="239" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 14.—Crater of Vesuvius in 1756.</span> - (From a drawing made on the spot)</div> -</div> - -<p>But in the year 79 a terrible and unexpected eruption -occurred, by which a vast, crateral hollow was -formed in the midst of Vesuvius, and all the southern -side of the great rim surrounding this crater was -broken down. Under the materials ejected during this -eruption, the cities of Pompeii, Herculaneum, and -Stabiæ were overwhelmed and buried.</p> - -<p>Numerous descriptions and drawings enable us to -understand how in the midst of the vast crater formed -in the year 79 the modern cone has gradually been -built up. Fresh eruptions are continually increasing -the bulk, or raising the height of the Vesuvian cone.</p> - -<p>The accompanying drawings made by Sir William -Hamilton enable us to understand the nature of -<span class="pagenum" id="Page_85">- 85 -</span> -the changes which have been continually taking place -at the summit of Vesuvius. The drawing <a href="#fig14">fig. 14</a> -shows the appearance presented by the crater in the -year 1756.</p> - -<div class="figcenter" id="fig15" style="width: 420px;"> - <img src="images/fig15.png" width="420" height="246" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 15.—The Summit of Vesuvius in 1767.</span> - (From an original drawing.</div> -</div> - -<div class="sidenote">VESUVIUS IN MODERN TIMES.</div> - -<p>At this time we see that inside the crater a series -of cones had been built up one within the other from -which lava issued, filling the bottom of the crater and -finding its way through a breach in its walls, down the -side of the cone. It is evident that the ejected materials -falling on the sides of the innermost cone would -tend to enlarge the latter till its sides became confluent -with the cone surrounding it, and if this action went -on long enough, the crater would be entirely filled up -and a perfect cone with only a small aperture at the -top would be produced. But from time to time, grand -and paroxysmal outbursts have occurred at Vesuvius, -<span class="pagenum" id="Page_86">- 86 -</span> -which have truncated the cone, and sometimes formed -great, cup-shaped cavities, reaching almost to its base, -like that shown in <a href="#fig13">fig. 13</a>.</p> - -<p>In 1767 the crater of Vesuvius, as shown in <a href="#fig15">fig. 15</a>, -contained a single small cone in a state of constant -spasmodic outburst, like that of Stromboli.</p> - -<div class="figcenter" id="fig16" style="width: 458px;"> - <img src="images/fig16.png" width="458" height="238" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 16.—Summit of Vesuvius in 1848.</span></div> -</div> - -<p>In 1843, we find that the crater of Vesuvius contained -three such small cones arranged in a line along -its bottom as depicted in <a href="#fig16">fig. 16</a>.</p> - -<p>These drawings of the summit of Vesuvius give a -fair notion of the changes which have been continually -going on there during the whole of the historical period. -Ever and anon a grand outburst, like that of 1822, has -produced a vast and deep crater such as is represented -in <a href="#fig13">fig. 13</a>, and then a long continuance of quiet and -<span class="pagenum" id="Page_87">- 87 -</span> -regular ejections has built up within the crater small -cones like those shown in -figs. <a href="#fig14">14</a>, <a href="#fig15">15</a> and <a href="#fig16">16</a>, till at -last the great crater has been -completely filled up, and the -cone reconstructed.</p> - -<div class="figcenter" id="fig17" style="width: 666px;"> - <img src="images/fig17.png" width="666" height="172" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 17.—Outlines of Vesuvius, - showing its Form at different periods of its history.</span></div> -</div> - - -<div class="sidenote">CHANGES IN OUTLINE OF VESUVIUS.</div> - -<p>In the series of outlines -in <a href="#fig17">fig. 17</a>, we have endeavoured to illustrate the succession -of changes which -has taken place in Vesuvius during historical times. -In the year 79 one side -of the crater-wall of the -vast mountain-mass was -blown away. Subsequent -ejections built up the present -cone of Vesuvius within the -great encircling crater-wall -of Somma, and the form of -this cone and the crater at -its summit have been undergoing -continual changes -during the successive eruptions of eighteen centuries.</p> - -<p>What its <i>future</i> history -may be we can only conjecture -from analogy. It may -be that a long continuance -<span class="pagenum" id="Page_88">- 88 -</span> -of eruptions of moderate energy may gradually raise the -central cone till its sides are confluent with those of -the original mountain; or it may be that some violent -paroxysm will entirely destroy the modern cone, reducing -the mountain to the condition in which it was -after the great outburst of 79. On the other hand, -if the volcanic forces under Vesuvius are gradually -becoming extinct (but of this we have certainly no -evidence at present), the mountain may gradually sink -into a state of quiescence, retaining its existing form.</p> - -<p>The series of changes in the shape of Vesuvius, -which are proved by documentary evidence to have -been going on during the last 2,000 years, probably -find their parallel in all active volcanoes. In all of -these, as we shall hereafter show, the activity of the -vents undergoes great vicissitudes. Periods of continuous -moderate activity alternate with short and -violent paroxysmal outbursts and intervals of complete -rest, which may in some cases last for hundreds or -even thousands of years. During the periods of continuous -moderate activity, the crater of the volcano is -slowly filled up by the growth of smaller cones within -it; and the height of the mountain is raised. By the -terrible paroxysmal outbursts the mountain is often -completely gutted and its summit blown away; but -the materials thus removed from the top and centre of -the mass are for the most part spread over its aides, -so that its bulk and the area of its base are thereby -increased. During the intervals of rest, the sides of -<span class="pagenum" id="Page_89">- 89 -</span> -the mountain which are so largely composed of loose -and pulverulent materials are washed downwards by -rains and driven about by winds. Thus all volcanoes -in a state of activity are continually growing in size -every ejection, except in the case of those where the -materials are in the finest state of subdivision, adding -to their bulk; the area of their bases being increased -during paroxysmal outbursts, and their height during -long-continued moderate eruptions.</p> - -<div class="sidenote">DEVIATIONS FROM CONICAL FORM.</div> - -<p>We have pointed out that the conical form of volcanic -mountains is due to the slipping of the falling -materials over one another till they attain the angle -at which they can rest. There are, however, some -deviations from this regular conical form of volcanoes -which it may be well to refer to.</p> - -<p>The quantity of rain which falls during volcanic -eruptions is often enormous, owing to the condensation -of the great volumes of steam emitted from the vent. -Consequently the falling lapilli and dust often descend -upon the mountain, not in a dry state but in the condition -of a muddy paste. Many volcanic mountains -have evidently been built up by the flow of successive -masses of such muddy paste over their surfaces. Some -volcanic materials when mixed with water have the -property of rapidly 'setting' like concrete. The -ancient Romans and modern Italians, well acquainted -with this property of certain kinds of volcanic dust and -lapilli, have in all ages employed this 'puzzolana,' as it -is called, as mortar for building. The volcanic muds -<span class="pagenum" id="Page_90">- 90 -</span> -have often set in their natural positions, so as to form -a rock, which, though light and porous, is of tolerably -firm consistency. To this kind of rock, of which -Naples and many other cities are built, the name of -'tuff' or 'tufa' is applied. A similar material is known -in Northern Germany as 'trass.'</p> - -<p>The cause of the 'setting' of puzzolana and tufa -is that rain-water containing a small proportion of carbonic -acid acts on the lime in the volcanic fragments, -and these become cemented together by the carbonate -of lime and the free silica, which are thus produced in -the mass.</p> - -<p>When a strong wind is blowing during a volcanic -outburst, the materials may be driven to one side of -the vent, and accumulate there more rapidly than on -the other. Thus lop-sided cones are formed, such as -may frequently be observed in some volcanic districts. -In areas where constant currents of air, like the trade-winds, -prevail, all the scoria-cones of the district may -thus be found to be unequally developed on opposite -sides, being lowest on those from which the prevalent -winds blow, and highest on the sides towards which -these winds blow.</p> - -<div class="sidenote">ANGLE OF SLOPE IN VOLCANIC CONES.</div> - -<p>The examination of any careful drawing, or better -still of the photograph, of a volcanic cone, will prove -that the profile of such cones is not formed by straight -lines, but by curves often of a delicate and beautiful -character. The delineations of the sacred volcano of -Fusiyama, which are so constantly found in the productions -<span class="pagenum" id="Page_91">- 91 -</span> -of Japanese artists, must have familiarised -everyone with the elegant curved lines exhibited by -the profiles of volcanoes. The upper slope of the -mountain is comparatively steep, often exhibiting -angles of 30° to 35°, but this steepness of slope gradually -diminishes, till it eventually merges in the -surrounding plains. The cause of this elegant form -assumed by most volcanic mountains is probably two-fold. -In the first place we have to remember that the -materials falling upon the flanks of the mountain -differ in size and shape, and some will rest on a steeper -slope than others. Thus, while some of the materials -remain on the upper part of the mountains, others are -rolling outwards and downwards. Hence we find that -those cones which are composed of uniform materials -have straight sides. But in some cases, we shall see -hereafter, there has certainly been a central subsidence -of the mountain mass, and it is this subsidence which -has probably given rise to the curvature of its flanks.</p> - -<p>We have hitherto considered only the methods by -which the froth or foam, which accumulates on the -surface of fluid lava, is dispersed. But in many cases -not only is this scum of the lava ejected from the -volcanic vent by the escaping steam, but the fluid lava -itself is extruded forcibly, and often in enormous -quantities.</p> - -<p>The lava in a volcanic vent is always in a highly -heated, usually incandescent, condition. Seen by night, -its freshly exposed surface is glowing red, sometimes -<span class="pagenum" id="Page_92">- 92 -</span> -apparently white-hot. But by exposure to the atmosphere -the surface is rapidly chilled, appearing dull red -by night, and black by day. Many persons are surprised -to find that a flowing stream of lava presents the appearance -of a great mass of rough cinders, rolling along -with a rattling sound, owing to the striking of the clinker-like -fragments against each other. When viewed -by night, the gleaming, red light between these rough, -cindery masses betrays the presence of incandescent -materials below the chilled surface of the lava-stream.</p> - -<p>No fact in connection with lavas is more striking -than the varying degrees of liquidity presented by -them in different cases. While some lava-streams -seem to resemble rivers, the material flowing rapidly -along, filling every channel in its course, and deluging -the whole country around, others would be more fitly -compared to glaciers, creeping along at so slow a rate -that the fact of their movement can only be demonstrated -by the most careful observation. Even when -falling over a precipice such lavas, owing to their imperfect -liquidity, form heavy, pendent masses like a -'guttering' candle, as is shown by <a href="#fig18">fig. 18</a>, which is -taken from a drawing kindly furnished to me by Capt. -S. P. Oliver, R.A. The causes of these differences in -the rate of motion of lava-streams we must proceed to -consider.</p> - -<p><span class="pagenum" id="Page_93">- 93 -</span></p> - -<div class="figcenter" id="fig18" style="width: 436px;"> - <img src="images/fig18.png" width="436" height="313" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 18.—Cascade of Lava tumbling over a cliff in the Island - of Bourbon.</span></div> -</div> - -<div class="sidenote">TEMPERATURE OF LAVA-STREAMS.</div> - -<p>There can be no doubt that the temperature of -lavas varies greatly in different cases. This is shown -by the fact that while some lavas are in a state of complete -fusion, similar to that of the slags of furnaces, -and like the latter, such lavas on cooling form a glassy -mass, others consist of a liquid magma in which a larger -or smaller number of crystals are found floating. In -these latter cases the temperature of the magma must -be below the fusing-point of the minerals which exist -in a crystalline condition in its midst. It has indeed -been suggested that the whole of the crystals in lavas -are formed during the cooling down of a completely -fused mass; but no one can imagine that the enclosed -crystals of quartz, felspar, leucite, olivine, &c., have -been so formed, such crystals being sometimes more -than an inch in diameter. The microscopic examination -<span class="pagenum" id="Page_94">- 94 -</span> -of lavas usually enables us to discriminate between -those complete crystals which have been formed at great -depths and carried up to the surface, and the minute -crystalline particles and microliths which have been -developed in the glassy mass during cooling. Crystals -of the former class, indeed, exhibit abundant evidence, -in their liquid cavities and other peculiarities, that they -have not been formed by simple cooling from a state -of fusion, but under the combined action of heat, -the presence of water and various gases, and intense -pressure.</p> - -<p>As we have already seen, the different lavas vary -greatly in their degrees of fusibility. The basic lavas, -containing a low percentage of silica, are much more -fusible than the acid lavas, which contain a high percentage -of silica. When the basic lavas are reduced to -a complete state of fusion their liquidity is sometimes -very perfect, as is the case at Kilauea in Hawaii, where -the lava is thrown up into jets and fountains, falling in -minute drops, and being drawn out into fine glassy -threads. On the other hand, the less fusible acid lavas -appear to be usually only reduced to the viscous or -pasty condition, which artificial glasses assume long -before their complete fusion. Of this fact I have found -many proofs in the Lipari Islands, where such glassy, -acid lavas abound. In <a href="#fig06">fig. 6</a> (page 43) a lava-stream -is represented on the side of the cone of Vulcano.</p> - -<p><span class="pagenum" id="Page_95">- 95 -</span></p> - -<div class="sidenote">IMPERFECTLY FLUID LAVAS.</div> - -<p>This lava is an obsidian—that is to say, it is of -the add type and completely glassy—but its liquidity -must have been very imperfect, seeing that the stream -has come to a standstill before reaching the bottom of -a steep slope of about 35°. In <a href="#fig19">fig. 19</a> there is given a -side view of the same stream of obsidian, from which -it will be seen that it has flowed slowly down a steep -slope and heaped itself up at the bottom, as its fluidity -was not complete enough to enable it to move on a -slighter incline. An examination of the interior of -such imperfectly fluid lavas affords fresh proofs of the -slow and tortuous movements of the mass. Everywhere -we find that the bands of crystallites and sphærulites -are, by the movement of the mass, folded and crumpled -and puckered in the most remarkable manner, as is -illustrated in figs. <a href="#fig20">20</a> and <a href="#fig21">21</a>. Similar appearances -occur again and again among the vitreous and semi-vitreous -acid lavas of Hungary.</p> - -<div class="figcenter" id="fig19" style="width: 475px;"> - <img src="images/fig19.png" width="475" height="284" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 19.—Lava-stream (obsidian) in the Island of Vulcano - showing the imperfect liquidity of the mass.</span></div> -</div> - -<p><span class="pagenum" id="Page_96">- 96 -</span></p> - -<div class="figcenter" id="fig20" style="width: 303px;"> - <img src="images/fig20.png" width="303" height="249" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 20.—Interior of a Rhyolitic Lava-stream in the Island of - Lipari, showing broad sigmoidal folds produced by the slow - movements of the mass.</span></div> -</div> - -<div class="figcenter" id="fig21" style="width: 297px;"> - <img src="images/fig21.png" width="297" height="236" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 21.—Interior of a Rhyolitic Lava-stream in the Island of - Lipari, showing the complicated crumplings and puckerings - produced by the slow movements of the mass.</span></div> -</div> - -<div class="sidenote">RATE OF MOVEMENT OF VESUVIAN LAVAS.</div> - -<p>But, although the temperature of lava-streams and -the fusibility of their materials may in some cases -<span class="pagenum" id="Page_97">- 97 -</span> -account for their condition of either perfect liquidity -or viscidity, it is clear that in other instances there -must be some other cause for this difference. Thus it -has been found that at Vesuvius the lavas erupted in -modern times have all a striking similarity to one -another in chemical composition, in the minerals which -they contain, and in their structure. They are all basic -lavas, which when examined by the microscope are -seen to consist of a more or less glassy magma, in the -midst of which numerous crystals of augite, leucite, -olivine, magnetite, and other minerals are scattered. -Yet nothing can be more strikingly different than -the behaviour of the lavas poured out from Vesuvius at -various periods. In some cases the lava appears to be -in such a perfectly liquid condition that, issuing from -the crater, it has been described as rushing down the -slope of the cone like a stream of water, and such exceedingly -liquid lavas have in some cases flowed to the -distance of several miles from the base of the mountain -in a very short time. But other Vesuvian lavas -have been in such a viscid condition that their rate of -movement has been so extremely slow as to be almost -imperceptible. Such lava-streams have continued in -movement during many years, but the progress has -been so slow (often only a few inches in a day) that -it could only be proved by means of careful measurements.</p> - -<p>If we examine some of these Vesuvian lavas which -have exhibited such striking differences in their rate of -<span class="pagenum" id="Page_98">- 98 -</span> -flow, we shall find that they present equally marked -differences in the character of their surfaces. The lava-current -of 1858 was a remarkable example of a slow-flowing -stream, and its surface, as will be seen in <a href="#fig22">fig. 22</a>, -which is taken from a photograph, has a very marked -and peculiar character. A tenacious crust seems to have -formed on the surface, and by the further motion of -the mass this crust or scum has been wrinkled and -folded in a very remarkable manner. Sometimes this -folded and twisted crust presents a striking resemblance -to coils of rope. Precisely similar appearances may be -observed on the surface of many artificial slags when -they flow from furnaces, and are seen to be due to the -same cause, namely, the wrinkling up of the chilled -surface-crust by the movement of the liquid mass below. -Lavas which present this appearance are frequently called -'ropy lavas'; an admirable example of them is afforded -in the lava-cascade of the Island of Bourbon represented -in <a href="#fig18">fig. 18</a> (page 93).</p> - -<p>But lavas in which the rate of flow has been very -rapid, exhibit quite a different kind of surface to -that of the ropy lavas. The Vesuvian lava-stream -of 1872 was remarkable for the rapidity of its flow, -and its surface presents a remarkable contrast to that -of the slow-moving lava of 1858. The surface of the -lava-current of 1872 is covered with rough cindery -masses, often of enormous dimensions, and it is exceedingly -difficult to traverse it, as the ragged projecting -fragments tear the boots and lacerate the skin. The -appearance presented by this lava-stream is illustrated -by <a href="#fig23">fig. 23</a>, which is also taken from a photograph.</p> - -<div class="figcenter" id="fig22" style="width: 459px;"> - <img src="images/fig22.png" width="459" height="674" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 22.—Vesuvian Lava-stream of 1858, - exhibiting the peculiar 'Ropy' Surfaces of Slowly - Moving Currents.</span><br /><br /> - (<i>From a Photograph.</i>)</div> -</div> - - -<div class="figcenter" id="fig23" style="width: 460px;"> - <img src="images/fig23.png" width="460" height="675" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 23.—Vesuvian Lava-stream of 1872, - exhibiting the Rough Cindery Surfaces characteristic of - Rapidly Flowing Currents.</span><br /><br /> - (<i>From a Photograph.</i>)</div> -</div> - - -<p><span class="pagenum" id="Page_99">- 99 -</span></p> - -<div class="sidenote">VESUVIAN LAVA-STREAM OF 1872.</div> - -<p>Now it is found that those lava-streams which move -slowly and present ropy surfaces give off but little -steam during their flow, while those lava-streams which -flow more rapidly and present a rough and cindery -appearance give off vast quantities of steam. The -extraordinary amount of vapour given off from the lava-streams -which flowed from Vesuvius in 1872 is illustrated -in the photograph copied in <a href="#fig05">fig. 5</a> (facing page 24), -in which the three lava-currents are each seen to be -surmounted by enormous vapour-clouds rising to the -height of several thousands of feet above them, and -mingling with the column that issued from the central -vent. By the escape of this enormous quantity of steam -the surface of the lava was thrown into rugged cindery -projections, and in some places little cones were formed -upon it, which threw out small scoriæ and dust. The -quantity of vapour was, in fact, so great, that little -parasitical volcanoes were formed on the surface of the -lava-stream. Some of these miniature volcanoes were -of such small dimensions that they were carried away -on boards to be employed as illustrations in the lecture-rooms -of the University of Naples.</p> - -<p>The arrangement of the materials forced out from -fissures on the surfaces of lava-streams by the disengaged -vapours and gases depends on the degree of -fluidity of the lava, and the force of the escaping steam-jets. -In very viscous lavas the materials may issue -<span class="pagenum" id="Page_100">- 100 -</span> -quietly, forming great concentric masses like coils of -rope; such were described by Mr. Heaphy as occurring -in New Zealand (see <a href="#fig24">fig. 24</a>).</p> - -<div class="figcenter" id="fig24" style="width: 384px;"> - <img src="images/fig24.png" width="384" height="70" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 24.—Concentric Folds on mass of cooled Lava.</span></div> -</div> - -<p>In other cases the lava, if somewhat more liquid, may -in issuing quietly without great outbursts of steam, -accumulate in great bottle-shaped masses, which have -been compared to 'petrified fountains.' Cases of this -kind have been described by Professor Dana as occurring -on the slopes of Hawaii (see <a href="#fig25">fig. 25</a>).</p> - -<div class="figcenter" id="fig25" style="width: 314px;"> - <img src="images/fig25.png" width="314" height="284" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 25.—Mass of cooled Lava formed over a spiracle on the - slopes of Hawaii.</span></div> -</div> - -<p><span class="pagenum" id="Page_101">- 101 -</span></p> - -<div class="sidenote">MINIATURE CONES ON LAVA-STREAMS.</div> - -<p>When the steam escapes with explosive violence -from a spiracle ('bocca') on the surface of a lava-stream, -minute cinder cones, like those described as being -formed in 1872, are the result. <a href="#fig26">Fig. 26</a> represents a -group of miniature cones thrown up on the Vesuvian -lava-stream of 1855: it is taken from a drawing by -Schmidt.</p> - -<div class="figcenter" id="fig26" style="width: 448px;"> - <img src="images/fig26.png" width="448" height="224" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 26.—Group of small Cones thrown up on the Vesuvian - Lava-current of 1856.</span></div> -</div> - -<p>Some of these appear like burst blisters or bubbles, -while others are built up of scoriaceous masses which -have been ejected from the aperture and have become -united while in a semi-fluid condition. Other examples -of these spiracles or bocche on the surfaces of lava-currents -may be seen in the figs. <a href="#fig22">22</a> and <a href="#fig23">23</a>, which are -copied from photographs.</p> - -<p>The facts we have described all point to the conclusion -that the presence of large quantities of water -imprisoned in a mass of lava contributes greatly to its -mobility. And this conclusion is supported by so many -<span class="pagenum" id="Page_102">- 102 -</span> -other considerations that it is now very generally accepted -by geologists. The condition of this imprisoned -water in lavas is one which demands further investigation -at the hands of physicists. It has been suggested, -with some show of reason, that the water may -exist in the midst of the red-hot lava as minute particles -in the curious 'spheroidal condition' of Boutigny, -and that these flash into steam as the lava flows along.</p> - -<p>Lava, when extruded from a volcanic crater in a -more or less completely fluid state, flows down the side -of the cone, and then finds its way along any channel -or valley that may lie in its course, obeying in its -movements all the laws of fluid bodies. The lava-currents -thus formed are sometimes of enormous -dimensions, and may flood the whole country for many -miles around the vent.</p> - -<p>Lava-streams have been described, which have -flowed for a distance of from fifty to a hundred miles -from their source, and which have had a breadth varying -from ten to twenty miles. Some lava-streams -have a thickness of 500 feet, or even more. These -measures will give some idea of the enormous quantities -of material brought from the earth's interior by -volcanic action and distributed over its surface. The -mass of lava which flowed out during an eruption off -Reykjanes in Iceland, in the year 1783, has been calculated -to be equal in bulk to Mont Blanc.</p> - -<p>There are many parts of the earth's surface, such -as the Western Isles of Scotland and the North-east of -<span class="pagenum" id="Page_103">- 103 -</span> -Ireland, the Deccan of India, and large tracts in the -Rocky Mountains, where successive lava-sheets have -been piled upon one another to the height of several -thousands of feet, and cover areas of many hundreds or -even thousands of square miles.</p> - -<div class="sidenote">FEATURES OF LAVA-STREAMS.</div> - -<p>The more fusible basic lavas are as a general rule -more liquid in character than any others, and it is these -very liquid lavas that are usually found forming plateaux -built up of successive lava-streams. The less liquid -lavas, like those of Hungary and Bohemia, are not -usually found flowing to such distances from the vent, -but form dome-shaped mountain-masses.</p> - -<p>Lava-streams usually exhibit in their upper and -under surfaces a scoriaceous texture due to the escape -of steam from the upper surface, portions of the cindery -masses so formed falling off from the end of the stream, -and being rolled over by the stream so as to form its -base. The thickness of this scoriaceous upper and -lower part of a lava-stream varies according to the -quantity of steam imprisoned in it; but all thick lava-streams -have a compact central portion which is composed -of hard, solid rock. Very good examples of -the internal structure of lava-streams may sometimes -be examined in the sea-cliffs of volcanic islands. In -<a href="#fig27">fig. 27</a> we have given a copy of a drawing made while -sailing round the shores of Vulcano. The scoriaceous -portions of lava-streams are sometimes employed, as at -Volvic in the Auvergne, as a building material, or as at -Neidermendig in the Eifel and in Hungary for mill-stones; -<span class="pagenum" id="Page_104">- 104 -</span> -the compact portions are employed for building -and paving, and for road metal. The rock of some of -the modern lava-streams of Vesuvius is largely quarried -for paving the streets of Naples.</p> - -<p>This solid portion of the lava-streams in slowly -cooling down from its highly-heated condition undergoes -contraction, and in consequence is rent asunder -by a number of cracks. Sometimes these cracks -assume a wonderfully regular arrangement, and the -rock may be broken up into very symmetrical masses.</p> - -<div class="figcenter" id="fig27" style="width: 476px;"> - <img src="images/fig27.png" width="476" height="139" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 27.—Natural section of a Lava-stream in the Island of - Vulcano, showing the compact central portion and the - scoriaceous upper and under surfaces.</span></div> -</div> - -<div class="sidenote">COLUMNAR STRUCTURE OF LAVAS.</div> - -<p>If we imagine a great sheet of heated material, -like a lava-stream, slowly cooling down, it is evident that -the contraction which must take place in it will tend -to produce fissures breaking up the mass into prisms. -A little consideration will convince us what the form of -these prisms must be. There are only three regular -figures into which a surface can be divided, namely, -equilateral triangles, squares, and regular hexagons; -the first being produced by the intersection of sets of -six lines radiating at angles of 60° from certain centres; -<span class="pagenum" id="Page_105">- 105 -</span> -the second by the intersection of sets of <i>four</i> lines -radiating from centres at angles of 90°; and the third -from sets of <i>three</i> lines radiating from centres at an -angle of 120°. It is evident that a less amount of -contractile force will be required to produce the sets -of <i>three</i> cracks rather than those of four or six cracks; -or, in other words, the contractile force in a mass will -be competent to produce the cracks which give rise to -hexagons rather than those which form squares or triangles. -This is no doubt the reason why the prisms -formed by the cooling of lava, as well as those produced -during the drying of starch or clay, are hexagonal in -form.</p> - -<p>The hexagonal prisms or columns formed by contraction -during the consolidation of lavas vary greatly -in size, according to the rate of cooling, the nature of -the materials, and the conditions affecting the mass. -Sometimes such columns may be found having a -diameter of eight or ten feet and a length of five hundred -feet, as in the Shiant Isles lying to the north of -the Island of Skye; in other cases, as in certain volcanic -glasses, minute columns, an inch or two in length and -scarcely thicker than a needle, are formed; and examples -of almost every intermediate grade between these -two extremes may sometimes be found. The largest -columns are those which are formed in very slowly -cooling masses.</p> - -<p>The columnar structure is exhibited by all kinds of -lava, and indeed in other rock-masses which have been -<span class="pagenum" id="Page_106">- 106 -</span> -heated by contact with igneous masses and gradually -cooled. The rocks which display the structure in -greatest perfection, however, are the basalts.</p> - -<p>Mr. Scrope first called attention to the fact that the -upper and lower portions of lava-streams sometimes -cool in very different ways, and hence produce columns -of dissimilar character. The lower portion of the mass -parts with its heat very slowly, by conduction to the -underlying rocks, while the upper portions radiate heat -more irregularly into the surrounding atmosphere. -Hence we often find the lower portions of thick lava-streams -to be formed of stout, vertical columns of great -regularity; while the upper part is made up of smaller -and less regular columns, as shown in <a href="#fig28">fig. 28</a>.</p> - -<div class="figcenter" id="fig28" style="width: 395px;"> - <img src="images/fig28.png" width="395" height="194" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 28.—Section of a Lava-stream exposed on the side of the - river Ardèche, in the south-west of France.</span></div> -</div> - -<p>The remarkable grotto known as Fingal's Cave in -the Island of Staffa has been formed in the midst of a -lava-stream such as we have been describing; the -thick vertical columns, which rise from beneath the -level of the sea, are divided by joints and have been -<span class="pagenum" id="Page_107">- 107 -</span> -broken away by the action of the sea; in this way a -great cavern has been produced, the sides of which are -formed by vertical columns, while the roof is made up -of smaller and interlacing ones. The whole structure -bears some resemblance to a Gothic cathedral; the sea -finding access to its floor of broken columns, and permitting -the entrance of a boat during fine weather. -Similar, though perhaps less striking, structures are -found in many other parts of the globe wherever basaltic -and other lava-streams exhibit the remarkable columnar -structure as the result of their slow cooling. Portions of -basaltic columns are often employed for posts by the -road-sides, as in Central Germany and Bohemia, or for -paving stones, as in Pompeii and at the Monte Albano -near Rome.</p> - -<div class="figright" id="fig29" style="width: 155px;"> - <img src="images/fig29.png" width="93" height="157" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 29.—Portion of a Basaltic - Column from the Giant's Causeway, - exhibiting both the ball-and-socket - and the tenon-and-mortise structures.</span></div> -</div> - -<div class="sidenote">OTHER JOINT-STRUCTURES IN LAVAS.</div> - -<p>Occasionally basaltic lava-streams exhibit other curious -structures in addition to the columnar. Thus -some basaltic columns are -found divided into regular -joints by equidistant, curved -surfaces, the joints thus -fitting into one another by -a kind of ball-and-socket -arrangement. Sometimes -we find processes projecting -from the angles of the -curved joint-surfaces, which -cause the blocks to fit together -as with a tenon and mortise. This kind of -<span class="pagenum" id="Page_108">- 108 -</span> -structure is admirably displayed at the Giant's Causeway, -Co. Antrim, in the North of Ireland. A portion -of a basaltic column from this locality is represented -in <a href="#fig29">fig. 29</a>.</p> - -<div class="figcenter" id="fig30" style="width: 352px;"> - <img src="images/fig30.png" width="352" height="340" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 30.—Vein of green Pitchstone, at Chiaja di Luna in the - Island of Ponza, breaking up into regular columns, and into - spherical masses with a concentric series of joints.</span></div> -</div> - -<p>While the ordinary columnar structures are very -common in basalts, the ball-and-socket and tenon-and-mortise -structures are exceedingly rare. The question -of the mode of origin of these remarkable structures -has given rise to much discussion, and the -opinions of geologists and physicists are by no means -unanimous upon the subject.</p> - -<p>Sometimes we find masses of lava traversed by curved -<span class="pagenum" id="Page_109">- 109 -</span> -joints, and occasionally we find curious combinations -of curved and plane joints, giving rise to appearances -scarcely less remarkable than those presented by the -columns of the Giant's Causeway. Some of the more -striking examples of this kind have been described and -explained by Professor Bonney.</p> - -<div class="figcenter" id="fig31" style="width: 462px;"> - <img src="images/fig31.png" width="462" height="258" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 31.—Illustration of the 'Perlitic structure' in glassy Rocks.</span><br /> - a. Perlltic structure, as seen in a lava from Hungary.<br /> - b. The same structure, artificially produced in Canada Balsam during cooling.<br /> - </div> -</div> - - -<div class="sidenote">PERLITIC-STRUCTURE IN LAVAS.</div> - -<p>In the Ponza Islands there occurs a remarkable -example of a columnar pitchstone, which is also traversed -by a member of curved concentric joints, causing the -rock to break up into pieces like the coats of an onion. -This remarkable rock-mass is represented in <a href="#fig30">fig. 30</a>.</p> - -<p>A very similar structure is often seen in certain -glassy lavas, when they are examined in thin sections -under the microscope. Such glassy lavas exhibit the -<span class="pagenum" id="Page_110">- 110 -</span> -peculiar lustre of mother-of-pearly doubtless in consequence -of the interference of light along the cracks. -Lavas exhibiting this character are known to geologists -as 'perlites.' The perlitic structure has been produced -artificially by Mr. Grenville Cole in Canada Balsam, and -by MM. Fonqué and Michel Lévy, in chemically -deposited silica. See <a href="#fig31">fig. 31</a>.</p> - -<p>A thick lava-stream must take an enormous period -to cool down—probably many hundreds or even thousands -of years. It is possible to walk over lava-streams -in which at a few inches below the surface the rock is -still red-hot, so that a piece of stick is lighted if -thrust into a crack. Lava is a very bad conductor of -heat, and loose scoriæ and dust are still worse conductors. -During the eruption of Vesuvius in 1872, -masses of snow which were covered with a thick layer -of scoriæ, and afterwards by a stream of lava, were found -three years afterwards consolidated into ice, but not -melted. The city of Catania is constantly supplied -with ice from masses of snow which have been buried -under the ejections of Etna.</p> - -<p>During the cooling down of lavas, the escape of -steam and various gases gives rise to the deposition of -many beautiful crystalline substances in the cavities -and on the surfaces of the lava. Deposits of sulphur, -specular-iron, tridymite, and many other substances -are often thus produced, and the colour and appearance -of the rock-masses are sometimes completely disguised -by these surface incrustations, or by the decomposition -<span class="pagenum" id="Page_111">- 111 -</span> -of the materials of the lava by the action of the add -gases, and vapours upon it.</p> - -<div class="sidenote">SINKING OF SURFACES OF LAVA-STREAMS.</div> - -<p>Very frequently the surface of a lava-stream becomes -solid, while the deeper portions retain their fluid condition; -under such circumstances the central portions -may flow away, leaving a great hollow chamber or cavern. -In consequence of this action, we not unfrequently find -the upper surface of a lava-current exhibiting a depression, -due to the falling in of the solidified upper -portions when the liquid lava has flowed away and left -it unsupported, as in <a href="#fig32">fig. 32</a>.</p> - -<div class="figcenter" id="fig32" style="width: 390px;"> - <img src="images/fig32.png" width="390" height="75" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 32.—Transverse section of a Lava stream.</span> -(The dotted line indicates the original surface.)</div> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_112">- 112 -</span></p> - -<h2 class="nobreak" id="CHAPTER_V">CHAPTER V<br /> - -<span class="smaller">THE INTERNAL STRUCTURE OF VOLCANIC MOUNTAINS.</span></h2> -</div> - - -<p class="p0"><span class="smcap">Near</span> the high-road which passes between the towns -of Eger and Franzenbad in Bohemia, there rises a small -hill known as the Kammerbühl (see <a href="#fig33">fig. 33</a>), which has -attracted to itself an amount of interest and attention -quite out of proportion to its magnitude or importance. -During the latter part of the last century and the -earlier years of the present one, the fiercest controversies -were waged between the partisans of rival -schools of cosmogony over this insignificant hill; some -maintaining that it originated in the combustion of -a bed of coal, others that its materials were entirely -formed by some kind of 'aqueous precipitation,' and -others again that the hill was the relic of a small -volcanic cone.</p> - -<p>Among those who took a very active part in this -controversy was the poet Goethe, who stoutly maintained -the volcanic origin of the Kammerbühl, styling -it 'a pocket edition of a volcano.' To Goethe belongs -the merit of having suggested a Very simple method -by which the controversies concerning this hill might -<span class="pagenum" id="Page_113">- 113 -</span> -be set at rest: he proposed that a series of excavations -should be undertaken around the hill, and a tunnel -driven right under its centre.</p> - -<div class="figcenter" id="fig33" style="width: 484px;"> - <img src="images/fig33.png" width="484" height="315" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 33.—The Kammerbühl of Kammerberg, Bohemia.</span><br /> - (As seen from the south-west)</div> -</div> - -<div class="sidenote">THE KAMMERBÜHL.</div> - -<p>The poet's friend, Count -Caspar von Sternberg, determined to put this project -into execution. This series of excavations, which was -completed in 1837, has for ever set at rest all doubts -as to the volcanic origin of the Kammerbühl. A plug -of basalt was found filling the centre of the mass, and -connected with a small lava-stream flowing down the -side of the hill; while the bulk of the hill was shown -to be composed of volcanic scoriæ and lapilli. The -<span class="pagenum" id="Page_114">- 114 -</span> -section <a href="#fig34">fig. 34</a> will illustrate the structure of the hill -as revealed by these interesting excavations.</p> - -<div class="figcenter" id="fig34" style="width: 494px;"> - <img src="images/fig34.png" width="494" height="158" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 34.—Section of the Kammerbühl, in Bohemia.</span><br /> - <i>a a.</i> Metamorphic rocks. <i>b.</i> Basaltic scoriæ. <i>c.</i> Solid plug of basalt rising - through the centre of the volcanic pile, <i>d d.</i> Lava-stream composed of the - same rock. <i>e e.</i> Alluvial matter surrounding the old volcano.<br /> - - (The dotted lines indicate the probable former outline of the volcano.)</div> -</div> - -<div class="sidenote">VOLCANOES DISSECTED BY DENUDATION.</div> - -<p>It can of course very seldom happen that actual -mining operations, like those undertaken in the case of -the Kammerbühl, will be resorted to in order to determine -the structure of volcanic mountains. Geologists -have usually to avail themselves of less direct, but by -no means less certain, methods than that of making -artificial excavations in order to investigate the earth's -crust. Fortunately it happens that what we cannot -accomplish ourselves, nature does for us. The action -which we call 'denudation' serves as a scalpel to dissect -volcanic mountains for us, and to expose their inner -recesses to our view. Many portions of the earth's -surface are complete museums crowded with volcanic -'subjects,' exhibiting every stage of the process of -dissection. In some, rains and winds have stripped off -the loose covering of cinders and dust, and exposed the -<span class="pagenum" id="Page_115">- 115 -</span> -harder and more solid parts—the skeleton of the mountain. -In others, the work of destruction has proceeded -still further, and slowly wearing rivers or the waves of -the sea may have cut perfect, vertical sections of the -mountain-mass. Sometimes the removal of the materials -of the volcanic mountain has gone on to such an -extent that its base and ground-plan are fully exposed. -It only requires the necessary skill in piecing together -our observations on these dissected volcanoes, in order -to arrive at just views concerning the 'comparative -anatomy' of volcanoes. As the knowledge of the -structure of animals remained in the most rudimentary -condition until the practice of dissection was commenced, -so our knowledge of volcanoes was likewise -exceedingly imperfect till geologists availed themselves -of the opportunities afforded to them of studying naturally -dissected volcanic mountains.</p> - -<p>In some cases we may find that the sea has encroached -on the base of a volcanic hill, till one half of -it has been washed away, and the structure of the mass -to its very centre is exposed to our view. Thus in -<a href="#fig06">fig. 6</a> (page 43), it will be seen that there lies in front -of Vulcano a peninsula called Vulcanello, consisting of -three volcanic cones, united at their base, with the lava-streams -which have flowed from them. One half of -the cone on the left-hand side of the picture has been -completely washed away by the sea, and a perfect section -of the internal structure of the cone is exposed. -The appearances presented in this section are shown in -<span class="pagenum" id="Page_116">- 116 -</span> -the sketch, <a href="#fig35">fig. 35</a>. Some portions of the face of this -section are concealed by the heaps of fragments which -have fallen from it, but enough is visible to convince -us that three kinds of structures go to make up the -cone. In the first place, we have the loose scoriæ and -lapilli, which in falling through the air have arranged -themselves in tolerably regular layers upon the sides -of the cone.</p> - -<div class="figcenter" id="fig35" style="width: 465px;"> - <img src="images/fig35.png" width="465" height="259" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 35.—Natural section or a Volcanic Cone in the Island of - Vulcano.</span><br /> - <i>a.</i> Crater. <i>b b.</i> Lava-streams. <i>c.</i> Dykes which have clearly formed the ducts, - through which the lava has risen to the crater. <i>d d.</i> Stratified volcanic scoriæ. - <i>e.</i> Talus of fallen materials.</div> -</div> - -<p>In the second place, we have lava-streams -which have been ejected from the crater or from fissures -on the flanks of the cone, and flowed down its sides. -And thirdly, we find masses of lava filling up cracks in -the cone; these latter are called 'dykes.' Of these -three kinds of structures most volcanic mountains are -<span class="pagenum" id="Page_117">- 117 -</span> -built up, but in different cases the part played by these -several elements may be very unequal. Sometimes -volcanoes consist entirely of fragmentary materials, -at others they are made up of lavas only, while in the -majority of cases they have been formed by alternations -of fragmentary and fluid ejections, the whole -being bound together by dykes, which are masses of -lava injected into the cracks formed from time to time -in the sides of the growing cone.</p> - -<p>If we direct our attention in the first place to the -fragmentary ejections, we shall find that they affect a -very marked and peculiar arrangement, which is best -exhibited in those volcanic cones composed entirely of -such materials.</p> - -<div class="sidenote">INTERNAL STRUCTURE OF VOLCANIC CONES.</div> - -<p>Everyone who examines volcanoes for the first time -will probably be struck by the regular stratification of -materials of which they are composed. Thus the tuffs -covering the city of Pompeii are found to consist of -numerous thin layers of lapilli and volcanic dust, -perfectly distinct from one another, and assuming even -the arrangement which we usually regard as characteristic -of materials that have been deposited from a -state of suspension in water. The fragmentary materials -in falling through the air are sorted, the finer -particles being carried farther from the vent than the -larger and heavier ones. The force of different volcanic -outbursts also varies greatly, and sometimes materials -of different character are thrown out during successive -ejections. These facts will be illustrated by <a href="#fig36">fig. 36</a>, -<span class="pagenum" id="Page_118">- 118 -</span> -which is a drawing of a section exposed in a quarry -opened in the side of the Kammerbühl. In this section -we see that the falling scoriæ have been arranged in -rudely parallel beds, but the regular deposition of these -has been interrupted by the ejection of masses of burnt -slate torn from the side of the vent, probably during -some more than usually violent paroxysm of the volcano. -In those volcanoes which are built up of tuffs and -materials which have fallen in the condition of a muddy -paste, the perfect stratification of the mass is often very -striking indeed, and large cones are found built up of -thin uniformly-spread layers of more or less finely-divided -materials, disposed in parallel succession. Such -finely-stratified tuff-cones abound in the district of the -Campi Phlegræi.</p> - -<div class="figcenter" id="fig36" style="width: 325px;"> - <img src="images/fig36.png" width="325" height="211" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 36.—Section in the side of the Kammerbühl, Bohemia.</span><br /> - <i>a a.</i> stratified basaltic scoriæ. <i>b b.</i> Bands made up of fragments of burnt slate. - <i>c.</i> Stratified basaltic scoriæ. <i>d d.</i> Pseudo-dykes occupying lines of fault.</div> -</div> - -<p><span class="pagenum" id="Page_119">- 119 -</span></p> - -<div class="sidenote">ARRANGEMENT OF FRAGMENTAL MATERIALS.</div> - -<p>If, in consequence of any subterranean movements, -fissures are produced in the sides of the cones formed -of fragmentary materials, these often become gradually -filled with loose fragments from the sides of the fissure, -and in this manner 'pseudo-dykes' are formed. An -example of such pseudo-dykes is represented in <a href="#fig36">fig. 36</a>, -where the beds composing the volcanic cone of the -Kammerbühl are seen to have been broken across or -faulted, and the fissures produced in the mass have -been gradually filled with loose fragments.</p> - -<p>It is not difficult to imitate, on a small scale, the -conditions which exist at those volcanic vents from -which only fragmentary materials are ejected. If we -take a board having a hole in its centre, into which a -pipe is inserted conveying a strong air-blast, we shall, -by introducing some light material like bran or sawdust -into this pipe cause an ejection of fragments, -which will, when the board is placed horizontally, -fall around the orifice of the pipe and accumulate there -in a conical heap (<a href="#fig37">fig. 37</a>). It will be found necessary, -as was shown by Mr. Woodward, who performed the -experiment before the Physical Society, to adopt some -contrivance, such as a screw, for forcing the material -into the air-pipe. If we alternately introduce materials -of different colours, like mahogany- and deal-sawdust -into the pipe, these materials will be arranged in layers -which can be easily recognised, and the mode of accumulation -of the mass will be evident. By means of a -sheet of tin or cardboard we may divide this miniature -volcanic cone vertically into two portions, and if we -<span class="pagenum" id="Page_120">- 120 -</span> -sweep one of these away the internal structure of the -other half will be clearly displayed before our eyes.</p> - -<p>In this way we shall find that the conical heap of -sawdust with the hole in its centre has a very peculiar -and definite arrangement of its materials. It is made up -of a number of layers each of which slopes in opposite -directions, towards the centre of ejection and away -from that centre. These layers are thickest along the -line of the circle where the change in slope takes place, -and they thin away in the direction of the two opposite -slopes.</p> - -<div class="figcenter" id="fig37" style="width: 452px;"> - <img src="images/fig37.png" width="452" height="280" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 37.—Experimental illustration of the mode of Formation - of Volcanic Cones composed of fragmental materials.</span></div> -</div> - -<div class="sidenote">CAUSE OF THIS ARRANGEMENT.</div> - -<p>The cause of this peculiar arrangement of the -materials is evident. The sawdust thrown up by the -air blast descends in a shower and tends to accumulate -in a circular heap around the orifice, the area of this -<span class="pagenum" id="Page_121">- 121 -</span> -circular heap being determined by the force of the -blast. Within this circular area, however, the quantity -of falling fragments is not everywhere the same; along -a circle surrounding the vent at a certain distance, the -maximum number of falling fragments will be found to -descend, and here the thickest deposit will take place. -As this goes on, a circular ridge will be formed, with -slopes towards and away from the centre of injection. -As the ridge increases in height, the materials will tend -to roll down either one slope or the other, and gradually -a structure of the form shown in the figure will be piled -up. The materials sliding down the outer slope will -tend to increase the area of the base of the cone, while -those which find their way down the inner slope will -fall into the vent to be again ejected.</p> - -<div class="figcenter" id="fig38" style="width: 445px;"> - <img src="images/fig38.png" width="445" height="167" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 38.—Natural section of a Tuff-cone forming the Cape of - Misenum, and exhibiting the peculiar internal Arrangement - characteristic of volcanoes composed of fragmentary - materials.</span></div> -</div> - -<p>Volcanic cones composed of scoriæ, dust, &c. are -found to have exactly the same internal structure as is -exhibited by the miniature cone of sawdust. The more -<span class="pagenum" id="Page_122">- 122 -</span> -or less regular layers of which they are made up dip in -opposite directions, away from and towards the vent, -and thin out in the direction of their dip (see <a href="#fig38">fig. 38</a>). -In small cones the crater or central cavity is of considerable -size in proportion to the whole mass, but as -the cone grows upwards and outwards, the dimensions -of the crater remain the same, while the area of the -base and the height of the cone are continually increasing. -This is the normal structure of volcanic cones -formed of fragmentary materials, though, as we shall -hereafter show, many irregularities are often produced -by local and temporary causes.</p> - -<div class="figcenter" id="fig39" style="width: 482px;"> - <img src="images/fig39.png" width="482" height="218" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 39.—Section of a small Scoria-cone formed within the - crater of Vesuvius in the year 1885, illustrating the - filling-up of the central tent of the cone by subsequent - ejections.</span></div> -</div> - -<p>In some cases the central vent of a volcanic scoria-cone -may be filled up by subsequent ejections. A -beautiful example of this kind was observed by Abich, -<span class="pagenum" id="Page_123">- 123 -</span> -in the case of a small cone formed within the crater of -Vesuvius in 1835, and is represented in <a href="#fig39">fig. 39</a>.</p> - -<div class="figcenter" id="fig40" style="width: 438px;"> - <img src="images/fig40.png" width="438" height="135" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 40.—Volcanic Cones composed of Scoriæ, and breached on - one side by the outflow of lava-currents.</span></div> -</div> - -<div class="sidenote">BREACHED CONES.</div> - -<p>Many cones formed in the first instance of scoriæ, -tuff, and pumice may give rise to streams of lava, before -the vent which they surround sinks into a state -of quiescence. In these cases, the liquid lava in the -vent gives off such quantities of steam that masses of -froth or scoriæ are formed, which are ejected and -accumulate around the orifice. When the force of the -explosive action is exhausted, the lava rises bodily in -the crater, which it more or less completely fills. But, -eventually, the weaker side of the crater-wall yields -beneath the pressure of the liquid mass, and this part -of the crater and cone is swept away before the advancing -lava-stream. Examples of such 'breached -cones' abound in Auvergne and many other volcanic -districts (see <a href="#fig40">fig. 40</a>). A beautiful example of a cone -formed of pumice, which has been breached by the -outflow of a lava-stream of obsidian, occurs in the -Lipari Islands, at the Rocche Rosse. It is this locality -<span class="pagenum" id="Page_124">- 124 -</span> -which supplies the whole world with pumice (see -<a href="#fig41">fig. 41</a>).</p> - -<div class="figcenter" id="fig41" style="width: 696px;"> - <img src="images/fig41.png" width="696" height="299" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 41.—Campo Bianco, in the Island of Lipari. - A Pumice-cone breached by the Outflow of an Obsidian Lava-current.</span></div> -</div> - -<p>It is often surprising to find how volcanic cones composed -of loose materials, such as tuffs, scoriæ, or pumice, -retain their distinctive forms, and even the sharpness -of their outlines, during enormous periods of time. -Thus, in the scoria-cones which abound in the Auvergne, -and were, in all probability, formed before the historical -period, the sharp edges of the craters appear to have -suffered scarcely any erosion, and the cones are as perfect -in their outlines as though formed but yesterday. -It is probable that the facility with which these cindery -heaps are penetrated by the rain which falls upon them -is the cause why they are not more frequently washed -away.</p> - -<div class="figcenter" id="fig42" style="width: 462px;"> - <img src="images/fig42.png" width="462" height="112" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 42.—Volcanic Cones in Auvergne which have suffered to - some extent from atmospheric denudation.</span></div> -</div> - -<p>Sometimes, however, scoria-cones are found reduced -by atmospheric waste to mere heaps of cinders, in -which the position of the crater is indicated only by a -slight depression, as in <a href="#fig42">fig. 42</a>.</p> - - -<p><span class="pagenum" id="Page_125">- 125 -</span></p> - -<div class="sidenote">CONES COMPOSED OF LAVA.</div> - -<p>When but little explosive action takes place at the -volcanic vent, and only fluid lava is ejected, mountains -are formed differing very greatly in character from the -cones composed of fragmentary materials.</p> - -<p>If the lavas be of very perfect liquidity, like those -erupted in the Sandwich Islands, they flow outwards -around the vent to enormous distances. By the accumulation -of materials during successive outbursts, a -conical mass is built up which has but a slight elevation -in proportion to the area of its base. Thus in Hawaii -we find great volcanic cones, composed of very fluid -lavas, which have a height of nearly 14,000 feet with a -diameter of base of seventy miles. In these Hawaiian -mountains the slope of the sides rarely exceeds 6° to 8°.</p> - -<p>But if, on the other hand, the lavas be of much -more viscid consistency, the character of the volcanic -cones which are produced by their extrusion will be -very different. The outwelling material will tend to -accumulate and heap itself up around the vent. By -successive ejections the first-formed shell is forced upwards -and outwards, and a steep-sided protuberant mass -is formed, exhibiting in its interior a marked concentric -arrangement. Dr. Ed. Reyer, of Grätz, has devised a -very ingenious method for reproducing on a miniature -scale the characteristic features of these eruptions of -viscid lavas. He takes a quantity of plaster of Paris -reduced to a pasty consistence, which he forces through -a hole in a board. The plaster accumulates in a great -rounded boss about the orifice through which it has -been forced. If the plaster have some colouring matter -introduced into it, the mass, on being cut across, will -<span class="pagenum" id="Page_126">- 126 -</span> -exhibit in the disposition of its colour-bands the kind -of action which has gone on during its extrusion, <a href="#fig43">fig. 43</a>.</p> - -<div class="figcenter" id="fig43" style="width: 435px;"> - <img src="images/fig43.png" width="435" height="195" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 43.—Experimental illustration of the Mode of Formation - of volcanic cones composed of viscid lavas.</span></div> -</div> - -<div class="figcenter" id="fig44" style="width: 441px;"> - <img src="images/fig44.png" width="441" height="125" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 44.—The Grand Puy of Sarcoui, composed of trachyte, rising - between two breached scoria-cones (Auvergne).</span></div> -</div> - -<p>There are many volcanic cones which exhibit clear -evidence of having thus been formed by the extrusion -of a viscid mass of lava through a volcanic fissure. -Among such we may mention the domitic Puys of -Auvergne, <a href="#fig44">fig. 44</a>, many andesitic volcanoes in Hungary, -the phonolite hills of Bohemia, and the so-called -'mamelons' of the Island of Bourbon. See figs. <a href="#fig45">45</a> and -<a href="#fig46">46</a>. When the interior of these masses is exposed by -<span class="pagenum" id="Page_127">- 127 -</span> -natural or artificial sections, they are all found to -exhibit the onion-like structure which occurs in the -plaster models.</p> - -<div class="sidenote">INTERNAL STRUCTURE OF LAVA-CONES.</div> - -<div class="figcenter" id="fig45" style="width: 293px;"> - <img src="images/fig45.png" width="293" height="186" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 45.—Volcanic Cone (Mamelon) composed of very viscid lava. - (Island of Bourbon.)</span></div> -</div> - -<div class="figcenter" id="fig46" style="width: 400px;"> - <img src="images/fig46.png" width="400" height="199" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 46.—Another Mamelon in the Island of Bourbon, with a - crater at its summit.</span></div> -</div> - -<p>But while some volcanoes are composed entirely of -the fragmentary ejections and others are wholly formed -by successive outflows of lava, the majority of volcanoes, -especially those of larger dimensions, are built -up of alternations of these different kinds of materials.</p> - -<p><span class="pagenum" id="Page_128">- 128 -</span></p> - -<div class="figcenter" id="fig47" style="width: 440px;"> - <img src="images/fig47.png" width="440" height="303" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 47.—Cliff-section in the Island of Madeira, showing how - a composite volcano is built up of lava-streams, beds of - scoriæ, and dykes.</span></div> -</div> - -<div class="sidenote">NATURAL SECTIONS OF CONES.</div> - -<p>The structure of these composite cones may be -understood by an inspection of the accompanying <a href="#fig47">fig. 47</a>, -which shows the appearances presented in a cliff -on the coast of the Island of Madeira. We see that the -mass is made up of numerous layers of volcanic scoriæ, -alternating with sheets of lava. The latter, which are -represented in transverse section in the drawing, are -seen to thin out on either side, and to vary greatly in -breadth. Besides the alternating masses of scoriæ and -the lava-sheets, there are seen in the section, bands of -a bright-red colour, which are represented in the drawing -by black lines. These are layers of soil, or volcanic -dust, which, by the passage of a lava-stream over -<span class="pagenum" id="Page_129">- 129 -</span> -their surface, have been burnt so as to acquire a brick-red -colour. These bands of red material, to which the -name of 'laterite' has been frequently applied, very -commonly occur in sections of composite volcanic cones. -Crossing the whole of the horizontally-disposed masses -in the section, we find a number of 'dykes,' which are -evidently great cracks filled with lava from below. -Some of these run vertically through the cliffs, others -obliquely. In some cases the lava, rising to fill a dyke, -has flowed as a lava-stream at the surface. Last of all, -we must call attention to the fact that the section exhibits -evidence of great movements having taken place -subsequently to the accumulation of the whole of the -materials. A great crack has been produced, on one -side of which the whole mass has subsided bodily, -giving rise to the phenomenon which geologists call -a 'fault.'</p> - -<p>In the section, <a href="#fig27">fig. 27</a>, p. 104, copied from a drawing -of a sea-cliff in the Island of Vulcano, a transverse -section of a lava-stream is represented on a somewhat -larger scale. The upper and under surface of the lava-stream -is seen to have a scoriaceous structure, but the -thick central mass is compact, and divided by regular -joint-planes. This section also illustrates the fact that, -before the lava-stream flowed down the sides of the -mountain, a valley had been cut by meteoric agencies -on the flanks of the volcano, the dykes which traverse -the lower beds of tuff being abruptly truncated.</p> - -<p>In mountain ravines, upon the slopes of ancient -<span class="pagenum" id="Page_130">- 130 -</span> -volcanoes, and in the cliffs of volcanic islands, we are -often able to study the way in which these great mountain -masses are built up of alternating lava-currents, -beds of volcanic agglomerate, scoriæ, tuff and dust, and -intersecting dykes. In <a href="#fig48">fig. 48</a>, the features above -described are illustrated by a section in the sides of -the great volcano of Mont Dore.</p> - -<div class="figcenter" id="fig48" style="width: 416px;"> - <img src="images/fig48.png" width="416" height="295" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 48.—Section seen at the cascade. Bains du Mont Dore.</span></div> -</div> - -<div class="figcenter" id="fig49" style="width: 451px;"> - <img src="images/fig49.png" width="451" height="146" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 49.—Section in the Island of Ventotienne, showing a great - stream of andesitic lava overlying stratified tuffs.</span></div> -</div> - -<p><span class="pagenum" id="Page_131">- 131 -</span></p> - -<div class="sidenote">SECTIONS IN THE PONZA ISLANDS.</div> - -<p>In figs. <a href="#fig49">49</a>, <a href="#fig50">50</a>, <a href="#fig51">51</a>, and <a href="#fig52">52</a>, we have given -drawings of portions of the sea-cliffs in several of -the Ponza Islands, a small volcanic group off the -Italian coast.</p> - -<div class="figcenter" id="fig50" style="width: 467px;"> - <img src="images/fig50.png" width="467" height="201" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 50.—Cliff on the south side of the Island of San - Stephano.</span><br /> - <i>a.</i> Trachyte lava-stream, with a scoriaceous upper surface overlaid by stratified - tuffs, <i>b</i>.</div> -</div> - -<div class="figcenter" id="fig51" style="width: 480px;"> - <img src="images/fig51.png" width="480" height="221" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 51.—The headland of Monte della Guardia, in the Island - of Ponza.</span><br /> - <i>a.</i> Columnar trachyte. <i>b.</i> Stratified tuffs. <i>c.</i> Pumiceous agglomerates. - <i>d.</i> Dyke of rhyolite.</div> -</div> - -<p><span class="pagenum" id="Page_132">- 132 -</span></p> - -<div class="figcenter" id="fig52" style="width: 480px;"> - <img src="images/fig52.png" width="480" height="232" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 52.—Western side of the same headland, as seen from the - north side of Luna Bay.</span><br /> - <i>a.</i> Trachyte lava. <i>b.</i> Stratified tuffs. <i>c.</i> Dykes of rhyolite, with their edges passing - into pitchstone. <i>d.</i> Pumiceous agglomerate.</div> -</div> - -<div class="figcenter" id="fig53" style="width: 470px;"> - <img src="images/fig53.png" width="470" height="253" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 53.—Sea-cliff at Il Capo, the north-east point of Salina - showing stratified agglomerates traversed by numerous - dykes, the whole being unconformably overlaid by stratified - aqueous deposits.</span></div> -</div> - -<p><a href="#fig53">Fig, 53</a> represents a cliff-section in the island of -Salina, one of the Liparis, exhibiting evidence that a -<span class="pagenum" id="Page_133">- 133 -</span> -series of volcanic agglomerates traversed by dykes of -Andesite have been denuded and covered by a recent -stratified deposit.</p> - -<div class="sidenote">PART PLAYED BY DYKES IN CONE-BUILDING.</div> - -<p>In the formation of these great composite cones, a -minor but by no means insignificant part is played by -the dykes, or lava-filled fissures, which are seen -traversing the mass in all directions. That dyke-fissures -often reach the surface of a volcanic cone, and -that the material which injects them then issues as a -lava-stream, is illustrated by <a href="#fig54">fig. 54</a>. The formation -of these cracks in a volcanic cone, and their injection -by liquid lava, must of course distend the mountainous -mass and increase its volume. If we visit the great -crater-walls of Somma in Vesuvius, and of the Val del -Bove in Etna, we shall find that the dykes are so -numerous that they make up a considerable portion of -the mass. When the loose scoriæ and tuffs are removed -by denudation, these hard dykes often stand -up prominently like great walls, as represented in -<a href="#fig55">fig. 55</a>. Even in such cases as these, however, it is -doubtful whether the bulk of all the dykes put together -exceeds one-tenth of that of the lavas and fragmentary -materials.</p> - -<div class="figcenter" id="fig54" style="width: 434px;"> - <img src="images/fig54.png" width="434" height="119" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 54.—Section observed in the Val del Bove, Etna, showing - a basaltic dyke, from the upper part of which a lava-current has flowed.</span></div> -</div> - -<p><span class="pagenum" id="Page_134">- 134 -</span></p> - -<div class="figcenter" id="fig55" style="width: 428px;"> - <img src="images/fig55.png" width="428" height="260" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 55.—Basaltic Dykes projecting from masses of stratified - scoriæ in the sides of the Val del Bove, Etna.</span></div> -</div> - -<p>Hence we are led by an examination of the internal -structure of volcanic mountains to conclude that scoriæ- -and tuff-cones, and cones formed of very liquid lavas, -increase by an <i>exogenous</i> mode of growth, all new -materials being added to them from without; in the -cones formed of very viscid lavas, on the other hand, -the growth is <i>endogenous</i>, taking place by successive -accretions within it. The composite cones owe their -origin to both the <i>exogenous</i> and the <i>endogenous</i> -modes of growth, but in a much greater degree to the -former than the latter. The layers of scoriæ, tuff, and -dust, and the successive lava-streams are added to the -<span class="pagenum" id="Page_135">- 135 -</span> -mass from without, and the lava forming the dykes -from within it.</p> - -<div class="sidenote">THEORY OF ELEVATION CRATERS.</div> - -<p>There are doubtless cases in which, when a tuff-cone -is formed, a mass of very viscid lavas is extruded into -its interior, and the mass is distended like a gigantic -bubble. But inasmuch as the very viscid lavas do not -appear to give rise to scoriæ to anything like the same -extent as the more liquid kinds, such 'cupolas,' as -they have been called by some German geologists, are -probably not very numerous, and may be regarded as -constituting the exception rather than the rule. The -idea which was formerly entertained by some geologists -that all great volcanic mountains were formed of masses -originally deposited in a horizontal position, and subsequently -blown up into a conical form, has been effectually -disposed of by the observations of Lyell and Scrope.</p> - -<p>The condition of the great fluid masses which -underlie volcanic vents is another point on which -much light has been thrown by the study of naturally-dissected -volcanoes. In some cases, as was shown by -Hochstetter during his admirable researches among -the New Zealand volcanoes, the rising lavas form a -great chamber for themselves in the midst of a volcanic -cinder-cone, taking the place of loose materials -which are re-ejected from the vent, or have been -re-fused and absorbed into the mass of lava itself. -From this central reservoir of lava, eruptions are kept -up for some time, but when the volcano sinks into a -state of quiescence the lava slowly consolidates. In -<span class="pagenum" id="Page_136">- 136 -</span> -such slowly solidified masses of lava, very beautiful -groups of radiating columns are often exhibited -Northern Germany abounds with examples of such -basaltic masses, which have once formed the centres of -great cinder-cones; but in consequence of the removal -of the loose materials and the surrounding strata by -denudation, these central reservoirs of the volcanoes -have been left standing above the surface, and exhibit -the peculiar arrangements of the columns formed in -them during the process of cooling.</p> - -<div class="sidenote">INTRUSIVE LAVA-SHEETS.</div> - -<p>But in the majority of the more solidly-built composite -volcanoes no such liquid reservoir can be formed -within the volcanic cone itself. Under these circumstances, -the lavas, especially those of more liquid -character, tend to force passages for themselves among -the rocks through which they are extruded. Wherever -a weak point exists, there such lavas will find their -way, and as the planes of stratification in sedimentary -rocks constitute such weak places, we constantly find -sheets of lava thus inserted between beds of aqueous -origin. The areas over which these intrusive sheets -of rock sometimes extend may be very great, but the -more fusible, basic lavas (basalt, &c.) usually form -much more widely-spreading sheets than the less -fusible, acid lavas. In some cases these great intrusive -sheets are found extending to a distance of twenty or -thirty miles from the centre at which they were -ejected, and they often follow the bedding of the strata -with which they are intercalated in so regular a manner, -<span class="pagenum" id="Page_137">- 137 -</span> -that it is difficult for an observer to believe at first -sight that they can have been formed in the way -which we have described. A closer examination will -generally reveal the fact that while these intrusive -lava-sheets retain their parallelism with the strata -among which they have been intruded, over considerable -areas, yet they sometimes break across, or send -offshoots into them, as shown in <a href="#fig56">fig. 56</a>. In all cases, -too, the rocks lying above and below such sheets will -be found to be more or less baked and altered, and -this affords a very convincing evidence of the intrusion -of the igneous mass between the strata so -altered.</p> - -<div class="figcenter" id="fig56" style="width: 451px;"> - <img src="images/fig56.png" width="451" height="178" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 56.—Sheets of Igneous Rock (Basalt) intruded between - beds of sandstone, clay, and limestone. (Island of Skye.)</span></div> -</div> - -<p>That in the case of most great volcanic mountains, -or systems of mountains, vast reservoirs of liquid lava -must exist in the earth's crust far below the surface, -there can be little room for doubt. Whether such -fluid masses are in direct or indirect communication -with a great central reservoir, even supposing such to -<span class="pagenum" id="Page_138">- 138 -</span> -exist, is a totally different question. In many cases -the outburst of volcanoes in more or less close proximity -has been observed to take place simultaneously, while -in others the commencement of the eruption of one -volcano has coincided with the lapse into quiescence -of another in its vicinity. On the other hand, the remarkable -case of the volcanoes of Hawaii seems to indicate -that two vents in close proximity may be supplied -from perfectly distinct reservoirs of lava. The active -craters of Mauna Loa and Kilauea are situated at the -heights of 14,000 and 4,000 feet respectively above the -sea level; yet the former is sometimes in a state of -violent activity, with which the latter shows no signs -of sympathy whatever. We shall, in a future chapter, -adduce evidence that the liquid lavas in underground -reservoirs may undergo various stages of change in the -enormous periods of time during which habitual volcanic -vents are supplied from them.</p> - -<p>We have already shown that the character assumed -by a mass of fused material in cooling varies greatly -according as the cooling takes place rapidly at the -surface or slowly under enormous pressure. In the -former case a glassy base is formed containing a greater -or smaller number of crystallites or embryo crystals, in -the latter the whole rock is converted into a mass of -fully-developed crystals.</p> - -<div class="sidenote">CONSOLIDATION OF LAVAS AT GREAT DEPTHS.</div> - -<p>The lavas which are poured out at the surface consist, -as we have seen, of a glassy magma in which a -greater or smaller number of crystals are found which -<span class="pagenum" id="Page_139">- 139 -</span> -have been borne up from below. The great dykes and -intrusive sheets consist for the most part of a mass of -small or imperfectly developed crystals in which a -number of large and perfectly formed crystals are embedded. -Such rocks are said to have a 'porphyritic' -structure. The rocks formed by the consolidation of -the liquid masses in the underground reservoirs are -found to be perfectly crystallised, the crystals impressing -one another on every side and making up the whole -mass to the exclusion of any paste or magma between -them. The crystals in those rocks which have consolidated -at these vast depths exhibit evidence, in their -enclosed watery solutions and liquefied carbonic acid, -of the enormous pressures under which they must have -been consolidated. The lavas, the more or less porphyritic -rocks of the dykes and sheets, and the perfectly -crystalline (granitic) rocks of the underground reservoirs -pass into one another, however, by the most insensible -gradations.</p> - -<p>We sometimes find examples of volcanoes which, by -the action of denuding forces, have had their very -foundations exposed to our view. Such examples occur -in the Western Isles of Scotland, in the Euganean Hills -near Padua in Northern Italy, and in many other parts -of the earth's surface. In these cases we are able to -trace the ground-plan of the volcanic pile, and to study -the materials which have consolidated deep beneath -the surface in the very heart of the mountain.</p> - -<p>In studying these 'basal wrecks' of old volcanoes -<span class="pagenum" id="Page_140">- 140 -</span> -it is always necessary to bear in mind that the appearance -and general characters of a volcanic rock may be -completely disguised by chemical changes going on -within it. It is through want of attention to this fact -that so many mistakes were made by the Wernerian -school of geologists who declared that they could find -no analogy between the basaltic rocks of the globe and -the products of active volcanoes, and were hence led to -refer the origin of the former to some kind of 'aqueous -precipitation.'</p> - -<p>Many of the hard and crystalline marbles which are -employed as ornamental stones were originally loose -masses of shells and corals, as we easily perceive when -we examine the polished faces. But these incoherent -heaps of organic <i>débris</i> have been converted into a compact -and solid rock in consequence of the mass being -penetrated by water containing carbonate of lime in solution. -Crystals of this substance were deposited in every -cavity and interstice of the mass, and thus the accumulation -of separate organisms was gradually transformed -to a material of great solidity and hardness.</p> - -<div class="sidenote">FORMATION OF AMYGDALOIDS.</div> - -<p>In precisely the same way loose heaps of scoriæ, -lapilli, or pumice may, by the passage through them of -water containing various substances in solution, have -their vesicles filled with crystals, and thus be converted -into the hardest and most solid of rock-masses. Similarly -the scoriaceous portions of lava-streams have their -vesicles filled with crystalline substances deposited from -a state of solution, and are thus converted into a solid -<span class="pagenum" id="Page_141">- 141 -</span> -mass which may at first sight appear to offer but little -resemblance to the vesicular materials of recent lava-streams. -To these vesicular rocks which have their -cavities filled with crystalline substances geologists -apply the name of amygdaloids (<i>L. amygdalus</i>, an -almond). The cavities in lava-rocks are usually more -or less elongated, owing to the movement of the mass -while in a still plastic state, and the crystalline materials -filling these cavities take the almond-like shape; -hence the name.</p> - -<p>When the amygdaloids and altered fragmentary -ejections of volcanoes are studied microscopically, their -true character is at once made manifest. The exposure -of faces of these altered volcanic rocks to the -weathering influences of the atmosphere, in many -cases also causes their true nature to be revealed, -the crystalline materials filling the interstices and -vesicles of the mass are dissolved away by the rain-water -containing carbonic acid, and the rock regains its -original cavernous structure and appearance. But this -repeated passage of water through volcanic rock-masses -may result in the removal of so large a portion of their -materials that the remainder crumbles down into the -condition of a clay or mud.</p> - -<p>In the basal wrecks of volcanoes, of which we have -spoken, we usually find only small and fragmentary -remains of the great accumulations of loose and scoriaceous -materials which originally constituted the bulk -of the mountain mass. In the centre of the ground-plan -<span class="pagenum" id="Page_142">- 142 -</span> -of such a denuded volcano we find great masses -of highly crystalline or granitic rock, which evidently -occupy vast fissures broken through the sedimentary -or other rocks upon which the volcanic pile has been -reared. These highly crystalline rocks exhibit, as we -have shown, clear evidence of having been consolidated -from a state of fusion with extreme slowness and under -enormous pressure, but their ultimate chemical composition -is identical with that of the lavas which have -been ejected from the volcano.</p> - -<p>When, as frequently happens, the volcano, after -pouring out one kind of lava for a certain period, has -changed the nature of its ejections, and given rise to -materials of different composition, we find clear evidence -of the fact in studying the basal wreck or -ground-plan of the volcano. A great intrusive crystalline -mass, of the same chemical composition as the -first-extruded lava, is found to be rent asunder and -penetrated by a similarly crystalline mass having the -composition of the lavas of the second period. Thus, -in the volcanoes of the Western Isles of Scotland, which -are reduced by the action of denudation to this condition -of basal wrecks, we find that rhyolites, trachytes, -and andesites were ejected during the earlier periods -of their history, and basalts during the later periods.</p> - -<div class="figcenter" id="fig57" style="width: 444px;"> - <div class="figcaption"><span class="smcap">Fig. 57.—Plan of the Dissected Volcano of Mull, - in the Inner Hebrides.</span></div> - <img src="images/fig57.png" width="444" height="494" alt="" /> -</div> - - -<div class="figcenter" id="fig58" style="width: 460px; padding-top: 12px;"> -<div class="figcaption"><span class="smcap">Fig. 58.—Section of the Volcano along the line <i>A B</i>.</span></div> - - <img src="images/fig58.png" width="460" height="136" alt="" /> - -<table summary="mapkey"> -<tr> - <td class="tdl smaller"> - <i>a</i> Rocks on which the Volcano has been built up.<br /> - <i>b</i> Great intrusive masses of acid and intermediate rocks.<br /> - <i>c</i> Lara currents of basalt which have flowed from <i>d</i>.<br /> - <i>d</i> Intrusive masses of gabbros & dolerite.<br /> - <i>e</i> Lava currents which have flowed from <i>b</i>.<br /> - <i>f</i> Volcanic tuffs and agglomerates.</td> -</tr> -</table> -</div> - -<p><span class="pagenum" id="Page_143">- 143 -</span></p> - -<div class="sidenote">ANCIENT VOLCANO OF MULL.</div> - -<p>We perceive on studying the ground-plan of these volcanoes -that great masses of granite, syenite, and diorite—the -crystalline representatives of the first-extruded -lavas—are penetrated by intrusions of gabbro—the -granitic form of the later-ejected lavas. These features -are admirably illustrated by the ruined volcano now -constituting the Island of Mull, one of the Inner -Hebrides, a plan of which is given in <a href="#fig57">fig. 57</a>, and a section -in <a href="#fig58">fig. 58</a>. This volcano probably had a diameter -at its base of nearly thirty miles, and a height of from -10,000 to 12,000 feet, but is now reduced to a group of -hills few of which exceed 3,000 feet in height.</p> - -<p>From these great intrusive masses of highly crystalline -rocks there proceed in every direction great spurs -or dykes, which are evidently the radiating fissures -formed during the outwelling of igneous materials -from below, injected by these fluid substances. The -rock forming these dykes is often less perfectly crystalline -than that which constitutes the centre of the mass, -and we may indeed detect among the materials of these -dykes examples of every variety of structure, from the -perfectly crystalline granite to the more or less glassy -substance of lavas. Besides the vertical or oblique -dykes we also find horizontal sheets, which, passing -from these central masses, have penetrated between -the surrounding strata, often, as we have seen, to -enormous distances.</p> - -<p>For the sake of simplicity, we have spoken of these -ground-plans, or basal wrecks of volcanoes, as constituting -a flat plain; as a matter of fact, however, the -unequal hardness of the materials composing volcanic -mountains causes them to assume, under the influence -of denuding agencies, a very rugged and uneven surface. -<span class="pagenum" id="Page_144">- 144 -</span> -The hard crystalline materials filling the central -vent stand up as great mountain groups; each large -dyke, by the removal of the surrounding softer materials, -is left as a huge wall-like mass, while the remnants -of lava-streams are seen constituting a number -of isolated plateaux.</p> - -<p>The great Island of Skye is the basal wreck of -another volcano which was also in eruption during -Tertiary times; probably, many millions of years ago. -This immense volcano had originally a diameter at its -base of about thirty miles, and a height of 12,000 to -15,000 feet, and must have been comparable to Etna -or Teneriffe in its dimensions. At the present time, -there is nothing left of this vast pile but the highly -crystalline granites and gabbros filling up the great -fissures through which the eruption of igneous materials -took place. These, worn by denudation into -rounded dome-like masses and wild rugged peaks, constitute -the Red Mountains and Coolin Hills of Skye, -which rise to the height of more than 3,000 feet above -the sea-level. From these great, central masses of -crystalline rocks, innumerable radiating dykes may be -found rising through the surrounding rock-masses, -with isolated patches of the scoriæ and lapilli ejected -from the volcano, which have here and there escaped -removal by denudation. Along what were the outskirts -of this great mountain-mass are found flat-topped hills, -built up of lava-streams, only small portions of which -have escaped removal by denudation.</p> - -<p><span class="pagenum" id="Page_145">- 145 -</span></p> - -<div class="sidenote">RESERVOIRS BENEATH VOLCANOES.</div> - -<p>But this wearing away of the structure of a volcanic -cone by the denuding forces may proceed even one -stage farther, and we may then have revealed for our -inspection and study the mass of originally fluid -materials, from which one or more volcanoes have been -fed, cooled and consolidated in their original reservoir. -There are many examples of masses of granitic or -highly crystalline rocks, having precisely the same -composition as the different varieties of lavas, which -are found lying in the midst of the sedimentary rocks, -and sending off into these rocks veins and dykes of the -same composition with themselves. No one who has -carefully studied the appearances presented by volcanic -mountains in different stages of dissection, by the -action of denuding forces, can avoid recognising these -great granitic masses as the cooled reservoirs from -which volcanoes have in all probability been supplied -during earlier periods of the earth's history.</p> - -<p>The eruption of these great masses of incandescent -rock, impregnated with water and acid gases, through -strata of limestone, sandstone, clay, coal, &c., may be -expected to produce striking and wonderful chemical -changes in the latter. Nor are we disappointed in -these anticipations. Whenever we examine the sedimentary -materials around volcanic vents, we find that, -in contact with the once-fused materials, they everywhere -exhibit remarkable evidences of the chemical action to -which they have been subjected. The limestones are -converted into statuary marble, the sandstones pass -<span class="pagenum" id="Page_146">- 146 -</span> -into quartzite, the days assume the hardness and lustre -of porcelain, while the coals have lost their volatile -ingredients and assumed a form like coke or graphite. -And these changes are found to extend in many cases -to the distance of many hundreds of yards from the -planes of junction between the igneous and the sedimentary -materials.</p> - -<p>Among the most interesting effects resulting from -the extrusion of masses of incandescent rock, charged -with water and various gases, through beds of limestone, -clay, sandstone, &c., we may mention the production -of those beautiful crystalline minerals which -adorn our museums and are so highly prized as gems. -By far the larger part of these beautiful minerals have -been formed, directly or indirectly, by volcanic agencies.</p> - -<p>These gems and beautiful minerals are, for the most -part, substances of every-day occurrence, which entirely -owe their beauty to the crystalline forms they have -assumed. The diamond is crystallised carbon, the ruby -and sapphire are crystallised alumina, the amethyst and -a host of other gems are crystallised silica; and in almost -all cases the materials of gems are common and -widely diffused, it is only in their finely crystalline -condition that they are rare and therefore valuable.</p> - -<div class="sidenote">FORMATION OF VOLCANIC MINERALS.</div> - -<p>Crystals are formed during the slow deposition of a -substance, either by the evaporation of a liquid in which -it is dissolved, by its volatilisation, or its cooling from -a state of fusion. In many cases it can be shown that -the formation of large and regular crystals is aided if -<span class="pagenum" id="Page_147">- 147 -</span> -the work goes on with extreme slowness and under -great pressure. By sealing up various substances in -tubes containing water which can be kept at a high -temperature, minute crystals of many well-known minerals -have been artificially formed by chemists. Part of -the water converted into steam has formed a powerful -spring, which, reacting upon the remainder of the -liquid in the tube, has subjected it to enormous pressure, -and under these conditions of extreme pressure and -temperature, chemical actions take place of which we -have no experience under ordinary circumstances. -The experiments of Mr. Hannay seem to prove that -when carbon is separated from certain organic substances -at a high temperature and under great pressure, -it may crystallise in the form of the diamond. And -the recent discovery of diamonds in the midst of -materials filling old volcanic vents in South Africa -seems to show that this was in many cases the mode -in which the gem was originated. Even under the -conditions which prevail at the earth's surface, however, -minute and unnoticed chemical actions taking place -during long periods of time, produce most remarkable -results. This has been well illustrated by M. Daubrée, -who has shown that in the midst of masses of concrete -which the Romans built up around the hot springs of -Plombières and other localities, many crystalline -minerals have been formed, in the course of 2,000 -years, by the action of the waters upon the ingredients -of the concrete.</p> - -<p><span class="pagenum" id="Page_148">- 148 -</span></p> - -<p>But most of the crystals of minerals which have -been thus artificially formed are of minute, indeed often -of microscopic, dimensions. In the underground reservoirs -beneath volcanoes, however, we have all the -necessary conditions for the formation of crystals of -minerals on a far grander scale. High temperatures, -pressures far greater than any we can command at -the earth's surface, the action of superheated steam -and many acid gases on the various constituents of both -igneous and sedimentary rocks, and, above all, time of -almost unlimited duration; these constitute such a set -of conditions as may fairly be expected to result in the -formation of crystals, similar to those artificially produced -but of far greater size and beauty.</p> - -<p>If we visit those parts of the earth's surface where -great masses of fused volcanic rock have slowly cooled -down in contact with sedimentary materials, we shall -not be disappointed in our expectations. Diamonds, -rubies, sapphires, emeralds, topazes, garnets, and a -host of equally beautiful, if less highly prized, crystalline -substances, are found in such situations, lying in the -subterranean chemical laboratories in which they have -been formed, but now, by the action of denuding forces, -revealed to our view.</p> - -<p>In some cases it is not necessary to penetrate to -these subterranean laboratories in order to find these -beautiful gems and other crystallised minerals; for the -steam jets which issue from volcanic fissures carry -up fragments of rock torn from the side of the vent, -<span class="pagenum" id="Page_149">- 149 -</span> -and in the cavities and fissures of such ejected masses -beautiful crystallised products are often found. Such -rock-fragments containing minerals finely crystallised are -found abundantly on the flanks of Vesuvius and other -active volcanoes, and among the materials of the -Laacher See and other extinct volcanoes.</p> - -<div class="sidenote">FORMATION OF MINERAL-VEINS.</div> - -<p>But it is not only the finely crystallised minerals -and gems which we owe to volcanic action. The various -metallic minerals have nearly all been brought from -deep-seated portions of the earth's crust and deposited -upon the sides of rock-fissures by the agency of the -same volcanic forces. It is these forces which have, in -the first instance, opened the cracks through the solid -rock masses; and, in the second place, have brought -the metallic sulphides, oxides, and salts—either in -fusion, in solution, or in a vaporised condition—from -the deep-seated masses within the earth, causing them -to crystallise upon the sides of the fissures, and thus -form those metallic lodes and veins which are within -reach of our mining operations.</p> - -<p>There is still one other important class of minerals -which owe the existence, though indirectly, to volcanic -agencies. The cavities of igneous rocks, especially -the vesicles formed by the escape of steam, constitute, -when filled with water, laboratories in which -complicated chemical reactions take place. The materials -of the lava are gradually dissolved and re-crystallised -in new combinations. By this means the most -beautiful examples of such minerals as the agates, the -<span class="pagenum" id="Page_150">- 150 -</span> -onyxes, the rock-crystals, the Iceland-spars, and the -numerous beautiful crystals classed together as 'Zeolites' -have been formed. No one can visit a large -collection of crystalline minerals without being struck -with the large number of beautiful substances which -have thus been formed as secondary products from -volcanic materials.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_151">- 151 -</span></p> - -<h2 class="nobreak" id="CHAPTER_VI">CHAPTER VI.<br /> - -<span class="smaller">THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC VENTS</span></h2> -</div> - - -<p class="p0">From what has been said in the preceding chapters it -will be seen that while some of the materials ejected -from volcanic vents are, by the movements of the air -and ocean, distributed over every part of the face of -the globe, another, and by far the larger, part of the -matter so ejected, accumulates in the immediate vicinity -of the vent itself. By this accumulation of erupted -materials, various structures are built up around the -orifices from which the ejections take place, and the -size and character of these structures vary greatly in -different cases, according to the quantity and nature of -the ejected materials, and the intensity of the eruptive -forces by which they were thrown from the orifice. We -shall proceed in the present chapter to notice the chief -varieties in the forms and characters of the heaps of -materials accumulated round volcanic vents.</p> - -<p>These heaps of materials vary in size from masses -no bigger than a mole-heap up to mountains like Etna, -Teneriffe, and Chimborazo. The size of volcanic mountains -<span class="pagenum" id="Page_152">- 152 -</span> -is principally determined by the conditions of the -eruptive action at the vent around which they are -formed. If this action exhausts itself in a single effort, -very considerable volcanic cones, like the Monte Nuovo -with many similar hills in its vicinity, and the Puys of -Auvergne, may be formed; but if repeated eruptions -take place at longer or shorter intervals from the same -vent, there appears to be scarcely any limit to the size -of the structures which may, under such conditions, be -formed. It is by this repeated action from the same -volcanic vent going on for thousands or even millions -of years, that the grandest volcanic mountains of the -globe have been built up. Such volcanoes have sometimes -a diameter at their base of from 30 to 100 miles, -and an elevation of from 10,000 to 25,000 feet.</p> - -<p>The <i>form</i> of volcanic mountains is determined in -part by the nature of the materials ejected, and in part -by the character of the eruptive action.</p> - -<p>From what has been said in the preceding chapter, -it will be gathered that the volcanoes built up by ejections -of fragmentary materials differ in many striking -particulars from those formed by the outwelling of lavas -from volcanic vents. In a less degree, the volcanoes -composed of the same kind of volcanic materials also -vary among themselves.</p> - -<div class="sidenote">CHARACTERS OF SCORIA-CONES.</div> - -<p>When masses of scoriæ in a semi-fluid condition are -thrown to only a little distance above the volcanic vent, -so that they have not time to assume a perfectly solid -condition before they fall round the vent, the rugged -<span class="pagenum" id="Page_153">- 153 -</span> -masses of lava unite to form heaps of most irregular -shape. In such cases, the falling fragments being in a -semi-plastic state, stick to the masses below, and do not -tend to roll down the sides of the heap. Irregular heaps -of such volcanic scoriæ abound on the surfaces of lava-streams, -being piled up around each 'bocca' or vent -which the steam-jets escaping from the lava-currents -form at their surfaces. Such irregular accumulations -of scoriæ were observed on the lavas of Vesuvius during -the eruptions of 1822, 1855, and 1872, and have also -been described in the case of many other volcanoes. In -<a href="#fig26">fig. 26</a> (p. 101) we have given representations of a group -of such irregular scoria-cones which was observed by -Schmidt on the Vesuvian lava of 1855. It will be seen -from this drawing that there is scarcely any limit to the -steepness of the sides of such scoria-heaps, in which -the materials are in an imperfectly solidified condition -when they reach the ground.</p> - -<p>But in the majority of cases, the scoriæ ejected from -volcanic vents are thrown to a great height, and are in -a more or less perfectly solidified condition when they -fell to the ground again. In such cases the fragments -obey the ordinary mechanical laws of falling bodies, -rolling and sliding over one another, till they acquire -a slope which varies according to the size and -form of the fragments. In this way the great conical -mounds are formed which are known as 'cinder-cones,' -or more properly as 'scoria-cones.' Scoria-cones -usually vary in the slope of their sides from 35° to -<span class="pagenum" id="Page_154">- 154 -</span> -40° and may differ in size from mere monticules to -hills a thousand feet or more in height. Scoria-cones -of this character abound in many volcanic districts, as the -Auvergne, where they may be numbered by thousands. -The materials forming such scoria-cones vary in size -from that of a nut to masses as large as a man's head, -and fragments of even larger dimensions are by no -means uncommon.</p> - -<p>When the lava in a volcanic vent is perfectly glassy, -instead of being partially crystalline in structure, we -find not scoriæ but pumice ejected. In such cases, as -in the Lipari Islands for example, we see cones entirely -built up of pumice. Such pumice-cones resemble in -the angle of their slope (see <a href="#fig41">fig. 41</a>, facing p. 124), -the ordinary scoria-cones, but are of a brilliant white -colour, appearing as if covered with snow.</p> - -<div class="sidenote">PRESERVATION OF SCORIA-CONES.</div> - -<p>Ordinary scoriæ are usually of a black colour when -first ejected, but after a short time the black oxide of -iron (magnetite) which they contain, attracts the oxygen -of the air and moisture, and assumes the reddish-brown -colour of iron-rust. Under such circumstances the -heaps of black material gradually acquire the red-brown -colour which is characteristic of so many of the -scoria-cones around Etna, and in the Auvergne and the -Eifel. The moisture of the air, and the rain falling -upon these loose cindery heaps, cause them to decompose -upon their surfaces; the action is facilitated by -the growth of the lower forms of vegetation, such as -mosses and lichens, and thus at last a soil is produced -<span class="pagenum" id="Page_155">- 155 -</span> -on the surfaces of these conical piles of loose materials -which may support an abundant vegetation. Cinder- or -scoria-cones are not uncommonly found retaining in -a most perfect manner their regular, conical form, the -lips of their craters being sharp and unbroken as if -the cone were formed but yesterday, while their slopes -may nevertheless be covered with a rich soil supporting -abundant grass and forest-trees. It may at first -sight seem difficult to understand how a loose mass of -scoriæ could have so long withstood the action of the -rain and floods, retaining so perfectly its even slopes -and sharp ridges. A little consideration will, however, -convince us that it is the very loose and pervious nature -of the materials of which scoria-cones are composed, -which tends to their perfect preservation. The rain at -once sinks into their mass, before it has time to form -rivulets and streams which would wear away their surfaces -and destroy the regularity of their outlines.</p> - -<p>Scoria- and pumice-cones are frequently found to -be acted upon by acid vapours to such an extent that -the whole of the materials is reduced to a white pulverulent -mass. In these cases the oxides of iron and -the alkalis have united with the sulphuric or hydrochloric -or carbonic acids, the compounds being carried -away in solution by the rain-water falling on the mass; -the materials left are silica, the hydrated silicate of -alumina, and hydrated sulphate of lime (gypsum), all of -which are of a white colour.</p> - -<p>Cinder- or scoria-cones, and pumice-cones, are often -<span class="pagenum" id="Page_156">- 156 -</span> -found raised by the action of winds to a greater elevation -on one side than the other, in the manner already -described. One side of the cone is often seen to be -more or less completely swept away by an outwelling -stream of lava, and thus breached cones are formed -(see <a href="#fig40">fig. 40</a>, p. 123). Not unfrequently we find a number -of cones which are united more or less completely -at their bases, as in Vulcanello (<a href="#fig06">fig. 6</a>, p. 43), the -several vents being so near together that their ejections -have mingled with one another. Cones composed -entirely of fragmentary materials often show an approach -to the beautifully curved slopes which we have -described as being so characteristic of volcanoes, as -may be seen in <a href="#fig41">fig. 41</a>, facing p. 124. In the case of -scoria- and pumice-cones this curvature is probably due -to the rolling downnwards and outwards of the larger -fragments.</p> - -<p>We have already pointed out that with the scoriæ -there are often ejected fragments torn from the sides -of the volcanic vents. Sometimes such fragments are -so numerous as to make up a considerable portion of -the mass of the volcanic cones. Thus in the Eifel we -find hills, of by no means insignificant size, completely -built up of small scoriæ and broken fragments of slate -torn from the rocks through which the volcanic fissures -have been opened. Occasionally we see that few or -no scoriæ have been ejected, and the volcanic vents are -surrounded simply by heaps of burnt slate.</p> - -<p>The smaller fragmentary materials ejected from -<span class="pagenum" id="Page_157">- 157 -</span> -volcanic vents—such as lapilli and dust—rest in heaps, -having a different angle of slope from those formed by -scoriæ. In many cases, as we have seen, such finely-divided -materials descend in the condition of mud, -which flows evenly over the surface of the growing -cone and consolidates in beds of very regularly stratified -'tufa' or 'tuff.'</p> - -<div class="sidenote">CHARACTERS OF TUFF-CONES.</div> - -<p>The 'tuff-cones' thus formed differ in many important -respects from the scoria-cones already described. -The slope of their sides varies from 15° to 30°, and -is almost always considerably less than in scoria- and -pumice-cones. The tuff-cones undergo much more -rapid degradation from rain and moisture than do the -scoria-cones; for, though the materials of the former -'set,' as we have seen, into a substance of considerable -hardness, yet this substance, being much less pervious -to water than the loose scoria heaps, permits of the -formation of surface-streams which furrow and wear -away the sides of the cones. Sometimes the sides of -the crater are found to be almost wholly removed by -atmospheric denudation, and only a shallow depression -is found occupying the site of the crater; such a case -is represented in <a href="#fig59">fig. 59</a>. We not unfrequently find -the whole slopes of such cones to be traversed by a -series of radiating grooves passing from the summit to -the base of the mountains, these channels being formed -by water, which has collected into streams, flowing -down the slopes of the mountains. The volcanic cone, -under these circumstances, frequently presents the -<span class="pagenum" id="Page_158">- 158 -</span> -appearance of a partially opened umbrella. Owing to -the impervious character of the materials composing -tuff-cones, their craters are frequently found to be -occupied by lakes.</p> - -<div class="figcenter" id="fig59" style="width: 447px;"> - <img src="images/fig59.png" width="447" height="192" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 59.—Summit of the volcano of Monte Sant' Angelo in Lipari -exhibiting a crater with walls worn down by denudatioh.</span></div> -</div> - -<p>Tufas have usually a white or yellowish-brown -colour, and these are the colours exhibited by the -cones composed of this material before they become -covered by vegetation. Tufas scoriæ, and lavas usually -crumble down to form a very rich soil, and many of the -choicest wines are produced from grapes grown on the -fertile slopes of volcanic mountains. When, however, -as not unfrequently happens, the materials are finely -divided and incoherent, they are so easily driven about -by the winds that cultivation of any kind is rendered -almost impossible. In the Islands of Stromboli and -Vulcano the gardens have to be surrounded by high -fences to prevent them from being overwhelmed by the -ever-shifting masses of volcanic sand.</p> - -<p><span class="pagenum" id="Page_159">- 159 -</span></p> - -<div class="sidenote">CHARACTERS OF LAVA-CONES.</div> - -<p>There are some cones which are composed in part -of scoriæ and in part of tufa. Hence we are sometimes -at a loss whether to group them with the one -class of cones or the other. But in the majority of -cases, scoria- and tuff-cones present the sufficiently -well-marked and distinctive characters which we have -described.</p> - -<p>Lava-cones differ quite as greatly in their forms as -do the cones composed of fragmentary materials, the -variations being principally determined by the degree -of liquidity of the lavas.</p> - -<p>We sometimes find that outwelling masses of lava, -when issuing in small quantities from a vent, accumulate -in cauliflower-shaped masses, or sometimes in the -form of a column, or bottle. Professor J. D. Dana -describes many such fantastically-formed masses of -lava as being found in Hawaii, one of which is represented -in <a href="#fig25">fig. 25</a> (p. 100).</p> - -<p>When the lava issues from the vent in great quantities -it tends to flow on all sides of it, and to build -up a great conical heap above the orifice. If the lava -be very liquid it flows to great distances, resting at a -very slight slope. Thus we find that the volcanoes of -Hawaii have been built up of successive ejections of -very liquid lava, which have formed cones having a -slope of only 6° to 8°, but of such enormous dimensions -that the diameter of their bases is seventy miles and -their height 14,000 feet.</p> - -<p><span class="pagenum" id="Page_160">- 160 -</span></p> - -<div class="figcenter" id="fig60_neg" style="width: 431px;"> - <a href="images/fig60.png"><img src="images/fig60_neg.png" width="431" height="161" alt="" /></a> - <div class="figcaption"><span class="smcap">Fig. 60.—Outlines of Lava-cones.</span><br /> - 1. Mauna Loa, in Hawaii. Composed of very fluid lava. - 3. The Schlossberg of Teplitz, Bohemia. Composed of very imperfectly fluid or - viscid lava.<br /> - Click on image to see original negative view.</div> -</div> - -<p>If, on the other hand, the lava be viscid, or very -imperfectly liquid in character, it tends to accumulate -immediately around the vent; fresh ejections force -the first extruded matter outwards, in the manner -so well illustrated by Dr. Reyer's experiments, and at -last a more or less steep-sided bulbous mass is formed -over the vent. Such bulbous masses, composed of imperfectly -fluid lavas, occur in many volcanic districts, -and constitute hills of considerable size. From the -tendency of matters thus extruded to choke up the -vents, however, these volcanoes composed of viscid -lavas cannot be expected to attain the vast dimensions -reached by some of those composed of very liquid -lavas. The difference in the forms of lava-cones composed -of very fluid or of somewhat viscid materials is -illustrated in <a href="#fig58">fig. 58</a>. When the interior of such steep-sided -volcanic mountains composed of viscid materials -is exposed by the action of denuding forces, the peculiar -internal structure we have described is displayed by -<span class="pagenum" id="Page_161">- 161 -</span> -them. In the Chodi-Berg of Hungary, a great bulbous -mass of andesitic rock, this endogenous structure is -admirably displayed. It is also well seen in the excavation -of the hill of the Grand Sarcoui, a similar -mass, composed of altered trachyte, which has been -erupted in the midst of a scoria-cone in the Auvergne. -See <a href="#fig44">fig. 44</a> (p. 126).</p> - -<div class="sidenote">CHARACTERS OF COMPOSITE CONES.</div> - -<p>Most of the great volcanic mountains of the globe -belong to the class of 'composite cones,' and are built -up by alternate ejections of fluid lava and fragmentary -materials. The slope of the sides in such composite -cones is subject to a wide range of variation, being -determined in part by the degree of liquidity of the -lavas, in part by the nature of the fragmentary materials -ejected, and in part by the proportions which the -fragmentary and lava-ejections bear to one another.</p> - -<p>But there is another set of causes which tends to -modify the form and character of these composite, -volcanic cones. As we have already pointed out, the -sides of such cones are liable to be rent asunder from -time to time, and the fissures so produced are injected -with masses of liquid lava from below. These fissures, -rent in the sides of volcanic cones, often reach the -surface and eruptive action takes place, giving rise to -the formation of a cone, or series of cones, upon the -line of the fissure (<a href="#fig61">fig. 61</a>). Such small cones thrown -up on the flanks of a great volcanic mountain are -known as 'parasitic cones'; though subordinate to the -great mountain mass, they may be in themselves of -<span class="pagenum" id="Page_162">- 162 -</span> -considerable dimensions. Among the hundreds of -parasitic cones which stud the flanks of Etna, there -are some which are nearly 800 feet in height.</p> - -<div class="figcenter" id="fig61" style="width: 440px;"> - <img src="images/fig61.png" width="440" height="194" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 61.—Diagram illustrating the formation of Parasitic Cones - along lines of fissure formed on the flanks of a great - volcanic mountain.</span></div> -</div> - -<div class="figcenter" id="fig62" style="width: 433px;"> - <img src="images/fig62.png" width="433" height="153" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 62.—Outline of Etna, as seen from Catania.</span></div> -</div> - -<div class="sidenote">FORMATION OF PARASITIC CONES.</div> - -<p>The building up of parasitic cones upon the flanks -of a volcanic mountain tends, of course, to destroy its -regular conical form. This may be well seen in Etna, -which, by the accumulation of materials upon its flanks, -has become a remarkably 'round-shouldered' mountain. -(See figs. <a href="#fig62">62</a> and <a href="#fig63">63</a>.) At the same time it must -<span class="pagenum" id="Page_163">- 163 -</span> -be remembered that materials erupted from the central -vent tend to fill up the hollows between these parasitic -cones, and thus to restore to the mountain its regularly -conical form.</p> - -<div class="figcenter" id="fig63" style="width: 452px;"> - <img src="images/fig63.png" width="452" height="183" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 63.—Outline of Etna, as seen from the Val del Bronte.</span></div> -</div> - -<div class="figcenter" id="fig64" style="width: 409px;"> - <a href="images/fig64_lg.png"><img src="images/fig64.png" width="409" height="285" alt="" /></a> - <div class="figcaption"><span class="smcap">Fig. 64.—Plan of the Volcano forming the Island of Ischia.</span><br /> - Click on image to view larger sized illustration.</div> -</div> - -<table class="smaller" summary="labels"> -<tr> - <td class="tdl" colspan="4"><i>a, a, a.</i> The semi-circular crater-ring of Epomeo.</td> -</tr> -<tr> - <td class="tdl" colspan="4"><i>b, c, d.</i> Lava-currents which have flowed from the principal crater.</td> -</tr> -<tr> - <td class="tdl" colspan="4"><i>e, f, g, h.</i> Plateaux formed by ancient lava-currents.</td> -</tr> -<tr> - <td class="tdl"><i>k.</i></td> - <td class="tdl">Montagnone.</td> - <td class="tdl" rowspan="6"><img src="images/bracer_70.png" width="10" height="70" alt="" /></td> - <td class="tdl" rowspan="6">Parasitic cones and craters on the slopes of the mountain.</td> -</tr> -<tr> - <td class="tdl"><i>l.</i></td> - <td class="tdl">Monte Rotaro.</td> -</tr> -<tr> - <td class="tdl"><i>m.</i></td> - <td class="tdl">Monte Tabor.</td> -</tr> -<tr> - <td class="tdl"><i>n.</i></td> - <td class="tdl">Castiglione.</td> -</tr> -<tr> - <td class="tdl"><i>o.</i></td> - <td class="tdl">Lago di Bagno.</td> -</tr> -<tr> - <td class="tdl"><i>p.</i></td> - <td class="tdl">The Cremate.</td> -</tr> -<tr> - <td class="tdl" colspan="4"><i>r.</i> Lava-stream of the Arso, which flowed from the Cremate in 1301.</td> -</tr> -<tr> - <td class="tdl" colspan="4"><i>x, x, x.</i> Raised beaches on the shores of the island, showing that it has - recently undergone elevation.</td> -</tr> -</table> - -<p><span class="pagenum" id="Page_164">- 164 -</span></p> - -<p>The Island of Ischia is a good example of a great -volcanic cone the flanks of which are covered with numerous -small parasitic cones. While the great central -volcano has evidently been long extinct, and one side of -its crater-wall is completely broken down, some of the -small parasitic cones around its base have been formed -within the historical period—one of them as recently -as the year 1302. <a href="#fig64">Fig. 64</a> is a plan of the Island of -Ischia, showing the numerous parasitic cones scattered -over the slopes of the principal cone.</p> - -<div class="figcenter" id="fig65" style="width: 429px;"> - <img src="images/fig65.png" width="429" height="135" alt="" /> - <div class="figcaption"><span class="smcap">Fig, 65.—A primary Parasitic Cone with a secondary one at its - base—Ischia.</span><br /> - <i>a.</i> Monte Rotaro. <i>b.</i> Monte Tabor. <i>c.</i> Lava-stream flowing from the latter.</div> -</div> - -<p>In one case we find that a parasitic cone, the Monte -Rotaro, has itself a similar smaller cone, which is parasitic -to it, at its foot; this secondary parasitic cone -gives off a small lava-stream of trachyte, which has -flowed down to the sea. (See <a href="#fig65">fig. 65</a>.)</p> - -<p><span class="pagenum" id="Page_165">- 165 -</span></p> - -<div class="figcenter" id="fig66" style="width: 449px;"> - <img src="images/fig66.png" width="449" height="201" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 66.—Scoria-cone near Auckland, New Zealand, with a - lava-current flowing from it.</span><br /> - The strata beneath the volcanic cone are exposed in the sea-cliff, and exhibit proofs - of depression having taken place.</div> -</div> - -<div class="figcenter" id="fig67" style="width: 464px;"> - <img src="images/fig67.png" width="464" height="140" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 67.—Section of rocks below the ancient triassic volcano - of Predazzo in the Tyrol.</span><br /> - The position of the strata <i>a b c</i>, etc., indicates a central subsidence.</div> -</div> - -<div class="sidenote">SUBSIDENCE BENEATH VOLCANIC VENTS.</div> - -<p>Most great volcanic mountains exhibit a tendency -towards a subsidence of their central portions, which -may take place either during or subsequently to their -period of activity. When we examine the strata upon -which a volcano has been built up, but which are now -exposed to our study by denuding forces, we usually -find that they incline towards the centre of the eruptive -activity. (See figs. <a href="#fig66">66</a> and <a href="#fig67">67</a>.) Two causes may -contribute to bring about this result. In the first -instance, we must remark that the piling up of materials -around the volcanic vent causes the subjacent -<span class="pagenum" id="Page_166">- 166 -</span> -strata to be subjected to a degree of pressure far is -excess of that which acts upon the surrounding rocks. -And secondly, it must be borne in mind that the continual -removal of material from below the mountain -must tend to the production of hollows, into which -the overlying strata will sink. The effect of this central -subsidence is to give to the flanks of volcanic -cones those beautifully curved outlines which constitute -so striking a feature in Vesuvius (see <a href="#fig17">fig. 17</a>, p. 87), -Fusiyama (see <a href="#fig77_neg">fig. 77</a>, No. 1, facing p. 178), and many -other volcanic mountains.</p> - -<p>There seems, at first sight, to be scarcely any limit -to the dimensions which these great composite volcanic -cones may attain: the lateral eruptions tending to -enlarge the area of the base of the mountain, and, -by the injection of the fissures, to knit together and -strengthen its structure, while the central eruptions -continually increase the elevation of the mass. Great, -however, as is the force which is concerned in the production -of our terrestrial volcanoes, it has its limits; -and, at last, the piling up of materials will have gone -on to such an extent, that the active forces beneath -the volcano are no longer competent either to raise -materials to the elevated summit of the mountain or -to tear asunder its strengthened and fortified flanks. -Under these circumstances, the volcanic forces, if they -have not already exhausted themselves, will be compelled -to find weak places in the district surrounding -the volcano, at which fissures may be produced and the -phenomena of eruption displayed.</p> - -<p><span class="pagenum" id="Page_167">- 167 -</span></p> - -<div class="sidenote">SHIFTING OF VOLCANIC FOCI.</div> - -<p>Some volcanic cones exhibit evidence that during -the series of eruptions by which they have gradually -been built up, the centre of volcanic action has shifted -to another point within the mountain. Thus Lyell has -shown, in the case of Etna, that during the earlier -periods in the history of the mountain the piling up -of materials went on around a centre which is now -situated at a distance of nearly four miles from the -present focus of eruptive activity. Some of our old -British volcanoes, of which the denuded wrecks exist -in the Western Isles of Scotland, show similar evidence -of a shifting of the axis of eruption.</p> - -<p>One of the most conspicuous features of a volcanic -cone is the great depression or crater found at its -summit. In describing the internal structure of volcanic -cones, we have seen how these craters are produced -and acquire their inverted conical form, by the -slipping and rolling back of materials towards the -centre of eruptive action.</p> - -<p>Almost all volcanic cones exhibit craters, but in -those which are formed entirely by the outwelling of -viscid lavas the central depression is often slight and -inconspicuous, and occasionally altogether wanting. It -frequently happens, however, that eruptive action has -ceased at the centre of a volcano, and its summit-crater -may by denudation be entirely destroyed, while -new and active craters are formed upon its flanks. -Stromboli furnishes us with an admirable example of -this kind (see <a href="#fig01">fig. 1</a>, facing p. 10). Other volcanoes -<span class="pagenum" id="Page_168">- 168 -</span> -may exhibit several craters, one at the summit of the -mountain and others upon its flanks. Of this we find -a good example in Vulcano (<a href="#fig06">fig. 6</a>, p. 43).</p> - -<div class="figcenter" id="fig68" style="width: 444px;"> - <img src="images/fig68.png" width="444" height="233" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 68.—Cotopaxi (19,600 feet), as seen from a distance of - ninety miles.</span></div> -</div> - -<p>When a volcano has been built up by regular and -continuous eruptions from the same volcanic vent, the -size of the crater remains the same, while the volcano -continually grows in height and in the diameter of its -base. The size of the crater will be determined by the -eruptive force at the volcanic centre, the size of the -mountain by the duration of the volcanic activity and -the quantity of material ejected. In the earliest stage -of its history, such a volcano will resemble Monte -Nuovo, which has a crater reaching down almost to the -base of the mountain; in the later stages of its history, -such a volcano will resemble Cotopaxi (<a href="#fig68">fig. 68</a>) and -Citlaltepetl (<a href="#fig69">fig. 69</a>), in which the crater, though of far -<span class="pagenum" id="Page_169">- 169 -</span> -greater absolute dimensions than that of Monte Nuovo, -bears but a small proportion to the vast cone at the -summit of which it is situated.</p> - -<div class="figcenter" id="fig69" style="width: 440px;"> - <img src="images/fig69.png" width="440" height="266" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 69.—Citlaltepetl, or the Pic d'Orizaba, in Mexico (17,370 - feet), as seem from the forest of Xalapa.</span></div> -</div> - -<div class="sidenote">ORIGIN OF VOLCANIC CRATERS.</div> - -<p>In the great majority of volcanoes, however, eruptive -action does not go on by any means regularly and -continuously, but terrible paroxysmal outbursts occur, -which suddenly enlarge the dimensions of the crater to -an enormous extent.</p> - -<p>In the year 1772, there occurred a volcanic eruption -in the Island of Java, which is perhaps the most violent -and terrible that has happened within the historical -period. A lofty volcanic cone, called Papandayang, -9,000 feet high, burst into eruption, and, in a single -night, 30,000,000,000 cubic feet of materials were -<span class="pagenum" id="Page_170">- 170 -</span> -thrown into the atmosphere, falling upon the country -around the mountain where no less than forty villages -were buried. After the eruption, the volcano was found -to have been reduced in height from 9,000 to 5,000 -feet, and to present a vast crater in its midst, which -had been formed by the ejection of the enormous mass -of materials.</p> - -<p>Many similar cases might be cited of the removal -of a great part of a mountain-mass by a sudden, -paroxysmal outburst. In some cases, indeed, the whole -mass of a mountain has been blown away during a -terrific eruption, and the site of the mountain is now -occupied by a lake. This is said to have been the case -with the Island of Timor, where an active volcano, -which was visible from a distance of 300 miles at sea, -has entirely disappeared.</p> - -<p>The removal of the central portion of great volcanic -mountains by explosive action, gives rise to the -formation of those vast, circular, crater-rings of which -such remarkable examples occur in many volcanic districts. -These crater-rings present a wall with an outer -slope agreeing with that of the volcanic cone of which -they originally formed a part, but with steep inner -cliffs, which exhibit good sections of the beds of tuff, -ash, and lava with the intersecting dykes of which -the original volcano was built up. Near Naples, one -of these crater-rings, with sloping outer sides and -steep inner ones, is employed to form the royal game-preserve -of Astroni, the only entrance to the crater -being closed by gates.</p> - -<p><span class="pagenum" id="Page_171">- 171 -</span></p> - -<div class="sidenote">FORMATION OF CRATER-LAKES.</div> - -<p>As these crater-rings are usually composed of materials -more or less impervious to water, they often become -the site of lakes. The beautiful circular lake of Laach, -in the Rhine Provinces, with the numerous similar -examples of Central Italy—Albano, Nemi, Bracciano, -and Bolsena—the lakes of the Campi Phlegræi (Agnano, -Avernus, &c.), and some similar lakes in the Auvergne, -may be adduced as examples of crater-rings which have -become the site of lakes.</p> - -<div class="figcenter" id="fig70" style="width: 444px;"> - <img src="images/fig70.png" width="444" height="233" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 70.—Lac Paven, in the Auvergne.</span><br /> - <i>a.</i> Scoriæ. <i>b.</i> Basalt.</div> -</div> - -<p>One of the most beautiful of the crater-lakes in the -Auvergne is Lac Paven (<a href="#fig70">fig. 70</a>), which lies at the foot of -a scoria-cone, Mont Chalme, and is itself surrounded by -masses of ejected materials. The crater-lake of Bagno, -in Ischia (<a href="#fig71">fig. 71</a>), has had a channel cut between it -and the sea, so that it serves as a natural harbour. -The lake of Gustavila, in Mexico (<a href="#fig72">fig. 72</a>), is an example -of a crater-lake on a much larger scale.</p> - -<p><span class="pagenum" id="Page_172">- 172 -</span></p> - -<p>In many of these crater-rings the diameter of the -circular space enclosed by them is often very great -indeed as compared with the height of the walls.</p> - -<div class="figcenter" id="fig71" style="width: 403px;"> - <img src="images/fig71.png" width="403" height="151" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 71.—The crater-lake called Lago del Bagno, in Ischia, - converted into a harbour.</span></div> -</div> - -<div class="figcenter" id="fig72" style="width: 485px;"> - <img src="images/fig72.png" width="485" height="168" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 72.—Lake of Gustavila, in Mexico.</span><br /> - (The terraces round the lake have been artificially formed.)</div> -</div> - -<div class="sidenote">DIMENSIONS OF CRATER-LAKES.</div> - -<p>Two of the largest crater-rings in the world are -found in Central Italy, and are both occupied by lakes, -the circular forms of which must strike every observer. -The Lago di Bracciano, which lies to the north-west of -Rome, is a circular lake six and a half miles in diameter, -surrounded by hills which at their highest point rise to -the height of 1,486 feet above the sea, while the surface -<span class="pagenum" id="Page_173">- 173 -</span> -of the waters of the lake is 640 feet above the sea-level. -The Lago di Bolsena is somewhat less perfectly -circular in outline than the Lago di Bracciano; it has a -length from north to south of ten-and-a-quarter miles -and a breadth from east to west of nine miles; the surface -of the waters of this lake is 962 feet above that of -the waters of the Mediterranean. The lake of Bolsena, -like that of Bracciano, is surrounded by hills composed -of volcanic materials; the highest points of this ring -of hills rise to elevations of 684, 780, and 985 feet -respectively above the waters of the lake.</p> - -<p>In these great circular lakes of Bolsena and Bracciano, -as well as in the smaller ones of Albano, Nemi, -and the lakes of Frascati in the same district, the vast -circular spaces enclosed by them, the gradual outer -slope of the ring, and the inner precipices which bound -the lake, all afford evidence of the explosive action to -which they owe their origin.</p> - -<p>But while the vast crater-rings we have mentioned -are frequently found to be occupied by lakes, there are -many other similar crater-rings which remain dry, either -from the materials of which they are composed being -of more pervious character, or from rivers having cut a -channel through the walls of the crater, in this way -draining off its waters.</p> - -<p>Thus in the Campi Phlegræi, while we have the -craters of Agnano and Avernus forming complete circular -lakes, Astroni has only a few insignificant lakelets -on its floor, and the Pianura, the Piano di Quarto, -<span class="pagenum" id="Page_174">- 174 -</span> -which have each a diameter of three or four miles, with -many others, remain perfectly dry. In the vicinity of -the great crater-lakes of Central Italy we find the crater-ring -of the Vallariccia, which has evidently once been -a lake but is now drained, its floor being covered with -villages and vineyards.</p> - -<div class="sidenote">CRATER-RINGS SURROUNDING CONES.</div> - -<p>A comparison of these vast crater-rings leads us to the -conclusion that in the majority of cases, if not in every -instance, they are composed almost entirely of volcanic tuff -and dust. In the case of the more solidly-built composite -volcanic cones, the volcanic forces, as we have seen, -produce fissures in the mass, and along these fissures -parasitic cones are thrown up, the tension of the mass -of imprisoned vapours below the mountain being thus -from time to time relieved. But in the case of a volcanic -cone composed of loose fragmentary materials, such -temporary relief is impossible. The cracks, as soon as -they originate, will be filled up and choked by the falling -in of materials from above and at their sides. In -this way the eruptive action will be continually repressed, -till at last the imprisoned vapours acquire such -a high state of tension that the outburst, when it occurs, -is of the most terrible character, and the whole central -mass of the volcano is blown into the air. It may often -seem surprising that the ejection of such vast masses of -material from the centre of a volcanic cone does not -effect more in the way of raising the height of the -crater-walls. But it must be remembered that, in the -case of craters of such vast area, the majority of the -<span class="pagenum" id="Page_175">- 175 -</span> -ejected materials must fall back again within its circumference. -By repeated ejections these materials will -at last be reduced to such an extreme state of comminution -that they can be borne away by the winds, and -spread over the country to the distance of hundreds -or thousands of miles. After great volcanic outbursts -enormous areas are thus found covered with fine volcanic -dust to the depth of many inches or feet.</p> - -<div class="figcenter" id="fig73" style="width: 432px;"> - <img src="images/fig73.png" width="432" height="214" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 73.—Peak of Teneriffe in the Canary Islands (12,182 ft.), - surrounded by great crater-rings.</span></div> -</div> - -<p>Sometimes, as in the case of the Lago di Bracciano, -the eruptive forces appear to have entirely exhausted -themselves in the prodigious outburst by which the -great crater was produced. But in other cases, as in -that of the Lago di Bolsena, the eruptive action was -resumed at a later date, and small tuff-cones were -thrown up upon the floor of the crater; these now -rise as islands above the surface of the lake. In other -cases, again, the eruptive action was resumed after the -<span class="pagenum" id="Page_176">- 176 -</span> -formation of the great crater-ring, with such effect that -bulky volcanic cones were built up in the midst of the -crater-ring which surrounds them like a vast wall; -examples of this are exhibited in the extinct volcanoes -of Rocca Monfina and Monte Albano. Some of the -grandest volcanoes of the globe, such as Teneriffe (<a href="#fig73">fig. 73</a>), -the volcanoes of Mauritius and Bourbon (figs. <a href="#fig74">74</a> -and <a href="#fig75">75</a>), and many others that might be cited, are thus -found to be surrounded by vast crater-rings. Vesuvius -itself is surrounded by the crater-ring of Somma (<a href="#fig76">fig. 76</a>).</p> - -<div class="figcenter" id="fig74" style="width: 489px;"> - <img src="images/fig74.png" width="489" height="207" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 74.—The volcano of Bourbon, rising in the midst of a - crater-ring four miles in diameter.</span></div> -</div> - -<div class="figcenter" id="fig75" style="width: 429px;"> - <img src="images/fig75.png" width="429" height="201" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 75.—The volcano of Bourbon, as seen from another point of - view, with three concentric crater-rings encircling its base.</span></div> -</div> - -<p><span class="pagenum" id="Page_177">- 177 -</span></p> - -<div class="sidenote">BASALTIC CONES IN TRACHYTIC CRATER-RINGS.</div> - -<p>This formation of cone within crater, often many -times repeated, is very characteristic of volcanoes. The -craters mark sudden and violent paroxysmal outbursts, -the cones are the result of more moderate but long-continued -ejection. Sometimes, as at Vesuvius in 1767 -(<a href="#fig15">fig. 15</a>, p. 85), we find a nest of craters and cones which -very strikingly exemplifies this kind of action.</p> - -<div class="figcenter" id="fig76" style="width: 447px;"> - <img src="images/fig76.png" width="447" height="149" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 76.—Vesuvius, as seen from Sorrento, half encircled by - the crater-ring of Somma.</span></div> -</div> - -<p>We shall point out, hereafter, that at most volcanic -centres the ejection of trachytic lavas precedes that of -the basaltic lavas. Now it is these trachytic lavas which -principally give rise to the formation of the light lapilli -of which tuff-cones are formed. Hence it is that we -so frequently find, as in the case of Vesuvius, Rocca-Monfina, -and many other volcanoes, that a great crater-ring, -largely composed of tuffs, encloses a cone built -up of more basic lavas.</p> - -<p><span class="pagenum" id="Page_178">- 178 -</span></p> - -<p>In <a href="#fig77_neg">fig. 77</a> we have shown by a series of outline -sections the various forms assumed by volcanoes in -consequence of the different kinds of eruptive action -going on in them:—</p> - -<p>1. Is an outline of Fusiyama, an almost perfect -cone, with a small crater at its summit. The sides of -this volcano admirably illustrate the beautiful double -curves characteristic of volcanic cones.</p> - -<p>2. Hverfjall in Iceland, a volcanic cone with a large -crater, reaching almost to its base.</p> - -<p>3. The crater-lake of Bracciano, in which the area -of the crater is out of all proportion to the height of -the crater-walls.</p> - -<p>4. Rocca-Monfina, in Southern Italy, a tuff-cone of -large dimensions, in the midst of which an andesitic -lava-cone has been built up.</p> - -<p>5. Teneriffe, in the Canary Islands, in which a perfect -volcanic cone has been built up in the centre of an -encircling crater-ring.</p> - -<p>6. Vulcano, in the Lipari Islands, in which, by the -shifting of the centre of volcanic activity along a line -of fissure, a series of overlapping volcanic cones has -been produced.</p> - -<div class="figcenter" id="fig77_neg" style="width: 438px;"> - <a href="images/fig77.png"><img src="images/fig77_neg.png" width="438" height="620" alt="" /></a> - <div class="figcaption"><span class="smcap">Fig. 77.—Outlines of various Volcanoes, illustrating the - different relations of the craters to cones.</span><br /> - Click on image to see original negative view.</div> -</div> - -<p><span class="pagenum" id="Page_179">- 179 -</span></p> - -<div class="sidenote">SUBMARINE VOLCANOES.</div> - -<p>While speaking of the varieties of form assumed by -volcanic cones and craters, we must not forget to notice -the effects which are produced by denuding forces upon -them. In the case of submarine volcanoes, like the -celebrated island called by the English Graham Isle, by -the French Isle Julie, and by the Germans the Insel -Ferdinandez (<a href="#fig78">fig. 78</a>), which was thrown up off the -coast of Sicily in 1831, it was evident that volcanic -outbursts taking place at some depth below the level -of the sea gradually piled up a cone of scoriæ with a -crater in its midst. By constant accessions to its -mass, this scoria-cone was eventually raised above -the sea-level, but the action of the waves upon the -loose materials soon destroyed the crater-walls and -eventually reduced the island to a shoal. It is evident -that in all cases in which eruptions take place beneath -the sea-level, and the loose materials are exposed during -their accumulation to the beating of the sea-waves, the -form of the volcanic cone so produced will be greatly -modified by the interaction of the two sets of opposed -causes, the eruptive forces from below and the distributive -action of the sea-waves.</p> - -<div class="figcenter" id="fig78" style="width: 422px;"> - <img src="images/fig78.png" width="422" height="205" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 78.—Island thrown up in the Mediterranean Sea in July - and August 1881.</span><br /> - (The view was taken in the month of September, after the sides of the - crater had been washed away by the waves.)</div> -</div> - -<p><span class="pagenum" id="Page_180">- 180 -</span></p> - -<p>Craters when once formed are often rent across, -along the line of the fissure above which they are -thrown up. Thus the crater of Vesuvius was in 1872 -rent completely asunder on one side, so that it was -possible to climb through the fissure thus produced and -reach the bottom of the crater. Streams flowing down -the sides of the crater, and escaping through such a rent, -may in the end greatly modify the form and disguise -the characters of a volcanic crater. Of this kind of -action we have a striking example in the Val del Bove -of Etna.</p> - -<p>Volcanoes, as we shall point out in the sequel, are -after their extinction frequently submerged beneath the -waters of the ocean. The sea entering the craters, -eats back their cliff-like sides and enlarges their areas. -Such denuded waters are called 'calderas,' the channels -into them 'barrancos.'</p> - -<p>Sometimes the action of the waves upon a partially -submerged volcano has led to the cutting back of its -slopes into steep cliffs, at the same time that the crater-ring -is enlarged. In such cases we have left a more -or less complete rocky ring, composed of alternating -lavas and fragmentary materials. Of such a ruined -crater-ring, the Island of St. Paul in the South Atlantic -affords an admirable example.</p> - -<p>When the action of denudation has gone still further, -all the lavas and tuffs composing the cone may be -completely removed and nothing left but masses of the -hard and highly-crystalline rocks which have cooled -<span class="pagenum" id="Page_181">- 181 -</span> -down slowly in the heart of the volcano. An example -of this kind is afforded to us by St. Kilda, the remotest -member of the British Archipelago.</p> - -<p>But although the majority of volcanic craters are -clearly formed by explosive action, there are some -craters, like those of Kilauea in Hawaii, which probably -owe their origin to quite a different set of causes. In -this case the explosive action at the vent is but slight, -and the crater, which is of very irregular form, appears -to have originated in a fissure, which has been slowly -enlarged by the liquid lavas encroaching upon and -eating away its sides. Such craters as these, however, -appear to be comparatively rare.</p> - -<p>Besides the great volcanic mountains composed of -lava, scoriæ, tuff and ash, there are other structures -which are formed around volcanic vents even when -these do not eject molten rock-masses. The water -which issues in these cases either as steam or in a -more or less highly heated condition frequently carries -materials in suspension or solution, and these sometimes -accumulate in considerable quantities around the -vent.</p> - -<div class="sidenote">FORMATION OF MUD-VOLCANOES.</div> - -<p>When fissures are formed in the midst of loose -argillaceous materials, such as are frequently produced -by the decomposition of volcanic rocks, the waters -which issue through them are sometimes so charged -with muddy matter that this accumulates to form cones -having all the general characters of volcanic mountains, -and which occasionally rise to the height of 250 feet. -<span class="pagenum" id="Page_182">- 182 -</span> -The gases and vapours which issue from these 'mud-volcanoes' -are those which are known to be emitted from -volcanic vents at which the action going on is not very -intense. Daubeny and others have suggested that -these mud-volcanoes may be the result of actions which -have little or no analogy with those which take place -at ordinary volcanic vents, and that the combustion of -subterranean beds of sulphur and similar causes would -be quite competent to their production. But inasmuch -as these mud-volcanoes are almost always situated -in regions in which the more powerful volcanic action -has only recently died away, and the gases and vapours -emitted by them are very similar in character to those -which issue from volcanoes, there does not appear to -be any good reason for doubting that they should be -classed as truly volcanic phenomena.</p> - -<p>Mud-volcanoes are found in Northern Italy near Modena, -in Sicily near Girgenti, on the shores of the Sea of -Azof and the Caspian, in Central America, and in other -parts of the globe. The gas frequently escapes from -them with such violence that mud is thrown into the -air to the height of several hundreds of feet. Sometimes -this gas is inflammable, consisting of sulphuretted -hydrogen, hydrogen, or some hydrocarbons, and these -gases occasionally take fire, so that true flames issue -from these mud-volcanoes. In other cases the mud-volcanoes -appear to be formed by either hot or cold -springs containing large quantities of suspended materials, -and the liquid mud issues from the vent without -any violent eruptive action.</p> - -<p><span class="pagenum" id="Page_183">- 183 -</span></p> - -<div class="figcenter" id="fig79_neg" style="width: 465px;"> - <a href="images/fig79.png"><img src="images/fig79_neg.png" width="465" height="178" alt="" /></a> - <div class="figcaption"><span class="smcap">Fig. 79.—Sinter-cones surrounding the orifices of Geysers.</span><br /> - 1. Basin of the Great Geyser, Iceland. 2. Hot spring cone. 3. Old Faithful. - 4. The Great Geyser. 5. Liberty Cap. (2, 3, 4 and 5 are in the - Yellowstone Park district of the Rocky Mountains.)<br /> - Click on image to see original negative view.</div> -</div> - -<div class="sidenote">FORMATION OF SINTER-CONES.</div> - -<p>The soluble materials which waters issuing from -volcanic vents deposit on their sides are chiefly silica -and carbonate of lime.</p> - -<p>Hot springs, whether intermittent or constant, -often contain large quantities of silica in solution. -The solution of this silica is effected, at the moment of -its separation from combination with the alkali or -alkaline earths, during the decomposition of volcanic -rocks, and is favoured by the presence of alkaline carbonates -in the water, and the high temperature and -the pressure under which it exists in the subterranean -regions. When the water reaches the surface and, -being relieved from pressure, begins to cool down the -silica is deposited. By this deposited silica the basins -around the geysers of Iceland are formed. Sometimes -conical structures are built up around the vents of hot -springs by the deposition of silica from their waters. -<span class="pagenum" id="Page_184">- 184 -</span> -Examples of this kind abound in the National Park of -Colorado, where they have received fanciful names, such -as the Beehive, Liberty Gap, &c. This deposited silica -is known to geologists as sinter. The forms of some -of the structures which surround the orifices of geysers -is shown in <a href="#fig79_neg">fig. 79</a>. The 'Liberty Cap' is an extinct -geyser-cone fifty feet high and twenty feet in diameter.</p> - -<p>Hot and cold springs rising in volcanic regions are -often highly charged with carbonic acid, and in passing -through calcareous rocks dissolve large quantities of -carbonate of lime. Upon exposure to the atmosphere, -the free carbonic acid escapes and the carbonate of lime -is deposited in the form known as 'travertine.' Such -springs occur in great numbers in many volcanic regions. -In the Auvergne great rock-masses occur -formed of carbonate of lime deposited from a state of -solution and taking the form of natural aqueducts and -bridges. In Carlsbad the numerous hot springs have -deposited masses of pisolitic rock (Strudelstein) which -have filled up the whole bottom of the valley, and upon -these deposits the town itself is mainly built. In Central -Italy the deposits of travertine formed by calcareous -springs are of enormous extent and thickness: -St. Peter's and all the principal buildings of Rome -being constructed of this travertine or 'Tibur-stone.'</p> - -<div class="sidenote">FORMATION OF SINTER-TERRACES.</div> - -<p>When springs charged with silica or carbonate of -lime rise upon the slope of a hill composed of loose -volcanic materials, they give rise to the remarkable -structures known as sinter- and travertine-terraces (see -<span class="pagenum" id="Page_185">- 185 -</span> -<a href="#fig80_neg">fig. 80</a>). The water flowing downwards from the vent -forms a hard deposit upon the lower slope of the hill, -while the continual deposition of solid materials within -the vent tends to choke it up. As a new vent -cannot be forced by the waters through the hard rock -formed below, it is originated a little higher up. Thus -the site of the spring is gradually shifted farther and -farther back into the hill. As deposition takes place -along the surfaces over which this water flows, terraces -are built up enclosing basins. Of structures of -this kind we have remarkable examples in the sinter-terraces -of Rotomahana in New Zealand and the travertine-terraces -of the Gardiner's River in the Yellowstone -Park district of the Rocky Mountains.</p> - -<div class="figcenter" id="fig80_neg" style="width: 461px;"> - <a href="images/fig80.png"><img src="images/fig80_neg.png" width="461" height="205" alt="" /></a> - <div class="figcaption"><span class="smcap">Fig. 80.—Diagram illustrating the mode of formation of - Travertine and Sinter Terraces on the sides of a hill of tuff.</span><br /> - Click on image to see original negative view.</div> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_186">- 186 -</span></p> - -<h2 class="nobreak" id="CHAPTER_VII">CHAPTER VII.<br /> - -<span class="smaller">THE SUCCESSION OF OPERATIONS TAKING PLACE -AT VOLCANIC CENTRES.</span></h2> -</div> - - -<p class="p0">That a volcanic vent, when once established, may -display intense activity during enormous periods of -time, there cannot be the smallest reason for doubting; -for the accumulation of materials around some existing -volcanic centres must certainly have been going on -during many thousands, perhaps millions, of years. -To us, whose periods of observation are so circumscribed, -it may therefore at first sight appear a hopeless -task to trace the 'life-history of a volcano,' to discover -the stages of its development, and to indicate the -various episodes which have occurred during the long -periods it has been in existence. But when it is remembered -that we have the opportunity of studying -and comparing hundreds of such volcanoes, exhibiting -every varying phase of their development, we shall -see that such an attempt is by no means so unpromising -as it at first sight appears to be. In the -present chapter, we shall give an account of the results -which have already been obtained by inquiries directed -to this object.</p> - -<p><span class="pagenum" id="Page_187">- 187 -</span></p> - -<div class="sidenote">CYCLES OF VOLCANIC PHENOMENA.</div> - -<p>There is not the smallest room for doubt that -during the past history of our globe, exhibitions of -subterranean energy have occurred at many different -parts of its surface. There is further evidence that -at the several sites where these displays of the volcanic -forces have taken place, the succession of the -outbursts has run through a regular cycle, gradually -increasing in intensity to a maximum, and then as -gradually dying away.</p> - -<p>A little consideration will show that the first portion -of this cycle of events is the one which it is most difficult -to examine and study. The products of the earlier -and feeble displays of volcanic activity, at any particular -centre, are liable to be destroyed, or masked, during -the ejection of overwhelming masses of materials in -the later stages of its more matured energy. That the -feeble displays of volcanic force now exhibited in some -localities will gradually increase in intensity in the -future, and eventually reach the grandest stage of development, -there can be no reason for doubting. But, -unfortunately, we are quite unable to discriminate -these feeble manifestations, which are the embryonic -stages in the development of grand exhibitions of the -volcanic forces, from slight outbursts which die away -and make no farther sign.</p> - -<p>From what has been proved concerning the true -nature of volcanic action, however, it is certain that -the first step towards the exhibition of such action, at -any particular locality, must be the production of an -<span class="pagenum" id="Page_188">- 188 -</span> -aperture in the earth's crust. Only by means of such -an aperture can the vapours, gases, and rocky materials -reach the surface, and give rise to the phenomena -there displayed. There is reason to believe that all -such apertures are really of the nature of fissures, or -cracks, which have been opened through the superjacent -strata by the efforts of the repressed subterranean forces.</p> - -<p>Some recent writers have, it is true, endeavoured -to draw a distinction between what they call 'fissure-eruptions,' -and eruptions taking place from volcanic -cones. But all volcanic outbursts are truly -'fissure-eruptions'—the subterranean materials finding their -way to the surface through great cracks, which, in a -more or less vertical position, traverse the overlying -rock-masses. It is true that in many cases portions of -these cracks soon get choked up, while other portions -become widened, and the volcanic energy is concentrated -at such spots. Thus the materials ejected from -these fissures are usually emitted in greatest quantities -at one or more points along the fissure, and a single -great volcanic vent, or a row of smaller vents, is established -upon the line at which the fissure reaches the -surface.</p> - -<p>We have seen that the amount of explosive action -taking place at different volcanic vents varies according -to the proportion of imprisoned water contained in the -lava. In the cases where there is much explosive -action, vast accumulations of scoriæ, lapilli, and dust -<span class="pagenum" id="Page_189">- 189 -</span> -take place, and cones of great size are built up; but -in those cases where the explosive action is small the -lavas flow quietly from the vent, and only small scoriæ-cones -are thrown up, these being probably soon swept -away by the lava-currents themselves or by denuding -agencies. But both kinds of eruption have equal -claims to be called 'fissure-eruptions.'</p> - -<div class="sidenote">FORMATION OF VOLCANIC FISSURES.</div> - -<p>In the expansive force of great masses of imprisoned -vapour, we have a competent cause for the -production of the fissures through which volcanic outbursts -take place. Such fissures are found traversing -the rocks lying above volcanic foci, and often extending -to distances of many miles, or even hundreds of miles, -from the centres of activity. Some of these cracks -are found to be injected with fused materials from -below, others have been more or less completely filled -with various minerals that have been volatilized, or -carried by superheated waters from the deeper regions -of the earth's crust. That many of the cracks thus -produced in the superjacent rocks, by the heaving -forces of imprisoned vapour seeking to escape, never -reached the surface, we have sufficient proof in many -mining regions.</p> - -<p>If we now transfer our attention from the deeper -portions of the earth's crust to the surface, we can -well understand how the attempts of the imprisoned -vapours to force a passage for themselves through the -solid rock-masses would lead to shocks and jars among -the latter. Each of these shocks or jars would give -<span class="pagenum" id="Page_190">- 190 -</span> -rise, in the surrounding portions of the earth's crust, -to those vibrations which we know as earthquakes. -The close connection between most earthquakes and -volcanic phenomena is a fact that does not admit of -the smallest doubt; and though it would be rash to -define all earthquakes as 'uncompleted efforts to establish -a volcano,' yet, in the efforts of the repressed -subterranean forces to find a vent by the production -of fissures in the overlying rock-masses, we have a -cause competent to the production of those shocks -which are transmitted to such enormous distances as -waves of elastic compression.</p> - -<p>We have seen that the production of the fissure -upon which the small volcano of Monte Nuovo was -thrown up was preceded by a succession of earthquakes, -which for a period of over two years terrified -the inhabitants of the district, and might have warned -them of the coming event. In the same manner, -doubtless, the period before the appearance of volcanic -phenomena in a new area would be marked by powerful -subterranean disturbances within it, due to the -efforts of the imprisoned vapours to force for themselves -a channel to the surface.</p> - -<div class="sidenote">NATURE OF FIRST EJECTIONS FROM FISSURES.</div> - -<p>In the case of Monte Nuovo, we have seen that the -fissure, when produced, emitted water—at first in a -cold, then in a boiling condition—and, eventually, steam -and scoriæ. It is probable that through the first -cracks which reached the surface, during the heaving -of the subterranean forces, water, charged with carbonic -<span class="pagenum" id="Page_191">- 191 -</span> -acid, flowed abundantly, and that these cold -springs, charged with carbonic acid and carbonate of -lime, would be succeeded by others which were hot -and contained silica in solution. In Hungary, the -Western Isles of Scotland, and many other volcanic -districts, we find abundant evidence that, before the -eruption of lavas in the area, great masses of travertine -and siliceous sinter were formed by the action of cold -and hot springs.</p> - -<p>As the volcanic action became more intense by the -more perfect opening of the fissures, the evolution of -carbonic add gas would be succeeded by the appearance -of sulphurous acid, sulphuretted hydrogen, boracic -acid, and hydrochloric acid, which recent studies have -shown to be successively emitted from volcanic vents -as the temperature within them rises. At last lava or -molten rock becomes visible within the fissures, and -the ejection of the frothy masses—scoriæ, pumice, -lapilli and dust—commences, and this is sometimes -succeeded by the outflow of currents of lava.</p> - -<p>That volcanoes originate upon lines of fissure in -the earth's crust we have the most convincing proofs. -Not only have such fissures been seen in actual course of -formation at Vesuvius, Etna, and other active volcanoes, -but a study of the volcanoes dissected by denudation -affords the most convincing evidence of the same fact. -The remarkable linear arrangement seen in groups of -volcanoes, which is conspicuous to the most superficial -observer, confirms this conclusion.</p> - -<p><span class="pagenum" id="Page_192">- 192 -</span></p> - -<div class="figcenter" id="fig81" style="width: 745px;"> - <img src="images/fig81.png" width="745" height="503" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 81.—Map of the volcanic group of the Lipari Islands, - illustrating the position of the lines of fissure on - which the volcanoes have been built up.</span></div> -</div> - -<p><span class="pagenum" id="Page_193">- 193 -</span></p> - -<div class="sidenote">SHIFTING OF VENTS ALONG FISSURES.</div> - -<p>We have described the action going on at Stromboli -as typical of that which occurs at all volcanic vents. -Stromboli is, however, one among a group of islands all -of which are entirely of volcanic origin. The volcanoes -of this group of islands, the Æolian or Lipari Islands, -are arranged along a series of lines which doubtless -mark fissures in the earth's crust. These fissures, as -will be seen by the accompanying map (<a href="#fig81">fig. 81</a>), radiate -from a centre at which we have proofs of the former -existence of a volcano of enormous dimensions. It -is a very interesting fact, which the studies of Prof. -Suess have established, that the earthquakes which -have so often desolated Calabria appear to have originated -immediately beneath this great centre of volcanic -activity.</p> - -<div class="figcenter" id="fig82" style="width: 277px;"> - <img src="images/fig82.png" width="277" height="155" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 82.—The Puy de Pariou in the Auvergne, illustrating the - shifting of the centre of eruption along a line of fissures.</span></div> -</div> - -<p>When two volcanic cones are thrown up on the same -line of fissure, their full development is interfered with, -and irregularities in their form and characters are the -consequence. In the plan (<a href="#fig82">fig. 82</a>) and the section -(<a href="#fig83">fig. 83</a>) an example is given of the results of such a -shifting of the centre of eruption along a line of fissure. -<span class="pagenum" id="Page_194">- 194 -</span> -By the second outburst, one-half of the first-formed -cone has been removed, and the second-formed overlaps -the first.</p> - -<div class="figcenter" id="fig83" style="width: 432px;"> - <img src="images/fig83.png" width="432" height="125" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 83.—Ideal section of the Puy de Pariou.</span></div> -</div> - -<p>Sometimes a number of scoria- or tuff-cones are -thrown up in such close proximity to one another along -a line of fissure, that they merge into a long irregular -heap on the summit of which a number of distinct -craters can be traced. An example of this kind was -furnished by the line of scoria-cones formed above the -fissure which opened on the flanks of Etna in 1865 -(see <a href="#fig84">fig. 84</a>).</p> - -<div class="figcenter" id="fig84" style="width: 448px;"> - <img src="images/fig84.png" width="448" height="170" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 84.—Fissure formed on the flanks of Etna during the -eruption of 1865.</span><br /> - <i>a.</i> Monte Frumento, an old parasitic cone. <i>b.</i> Line of fissure. - <i>c, c, c.</i> New scoria-cones thrown up on line of fissure. - <i>d.</i> Lava from same.</div> -</div> - -<p><span class="pagenum" id="Page_195">- 195 -</span></p> - -<div class="figcenter" id="fig85" style="width: 279px;"> - <img src="images/fig85.png" width="279" height="447" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 85.—Plan of the Island of Vulcano, based on the map of - the Italian Government.</span></div> -</div> - -<div class="sidenote">SHIFTING OF ERUPTIONS ALONG FISSURES.</div> - -<p>Even in the case of great composite cones, however, -we sometimes find proofs of the centre of eruption -having shifted its place along the line of fissure. No -better example of this kind could possibly be adduced -than that of the Island of Vulcano, with the peninsula -of Vulcanello, which is joined to it by a narrow isthmus -(see the map, <a href="#fig81">fig. 81</a>, p 192). In <a href="#fig85">fig. 85</a> we have -<span class="pagenum" id="Page_196">- 196 -</span> -given an enlarged plan of this island which will make -its peculiar structure more intelligible (see also the -section given in <a href="#fig77_neg">fig. 77</a>, No. 6, facing p. 178).</p> - -<p>The south-eastern part of the island consists of four -crater-rings, one half of each of Which has been successively -destroyed, through the shifting of the centre -of eruption towards the north-west, along the great -line of fissure shown in the general map (<a href="#fig81">fig. 81</a>). -The last formed of these four crater-rings is the one -which is now most complete, and culminates in Monte -Saraceno (1581 ft.), <i>a</i> in the plan, the highest point in -the island. The older crater-rings have been in part -removed by the inroads of the waters of the Mediterranean -on the shores of the island. In the centre of -the great crater, <i>b</i>, which we have just described, rises -the present active cone of Vulcano, 1,266 feet high, and -having a crater, <i>c</i>, about 600 yards in diameter and -more than 500 feet in depth. From this cone, a great -stream of obsidian, <i>e</i>, flowed in the year 1775, and a -small crater, <i>d</i>, the Fossa Anticha, has been opened in -the side of the cone. The continuation of the same -line of fissure is indicated by a ruined tuff-cone, <i>f</i>, -known as the Faraglione, and the three scoria-cones of -Vulcanello, <i>g, h</i>, which have been thrown up so close -to one another as to have their lower portions merged -in one common mass, as shown in <a href="#fig86">fig. 86</a>.</p> - -<div class="sidenote">SYSTEMS OF VOLCANIC FISSURES.</div> - -<p>Even in volcanoes of the largest dimensions we -sometimes find proofs of the centre of eruption having -shifted along the line of fissure. Lyell showed that -<span class="pagenum" id="Page_197">- 197 -</span> -such a change in the position of the central axis of the -volcano had taken place in Etna, and the same phenomenon -is exhibited in the clearest manner' by some of -the ancient volcanoes of the Inner Hebrides, which -have been dissected by the denuding forces.</p> - -<div class="figcenter" id="fig86" style="width: 451px;"> - <img src="images/fig86.png" width="451" height="266" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 86.—Vulcanello, with its three craters.</span><br /> - <i>a.</i> The most recently-formed and perfect crater, <i>b</i> and <i>c</i>. Older craters, the walls - of which have been partly removed by denudation, <i>e.</i> Lava-currents proceeding - from <i>b</i>. The section exposed in the cliff at <i>d</i> is represented in <a href="#fig35">fig. 35, p. 116</a>.</div> -</div> - -<p>In the case of the Lipari Islands, the fissures along -which the volcanic mountains have been thrown up -radiate from a common centre, and a similar arrangement -can be traced in many volcanic regions, especially -those in which a great central volcano has existed. In -other cases, however, as in the Campi Phlegræi, the -volcanic vents appear to be formed along lines which -assume a parallel arrangement, and this doubtless -marks the relative position of the original fissures -<span class="pagenum" id="Page_198">- 198 -</span> -produced in the earth's crust when these volcanoes were -formed. In some other cases we find evidences of the -existence of a principal fissure from the sides of which -smaller cracks originated. These three kinds of arrangements -of volcano-producing fissures are equally -well illustrated when we study those denuded districts, -in which, as we have seen, the ground-plans of volcanic -structures are revealed to our view.</p> - -<p>There is now good ground for believing that in -volcanic vents, at which long-continued eruptive action -takes place, the lavas of different chemical composition -make their appearance in something like a definite -order. It had been remarked by Scrope and other -geologists at the beginning of the present century, that -in many volcanic areas the acid or trachytic lavas -were erupted before the basic or basaltic.</p> - -<p>Von Richthofen, by his studies in Hungary and -the volcanic districts of the Rocky Mountains, has been -able to enunciate a law governing the natural order of -succession of volcanic products; and although some exception -to this law may be mentioned, it is found to -hold good for many other districts than those in which -it was first determined.</p> - -<p>In a great number of cases it has been found that -the first erupted rocks in a volcanic district are those -of intermediate composition which are known as andesites. -These andesites, which are especially characterised -by the nature of their felspar, sometimes contain -free quartz and are then known as quartz-andesites -<span class="pagenum" id="Page_199">- 199 -</span> -or dacites, from their abundance in Transylvania, the -old Roman province of Dacia.</p> - -<div class="sidenote">ORDER OF ERUPTION OF VOLCANIC PRODUCTS.</div> - -<p>Von Richthofen suggests that another class of volcanic -rocks, to which he gives the name of 'propylites,' -were in every case erupted before the andesites, and -in support of his views adduces the fact that in many -instances propylites are found underlying andesites. But -the propylites are, in chemical composition, identical -with the andesites, and like them present some varieties -in which quartz occurs, and others in which that mineral -is absent. In their microscopic characters the propylites -differ from the andesites and dacites only in the -fact that the former are more perfectly crystalline in -structure, being indeed in many cases quite undistinguishable -from the diorites or the plutonic representatives -of the andesites. The propylites also contain -liquid cavities, which the andesites and dacites as a -rule do not, and the former class of rocks, as Prof. Szabo -well points out, are usually much altered by the passage -of sulphurous and other vapours, in consequence of -which they frequently contain valuable metallic ores.</p> - -<p>The extrusion of these andesitic lavas is sometimes -accompanied, and sometimes preceded or followed, by -eruptions of trachytic lavas—that is, of lavas of intermediate -composition which have a different kind of -felspar from that prevailing in the andesites.</p> - -<p>In the final stages of the eruptive action in most -volcanic districts the lavas poured forth belong to the -classes of the rhyolitic or acid, and the basaltic or -basic lavas.</p> - -<p><span class="pagenum" id="Page_200">- 200 -</span></p> - -<p>These facts are admirably illustrated in the case of -the volcanic district of the Lipari Islands, to which we -have had such frequent occasion to refer. The great -central volcano of this district, which now in a ruined -condition constitutes a number of small islets (see the -map, <a href="#fig81">fig. 81</a>, p. 192), is composed of andesitic lavas. -The other great volcanoes thrown up along the three -radiating lines of fissure are composed of andesitic and -trachytic rocks. But all the more recent ejections of -the volcanoes of the district have consisted either of -rhyolites, as in Lipari and Vulcano, or of basalts, as in -Stromboli and Vulcanello.</p> - -<p>Von Richthofen and the geologists who most strongly -maintain the generalisations which he has made concerning -the order of appearance of volcanic products, go -much farther than we have ventured to do, and insist -that in all volcanic districts a constant and unvarying -succession of different kinds of lavas can be made out. -It appears to us, however, that the exceptions to the -law, as thus precisely stated, are so numerous as to -entirely destroy its value.</p> - -<p>The generalisation that in most volcanic districts the -first ejected lavas belong to the intermediate group of -the andesites and trachytes, and that subsequently the -acid rhyolites and the basic basalts made their appearance, -is one that appears to admit of no doubt, and is -found to hold good in nearly all the volcanic regions -of the globe which have been attentively studied.</p> - -<p>The Tertiary volcanic rocks of our own country, those -<span class="pagenum" id="Page_201">- 201 -</span> -of North Germany, Hungary, the Euganean Hills, the -Lipari Islands, and many other districts in the Old -World, together with the widespread volcanic rocks of -the Rocky Mountains in the New World, all seem to -conform to this general rule.</p> - -<div class="sidenote">THEORY OF VOLCANIC MAGMAS.</div> - -<p>In connection with this subject, it may be well to -refer to the ideas on the composition of volcanic rocks -which were enunciated by Bunsen, and the theoretic -views based on them by Durocher. Bunsen justly -pointed out that all volcanic rocks might be regarded -as mixtures in varying proportions of two typical kinds -of materials, which he named the 'normal trachytic' -and the 'normal pyroxenic' elements respectively. -The first of these corresponds very closely in composition -with the acid volcanic rocks or rhyolites, and the -second with the basic volcanic rocks or basalts. Durocher -pointed out that if quantities of these different -materials existed in admixture, the higher specific -gravity of the basic element would cause it gradually to -sink to the bottom, while the acid element would rise -to the top. Carrying out this idea still further, he propounded -the theory that beneath the earth's solid crust -there exist two magmas, the upper consisting of light -acid materials, the lower of heavy basic ones; and he -supposed that by the varying intensity of the volcanic -forces we may have sometimes one or the other magma -erupted and sometimes varying mixtures of the two.</p> - -<p>The study of volcanic rocks in recent years has not -lent much support to the theoretic views of Durocher -<span class="pagenum" id="Page_202">- 202 -</span> -concerning the existence of two universal magmas beneath -the earth's crust; and there are not a few facts -which seem quite irreconcilable with such a theory. -Thus we find evidence that in the adjacent volcanic districts -of Hungary and Bohemia, volcanic action was going -on during the whole of the latter part of the Tertiary -period. But the products of the contemporaneous volcanic -outbursts in adjacent areas were as different in -character as can well be imagined. The volcanic rocks -all over Hungary present a strong family likeness; the -first erupted were trachytes, then followed andesites and -dacites in great abundance, and lastly rhyolites and -basalts containing felspar. But in Bohemia, the lavas -poured out from the volcanoes during the same period -were firstly phonolites and then basalts containing nepheline -and leucite. It is scarcely possible to imagine -that such very different classes of lavas could have been -poured out from vents which were in communication -with the same reservoirs of igneous rock, and we are -driven to conclude that the Hungarian and Bohemian -volcanoes were supplied from different sources.</p> - -<div class="sidenote">SEPARATION OF LAVAS IN RESERVOIRS.</div> - -<p>But the undoubted fact that in so many volcanic -regions the eruption of andesitic and trachytic rocks, -which are of intermediate composition, is followed by -the appearance of the differentiated products, rhyolite -and basalt, which are of acid and basic composition respectively, -lends not a little support to the view that under -each volcanic district a reservoir of more or less completely -molten rock exists, and that in these reservoirs -<span class="pagenum" id="Page_203">- 203 -</span> -various changes take place during the long periods of -igneous activity. During the earlier period of eruption -the heavier and lighter elements of the contents of these -subterranean reservoirs appear to be mingled together; -but in the later stages of the volcanic history of the -district, the lighter or acid elements rise to the top, and -the heavier or basic sink to the bottom, and we have -separate eruptions of rhyolite and basalt. We even -find some traces of this action being carried still -further. Among the basalts ejected from the volcanoes -of Northern Germany, Bohemia, Styria, Auvergne, and -many other regions, we not unfrequently find rounded -masses consisting of olivine, enstatite, augite, and other -heavier constituents of the rock. These often form the -centre of volcanic bombs, and are not improbably portions -of a dense mass which may have sunk to the -bottom of the reservoirs of basaltic materials.</p> - -<p>In consequence of the circumstance that the eruption -of lavas of intermediate composition usually -precedes that of other varieties, we usually find the -central and older portions of great volcanoes to be -formed of andesites, trachytes, or phonolites, while the -outer and newer portions of the mass are made up of acid -or basic lavas. This is strikingly exemplified in the -great volcanoes of the Auvergne and the Western Isles -of Scotland, in all of which we find that great mountain -masses have, in the first instance, been built up by extrusions -of lava of the intermediate types, and that through -this central core fissures have been opened conveying -<span class="pagenum" id="Page_204">- 204 -</span> -basic lavas to the surface. From these fissures great -numbers of basaltic lava-streams have issued, greatly -increasing the height and bulk of the volcanic cones -and deluging the country all around.</p> - -<p>The lavas of intermediate composition—the andesites, -trachytes, and phonolites—possess, as we have already -seen, but very imperfect liquidity as they flow from the -volcanic vents. Hence we find them either accumulating -in great dome-shaped masses above the vent or -forming lava-streams which are of great bulk and thickness, -but do not flow far from the orifices whence they -issue. The more fusible basaltic lavas, on the other -hand, spread out evenly on issuing from a vent, and -sometimes flow to the distance of many miles from it. -This difference in the behaviour of the intermediate -and basic lavas is admirably illustrated in the volcanic -districts of the Auvergne and the Western Isles of -Scotland.</p> - -<p>In other cases, like Vesuvius, we find that great -volcanic cones of trachytic tuff have been built up, and -that these masses of fragmentary trachytic materials -have been surrounded and enclosed by the ejection, at -a later date, of great outbursts of basaltic lavas. In -still other cases, of which Rocca Monfina in Southern -Italy constitutes an excellent example, we find that a -great crater-ring of trachytic tuffs has been formed in -the first instance, and in the midst of this a cone, -composed of more basic materials, has been thrown up.</p> - -<p><span class="pagenum" id="Page_205">- 205 -</span></p> - -<div class="sidenote">EXCEPTIONS TO THE GENERAL LAW.</div> - -<p>In all these volcanoes we see the tendency towards -the eruption of intermediate lavas in the first instance, -and of basaltic and acid lavas at a later date. Valuable, -however, as are the early generalisations of Scrope, -and the more precise law enunciated by Von Richthofen -concerning the 'natural order of succession of volcanic -products,' we must not forget that there are to be -found a considerable number of exceptions to them. -There are some volcanic centres from which only one -kind of lava has been emitted, and this may be either -acid, basic, or intermediate in composition; and on -the other hand, there are districts in which various -kinds of lava have been ejected from the same vents -within a short period of time, in such a way as to defy -every attempt to make out anything like a law as to -the order of their appearance. Nevertheless the rules -which we have indicated appear to hold good in so -great a number of cases that they are well worthy of -being remembered, and may serve as a basis on which -we may reason concerning the nature of the action -going on beneath volcanic vents.</p> - -<p>From the study of the external appearances of volcanic -mountains, combined with investigations of those -which have been dissected by denudation, we are able -to picture to our minds the series of actions by which -the great volcanic mountains of the globe have been -slowly and gradually built up.</p> - -<p>In the first instance the eruptions appear to have -taken place at several points along a line of fissure, but -gradually all of these would become choked up except -<span class="pagenum" id="Page_206">- 206 -</span> -one which became the centre of habitual eruption. -From this opening, ejections, firstly of lavas of intermediate -composition, and afterwards of basic materials, -would take place, until a volcano of considerable dimensions -was built up around it. But at last a point would -be reached in the piling up of this cone, when the volcanic -forces below would be inadequate to the work -of raising the liquid lava through the whole length of -the continually upward-growing tube of the volcano. -Under these circumstances the expansive force of the -imprisoned steam would find it easier to rend asunder -the sides of the volcanic cone than to force the liquid -material to the summit of the mountain. If these -fissures reached the surface explosive action would take -place, in consequence of the escape of steam from the -glowing mass, and scoria-, tuff-, and lava-cones would -be formed above the fissure. In this way, as we have -already pointed out, the numerous 'parasitic cones' -which usually abound on the flanks of the greater -volcanic mountains have been formed. The extrusion of -these masses of scoriæ and lava on the flanks of the -mountain tends, not only to increase the bulk of the -mass, but to strengthen and fortify the sides. For by -the powerful expansive force at work below, every weak -place in the cone is discovered and a fissure produced -there; but by the extrusion of material at this fissure, -and still more by the consolidation of the lava in the -fissure, the weak place is converted into one of -exceptional strength.</p> - -<p><span class="pagenum" id="Page_207">- 207 -</span></p> - -<div class="sidenote">INTRUSIVE MASSES BENEATH VOLCANOES.</div> - -<p>As the sides of the cone are thus continually repaired -and strengthened they are rendered more capable -of withstanding the heaving forces acting from below, -and these forces can then only find vent for themselves -by again raising the liquefied lava to the central orifice -of the mountain. Many volcanoes, like Etna, exhibit -this alternation of eruptive action from the crater at -the summit of the mountain, and from fissures opened -upon its flanks, the former tending to raise the height -of the volcanic pile, the latter to increase its bulk.</p> - -<p>But at last a stage will be reached when the volcanic -forces are no longer able either to raise the lava -up the long column of the central vent on the one -hand, or to rend asunder the strongly-built and well-compacted -flanks of the mountain on the other. It is -probably under these conditions, for the most part, -that the lavas find their way between the masses of -surrounding strata and force them asunder in the way -that we have already described.</p> - -<p>In the case of the more fluid basaltic lavas, as was -pointed out so long ago by Macculloch, the liquefied -materials may find their way between the strata to -enormous distances from the volcanic centre. Such -extended flat sheets of igneous rock retain their -parallelism with the strata among which they are intruded -over large areas, and did not probably produce any -marked phenomena at the surface.</p> - -<p>But in the case of less fluid lavas, such as those of -intermediate or acid composition, for example, the -<span class="pagenum" id="Page_208">- 208 -</span> -effect would be far otherwise. Such lavas, not flowing -readily from the centre of eruption, would tend to -form great bulky lenticular masses between the strata -which they forced asunder, and, in so doing, could not -fail to upheave and fissure the great mountain-mass -above. Vast lenticular masses of trachytic rock, thus -evidently forced between strata, have been described -by Mr. G. K. Gilbert, as occurring in the Henry -Mountains of Southern Utah, and by him have been -denominated 'laccolites,' or stone-cisterns. Whether -the great basaltic sheets, like those described by -Macculloch, and those more bulky lenticular reservoirs -of rock of which Mr. Gilbert has given us such an -admirable account, were in all cases connected with -the surface, may well be a matter for doubt. It is -quite possible that, in some cases, liquefied masses of -rocky materials in seeking to force their way to the -surface only succeeded in thus finding a way for themselves -between the strata, and their energy was expended -before the surface was reached and explosive -action took place. But it is an undoubted fact that -beneath many of the old volcanoes, of which the internal -structure is now revealed to us by the action of -denuding forces, great intrusive sheets and laccolites -abound; and we cannot doubt that beneath volcanoes -now in a state of eruption, or in those which have but -recently become extinct, similar structures must be in -course of formation.</p> - -<div class="sidenote">EFFECTS OF INTRUSION BENEATH CONES.</div> - -<p>That great upheaving forces have operated on -<span class="pagenum" id="Page_209">- 209 -</span> -volcanoes, subsequently to the accumulation of their -materials, we have sufficient evidence in the Val del -Bove of Etna, the Caldera of Palma, the Corral of -Madeira, &c. In all of these cases we find a radial -fissure ('barranco') leading into a great crateral -hollow; and these radial fissures are of such width and -depth that their origin can only be referred to a disruptive -force like that which would be exercised by -the intrusion of masses of more or less imperfectly -fluid material between the subjacent strata. These -facts, of course, lend no countenance to the views -formerly held by many geologists, both in Germany and -France, that the materials of which volcanoes are built -up were deposited in an approximately horizontal position, -and were subsequently blown up like a gigantic -bubble. In Etna, Palma, and Madeira we find abundant -proofs that the mass existed as a great volcanic -cone before the production of the fissures (barrancos), -which we have referred to the force exercised during -the intrusion of great igneous masses beneath them.</p> - -<p>But besides the horizontally-disposed intrusive -sheets and laccolites, great, radiating, vertical fissures -are produced by the heaving forces acting beneath -those volcanic centres which have been closed up -and 'cicatrised' by the exudation from them of subterranean -materials. These vertical intrusions, which -we call dykes, like the horizontal ones, differ in character, -according to the nature of the materials of -which they are composed. Dykes of acid and intermediate -<span class="pagenum" id="Page_210">- 210 -</span> -lava are usually of considerable width, and -do not extend to great distances from the centres of -eruption. Dykes composed of the more-liquid, basic -lavas, on the other hand, may extend to the distance -of hundreds of miles from the central vent. The way -in which comparatively narrow, basaltic dykes are found -running in approximately straight lines for such -enormous distances is a very striking fact, and bears -the strongest evidence to the heaving and expanding -forces at work at volcanic centres, during and subsequently -to the extrusion of the igneous products at -the surface.</p> - -<p>These basaltic dykes occur in such prodigious -numbers around some volcanic vents, that the whole -of the stratified rocks in the immediate vicinity are -broken up by a complete network of them, crossing -and interlacing in the most complicated fashion. -Farther away from the vents, similar dykes are found -in smaller numbers, evidently radiating from the same -centre, and sometimes extending to a distance of -more than a hundred miles from it. Nowhere can we -find more beautiful illustrations of such dykes than -in the Western Isles of Scotland. When composed of -materials which do not so easily undergo decomposition -as the surrounding rocks, they stand up like -vast walls; but when, on the other hand, they are -more readily acted on by atmospheric moisture than are -the rocks which enclose them, they give rise to deep -trenches with vertical sides, which render the country -almost impassable.</p> - -<p><span class="pagenum" id="Page_211">- 211 -</span></p> - -<div class="sidenote">STRUCTURE OF INTRUSIVE MASSES.</div> - -<p>The lava consolidating in these horizontal intrusions -(sheets and laccolites), and the vertical intrusions -(dykes), is usually more crystalline in structure than -the similar materials poured out at the surface. In -the same dyke or sheet, when it is of great width, -we often find every variation—from a glassy material -formed by the rapid cooling of the mass where it is in -contact with other rocks, to the perfectly crystalline -or granitic varieties which form the centre of the intrusion. -It is in these dykes and other intrusions -that we find the most convincing evidence of the truth -of the conclusions, which we have enunciated in a -former chapter, concerning the dependence of the -structure of an igneous rock upon the conditions -under which it has consolidated. One material is -found, under varying conditions, assuming the characters -of obsidian, rhyolite, quartz-felsite, or granite; -another, under the same set of conditions, taking the -form of tachylyte, basalt, dolerite, and gabbro.</p> - -<p>That these great intrusive masses, sheets and dykes, -in their passage between the sedimentary rocks sometimes -find places where the overlying strata are of -such thinness or incoherence that the liquefied rocks are -able to force a way for themselves to the surface, we -have the clearest proof. In some dykes we find the -rock in their upper portions losing its compact character -and becoming open and scoriaceous, showing that -the pressure had been so far diminished as to allow of -the imprisoned water flashing into steam.</p> - -<p><span class="pagenum" id="Page_212">- 212 -</span></p> - -<p>All round great volcanoes which have become extinct -we frequently find series of small volcanic cones, -which have evidently been thrown up along the lines -where the great lava-filled fissures, which we have been -describing, have reached the surface and given rise to -explosive action there. The linear arrangement of -these small cones, which are thrown up in the plains -surrounding vast volcanic mountains that have become -extinct, is very striking. The numerous 'puys' of the -Auvergne and adjoining volcanic regions of Central -France are for the most part small scoria- and lava-cones -which were thrown up along great lines of fissure -radiating from the immense, central, volcanic mountains -of the district, after they had become extinct. These -scoria-cones and the small lava-streams which flow from -them, as was so well shown by Mr. Scrope, mark the -latest efforts of the volcanic forces beneath the district -before they finally sank into complete extinction. In -the Western Isles of Scotland, as I have elsewhere -shown, we can study the formation of these later-formed -cones in the plains around extinct volcanic mountains, -with the additional advantage of having revealed to -us, by the action of the denuding forces, their connection -with the great radiating fissures.</p> - -<p>It has been shown that the several stages in the -decline of each volcanic outburst is marked by the -appearance at the vent of certain acid gases. In the -same way, after the ejection of solid materials from a -volcanic vent has come to an end, certain gaseous -<span class="pagenum" id="Page_213">- 213 -</span> -substances continue to be evolved; and as the temperature -at the vents declines, the nature of the volatile substances -emitted from them undergoes a regular series -of changes.</p> - -<div class="sidenote">ORDER OF EMISSION OF VOLCANIC GASES.</div> - - -<p>M. Fouqué, by a careful series of analyses of the -gases which he collected at different gaseous vents, or -fumaroles as they are called, in the crater of Vulcano, -has been able to define the general relations which -appear to exist between the temperature at a volcanic -orifice and the volatile substances which issue from it. -He found that in fumaroles, in which the temperature -exceeded 360° centigrade, and in which in consequence -strips of zinc were fused by the stream of issuing gas, -the analysis of the products showed sulphurous acid -and hydrochloric add to be present in large quantities, -and sulphuretted hydrogen and carbonic acid in much -smaller proportions. Around these excessively heated -fumaroles, the lips of which often appear at night to -be red-hot, considerable deposits of sulphide of arsenic, -chloride of iron, chloride of ammonium, boracic acid, -and sulphur were taking place.</p> - -<p>It was found, however, that as the temperature of -the vent declined, the emission of the sulphurous acid -and hydrochloric acid diminished, and the quantity of -sulphuretted hydrogen and carbonic acid mingled with -them was proportionately increased.</p> - -<p>In the same way it appears to be a universal rule -that when a volcanic vent sinks into a condition of -temporary quiescence or complete extinction the powerfully -<span class="pagenum" id="Page_214">- 214 -</span> -acid gases, hydrochloric acid and sulphurous acid, -make their appearance in the first instance, and at a -later stage these are gradually replaced by sulphuretted -hydrogen and carbonic acid.</p> - -<p>Of these facts we find a very beautiful illustration -in the Campi Phlegræi near Naples. With the exception -of Monte Nuovo, the volcano which has most -recently been in a state of activity in that district is -the Solfatara. From certain apertures in the floor of -the crater of the Solfatara there issue continually watery -vapours, sulphurous acid, sulphuretted hydrogen, hydrochloric -acid, and chloride of ammonium. The action of -these substances upon one another, and upon the volcanic -rocks through which they pass, gives rise to the -formation of certain chemical products which, from a -very early period, have been collected on account of -their commercial value. The action of these add gases -upon the surrounding rocks is very marked; efflorescent -deposits of various sulphates and chlorides take -place in all the crevices and vesicles of the rock; sulphur -and sulphide of arsenic are also formed in considerable -quantities; and the trachytic tuffs, deprived of -their iron-oxide, alkaline earths and alkalies, which -are converted into soluble sulphates and chlorides, are -reduced to a white, powdery, siliceous mass. Many volcanoes, -which have sunk into a state of quiescence or -extinction like the Solfatara of Naples, exhibit the same -tendency to give off great quantities of the powerfully-acid -gases which act upon the surrounding rocks, and -<span class="pagenum" id="Page_215">- 215 -</span> -deprive them of their colour and consistency. Such -volcanoes are said by geologists to have sunk into the -'solfatara stage.'</p> - -<div class="sidenote">SOLFATARA-STAGE OF VOLCANOES.</div> - -<p>At the Lake of Agnano and some other points in the -Campi Phlegræi, however, we find fissures from which -the less-powerfully acid gases, sulphuretted hydrogen -and carbonic acid, issue. These gases as they, are -poured forth from the vents are found to be little, if at -all, above the temperature of the atmosphere. Sulphuretted -hydrogen is an inflammable gas, and in the so-called -salses and mud-volcanoes, at which it is ejected -in considerable quantities, it not unfrequently takes fire -and bums with a conspicuous flame. Carbonic acid on -account of its great density tends to accumulate in -volcanic fissures and craters rather than to mingle with -the surrounding atmosphere. At the so-called Grotto -del Cane, beside the Lago Agnano, it is the custom to -show the presence of this heavy and suffocating gas by -thrusting a dog into it, the poor animal being revived, -before life is quite extinct, by pouring cold water over -it. At the Büdos Hegy or 'stinking hill' of Transylvania, -carbonic acid and sulphuretted hydrogen are -emitted in considerable quantities, and it is possible -to take a bath of the heavy gas, the head being kept -carefully above the constant level of the exhalations.</p> - -<p>Although the stories of the ancient Avernian lake, -across which no bird could fly without suffocation, and -of the Guevo Upas, or Poison Valley of Java, which it -has been said no living being can cross, may not -<span class="pagenum" id="Page_216">- 216 -</span> -improbably be exaggerations of the actual facts, yet there -is a basis of truth in them in the existence of old volcanic -fissures and craters which evolve the poisonous -sulphuretted hydrogen and carbonic acid gases.</p> - -<p>Besides the gases which we have already named, -and which are the most common at and characteristic -of volcanic vents, there are some others which are not -unfrequently emitted. First among these we must -mention boracic acid, which, though not a remarkably -volatile substance, is easily carried along in a fine state -of division in a current of steam. At Monte Cerboli -and Monte Rotondo in Tuscany, great quantities of -steam jets accompanied by sulphuretted hydrogen and -boracic acid issue from the rocks, and these jets being -directed into artificial basins of water, the boracic acid -is condensed and is recovered by evaporation. We -have already noticed that boracic add is evolved with -the gases at Vulcano and other craters; and the part -which this substance plays in volcanic districts is shown -by the fact that many of the rocks, filling old subterranean -volcanic reservoirs, are found to be greatly -altered and to have new minerals developed in their -midst through the action upon them of boracic acid.</p> - -<p>Ammonia and various compounds of carbon, nitrogen, -and hydrogen are among the gases evolved from -volcanic vents. In some cases these gases may be produced -by the destructive distillation of organic materials -in the sedimentary rocks through which volcanic -outbursts take place. But it is far from impossible -<span class="pagenum" id="Page_217">- 217 -</span> -that under the conditions of temperature and pressure -which exist at the volcanic foci, direct chemical union -may take place between substances, which at the surface -appear to be perfectly inert in each other's presence.</p> - -<p>When the temperature at volcanic fissures is no -longer sufficiently high to cause water to issue in the -condition of vapour or steam, as is the case at the -'stufas' which we have described, it comes forth in -the liquid state. Water so issuing from old volcanic -fissures may vary in its temperature, from the boiling -point downwards.</p> - -<div class="sidenote">GEYSERS AND HOT-SPRINGS.</div> - -<p>When the water issues at a temperature little removed -from the boiling point, it is apt to give rise -to intermittent springs or geysers, the eruptions of -which exhibit a remarkable analogy with those of -ordinary volcanoes. Geysers may indeed be described -as volcanoes in which heated water, instead of molten -rock, is forced out from the vent by the escaping steam. -They occur in great abundance in districts in which the -subterranean action is becoming dormant or extinct, -such as Iceland, the North Island of New Zealand, -and the district of the National Park in the Rocky -Mountains.</p> - -<p>Many attempts have been made to explain the -exact mechanism by which the intermittent action of -geysers is produced, but it is not at all probable that -any one such explanation will cover all the varied phenomena -exhibited by them. Like volcanic outbursts, -<span class="pagenum" id="Page_218">- 218 -</span> -geyser eruptions doubtless originate in the escape of -bubbles of steam through a liquid mass, and this liberation -of steam follows any relief of pressure. In districts -where vast masses of lava are slowly cooling down from -a state of incandescence, and surface waters are finding -their way downwards while subterranean waters are -finding their way upwards, there can be no lack of the -necessary conditions for such outbursts. Sometimes -the eruptions of geysers take place at short and regular -intervals, at other times they occur at wide and irregular -intervals of time. In some cases the outbursts take -place spontaneously, and at others the action can be -hastened by choking up the vent with stones or earth.</p> - -<p>Other hot springs, like the Strudel of Carlsbad, rise -above the surface in a constant jet, while most of them -issue quietly and flow like ordinary springs.</p> - -<p>Although the violent and paroxysmal outbursts of -volcanic mountains arrest the attention, and powerfully -impress us with a sense of the volcanic activity -going on beneath the earth's surface, yet it may well -be doubted whether the quantity of heat, which the -earth gets rid of by their means, at all approaches in -amount that which is quietly dissipated by means of -the numerous 'stufas,' gaseous exhalations, and thermal -springs which occur in such abundance all over its -surface. For while the former are intermittent in their -action, and powerful outbursts are interrupted by long -periods of rest, the action of the latter, though feeble, -is usually continuous.</p> - -<p><span class="pagenum" id="Page_219">- 219 -</span></p> - -<div class="sidenote">EFFECTS OF HOT-SPRINGS.</div> - -<p>Most people may regard the hot spring of Bath as -a very slight manifestation of volcanic activity. This -spring issues at a constant temperature of 49° C, or -120° Fahr. As, however, no less than 180,000 gallons -of water issue daily from this source, we may well understand -how great is the amount of heat of which the -earth's crust is relieved by its agency. It may indeed -be doubted whether its action in this way is not -at least equal to that of a considerable volcano which, -though so much more violent, is intermittent in its -action.</p> - -<p>Nor are thermal springs by any means ineffective -agents in bringing materials from the interior of the -earth's crust and depositing it at the surface. The -Bath spring contains various saline substances, principally -sulphates and chlorides, in solution in its -waters. These are quietly carried by rivers to the sea, -and are lost to our view. The spring has certainly -maintained its present condition since the time of the -Romans, and I find that if the solid materials brought -from the interior of the earth during the last 2,000 -years had been collected, they would form a solid cone -equal in height to Monte Nuovo. Yet we usually regard -the Campi Phlegræi as a powerfully-active volcanic -district, and the subterranean action in our own country -as quite unworthy of notice.</p> - -<p>When we remember the fact that on the continent -of Europe the hot and saline springs may be numbered -by thousands, and that they especially abound in districts -<span class="pagenum" id="Page_220">- 220 -</span> -like Hungary, the Auvergne, the Rhine provinces, and -Central Italy, where volcanic action has recently become -extinct, we shall be able to form some slight idea of the -work performed by these agents, not only in relieving the -earth's crust of its superfluous heat, but in transporting -materials in a state of solution from the interior of that -crust and depositing them at the surface. The vast -deposits of siliceous sinter and of travertine also bear -witness to the effects produced by hot and mineral -springs.</p> - -<p>Nor is the work of these springs confined to the -surface. Mr. John Arthur Phillips has shown that -metallic gold and the sulphide of quicksilver (cinnabar) -have been deposited with the silica and other -minerals formed on the sides of a fissure from which -hot springs issue at the surface. There cannot be any -doubt that the metallic veins or lodes, which are the -repositories of most of the metals employed in the arts, -have been formed in cracks connected with great volcanic -foci, the transfer of the various sulphides, oxides, -and salts which fill the vein having been effected either -by solution, sublimation, or the action of powerful currents -of steam.</p> - -<p>As the igneous activity of the district declines, the -temperature of the issuing gases and waters diminishes -with it, until at last the volcanic forces appear to wholly -abandon that region and to be transferred to another.</p> - -<p>Yet even after all or nearly all indications of the -volcanic agencies cease to make themselves visible at -<span class="pagenum" id="Page_221">- 221 -</span> -the surface, occasional tremblings of the earth's crust -show that perfect equilibrium has not been restored -below, but that movements are taking place which -result in shocks that are transmitted through the -overlying and surrounding rock-masses as earthquake -vibrations.</p> - -<div class="sidenote">NATURE OF VOLCANIC CYCLES.</div> - -<p>Such is the cycle of changes which appears to take -place at each district of the earth's surface, as it -successively becomes the scene of volcanic activity.</p> - -<p>The invasion of any particular area of the earth's -surface by the volcanic forces appears to be heralded -by subterranean shocks causing earthquake vibrations. -Presently the origination of fissures is indicated by the -rise of saline and thermal springs, and the issuing of -carbonic acid and other gases at the surface. As the -subterranean activity becomes more pronounced, the -temperature of the springs and emitted gases is found -to increase, and at last a visible rent is formed at the -surface, exposing the incandescent materials below.</p> - -<p>From this open fissure which has thus been formed, -the gas and vapours imprisoned in the incandescent -rock-materials escape with such violence as to disperse -the latter in scoriæ and dust, or to cause them to well out -in great streams as lava-flows. Usually the action becomes -concentrated at one or several points at which -the ejected materials accumulate to form volcanic -cones.</p> - -<p>Sometimes the volcanic activity dies away entirely -after these cones are thrown up along the line of fissure, -<span class="pagenum" id="Page_222">- 222 -</span> -but at others some such centre becomes for a longer or -shorter time the habitual vent for the volcanic forces -in the district, and by repeated ejections of lavas and -fragmentary materials at longer or shorter intervals -the cone increases both in height and bulk.</p> - -<p>When the height of the cone has grown to a certain -extent, it becomes more easy for the volcanic energies -below to rend the sides of the cone than to raise the -molten materials to its summit. In this way lateral -or parasitic cones are thrown up on the flanks of the -volcanic mountain, the mass being alternately elevated -and strengthened by the ejections from the summit -and sides respectively.</p> - -<p>When the volcanic energies no longer suffice to -raise the fluid materials to the summit, nor to rend the -sides of the volcano, fissures with small cones may be -formed in the plains around the great central volcano.</p> - -<p>At last, however, this energy diminishes so far that -rock materials can no longer be forced to the surface, -the fissures become sealed up by consolidating lava, and -the volcanic cones fall into a condition of extinction -and decay.</p> - -<p>The existence of heated materials at no great depth -from the surface is indicated by the outburst of gases and -vapours, the formation of geysers, mud-volcanoes, and -ordinary thermal springs. But as the underlying rocks -cool down, the issuing jets of gas and vapour lose their -high temperature and diminish in quantity, the geysers -and mud-volcanoes become extinct, and the thermal -<span class="pagenum" id="Page_223">- 223 -</span> -springs lose their peculiar character or disappear, and -thus all manifestations of the igneous energies in the -district gradually die away.</p> - -<div class="sidenote">DURATION OF VOLCANIC CYCLES.</div> - -<p>Such a cycle of changes probably requires many -hundreds of thousands, or even many millions, of years -for its accomplishment; but by the study of volcanoes -in every stage of their growth and decline we are -able to reconstruct even the minutest details of their -history.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_224">- 224 -</span></p> - -<h2 class="nobreak" id="CHAPTER_VIII">CHAPTER VIII.<br /> - -<span class="smaller">THE DISTRIBUTION OF VOLCANOES UPON THE SURFACE -OF THE GLOBE.</span></h2> -</div> - - -<p class="p0">It is not by any means an easy task to frame an -estimate of the number of volcanoes in the world. -Volcanoes, as we have seen, vary greatly in their -dimensions—from vast mountain masses, rising to a -height of nearly 25,000 feet above the sea-level, to -mere molehills; the smaller ones being in many cases -subsidiary to larger, and constituting either parasitic -cones on their flanks, or 'puys' around their bases. -Volcanoes likewise exhibit every possible stage of development -and decay: while some are in a state of -chronic active eruption, others are reduced to the condition -of solfataras, and others again have fallen into -a more or less complete state of ruin through the -action of denuding forces.</p> - -<p>Even if we confine our attention to the larger -volcanoes, which merit the name of 'mountains,' and -such of these as we have reason to believe to be in a -still active condition, our difficulties will be diminished, -but not by any means removed. Volcanoes, as we have -<span class="pagenum" id="Page_225">- 225 -</span> -seen, may sink into a dormant condition that may -endure for hundreds or even thousands of years, and -then burst forth into a state of renewed activity; and -it is quite impossible, in many cases, to distinguish -between the conditions of dormancy and extinction. -Concerning certain small areas in Southern Europe, -Western Asia, and Northern Africa, historical records, -more or less reliable, extend back over periods of -several thousands of years; but with regard to the -greater part of the rest of the world we have no information -beyond a few hundred years, and there are -considerable areas which have been known only for far -shorter periods, while some are as yet quite unexplored. -In districts almost wholly uninhabited, or roamed over -by nomadic tribes, legend and tradition constitute our -only guides—and very unsafe ones they are—in the -attempt to determine what volcanoes have recently -been in a condition of activity.</p> - -<div class="sidenote">NUMBER OF ACTIVE VOLCANOES.</div> - -<p>We shall, however, probably be within the limits -of truth in stating that the number of great habitual -volcanic vents upon the globe, which we have reason -to believe are still in an active condition, is somewhere -between 300 and 350. Most of these active volcanic -vents are marked by more or less considerable mountains, -composed of the materials ejected from them. -If we include the mountains which exhibit the external -conical form, the crateral hollows, and other -features of volcanoes, but concerning the activity of -which we have no record or tradition, the number will -<span class="pagenum" id="Page_226">- 226 -</span> -fall little, if anything, short of 1,000. The mountains -composed of volcanic materials, but which have lost -through denudation the external form of volcanoes, are -still more numerous. The smaller temporary openings -which are usually subordinate to the habitual vents, -that have been active during the periods covered by -history and tradition, must be numbered by thousands -and tens of thousands. The still feebler manifestations -of the volcanic forces—such as are exhibited in -'stufas,' or steam-jets, geysers, or intermittent hot -springs, thermal and mineral waters, fumaroles, emitting -various gases, salses or spouting saline and muddy -springs, and mud volcanoes—may be reckoned by -millions. It is not improbable that these less powerful -manifestations of the volcanic forces, to a great extent -make up in number what they want in individual -energy; and the relief which they afford to the imprisoned -activities within the earth's crust may be -scarcely less than that which results from the occasional -outbursts at the 300 or 350 great habitual -volcanic vents.</p> - -<p>In taking a general survey of the volcanic phenomena -of the globe, no fact comes out more strikingly -than that of the very unequal distribution, in different -districts, both of the great habitual volcanic vents, and -of the minor exhibitions of subterranean energy.</p> - -<div class="sidenote">VOLCANOES OF THE CONTINENTS.</div> - -<p>Thus, on the whole of the continent of Europe, -there is but one habitual volcanic vent—that of -Vesuvius—and this is situated upon the shores of the -<span class="pagenum" id="Page_227">- 227 -</span> -Mediterranean. In the islands of the Mediterranean, -however, there are no less than six volcanoes; namely, -Stromboli and Vulcano, in the Lipari Islands; Etna, in -Sicily; Graham's Isle, a submarine volcano, off the -Sicilian coast; and Santorin and Nisyros, in the Ægean Sea.</p> - -<p>The African continent is at present known to contain -about ten active volcanoes—four on the west -coast, and six on the east coast; about ten other -active volcanoes occur on islands close to the African -coasts. In Asia, twenty-four active volcanoes are -known, but no less than twelve of these are situated -in the peninsula of Kamtschatka. No volcanoes are -known to exist in the Australian continent.</p> - -<p>The American continent contains a greater number -of volcanoes than the divisions of the Old World. -There are twenty in North America, twenty-five in -Central America, and thirty-seven in South America.</p> - -<p>Thus, taken altogether, there are about one hundred -and seventeen volcanoes situated on the great continental -lands of the globe, while nearly twice as many -occur upon the islands scattered over the various -oceans.</p> - -<p>Upon examining further into the distribution of -the continental volcanoes, another very interesting -fact presents itself. The volcanoes are in almost -every case situated either close to the coasts of the -continent, or at no great distance from them. There -are, indeed, only two exceptions to this rule. In the -<span class="pagenum" id="Page_228">- 228 -</span> -great and almost wholly unexplored table-land lying -between Siberia and Tibet four volcanoes are said to -exist, and in the Chinese province of Mantchouria -several others. More reliable information is, however, -needed concerning these volcanoes, situated, unlike all -others, at a great distance from the sea.</p> - -<p>It is a remarkable circumstance that all the oceanic -islands which are not coral-reefs are composed of -volcanic rocks; and many of these oceanic islands, as -well as others lying near the shores of the continents, -contain active volcanoes.</p> - -<p>Through the midst of the Atlantic Ocean runs a -ridge, which, by the soundings of the various exploring -vessels sent out in recent years, has been -shown to divide the ocean longitudinally into two -basins. Upon this great ridge, and the spurs proceeding -from it, rise numerous mountainous masses, -which constitute the well-known Atlantic islands and -groups of islands. All of these are of volcanic origin, -and among them are numerous active volcanoes. The -Island of Jan Mayen contains an active volcano, while -Iceland contains thirteen, and not improbably more; -the Azores have six active volcanoes, the Canaries -three; while about eight volcanoes lie off the west -coast of Africa. In the West Indies there are six -active volcanoes; and three submarine volcanoes have -been recorded within the limits of the Atlantic Ocean. -Altogether, no less than forty active volcanoes are -situated upon the great submarine ridges which traverse -the Atlantic longitudinally.</p> - -<p><span class="pagenum" id="Page_229">- 229 -</span></p> - -<div class="sidenote">VOLCANOES ON THE OCEANIC ISLANDS.</div> - -<p>But along the same line the number of extinct -volcanoes is far greater, and there are not wanting -proofs that the volcanoes which are still active are -approaching the condition of extinction. At a somewhat -earlier period of the earth's history the whole -line of the present Atlantic Ocean was in all probability -traversed by a chain of volcanoes on the very -grandest scale; but submergence has taken place, and -only a few portions of this great mountain range now -rise above the sea-level, forming the isolated islands -and island-groups of the Atlantic. Here and there -among these a still active volcano exists.</p> - -<p>But if the great medial chain of the Atlantic presents -us with an example of a chain of volcanic mountains -verging on extinction, we have in the line of -islands separating the Pacific and Indian Oceans an -example of a similar range of volcanic vents which are -in a condition of the greatest activity. In the peninsula -of Kamtschatka there are twelve active volcanoes, -in the Aleutian Islands thirty-one, and in the peninsula -of Alaska three. The chain of the Kuriles contains at -least ten active volcanoes; the Japanese Islands and -the islands lying to the south of Japan twenty-five. -The great group of islands lying to the south-east of -the Asiatic continent is at the present time the grandest -focus of volcanic activity upon the globe. No less -than fifty active volcanoes occur here. Farther south, -the same chain is probably continued by the four -active volcanoes of New Guinea, one or more -<span class="pagenum" id="Page_230">- 230 -</span> -submarine volcanoes, and several vents in New Britain, -the Solomon Isles, and the New Hebrides, the three -active volcanoes of New Zealand, and possibly by -Mount Erebus and Mount Terror in the Antarctic -region. Altogether, no less than 150 active volcanoes -exist in the chain of islands which stretch from -Behring's Straits down to the Antarctic circle; and if -we include the volcanoes on Indian and Pacific islands -which appear to be situated on lines branching from -this particular band, we shall not be wrong in the -assertion that this great system of volcanic mountains -includes at least one half of the habitually active vents -of the globe.</p> - -<p>A third series of volcanoes starts from near the -last in the neighbourhood of Behring's Straits, and -stretches along the whole western coast of the American -continent. In this great range there are about -eighty active volcanoes.</p> - -<div class="sidenote">LINEAR ARRANGEMENT OF VOLCANOES.</div> - -<p>In considering the facts connected with the distribution -of volcanoes upon the globe, the one which, by its -striking character, seems to demand our attention in -the first instance is that of the remarkable linear -arrangement of volcanic vents. We have already seen -that small scoria-cones are often thrown up on the -flanks, or at the base, of a great volcanic mountain, -along lines which are manifestly lines of fissure. In -the eruption of Etna, in 1865, and again in that of -1874, Professor Silvestri, of Catania, witnessed the -actual opening of great fissures on the north-east and -<span class="pagenum" id="Page_231">- 231 -</span> -north sides of the mountain: and along the bottom of -these cracks the glowing lava was clearly visible -(<a href="#fig84">fig. 84</a>, page 194). In the course of a few days, there were -thrown up a number of small scoria-cones along these -lines of fissure—those formed on the fissure of 1865 -being seven in number, and those on the fissure of 1874 -being no less than thirty-six in number. Precisely -familiar phenomena were witnessed upon the slopes of -Vesuvius, in 1760, when a fissure opened on the south -side of the mountain, and fifteen scoria-cones, which -are still visible, were thrown up along it.</p> - -<p>We have already considered the evidence pointing -to the conclusion that systems of volcanoes, like that -of the Lipari Islands, are similarly ranged along lines of -fissures, and there is equally good ground for believing -that the great linear bands of volcanoes, which, as we -have seen, stretch for thousands of miles, have had -their positions determined by great lines of fissure in -the earth's crust. While, however, the smaller fissures, -upon which rows of scoria-cones are thrown up, seem -to have been in many cases opened by a single effort -of the volcanic forces, the enormous fissures, which -traverse so large a portion of the surface of the globe, -are doubtless the result of numerous manifestations of -energy extending over vast periods of time.</p> - -<p>The greatest of these bands along which the volcanic -forces are so powerfully exhibited at the present day, is -the one which stretches from near the Arctic circle -at Behring's Straits to the Antarctic circle at South -<span class="pagenum" id="Page_232">- 232 -</span> -Victoria. The line followed by this volcanic band, which, -as we have seen, includes more than one half of the -active volcanoes of the globe, is a very sinuous one, -and it gives off numerous offshoots upon either side of -it. The great focus of this intense volcanic action may -be regarded as lying in the district between the islands -of Borneo and New Guinea. From this centre there -radiate a number of great lines, along which the -volcanic forces are exhibited in the most powerful -manner. The first of these extends northwards through -the Philippine Isles, Japan, the Kurile Islands, and -Kamtschatka, giving off a branch to the east, which -passes through the Aleutian Islands and the peninsula -of Alaska. This band, along which the volcanic forces -are very powerfully active, is continued towards the -south-east in the New Britain, the Solomon Islands, -Santa Cruz, the New Hebrides, New Zealand, and South -Victoria. East and west from the great central focus -there proceed two principal branches. The former of -these extends through the Navigator Islands and -Friendly Islands as far as Elizabeth Islands. The latter -passes through Java, and then turns north-westward -through Sumatra, the Nicobar Islands, the Andaman -Islands up to the coast of Burmah.</p> - -<p>The great band which we have been describing -exhibits the most striking examples of volcanic activity -to be found upon the globe. Besides the 150 or more -volcanoes which are known to have been in a state of -activity during the historical period, there are several -<span class="pagenum" id="Page_233">- 233 -</span> -hundred very perfect volcanic cones, many of which -appear to have but recently become extinct, if indeed, -they are not simply in a dormant condition. For long -distances these chains of volcanic cones are almost continuous, -and the only very considerable breaks in the -series are those between New Zealand and the New -Hebrides on the one hand, and between the former -islands and South Victoria on the other.</p> - -<div class="sidenote">GREAT VOLCANIC BANDS OF THE GLOBE.</div> - -<p>Much less continuous, but nevertheless very important, -is the great band of volcanoes which extends -along the western side of the great American continent, -and contains, with its branches, nearly a hundred active -volcanoes. On the north this great band is almost -united with the one we have already described by -the chain of the Aleutian and Alaska volcanoes. In -British Columbia about the parallel of 60° N. there -exist a number of volcanic mountains, one of which, -Mount St. Elias, is believed to be 18,000 feet in height, -and several of these have certainly been seen in a state -of eruption. Farther south in the part of the United -States, territories drained by the Columbia River, a number -of grand volcanic mountains exist, some of which -are probably still active, for geysers and other manifestations -of volcanic activity abound. From the southern -extremity of the peninsula of California an almost -continuous chain of volcanoes stretches through Mexico -and Guatemala, and from this part of the volcanic -band a branch is given off which passes through the -West Indies, and forms a connection with the great -<span class="pagenum" id="Page_234">- 234 -</span> -volcanic band of the Atlantic Ocean. In South America -the line is continued by the active volcanoes of Ecuador, -Bolivia and Chili, but at many intermediate points in -the chain of the Andes extinct volcanoes occur, which to -a great extent fill up the gaps in the series. A small -offshoot to the westward passes through the Galapagos -Islands. The great band of volcanoes which stretches -through the American continent is second only in importance, -and in the activity of its vents, to the band -which divides the Pacific from the Indian Ocean.</p> - -<p>The third volcanic band of the globe is that which -traverses the Atlantic Ocean from north to south. -This series of volcanic mountains is much more broken -and interrupted than the other two, and a greater -proportion of its vents are extinct. This chain, as we -shall show in a future chapter, attained its condition of -maximum activity during the distant period of the -Miocene, and now appears to be passing into a state of -gradual extinction. Beginning in the north with the -volcanic rocks of Greenland and Bear Island, we pass -southwards, by way of Jan Mayen, Iceland, and the -Faroe Islands, to the Hebrides and the north of Ireland. -Thence by way of the Azores, the Canaries and the -Cape de Verde Islands, with some active vents, we -pass to the ruined volcanoes of St. Paul, Fernando de -Noronha, Ascension, St. Helena, Trinidad and Tristan -d'Acunha. From this great Atlantic band two branches -proceed to the eastward, one through Central Europe, -where all the vents are now extinct, and the other -<span class="pagenum" id="Page_235">- 235 -</span> -through the Mediterranean to Asia Minor, the great -majority of the volcanoes along the latter line being -now extinct, though a few are still active. The vol -canoes on the eastern coast of Africa may be regarded -as situated on another branch from this Atlantic volcanic -band. The number of active volcanoes on this -Atlantic band and its branches, exclusive of those in -the West Indies, does not exceed fifty.</p> - -<div class="sidenote">LENGTH OF THE VOLCANIC BANDS.</div> - -<p>From what has been said, it will be seen that, not -only do the volcanoes of the globe usually assume a -linear arrangement, but nearly the whole of them can -be shown to be thrown up along three well-marked -bands and the branches proceeding from them. The -first and most important of these bands is nearly 10,000 -miles in length, and with its branches contains more -than 150 active volcanoes; the second is 8,000 miles -in length, and includes about 100 active volcanoes; -the third is much more broken and interrupted, extends -to a length of nearly 1,000 miles, and contains about -50 active vents. The volcanoes of the eastern coast of -Africa, with Mauritius, Bourbon, Rodriguez, and the -vents along the line of the Red Sea, may be regarded -as forming a fourth and subordinate band.</p> - -<p>Thus we see that the surface of the globe is covered -by a network of volcanic bands, all of which traverse it -in sinuous lines with a general north-and-south direction, -giving off branches which often run for hundreds -of miles, and sometimes appear to form a connection -between the great bands.</p> - -<p><span class="pagenum" id="Page_236">- 236 -</span></p> - -<p>These four bands of volcanic vents, running in a -general north-and-south direction, separate four -unequal areas within which the exhibitions of volcanic -activity are feeble or quite unknown. The two grandest -of the bands of volcanic activity, with their branches, -form an almost complete series encircling the largest -of the oceans.</p> - -<p>To this rule of the linear arrangement of the volcanic -vents of the globe and their accumulation along -certain well-marked bands, there are two very striking -exceptions, which we must now proceed to notice.</p> - -<p>In the very centre of the continent formed by -Europe and Asia, the largest unbroken land-mass of -the globe, there rises from the great central plateau -the remarkable volcanoes of the Thian Shan Range. -The existence of these volcanoes, of which only obscure -traditional accounts had reached Europe before the -year 1858, appears to be completely established by the -researches of the Russian traveller Semenof. Three -volcanic vents appear to exist in this region: the active -volcanoes of Boschan and Turfan or Hot-schen, and the -solfatara of Urumtsi. At a point situated about half-way -between these three volcanoes and the sea, another -active vent, that of Ujung-Holdongi, is said to exist. -Other volcanic phenomena have been stated to occur in -the great plateau of Central Asia, but the existence of -some at least of these appears to rest on very doubtful -evidence. The only accounts which we have of the -eruptions of these Thian Shan volcanoes are contained -<span class="pagenum" id="Page_237">- 237 -</span> -in Chinese histories and treatises on geography; and a -great service would be rendered to science could they -be visited by some competent explorer.</p> - -<div class="sidenote">EXCEPTIONALLY-SITUATED VOLCANOES.</div> - -<p>The second exceptionally-situated volcanic group is -that of the Sandwich Islands. While the Thian Shan -volcanoes rise in the centre of the largest unbroken -land-mass, and stand on the edge of the loftiest and -greatest plateau in the world, the volcanoes of the -Sandwich Islands rise almost in the centre of the largest ocean -and from almost the greatest depths in that ocean. -All round the Sandwich Islands the sea has a depth of -from 2,000 to 3,000 fathoms, and the island-group culminates -in several volcanic cones which rise to the -height of nearly 14,000 feet above the sea-level. The -volcanoes of the Sandwich Islands are unsurpassed in -height and bulk by those of any other part of the -globe.</p> - -<p>With the exception of the two isolated groups of -the Thian Shan and the Sandwich Islands, nearly all -the active volcanoes of the globe are situated near the -limits which separate the great land- and water-masses -of the globe—that is to say, they occur either on the -parts of continents not far removed from their coast-lines, -or on islands in the ocean not very distant from -the shores.</p> - -<p>The fact of the general proximity of volcanoes to -the sea, is one which has frequently been pointed out -by geographers, and may now be regarded as being -thoroughly established. Even the apparently anomalous -<span class="pagenum" id="Page_238">- 238 -</span> -case of the Thian Shan volcanoes is susceptible of -explanation if we remember the fact, now well ascertained -by geological researches, that as late certainly as -Pliocene times, a great inland sea spread over the -districts where the Caspian, the Sea of Aral, and many -other isolated lakes are now found. Upon the southern -shore of this sea rose the volcanoes of the Thian Shan, -some of which have not yet fallen into a state of -complete extinction.</p> - -<p>But although the facts concerning the general -proximity of volcanoes to the ocean may be admitted -to be thoroughly established, yet inferences are sometimes -hastily drawn from these facts which the latter, -if fairly considered, will not be found to warrant. It -is frequently assumed that we may refer all the remarkable -phenomena of volcanic action to the penetration -of sea-water to a mass of incandescent lava in the -earth's crust, and to the chemical or mechanical action -which would result from this meeting of sea-water and -molten rock. And this conclusion is supposed to find -support in the circumstance that many of the gases -and volatile substances emitted from volcanic vents are -such as would be produced by the decomposition of the -various salts contained in sea-water.</p> - -<p>This argument in favour of the production of volcanic -outbursts by the irruption of sea-water into subterranean -reservoirs, involves, as Mr. Scrope long ago -pointed out, a curious example of reasoning in a circle. -It is assumed, on the one hand, that the heaving -<span class="pagenum" id="Page_239">- 239 -</span> -subterranean movements, which give rise to the fissures by -which steam and other gases escape to the surface, -are the result of the passage of water to heated masses -in the earth's crust. But, on the other hand, it is supposed -that it is the production of these fissures which -leads to the influx of water to the heated materials. If -it is the passage of water through these fissures which -produces the eruptions, it may be fairly asked, what is -it that gives rise to the fissures? And if, on the other -hand, there exist subterranean forces competent to produce -the fissures, may they not also give rise to the eruptions -through the openings which they have originated? -Nor does the chemical argument appear to rest upon -any surer ground. It is true that many of the volatile -substances emitted from volcanic vents are such as -might be produced by the decomposition of sea-water, -but, upon the other hand, there are not a few substances -which cannot possibly be regarded as so produced, and, -all the materials may equally well be supposed to have -been originally imprisoned in the masses of subterranean lava.</p> - -<div class="sidenote">CAUSE OF PROXIMITY OF VOLCANOES TO SEA.</div> - -<p>The problem before us is this. Granting that it is -proved that active volcanoes are always in close proximity -to the ocean, are we to explain the fact by supposing -that the agency of sea-water is necessary to -volcanic outbursts, or by regarding the position of the -coast-lines as to some extent determined by the distribution -of volcanic action upon the surface of the -globe? The first supposition is the one which perhaps -<span class="pagenum" id="Page_240">- 240 -</span> -most readily suggests itself, but the latter, as we shall -hereafter show, is one in favour of which not a few -weighty arguments may be advanced.</p> - -<p>Another problem which suggests itself in connection -with the distribution of volcanoes is the following. Are -the great depressed tracts which form the bottom of -the oceans, like the elevated tracts which constitute -the continents, equally free from exhibitions of volcanic -energy?</p> - -<p>When we remember the fact that the area of the -ocean beds is two and three-quarter times as great as -that of the continents, it will be seen how important -this question of the existence of volcanoes at the bottom -of the ocean really is.</p> - -<p>The fact that recent deep-sea soundings have shown -the deepest parts of the ocean to be everywhere covered -with volcanic <i>débris</i> is by no means conclusive upon this -question; for, as we have seen, the ejections of sub-aerial -volcanoes are by the wind and waves distributed -over every part of the earth's surface.</p> - -<div class="sidenote">SUBMARINE ERUPTIONS.</div> - -<p>Submarine volcanic outbursts have occurred in -many parts of the globe, but it may well be doubted -whether any such outburst has ever commenced at the -bottom of a deep ocean, and has succeeded in building -up a volcanic cone reaching to the surface. Most, if -not all, of the recorded submarine outbursts have -occurred in the midst of volcanic districts, and the -volcanic cones have been built up in water of no -great depth. Indeed, when it is remembered that -<span class="pagenum" id="Page_241">- 241 -</span> -the pressure of each 1,000 fathoms of water is equivalent -to a weight of more than one ton on every square -inch of the ocean-bottom, it is difficult to imagine the -ordinary explosive action of volcanic vents taking place -at abysmal depths. If, however, fissures were opened -in the beds of the ocean, quiet outwellings of lava -might possibly occur.</p> - -<p>The solution of this problem of the probable -existence of volcanic outbursts on the floor of the -ocean can only be hoped for from the researches of -the geologist. The small specimens of the ocean-beds -brought up by deep-sea sounding-lines, taken at wide -distances apart, and including but a few inches from -the surface, can certainly afford but little information -upon the question. But the geologist has the opportunity -of studying the sea-bottoms of various -geological periods which have been upheaved and are -now exposed to his view. It was at one time supposed -by geologists that in the so-called 'trap-rocks' we -have great lava-sheets which must have been piled -upon one another, without explosive action. But the -more accurate researches of recent years have shown -that between the layers of 'trap-rock,' in every part of -the globe, traces of terrestrial surfaces and freshwater -deposits are found; and the supposed proofs of the -absence of explosive action break down no less signally -upon re-examination; for the loose, scoriaceous materials -would either be removed by denudation, or converted -into hard and solid rocks by the infilling of their -<span class="pagenum" id="Page_242">- 242 -</span> -vesicles and air-cavities with crystalline minerals. It -is not possible, among the representatives of former -geological periods, to point to any rocks that can be -fairly regarded as having issued from great submarine -fissures, and it is therefore fair to conclude that no -such great outbursts of the volcanic forces take plane -at the present day on the deep ocean-floors.</p> - -<p>In connection with the question of the relation -between the position of the volcanic bands of the -globe and the areas covered by the ocean, we may -mention a fact which deep-sea soundings appear to -indicate, namely, that the deepest holes in the ocean-floor -are situated in volcanic areas. Near Japan, the -soundings of the U.S. ship 'Tuscarora' showed that at -two points the depth exceeded 4,000 fathoms; and -the deepest sounding obtained by H.M.S. 'Challenger,' -amounting to 4,575 fathoms, was taken in the voyage -from New Gruinea to Japan, in the neighbourhood of -the Ladrone Islands. Depths nearly as great were -found in the soundings carried on in the neighbourhood -of the volcanic group of the West Indian Islands. -It must be remembered, however, that at present our -knowledge of the depths of the abysmal portions of -the ocean is very limited. A few lines of soundings, -often taken at great distances apart, are all we have to -guide us to any conclusions concerning the floors of -the great oceans, and between these lines are enormous -areas which still remain altogether unexplored. It -may be wise, therefore, to suspend our judgment upon -<span class="pagenum" id="Page_243">- 243 -</span> -such questions till more numerous facts have been -obtained.</p> - -<div class="sidenote">RELATIONS TO MOUNTAIN-CHAINS.</div> - -<p>Another fact concerning the distribution of volcanoes -which is worthy of remark is their relation to -the great mountain-ranges of the globe.</p> - -<p>Many of the grandest mountain-chains have bands -of volcanoes lying parallel to them. This is stinkingly -exhibited by the great mountain-masses which lie -on the western side of the American continent. The -Rocky Mountains and the Andes consist of folded and -crumpled masses of altered strata which, by the action -of denuding forces, have been carved into series of -ridges and summits. At many points, however, along -the sides of these great chains, we find that fissures -have been opened and lines of volcanoes formed, from -which enormous quantities of lava have flowed and -covered great tracts of country. At some parts of the -chain, however, the volcanoes are of such height and -dimensions as to overlook and dwarf the mountain-ranges -by the side of which they lie. Some of the -volcanoes lying parallel to the great American axis -appear to be quite extinct, while others are in full -activity.</p> - -<p>In the Eastern continent we find still more striking -examples of the parallelism between great mountain-chains -and the lands along which volcanic activity is -exhibited. Stretching in a more or less continuous -chain from east to west, through Europe and Asia, we -find the mountain-masses known in different parts of -<span class="pagenum" id="Page_244">- 244 -</span> -their course as the Pyrenees, the Alps, the Balkan, -the Caucasus, which form the axis of the Eastern -continent. These chains consist of numerous parallel -ridges, and give off branches on either side of them. -They are continued to the eastward by the Hindoo -Koosh and the Himalaya, with the four parallel ranges -that cross the great Central-Asian plateau. Now, on -either side of this grand axial system of mountains, we -find a great parallel band of volcanoes. The northern -volcanic band is constituted by the eruptive rocks of the -Auvergne, the Eifel, the Siebengebirge, Central Germany, -Bohemia, Hungary, and Transylvania, few, if any, -of the vents along this northern band being still active. -The remarkable volcanoes of the Thian Shan range -and of Mantchouria may not improbably be regarded -as a continuation of the same great series.</p> - -<p>The southern band of volcanoes, lying parallel to -the great mountain axis of the Old World, also consists -for the most part of extinct volcanoes, but includes -not a few vents which are still active. In this band -we include the extinct volcanoes of Spain and Sardinia, -the numerous extinct and active vents of the Italian -peninsula and islands, and those of the Ægean Sea and -Asia Minor. We may, perhaps, consider the scattered -volcanoes of Arabia and the northern part of the Indian -Ocean as a continuation of the same series. Both of -these bands may be regarded as offshoots from the -great mid-Atlantic volcanic chain, and the condition of -the vents, both in the principal band and its offshoots, -<span class="pagenum" id="Page_245">- 245 -</span> -is such as to indicate that they form parts of a system -which is gradually sinking into a state of complete -extinction.</p> - -<p>There are some other volcanic bands which exhibit -a similar parallelism with mountain chains; but, on the -other hand, there are some volcanoes between which -and the nearest mountain axes no such connection can -be traced.</p> - -<div class="sidenote">RELATION TO AREAS OF UPHEAVAL.</div> - -<p>There is yet one other fact concerning the mode of -distribution of volcanoes upon the surface of the globe, -to which we must allude. It was first established by -Mr. Darwin as one of the conclusions derived from the -valuable series of observations made by him during the -voyage of H.M.S. 'Beagle,' and relates to the position -of active volcanoes with respect to the portions of the -earth's crust which are undergoing upheaval or subsidence.</p> - -<p>From the relative position of the different kinds of -coral-reefs, and the fact that reef-forming corals cannot -live at a depth of more than twenty fathoms beneath -the sea-level, or above tide-mark, we are led to the conclusion -that certain areas of the earth's surface are undergoing -slow elevation, while other parts are as gradually -subsiding. This conclusion is confirmed by the -occurrence of raised beaches, which are sometimes found -at heights of hundreds, or even thousands, of feet above -the sea-level, and of submerged forests, which are not -unfrequently found beneath the waters of the ocean.</p> - -<p>By a study of the evidences presented by coral-reefs, -<span class="pagenum" id="Page_246">- 246 -</span> -raised beaches, submerged forests, and other -phenomena of a similar kind, it can be shown that certain -wide areas of the land and of the ocean-floor are at -the present time in a state of subsidence, while other -equally large areas are being upheaved. And the observations -of the geologist prove that similar upward -and downward movements of portions of the earth's -crust have been going on through all geological times. -Now, as Mr. Darwin has so well shown in his work -on 'Coral-Reefs,' if we trace upon a map the areas of -the earth's surface which are undergoing upheaval and -subsidence respectively, we shall find that nearly all -the active volcanoes of the globe are situated upon -rising areas, and that volcanic phenomena are conspicuously -absent from those parts of the earth's crust -which can be proved at the present day to be undergoing depression.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_247">- 247 -</span></p> - -<h2 class="nobreak" id="CHAPTER_IX">CHAPTER IX.<br /> - -<span class="smaller">VOLCANIC ACTION AT DIFFERENT PERIODS OF THE -EARTH'S HISTORY.</span></h2> -</div> - - -<p class="p0">It is only in comparatively recent times that the important -doctrine of geological continuity has come to -be generally accepted, as furnishing us with a complete -and satisfactory explanation of the mode of origin of -the features of our globe. The great forces, which are -ever at work producing modifications in those features, -operate so silently and slowly, though withal so surely, -that without the closest and most attentive observation -their effects may be easily overlooked; while, on the -other hand, there are so many phenomena upon our globe -which seem at first sight to bear testimony to the action -of sudden and catastrophic forces, very different to any -which appear to be at present at work, that the tendency -to account for all past changes by these violent actions -is a very strong one. In spite of this tendency, however, -the real potency of the forces now at work upon -the earth's crust has gradually made its way to recognition, -and the capability of these forces, when their -effects are accumulated through sufficiently long periods -<span class="pagenum" id="Page_248">- 248 -</span> -of time, to bring about the grandest changes, is now -almost universally admitted. The modern science of -geology is based upon the principle that the history of -the formation and development of the earth's surface-features, -and of the organisms upon it, has been continuous -during enormous periods of time, and that in -the study of the operations taking place upon the earth -at the present day, we may find the true key to the -changes which have occurred during former periods.</p> - -<p>In no branch of geological science has the doctrine -of continuity had to encounter so much opposition and -misconception as in that which relates to the volcanic -phenomena of the globe. For a long time students of -rocks utterly failed to recognise any relation between -the materials which have been ejected from active -volcanic vents and those which have been formed by -similar agencies at earlier periods of the earth's history. -And what was far worse, the subject became removed -from the sphere of practical scientific inquiry to that of -theological controversy, those who maintained the volcanic -origin of some of the older rocks being branded -as the worst of heretics.</p> - -<div class="sidenote">CONTROVERSY CONCERNING ORIGIN OF BASALT.</div> - -<p>With the theological aspects of the great controversy -concerning the origin of basalt and similar rocks—a -controversy which was carried on with such violence -and acrimony during the latter half of the eighteenth -century—we have here nothing to do. But it may not -be uninstructive to notice the causes of the strange -misconceptions which for so long a period stood in the -<span class="pagenum" id="Page_249">- 249 -</span> -way of the acceptance of rational views upon the subject.</p> - -<p>At this period but little had been done in studying -the chemical characters of aqueous and igneous rock-masses -respectively; and while, on the one hand, the -close similarity in chemical composition between the -ancient basalts and many modern lavas was not recognised, -the marked distinction between the composition -of such materials and most aqueous sediments -remained, on the other hand, equally unknown. Nor -had anything been yet accomplished in the direction -of the study of rock-masses by the aid of the microscope. -Hence there could be no appeal to those -numerous structural peculiarities that at once enable -us to distinguish the most crystalline aqueous rocks -from the materials of igneous origin.</p> - -<p>On the other hand, there undoubtedly exist rocks -of a black colour and crystalline structure, sometimes -presenting a striking similarity in general appearance -to the basalts, which contain fossils and are undoubtedly -of aqueous origin. Thus on the shore near -Portrush, in the North of Ireland, and in the skerries -which lie off that coast, there occur great rock-masses, -some of which undoubtedly agree with basalt in all -their characters, while others are dark-coloured and -crystalline, and are frequently crowded with <i>Ammonites</i> -and other fossils. We now know that the explanation -of these facts is as follows. Near where the town of -Portrush is now situated, a volcanic vent was opened -<span class="pagenum" id="Page_250">- 250 -</span> -in Miocene times through rocks of Lias shale. From -this igneous centre, sheets and dykes of basaltic lava -were given off, and in consequence of their contact with -these masses of lava, the Lias shales were baked and -altered, and assumed a crystalline character, though -the traces of the fossils contained in them were not -altogether obliterated. In the last century the methods -which had been devised for the discrimination of rocks -were so imperfect that no distinction was recognised -between the true basalt and the altered shale, and specimens -of the latter containing <i>Ammonites</i> found their -way to almost every museum in Europe, and were used -as illustrations of the 'origin of basalt by aqueous -precipitation.'</p> - -<p>Another source of the widely-spread error which -prevailed concerning the origin of basalt, was the failure -to recognise the nature of the alterations which take -place in the character of rock-masses in consequence of -the passage through them, during enormous periods of -time, of water containing carbonic acid and other active -chemical agents. The casual observer does not recognise -the resemblance which exists between certain -ornamental marbles and the loose accumulations of -shells and corals which form many sea-beaches; but -close examination shows that the former consist of -the same materials as the latter, bound together by a -crystalline infilling of carbonate of lime, which has -been deposited in all the cavities and interstices of the -mass. In the same way, as we have already seen, the -<span class="pagenum" id="Page_251">- 251 -</span> -vesicles and interstices of heaps of scoriæ may, by the -percolation of water through the mass, become so filled -with various crystalline substances, that its original -characters are entirely masked.</p> - -<p>But the progress of chemical and microscopic research -has effectually removed these sources of error. -Many rocks of aqueous origin, formerly confounded -with the basalts, have now been relegated to their -proper places among the different classes of rocks; -while, on the other hand, it has been shown that the -chemical and physical differences between the ancient -basalts and the modern basic lavas are slight and accidental, -and their resemblances are of the closest and -most fundamental character.</p> - -<div class="sidenote">VOLCANIC ORIGIN OF 'TRAP ROCKS.'</div> - -<p>The notion of the aqueous origin of basalt, which -was so long maintained by the school of Werner, has -now been entirely abandoned, and the so-called -'trap-rocks' are at the present day recognised as being as -truly volcanic in their origin as the lavas of Etna and -Vesuvius.</p> - -<p>There is, however, a vestige of this doctrine of -Werner, which still maintains its ground with obstinate -persistence. Many geologists in Germany who admit -that volcanic phenomena, similar to those which are -going on at the present day, must have occurred during -the Tertiary and the later Secondary periods, nevertheless -insist that among the earlier records of the world's -history we find no evidence whatever of such volcanic -action having taken place. By the geologists who hold -<span class="pagenum" id="Page_252">- 252 -</span> -these views it is asserted that while the granites and -other plutonic rocks were formed during the earlier -periods of the world's history, true volcanic products are -only known in connection with the sediment of the -later geological periods.</p> - -<p>Some geologists have gone farther even than this, -and asserted that each of the great geological periods -is characterised by the nature of the igneous ejections -which have taken place in it. They declare that granite -was formed only during the earliest geological periods, -and that at later dates the gabbros, diabases, porphyries, -dolerites and basalts, successively made their appearance, -and finally that the modern lavas were poured -out.</p> - -<p>A little consideration will suffice to convince us -that these conclusions are not based upon any good -evidence. The plutonic rocks, as we have already seen, -exhibit sufficient proofs in their highly crystalline -character, and in their cavities containing water, liquefied -carbonic acid, and other volatile substances, that they -must have been formed by the very slow consolidation -of igneous materials under enormous pressure. Such -pressures, it is evident, could only exist at great depths -beneath the earth's surface. Mr. Sorby and others have -endeavoured to calculate what was the actual thickness -of rock under which certain granites must have been -formed, by measuring the amount of contraction in -the liquids which have been imprisoned in the crystals -of these rocks. The conclusions arrived at are of a -<span class="pagenum" id="Page_253">- 253 -</span> -sufficiently startling character. It is inferred that the -granites which have been thus examined must have -consolidated at depths varying from 30,000 to 80,000 -feet beneath the earth's surface. It is true that in -arriving at these results certain assumptions have to -be made, and to these exception may be taken, but the -general conclusion that granitic rocks could only have -been formed under such high pressures as exist at -great depths beneath the surface, appears to be one -which is not open to reasonable doubt.</p> - -<p>If, then, granites and similar rocks were formed at -the depth of some miles, it is evident that they can -only have made their appearance at the surface by the -removal of the vast thickness of overlying rocks; and -the sole agency which we know of that is capable of -effecting the removal of such enormous quantities of -rock-materials, is denudation. But the agents of denudation—rain -and frost, rivers and glaciers, and sea-waves—though -producing grand results, yet work exceeding -slowly; and almost inconceivably long periods of time -must have elapsed before masses of rock several miles -in thickness could have been removed, and the subjacent -granites and other highly crystalline rocks have -been exposed at the surface.</p> - -<div class="sidenote">ANCIENT AND MODERN VOLCANIC ROCKS.</div> - -<p>It is an admitted fact that among the older geological -formations, we much more frequently find intrusions of -granitic rocks than in the case of younger ones. It is -equally true that among the sediments formed during -the most recent geological periods, no true granitic rocks -<span class="pagenum" id="Page_254">- 254 -</span> -have been detected. But if, as we insist is the case, -granitic rocks can only be formed at a great depth from -the surface, the £acts we have described are only just -what we might expect to present themselves under the -circumstances. The older a mass of granitic rock, the -greater chance there is that the denuding forces operating -upon the overlying masses, will have had an opportunity -of so far removing the latter as to expose the -underlying crystalline rocks at the surface. And, on -the other hand, the younger crystalline rocks are still, -for the most part, buried under such enormous thicknesses -of superincumbent materials that it is hopeless -for us to search for them. Nevertheless, it does occasionally -happen that, where the work of denudation -has been exceptionally rapid in its action, such crystalline -rocks formed during a comparatively recent geological -period, are exposed at the surface. This is the -case in the Western Isles of Scotland and in the -Pyrenees, where masses of granite and other highly -crystalline rocks are found which were evidently formed -during the Tertiary period.</p> - -<p>The granites which were formed in Tertiary times -present no essential points of difference from those -which had their origin during the earlier periods of the -earth's history. The former, like the latter, consist of -a mass of crystals with no imperfectly crystalline base -or groundmass between them; and these crystals include -numerous cavities containing liquids.</p> - -<p>Between the granites and the quartz-felsites every -<span class="pagenum" id="Page_255">- 255 -</span> -possible gradation may be found, so that it is impossible -to say where the one group ends and the other begins; -indeed, many of the rocks called 'granite-porphyries' -have about equal claims to be placed in either class. -Nor is the distinction between the quartz-felsites and -rhyolites any more strongly marked than that between -the former class of rocks and the granites; some of -the more crystalline rhyolites of Hungary being quite -undistinguishable, in their chemical composition, their -mineralogical constitution, and their microscopic characters, -from the quartz-felsites. The more crystalline -rhyolites are in turn found passing by insensible gradations -into the glassy varieties and finally into obsidian.</p> - -<div class="sidenote">RELATIONS BETWEEN GRANITE AND PUMICE.</div> - -<p>A piece of granite and a piece of pumice may at -first sight appear to present so many points of difference, -that it would seem quite futile to attempt to discover -any connection between them. Yet, if we analyse the -two substances, we may find that in ultimate chemical -composition they are absolutely identical. There is -nothing irrational, therefore, in the conclusion that the -same materials under different conditions may assume -either the characters of granite on the one hand, or -of pumice on the other; the former being consolidated -under circumstances in which the chemical and crystalline -forces have had the freest play and have used -up the whole of the materials to form crystallised -minerals, while the latter has cooled down and solidified -rapidly at the surface, in such a way that only incipient -crystallisation has occurred, and the glassy -<span class="pagenum" id="Page_256">- 256 -</span> -mass has been reduced to a frothy condition by the -escape of steam-bubbles from its midst This conclusion -receives the strongest support from the fact -that examples of every stage of the change, between -the glassy condition of pumice and the crystalline condition -of granite, may be detected among the materials -of which the globe is built up.</p> - -<p>There is still another class of facts which may be -adduced in support of the same conclusion. Many -lavas, as we have seen, contain crystals of much larger -dimensions than those constituting the mass of the -rock, which is then said to be 'porphyritic' in structure. -The porphyritically embedded crystals, when -carefully examined, are often seen to be broken and -injured, and to exhibit rounded edges, with other indications -of having undergone transport. When examined -microscopically, too, they often present the -cavities containing liquids which distinguish the crystals -of plutonic rocks. All the facts connected with -these porphyritic lavas point to the conclusion that -while the crystals in their groundmass have separated -from the liquefied materials near the surface, the large -embedded crystal, have been floated up from great -depths within the earth's crust, where they had been -originally formed.</p> - -<div class="sidenote">GRANITIC REPRESENTATIVES OF OTHER LAVAS.</div> - -<p>The careful consideration of all the facts of the case -leads to the conclusion that where pumice, obsidian, -and rhyolite are now being ejected at the surface, the -materials which form these substances are, at various -<span class="pagenum" id="Page_257">- 257 -</span> -depths in the earth's interior, slowly consolidating in the -form of quartz-felsite, granite-porphyry and granite. It -may be that we can nowhere point to the example of a -mass of rock which can be traced from subterranean -regions to the surface, and is, under such conditions, -actually seen to pass from the dense and crystalline -condition of granite to the vesicular and glassy form of -pumice; but great granitic masses often exhibit a more -coarsely crystalline condition in their interior, and the -offshoots and dykes which they give off not infrequently -assume the form of quartz-felsite; while, on -the other hand, the more slowly consolidated rocks -found in the interior of some rhyolite masses are not -distinguishable in any way from some of the true -quartz-felsites.</p> - -<p>That which is true of the lavas of acid composition -is equally true of the lavas of intermediate and basic -character. The andesites, the trachytes, the phonolites, -and the basalts have all their exact representatives -among the plutonic rocks, and these have a perfectly -crystalline or granitic structure. The plutonic and the -volcanic representatives of each of these groups are -identical in their chemical composition, and numerous -intermediate gradations can be found between the -most completely granitic and the most perfectly -vitreous or glassy types. In illustration of this fact, -we may again refer to the series of microscopic sections -of rocks given in the frontispiece.</p> - -<p>Another objection to the conclusion that the volcanic -<span class="pagenum" id="Page_258">- 258 -</span> -products of earlier periods of the earth's history were -identical in character with those which are being ejected -at the present day is based on the fact of the supposed -non-existence of the scoriaceous and glassy materials -which abound in the neighbourhood of the active volcanic -vents. Where, it is asked, do we find among the -older rocks of the globe the heaps of lapilli, dust, and -scoriæ, with the glassy and pumiceous rocks that now -occur so abundantly in all volcanic districts?</p> - -<p>In reply to this objection, we may point out that -these accumulations of loose materials are of such a -nature as to be capable of easy removal by denuding -agents, and that as they are formed upon the land they -will, if not already washed away by the action of rain, -floods, rivers, &c., run great risk of having their materials -distributed, when the land sinks beneath the -waters of the ocean and the surface is covered by new -deposits. With respect to the glassy rocks it must be -remembered that the action of water, containing carbonic -acid and other substances, in percolating through -such masses has a tendency to set up crystalline action, -and these glassy rocks easily undergo 'devitrification'; -it would therefore be illogical for us to expect glassy -rock-masses to retain their vitreous character through -long geological periods, during which they have been -subjected to the action of water and acid gases.</p> - -<p>But careful observation has shown that the scoriaceous -and vitreous rocks are by no means absent -among the igneous materials ejected during earlier -<span class="pagenum" id="Page_259">- 259 -</span> -periods of the earth's history. Their comparative -infrequency is easily accounted for when we remember, -in the first place, the ease with which such materials -would be removed by denuding forces, and in the -second place, the tendency of the action of percolating -water to destroy their characteristic features, by filling -up their vesicles with crystalline products and by -effecting devitrification in their mass.</p> - -<div class="sidenote">SIMILARITY OF ANCIENT AND RECENT LAVAS.</div> - -<p>If we go back to the very oldest known rock-masses -of the globe, those which are found underlying the -fossiliferous Cambrian strata, we find abundant evidence -that volcanic action took place during the period in -which these materials were being accumulated. Thus, -in the Wrekin, as Mr. Allport has so well shown, we find -clear proofs that before the long-distant period of the -Cambrian, there existed volcanoes which ejected scoriæ, -lapilli, and volcanic dust, and also gave rise to streams -of lava exhibiting the characteristic structures found -in glassy rocks. In these rocks, which have undergone -a curious alteration or devitrification, we still find all -those peculiar structures—the sphærulitic, the perlitic, -and the banded—so common in the rhyolites of Hungary, -with which rocks the Wrekin lavas, in their -chemical composition, precisely agree. Prof. Bonney, -too, has shown that the rocks of Charnwood Forest, -which are also probably of pre-Cambrian age, contain -great quantities of altered volcanic agglomerates, tuffs, -and ashes. I have found the sphærulitic, perlitic, and -banded structures exhibited by British lavas of the -<span class="pagenum" id="Page_260">- 260 -</span> -Cambrian, Silurian, Devonian and Carboniferous periods, -as well as in those of Tertiary age; and in -connection with these different lavas we find vast -accumulations, sometimes thousands of feet in thickness, -of volcanic agglomerates and tuffs which have undergone -great alteration.</p> - -<p>All these facts point to one conclusion—namely, -that during all past geological periods, materials similar -to those which are now being extruded from volcanic -vents were poured out on the earth's surface by analogous -agencies. If we could trace the lava-streams of -the present day down to the great subterranean reservoirs -from which their materials have been derived, -we should doubtless find that at gradually increasing -depths, where the pressure would be greater and the -escape of heat from the mass slower, the rocky materials -would by degrees assume more and more crystalline -characters. We should thus find obsidian or rhyolite -insensibly passing into quartz-felsite and finally -into granite; trachyte passing into orthoclase-porphyry -and syenite; and basalt passing into dolerite, augite-porphyry, -and gabbro.</p> - -<p>On the other hand, if we could replace the great -masses of stratified rocks which must once have overlain -the granites, syenites, diorites, and gabbros, we should -find that, as we approached the original surface, these -igneous materials would gradually lose their crystalline -characters, and when they were poured out at the surface -would take the forms of rhyolite, trachyte, andesite, -<span class="pagenum" id="Page_261">- 261 -</span> -and basalt—all of which might occasionally assume the -glassy forms known as obsidian or tachylyte.</p> - -<div class="sidenote">ALTERED FORMS OF ANCIENT LAVAS.</div> - -<p>But while we insist on the essential points of similarity -between the lavas poured out upon the surface of -the earth during earlier geological periods and those -which are being extruded at the present day, we must -not forget that by the action of percolating water and -acid gases, the mineral constitution, the structure, and -sometimes even the chemical composition of these -ancient lavas may undergo a vast amount of change. -In not a few cases these changes in the characters of a -lava may be carried so far that the altered rock bears -but little resemblance to the lava from which it was -formed, and it may be found desirable to give it a new -name. Among the rocks of aqueous origin we find -similar differences in the materials deposited at different -geological periods. Clay, shale and clay-slate have the -same composition, and the two latter are evidently only -altered forms of the first mentioned, yet so great is the -difference in their characters that it is not only allowable, -but desirable, to give them distinctive names.</p> - -<p>In the same way, among the deposits of the earlier -geological periods we find rocks which were doubtless -originally basalts, but in which great alterations have -been produced by the percolation of water through the -mass. The original rock has consisted of crystals of -felspar, augite, olivine, and magnetite distributed -through a glassy base. But the chemical action of -water and carbonic acid may have affected all the -<span class="pagenum" id="Page_262">- 262 -</span> -ingredients of the rock. The outward form of the felspar -crystals may be retained while their substance -is changed to kaolinite, various zeolites, and other minerals; -the olivine maybe altered to serpentine and other -analogous minerals; the magnetite changed to hydrous -peroxide of iron; the augite may be changed to uralite -or hornblende; and the surrounding glassy mass more -or less devitrified and decomposed. The hard, dense, -and black rock known as basalt has under these circumstances -become a much softer, earthy-looking mass -of a reddish-brown tint, and its difference from basalt -is so marked that geologists have agreed to call it by -another name, that of 'melaphyre.' Even in their ultimate -chemical compositions the 'melaphyres' differ -to some extent from the basalts, for some of the -materials of the latter may have been removed in -solution, and water, oxygen, and carbonic acid have -been introduced to combine with the remaining ingredients.</p> - -<p>But if we carefully study, by the aid of the microscope, -a large series of basalts and melaphyres, we shall -find that many rocks of the former class show the first -incipient traces of those changes which would reduce -them to the latter class. Indeed, it is quite easy to -form a perfect series from quite unaltered basalts to -the most completely changed melaphyres. Hence we -are justified in concluding that all the melaphyres were -originally basalts, just as we infer that all oaks were -once acorns.</p> - -<p><span class="pagenum" id="Page_263">- 263 -</span></p> - -<p>Now changes, similar to those which we have seen -to take place in the case of basaltic lavas, are exhibited -by the lavas of every other class, which have been exposed -to the influence of the same agencies,—namely, -the passage of water and acid gases. But inasmuch as -the minerals composing the basic lavas are for the most -part much more easily affected by such agencies than -are the minerals of acid lavas, the ancient basic rocks are -usually found in a much more highly altered condition -than are the acid rocks of equivalent age.</p> - -<div class="sidenote">NAMES GIVEN TO ALTERED LAVAS.</div> - -<p>We thus see that each of the classes of modern -lavas has its representative in earlier geological periods, -in the form of rocks which have evidently been derived -from these lavas, through alterations effected by the -agency of water and acid-gases that have permeated -their mass. Thus, while the basalts are represented -among the ancient geological formation by the melaphyres, -the andesites are represented by the porphyrites, -and the trachytes and rhyolites by different -varieties of felstones. And, as we can form perfect -series illustrating the gradual change from basalt to -melaphyre, so we can arrange other series demonstrating -the passage of andesites into porphyrites, and of -trachytes and rhyolites into felsites.</p> - -<p>It must be remembered, however, that these changes -do not take place in anything like determinate periods -of time. Occasionally we may find lavas of ancient -date which have undergone surprisingly little alteration, -and in other cases there occur lavas belonging to a -<span class="pagenum" id="Page_264">- 264 -</span> -comparatively recent period which exhibit very marked -signs of change.</p> - -<p>The alteration of the lavas and other igneous rocks -does not, however, stop with the production of the -melaphyres, porphyrites, and felstones. By the further -action of the water and carbonic acid of the atmosphere, -the basic lavas are reduced to the soft earthy mass -known as 'wacke,' and the intermediate and acid lavas -to the similar material known as 'claystone.' As the -passage of water and carbonic acid gas through these -rock-masses goes on, they are eventually resolved into -two portions, one of which is insoluble in water and -the other is soluble. The insoluble portion consists -principally of quartz, the crystals of which are almost -unattacked by water and carbonic acid, and the hydrated -silicate of alumina. All the sands and clays, which -together make up more than nine-tenths of the stratified -rocks of the globe, are doubtless derived, either -directly or indirectly, from these insoluble materials -separated during the decomposition of volcanic and -plutonic rocks. The soluble materials, which consist -of the carbonates, sulphates and chlorides of lime, -magnesia, soda, potash, and iron, give rise to the formation -of the limestones, gypsum, rock-salt, ironstones, and -other stratified masses of the earth's crust. We thus -see how the igneous materials of the globe, by their -decomposition, famish the materials for the stratified -rock-masses. The relations of the different plutonic -and volcanic rocks to one another and to the materials -<span class="pagenum" id="Page_265">- 265 -</span> -which are derived from them are illustrated in the -following table.</p> - -<div class="sidenote">RELATIONS OF ALTERED TO UNALTERED LAVAS.</div> - -<table summary="rocks"> -<tr> - <td class="tdc smaller" colspan="4">Plutonic Rocks</td> - <td class="tdc smaller" colspan="2">Unaltered<br />lavas</td> - <td class="tdc smaller" colspan="2">Altered lavas</td> - <td class="tdc smaller">Decomposed<br />Rocks</td> -</tr> -<tr style="height: 2em;"> - <td class="tdl">Granite</td> - <td><span style="font-size: 2em;">{</span></td> - <td><span class="wsnw">Quartz-felsite<br />('quartz-porphyry')</span></td> - <td><span style="font-size: 2em;">}</span></td> - <td><span class="wsnw">Rhyolite and<br />Obsidian</span></td> - <td rowspan="2"><img src="images/bracer_48.png" width="11" height="48" alt="" /></td> - <td rowspan="2">Felstone</td> - <td rowspan="3"><img src="images/bracer_48.png" width="11" height="48" alt="" /></td> - <td rowspan="3">Claystones</td> -</tr> -<tr> - <td class="tdl">Syenite</td> - <td><span style="font-size: 2em;">{</span></td> - <td class="tdl">Orthoclase-porphyry</td> - <td><span style="font-size: 2em;">}</span></td> - <td class="tdl">Trachyte</td> -</tr> -<tr> - <td class="tdl">Diorite</td> - <td><span style="font-size: 2em;">{</span></td> - <td class="tdl"><span class="wsnw">Hornblende-porphyry</span></td> - <td><span style="font-size: 2em;">}</span></td> - <td class="tdl">Andesite</td> - <td></td> - <td class="tdl">Porphyrite</td> -</tr> -<tr> - <td class="tdl">Miascite</td> - <td><span style="font-size: 2em;">{</span></td> - <td class="tdl">Liebnerite porphyry</td> - <td><span style="font-size: 2em;">}</span></td> - <td class="tdl">Phonolite</td> - <td></td> - <td class="tdc">?</td> - <td class="tdc">—</td> -</tr> -<tr> - <td class="tdl">Gabbro</td> - <td><span style="font-size: 2em;">{</span></td> - <td class="tdl">Augite-porphyry and<br />Dolerite</td> - <td><span style="font-size: 2em;">}</span></td> - <td class="tdl">Basalt</td> - <td></td> - <td class="tdl">Melaphyre</td> - <td></td> - <td class="tdc">Wacke</td> -</tr> -</table> - -<p>Some petrographers, indeed, have maintained the -principle that rocks belonging to widely separated geological -periods, even when they exhibit no essential -points of difference, should nevertheless be called by -distinct names. But such a system of classification is -calculated rather to hinder than to advance the cause -of science. If the palæontologist were to adopt the same -principle and give distinct names to the same fossil, -when it was found to occur in two different geological -formations, we can easily understand what confusion -would be occasioned, and how the comparison of the -fauna and flora of the different formations would be -thereby rendered impossible. But the naturalist, in his -diagnosis of a species, wisely confines himself to the -structure and affinities of the organism before him; and -in the same way the petrographer, in giving a name to -a rock, ought to be guided only by his studies of its -chemical composition, its mineralogical constitution, -and its structure, putting altogether out of view its -<span class="pagenum" id="Page_266">- 266 -</span> -geographical distribution and geological age. Only by -strict attention to this principle can we hope to arrive -at such comparisons of the rocks of different areas and -different periods, as may serve as the basis for safe -inductions.</p> - -<p>Before leaving this question of the relation which -exists between the igneous rocks of different ages, it -may be well to notice several facts that have been -relied upon, as proving that the several geological -periods are distinguished by characteristic igneous -products.</p> - -<p>It has frequently been asserted that the acid igneous -rocks are present in much greater quantities in connection -with the older geological formations than axe the -basic; while, on the other hand, the basic igneous -rocks are said to have been extruded in greater abundance -in the more recent geological periods. But in considering -this question it must not be forgotten that, -as a general rule, the basic rocks undergo decomposition -and disintegration far more rapidly than do the -acid rocks. In consequence of this circumstance the -chance of our finding their recognisable representatives -among the older formations, is much less in the case -of the former class of rocks than in the latter. As a -matter of fact, however, we do find great masses of -gabbro, diabase, and melaphyre associated even with -the oldest geological formations, while trachytes and -rhyolites abound in many volcanic districts where active -vents exist at the present day. Upon a general -<span class="pagenum" id="Page_267">- 267 -</span> -review of the subject, it may well be doubted whether -the supposed preponderance of acid igneous materials -in the earlier periods of the earth's history, and of -basic igneous materials during the later periods, rests -on any substantial basis of observation.</p> - -<div class="sidenote">AUGITIC AND HORNBLENDIC ROCKS.</div> - -<p>Another difference which has frequently been relied -upon, as distinguishing the older igneous rocks from -those of more recent date, is the supposed fact that the -former are characterised by the presence of hornblende, -the latter by the presence of augite. It may be admitted -that this distinction is a real one, but its -significance and value are greatly diminished when we -remember the relations which exist between the two -minerals in question. Hornblende and augite are -interesting examples of a dimorphous substance; in -chemical composition they are identical, or rather they -are liable to variation between the same limits, but in -their crystalline forms and optical characters they differ -from one another. It has been proved that hornblende is -the stable, and augite the unstable condition of the substance -in question. If hornblende be fused and allowed -to cool, it crystallises in the form of augite. On the -other hand, augite-crystals in rocks of ancient date are -found undergoing gradual change and passing into -hornblende. The mineral uralite has the outward form -of augite, but the cleavage and optical properties of -hornblende; and there are not wanting many facts -pointing to the conclusion that rocks which now contain -hornblende were originally augitic masses, in which -<span class="pagenum" id="Page_268">- 268 -</span> -the unstable mineral in their midst has been gradually -converted into the stable one.</p> - -<p>There are, however, two minerals which up to the -present time have been found in association only with -the older and newer rock-masses respectively. These -are <i>muscovite</i>, or the white form of mica, which occurs -in so many granites, but has not yet been discovered -in any modern representative of that rock; and leucite, -which is not yet known in rocks of older date than the -Tertiary.</p> - -<p>When we remember that muscovite would appear -to be a product of deep-seated igneous action, and is -only found in rock-masses that have been formed under -such conditions, we shall be the less surprised at its -non-occurrence in rocks of recent date, especially if we -bear in mind the fact that very few of the younger -granitic rocks have as yet been exposed at the surface -by denudation.</p> - -<p>With respect to leucite, on the other hand, it must -be remembered that it is a very unstable mineral which -appears to be easily changed into felspar. It is by no -means improbable, therefore, that some ancient igneous -rocks which now contain felspar were originally leucitic -rocks.</p> - -<p>To the view that the action of volcanic forces upon -the globe during past geological times was similar in -kind to that which we now observe going on around us, -still another objection has been raised. It has been -asserted that some of the deposits of igneous rock -<span class="pagenum" id="Page_269">- 269 -</span> -associated with the older geological formations are of -such a nature that they could not possibly have been -accumulated around volcanic vents of the kind which -we see in operation around us.</p> - -<div class="sidenote">VOLCANIC ORIGIN OF ANCIENT IGNEOUS ROCKS.</div> - -<p>Mr. Mallet has declared that the igneous products -of the Palæozoic period differ fundamentally in character -from those materials formed by volcanic action -during the later Secondary and the Tertiary periods. -Upon what observations these generalisations are based -he has given us no information, and the enormous -mass of facts which have been collected in recent years -concerning the structure of the lavas and fragmental -volcanic deposits of the pre-Cambrian, Cambrian, Silurian, -Devonian and Carboniferous periods, all point -to a directly opposite conclusion. The more carefully -we carry on our investigations concerning these ancient -lavas, by the aid of chemical analysis and microscopic -study, the more are we convinced of the essential -identity of the ancient and modern volcanic rocks, -both in their composition and their minute structure. -Of great masses of dust produced by crushing, such as -Mr. Mallet has supposed to have been formed during -the earlier geological periods, there is not the smallest -evidence; but we everywhere find proofs, when the -rocks are minutely examined, of the vesicular structure -so characteristic of materials produced by explosive -volcanic action.</p> - -<p>It has frequently been asserted that in the great -districts covered by basaltic lavas which we find in the -<span class="pagenum" id="Page_270">- 270 -</span> -Rocky Mountains of North America, in the Deccan of -India, in Abyssinia, and even in the Western Isles of -Scotland, we have proofs of the occurrence, during -earlier geological periods, of volcanic action very -different in character from that which at present takes place -on our globe. It has been asserted that the phenomena -observed in these districts can only be accounted for -by supposing that great fissures have opened in their -midst, from which lavas have issued in enormous floods -unaccompanied by the ordinary explosive phenomena -of volcanoes.</p> - -<p>It must be remembered, however, that none of the -districts in question have been subjected to careful -and systematic examination with a view to the discovery -of the vents from which these masses of lava have -issued, with the exception of that which occurs in our -own islands. In this case, in which superficial observers -have spoken of the district as being covered with horizontal -lava-sheets piled upon one another to the -depth of 3,000 feet, careful study of the rock-masses -has shown that the accumulations of basalt really consist -of a great number of lava-currents which have -issued at successive epochs covering enormous periods -of time. During the intervals between the emission -of these successive lava-currents the surfaces of the -older ones have been decomposed, and formed soils -upon which forests have grown up; they have been -eroded by streams, the valleys so formed being filled -with gravels; and lakes have been originated on their -<span class="pagenum" id="Page_271">- 271 -</span> -surfaces in which various accumulations have taken -place.</p> - -<div class="sidenote">TRULY-VOLCANIC ORIGIN OF LAVA PLATEAUX.</div> - -<p>It has been demonstrated, moreover, that the -basal-wrecks of no less than five volcanic mountains, -each of which must have rivalled Etna in its proportions, -existed within this area, and the connection of -the lava-currents, which have deluged the surrounding -tracts, with these great volcanoes has been clearly proved. -It is probable that when more careful and systematic -researches are carried on in the other districts, in which -widely-spread sheets of basaltic rocks exist, similar -volcanic vents will be discovered. It must also be -remembered that if such a country as Iceland were -subjected to long-continued denudation, the mountain -peaks and cones of loose materials would be worn away, -the whole island being thus reduced to a series of -plateaux composed of lava-sheets, the connection of -which with the crystalline materials filling the great -volcanic vents, a superficial observer might altogether -fail to recognise.</p> - -<p>But even where we cannot trace the former existence -of great volcanic mountains, like those which once rose -in the Hebrides, it would nevertheless be very rash to -conclude that the vast plateaux of lava-rock must have -been formed as gigantic floods unaccompanied by ordinary -volcanic action. Mr. Darwin has pointed out that -in crossing districts covered by lava, he was frequently -only able to determine the limits of the different currents -of which it was made up, by an examination of -the age of the trees and the nature of the vegetation -<span class="pagenum" id="Page_272">- 272 -</span> -which had sprung up on them. And everyone who -has travelled much in volcanic districts can confirm -this observation; what appears at first sight to be a -great continuous sheet of lava proves upon more careful -observation to be composed of a great number of -distinctly different lava-currents, which have succeeded -one another at longer or shorter intervals.</p> - -<p>We must remember, too, how various in kind are -the volcanic manifestations which present themselves -under different circumstances. Sometimes the amount -of explosive action at a volcanic vent is very great, and -only fragmental ejections take place, composed of the -frothy scum of the lava produced by the escape of gases -and vapours from its midst. But in other cases the -amount of explosive action may be small, and great -volumes of igneous materials may issue as lava-streams. -In such cases, only small scoriæ-cones would be formed -around the vents, and one half of such cones is commonly -swept away by the efflux of the lava-currents, -while the remainder may be easily removed by denuding -action or be buried under the lava-currents issuing -from other vents in the neighbourhood. Thus it -may easily come to pass that what a superficial observer -takes for an enormous mass of basaltic lava poured out -from a great fissure at a single effort, may prove upon -careful observation to be made up of innumerable lava-currents, -each of which is of moderate dimensions; and -it may further be found that these lava-currents, instead -of being the product of a single paroxysmal effort -<span class="pagenum" id="Page_273">- 273 -</span> -from one great fissure, have been accumulated by numerous -small outbursts taking place at wide intervals, -from a great number of minor orifices.</p> - -<div class="sidenote">SHIFTING OF VOLCANIC ACTION IN DIFFERENT AREAS.</div> - -<p>Having then considered the arguments which have -been adduced in support of the view that the volcanic -phenomena of former geological periods differ from -those which are still occurring upon the globe, we may -proceed to state the general conclusions which have been -drawn from the study of the volcanic rocks of the different -geological periods.</p> - -<p>From a survey of the volcanic rocks of different ages, -we are led to the interesting and important conclusion -that the scene of volcanic action has been continually -shifting to fresh areas at different periods of the -earth's history. We find repeated proofs that the -volcanic energy has made its appearance at a certain -part of the earth's crust, has gradually increased in -intensity to a maximum, and then as slowly declined. -But as these manifestations have died away at one part -of the earth's surface, they have gradually made their -appearance at another. In every district which has -been examined, we find abundant proofs that volcanic -energy has been developed at certain periods, has disappeared -during longer or shorter periods, and then -reappeared in the same area. And on the other hand, -we find that there is no past geological period in which -we have not abundant evidence that volcanic outbursts -took place at some portion of the earth's surface.</p> - -<p>To take the case of our own islands for example. -<span class="pagenum" id="Page_274">- 274 -</span> -We know that during the pre-Cambrian periods volcanic -outbursts occurred, traces of which are found both in -North and South Wales, in the Wrekin Chain in Shropshire, -in Charnwood Forest, and in parts of Scotland -and Ireland.</p> - -<p>In Cambro-Silurian times we have abundant proofs, -both in North Wales and the Lake district, that volcanic -action on the very grandest scale was taking place -during the Arenig and the older portion of the Llandeilo -periods, and again during the deposition of the Bala -or Caradoc beds. The lavas, tuffs, and volcanic agglomerates -ejected during these two periods have built up -masses of rock many thousands of feet in thickness. -Snowdon and Cader Idris among the Welsh mountains, -and some of the higher summits of the Lake district, -have been carved by denudation from the vast piles of -volcanic materials ejected during these periods.</p> - -<p>In Devonian or Old-Red-Sandstone times, volcanic -activity was renewed with fresh violence upon that -part of the earth's surface now occupied by the British -Islands. Along the line which now forms the Grampians -there rose a series of volcanoes of the very -grandest dimensions. Ben Nevis, and many others -among the higher Scotch mountains, have been carved -by denudation from the hard masses of granite, quartz-felsite, -and other plutonic rocks which formed the central -cores of these ancient volcanic piles. The remains -of the great lava-sheets, and of the masses of volcanic -agglomerate ejected from these grand Devonian volcanoes, -<span class="pagenum" id="Page_275">- 275 -</span> -make up hill-ranges of no mean altitude, like -the Sidlaws, the Ochils, and the Pentlands.</p> - -<div class="sidenote">ANCIENT BRITISH VOLCANOES.</div> - -<p>The volcanic action of the Devonian period was prolonged -into Carboniferous times, but was then evidently -diminishing gradually in violence. Instead of great -central volcanoes, such as existed in the earlier period, -we find innumerable small vents which threw out tuffs, -agglomerates and lavas, and were scattered over the -districts lying around the bases of the now extinct -Devonian volcanoes. In the central valley of Scotland -and in many parts of England, we find abundant proofs -of the existence of these small and scattered volcanic -vents during Carboniferous times. The well-known hill -of Arthur's Seat, which overlooks the city of Edinburgh, -and many castle-crowned crags of the Forth and Clyde -valleys, are the worn and denuded relics of these small -volcanoes. There are some indications which point to -the conclusion that the volcanic action of the Newer -Palæozoic epoch had not entirely died out in Permian -times, but the evidence upon this point is not altogether -clear and satisfactory.</p> - -<p>During nearly the whole of the Secondary or Mesozoic -periods the volcanic forces remained dormant in -the area of the British Isles. Some small volcanic outbursts, -however, appear to have occurred in Triassic -times in Devonshire. But in other areas, such as the -Tyrol, South-eastern Europe and Western America, the -Triassic, Jurassic, and Cretaceous periods were marked -by grand manifestations of volcanic activity.</p> - -<p><span class="pagenum" id="Page_276">- 276 -</span></p> - -<p>The volcanic forces which had during the long -Mesozoic periods deserted our part of the earth's surface, -appear to have returned to it in full rigour in the -Tertiary epoch. In the Newer-Palæozoic periods the -direction of the great volcanic band which traversed -our islands appears to have been from north-east to -south-west; but in Tertiary times a new set of fissures -were opened running from north to south. There is -evidence that during the Eocene or Nummulitic period, -the first indications of the subterranean forces having -gathered strength below the district were afforded by -the issue of calcareous and siliceous springs, and soon -fissures were opened which emitted scoriæ, tuffs, and -lavas. The intensity of the volcanic action gradually -increased till it attained its maximum in the Miocene -period, when a great chain of volcanic mountains -stretched north and south along the line of the Inner -Hebrides, the north-east of Ireland, and the sea which -separates Great Britain from Ireland. The basal-wrecks -of a number of these volcanoes can be traced -in the islands of Skye, Mull, Rum, and parts of the -adjoining mainland. We have already seen that along -this great band of volcanic action, which traverses the -Atlantic Ocean from north to south, a number of active -vents still exist, though their energy is now far less -intense than was the case in former times. The only -vestiges of the action of these now declining volcanic -forces, at present found in our islands, are the hot -springs of Bath and a few other warm and mineral -<span class="pagenum" id="Page_277">- 277 -</span> -springs; but in connection with this subject it must be -remembered that our country occasionally participates -in great earthquake-vibrations, like that which destroyed -Lisbon in the year 1759.</p> - -<div class="sidenote">ANCIENT VOLCANOES IN OTHER DISTRICTS.</div> - -<p>If we were to study any other part of the earth's -surface, we should arrive at precisely the same conclusion -as those to which we have been conducted by -our examination of the British Islands—namely, that -during past geological times the subterranean forces -had made themselves felt in the area, had gradually -attained a maximum, and then as gradually declined, -passing through all those varied cycles which we have -described in a former chapter. And we should also -find that these periods of volcanic activity alternated -with other periods of complete quiescence which were -of longer or shorter duration. But on comparing two -different districts, we should discover that what was a -period of volcanic activity in the one was a period of -repose in the other, and <i>vice versâ</i>.</p> - -<p>From these facts geologists have been led to the -conclusion which we have already enunciated—namely, -that the subterranean forces are in a state of continual -flux over the surface of the globe. At one point of the -earth's crust these forces gradually gather such energy -as to rend asunder the superincumbent rock-masses -and make themselves manifest at the surface in the -series of phenomena characteristic of volcanic action. -But after a longer or shorter interval of time—an interval -which must probably be measured by millions of -<span class="pagenum" id="Page_278">- 278 -</span> -years—the volcanic forces die out in that area to make -their appearance in another.</p> - -<p>Hence, although we may not be able to prove the -fact by any mathematical demonstration, a strong presumption -is raised in favour of the view that the -subterranean energy in the earth's crust is a constant -quantity, and that the only variations which take place -are in the locality of its manifestation.</p> - -<p>Upon this question whether the amount of this subterranean -energy within the earth's crust is at the present -time increasing, stationary, or declining, we are -not altogether destitute of evidence. There are some -considerations connected with certain astronomical -hypotheses, to which we shall hereafter have to refer, -that might lead us to entertain the view that the subterranean -activity was once far greater than it is at -present, and that during the long periods of the earth's -past history it has been slowly and gradually declining. -And those who examine the vast masses of igneous -materials which have been poured out from volcanic -vents during the earlier periods of the earth's history -may be inclined, at first sight, to point to them as -affording conclusive proof of this gradual decline.</p> - -<div class="sidenote">SUPPOSED DECLINE OF VOLCANIC ACTION.</div> - -<p>But a more careful study of the rocks in question -will probably cause a geologist to pause before jumping -to such a conclusion. If we look at the vast masses of -volcanic materials erupted in Miocene times in our own -island and in Ireland, for example, we might be led -to imagine that we have the indications of a veritable -<span class="pagenum" id="Page_279">- 279 -</span> -'Reign of Fire,' and that the evidence points to a -condition of things very different indeed from that -which prevails at the present day. But in arriving at -such a conclusion we should be neglecting a most important -consideration, the disregard of which has been -the fertile parent of many geological errors. Many -independent lines of evidence all point to the inference -that these volcanic ejections are not the result of one -violent effort, but are the product of numerous small -outbreaks which have been scattered over enormous -periods of time.</p> - -<p>When we examine with due care the lavas, tuffs, -and other volcanic ejections which constitute such -mountain-masses as those of the Hebrides, of the Auvergne, -and of Hungary, we find clear proofs that the -ancient Miocene volcanoes of these districts were clothed -with luxuriant forests, through which wild animals -roamed in the greatest abundance. The intervals between -the ejections of successive lava-streams were -often so great, that soils were formed on the mountain-slope, -and streams cut deep ravines and valleys in -them.</p> - -<p>The island of Java is situated near the very heart -of what is at the present day the most active volcanic -centre on the face of the globe, yet vegetable and -animal life flourish luxuriantly there, and the island -is one of the richest and most fertile spots upon the -face of the globe. Not all the terrors of occasional -volcanic outbreaks will ever drive the Neapolitan vine-dressers -<span class="pagenum" id="Page_280">- 280 -</span> -from the fertile slopes of Vesuvius, for its -periods of repose are long, and its eruptions are of -short duration.</p> - -<p>These considerations lead the geologist to conclude -that the evidence afforded by the ancient volcanic rocks -is clear and positive in support of the view that the -manifestations of the subterranean forces in the past -agree precisely in their nature and in their products -with those taking place around us at the present time. -On the question of great secular changes having -occurred in the amount of volcanic energy in past -geological periods, the evidence must be pronounced -negative, or at the best doubtful.</p> - -<p>But even if the geologist confesses himself unable -to establish the fact of any decline in the subterranean -energies during the vast periods of which he takes -cognisance, it must be remembered that such decline -may really be going on; for vast as was the duration of -the geological epochs, they probably constitute but a -fraction of those far grander periods which are required -by the speculations of the physical astronomer.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_281">- 281 -</span></p> - -<h2 class="nobreak" id="CHAPTER_X">CHAPTER X.<br /> - -<span class="smaller">THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF -NATURE.</span></h2> -</div> - - -<p class="p0">The first impression which is produced upon the mind, -when the phenomena of volcanic action are studied, is -that here we have exhibitions of destructive violence -the effects of which must be entirely mischievous and -disastrous to the living beings occupying the earth's -surface. A little consideration will convince us, however, -that the grand and terrible character of the displays -of volcanic energy have given rise to exaggerated -notions concerning their destructive effects. The fact -that districts situated over the most powerful volcanic -foci, like Java and Japan, are luxuriant in their productions, -and thickly inhabited, may well lead us to -pause ere we condemn volcanic action as productive only -of mischief to the living beings on the earth's surface. -The actual slopes of Vesuvius and Etna, and many other -active volcanoes, are abundantly clothed with vineyards -and forests and are thickly studded with populous -villages.</p> - -<p>As a matter of fact, the actual amount of damage -<span class="pagenum" id="Page_282">- 282 -</span> -to life and property which is effected by volcanic eruptions -is small. Usually, the inhabitants of the district -have sufficient warning to enable them to escape with -their lives and to carry away their most valuable possessions. -And though fertile tracts are covered by -loose dust and ashes, or by lava- and mud-currents, -yet the sterility thus produced is generally of short -duration, for by their decomposition volcanic materials -give rise to the formation of the richest and most -productive soils.</p> - -<p>Earthquakes, as we have already seen, are far more -destructive in their effects than are volcanoes. Houses -and villages, nay even entire cities, are, by vibrations -of portions of the earth's crust, reduced to heaps of -ruins, and famines and pestilences too frequently follow, -as the consequence of the disorganisation of our social -systems by these terrible catastrophes.</p> - -<p>It may well be doubted, however, whether the -annual average of destruction to life and property -caused by all kinds of subterranean action, exceeds that -produced either by floods or by hurricanes. Yet we -know that the circulation of water and air over our -globe are beneficial and necessary operations, and that -the mischief occasionally wrought by the moving bodies -of water and air is quite insignificant compared with -the good which they effect.</p> - -<p>In the same way, we shall be able to show that the -subterranean energies are necessary to the continued -existence of our globe as a place fitted for the habitation -<span class="pagenum" id="Page_283">- 283 -</span> -of living beings, and that the mischievous and -destructive effects of these energies bear but a small -and insignificant proportion to the beneficial results -with which they must be credited.</p> - -<div class="sidenote">LEVELLING ACTION OF DENUDING FORCES.</div> - -<p>We have had frequent occasion in the preceding -pages to refer to the work—slow but sure, silent -but effective—wrought by the action of the denuding -forces ever operating upon the surface of our globe. The -waters condensing from the atmosphere and falling upon -the land in the form of rain, snow, or hail, are charged -with small quantities of dissolved gases, and these -waters penetrating among the rock-masses of which the -earth's crust is composed, give rise to various chemical -actions of which we have already noticed such remarkable -illustrations in studying the ancient volcanic products -of our globe. By this action the hardest and -most solid rock-masses are reduced to a state of complete -disintegration, certain of their ingredients undergoing -decomposition, and the cementing materials which -hold their particles together being removed in a state of -solution. In the higher regions of the atmosphere this -work of rock-disintegration proceeds with the greatest -rapidity; for there the chemical action is reinforced by -the powerful mechanical action of freezing water. On -high mountain-peaks the work of breaking up rock-masses -goes on at the most rapid rate, and every craggy -pinnacle is swathed by the heaps of fragments which -have fallen from it. The Alpine traveller justly dreads -the continual fusillade of falling rock-fragments which -<span class="pagenum" id="Page_284">- 284 -</span> -is kept up by the ever-active power of the frost in these -higher regions of the atmosphere; and fears lest the -vibrations of his footsteps should loosen, from their -position of precarious rest, the rapidly accumulating -piles of detritus. No mountain-peak attains to any -very great elevation above the earth's surface, for the -higher we rise in the atmosphere the greater is the -range of temperature and the more destructive are the -effects of the atmospheric water. The moon, which is -a much smaller planet than our earth, has mountains -of far greater elevation; but the moon possesses neither -an atmosphere nor moisture on its surface, to produce -those levelling effects which we see everywhere going -on around us upon the earth.</p> - -<p>The disintegrated materials, produced by chemical -and mechanical actions of the atmospheric waters upon -rock-masses, are by floods, rivers, and glaciers, gradually -transported from higher to lower levels; and sooner or -later every fragment, when it has once been separated -from a mountain-top, must reach the ocean, where these -materials are accumulated and arranged to form new -rocks.</p> - -<p>Over every part of the earth's surface these three -grand operations of the disintegration of old rock-masses, -the transport of the materials so produced to -lower levels, and the accumulation of these materials to -form new rocks, is continually going on. It is by the -varied action of these denuding agents upon rocks of -unequal hardness, occupying different positions in -<span class="pagenum" id="Page_285">- 285 -</span> -relation to one another, that all the external features of -hills, and plains, and mountains owe their origin.</p> - -<p>It is a fact, which is capable of mathematical demonstration, -that by the action of these denuding -forces the surface of all the lands of the globe is being -gradually but surely lowered; and this takes place at -such a rate that in a few millions of years the whole -of the existing continents must be washed away and -their materials distributed over the beds of the oceans.</p> - -<div class="sidenote">NECESSITY FOR COMPENSATING AGENCIES.</div> - -<p>It is evident that there exists some agency by which -this levelling action of the denuding forces of the globe -is compensated; and a little consideration will show that -such compensating agency is found in the subterranean -forces ever at work within the earth's crust. The -effects of these subterranean forces which most powerfully -arrest our attention are volcanic outbursts and -earthquake shocks, but a careful study of the subject -proves that these are by no means the most important -of the results of the action of such forces. Exact observation -has proved that almost every part of the -earth's surface is either rising or falling, and the striking -and destructive phenomena of volcanoes and earthquakes -probably bear only the same relation to those -grand and useful actions of the subterranean forces, -which floods do to the system of circulating waters, and -hurricanes to the system of moving air-currents.</p> - -<p>If we ride in a well-appointed carriage with good -springs, upon a railway which is in excellent order, the -movement is almost imperceptible to us; and the rate -<span class="pagenum" id="Page_286">- 286 -</span> -of speed may be increased indefinitely, without making -itself apparent to our senses. The smallest impediment -to the evenness of the movement—such as that produced -by a small object placed upon the rails—at once -makes itself felt by a violent jar and vibration. How -perfectly insensible we may be of the grandest and -most rapid movements is taught us by the facts demonstrated -by the astronomer. By the earth's daily rotation, -we are borne along at a rate which in some places -amounts to over 1,000 miles an hour; and by its annual -revolution we are every hour transported through -a distance of 70,000 miles; yet concerning the fact -and direction of these movements we are wholly unconscious.</p> - -<p>In the case both of the railway train and of our -planet, we can only establish the reality of the movement, -and its direction and rate, by means of observations -upon external objects, which appear to us to have -a movement in the opposite direction. In the same -way we can only establish the fact of the movement of -portions of the earth's crust by noticing the changing -positions of parts of the earth's surface in relation to -the constant level of the ocean. When this is done we -find abundant proof that while some parts of the -earth's crust are rising, others are as undoubtedly -undergoing depression.</p> - -<div class="sidenote">POTENCY OF THE SUBTERRANEAN FORCES.</div> - -<p>We shall be able to form some idea of the vastness -of the effects produced by the subterranean forces, by -a very simple consideration. It is certain that during -<span class="pagenum" id="Page_287">- 287 -</span> -the enormous periods of time of which the records -have been discovered by the geologist, there have always -been continents and oceans upon the earth's -surface, just as at present, and it is almost equally -certain that the proportions of the earth's surface -occupied by land and water respectively, have not varied -very widely from those which now prevail. But, at the -same time, it is an equally well-established bet that -the denuding forces ever at work upon the earth's -surface would have been competent to the removal of -existing continents many times over, in the vast -periods covered by geological records. Hence we are -driven to conclude that the subterranean movements -have in past times entirely compensated for the waste -produced by the denuding forces ever at work upon our -globe. But this is not all. The subterranean forces -not only produce upheaval; in a great many cases the -evidences of subsidence are as clear and conclusive as -are those of upheaval in others. Hence we are driven -to conclude that the forces producing upheaval of portions -of the earth's crust are sufficient, not only to -balance those producing subsidence, but also to compensate -for the destructive action of denuding agents -upon the land-masses of the globe.</p> - -<p>It is only by a careful and attentive study and -calculation of the effects produced by the denuding -agents at work all around us, aided by an examination -of the enormous thicknesses of strata formed by the -action of such causes during past geological times, that -<span class="pagenum" id="Page_288">- 288 -</span> -we are able to form any idea of the reality and vastness -of the agents of change which are ever operating to -modify the earth's external features. When we have -clearly realised the grand effects produced on the surface -of the globe by these external forces, through the -action of its investing atmosphere and circulating -waters, then, and only then, shall we be in a position -to estimate the far greater effects resulting from the -internal forces, of which the most striking, but not the -most important, results are seen in the production of -volcanic eruptions and earthquake-shocks.</p> - -<p>Another series of facts which serve to convince the -geologist of the reality and potency of the forces ever -at work within the earth's crust, and the way in which -these have operated during past geological periods, is -found in the disturbed condition of many of the -stratified rock-masses of which it is composed. Such -stratified rock-masses, it is clear, must have been -originally deposited in a position of approximate -horizontality; but they are now often found in inclined -and even vertical positions; they are seen to be bent, -crumpled, puckered, and folded in the most remarkable -manner, and have not unfrequently been broken across -by dislocations—'faults'—which have sometimes displaced -masses, originally in contact, to the extent of -thousands of feet. The slate-rocks of the globe, moreover, -bear witness to the fact that strata have been -subjected to the action of lateral compression of enormous -violence and vast duration; while in the metamorphic -<span class="pagenum" id="Page_289">- 289 -</span> -rocks we see the effects of still more extreme -mechanical strains, which have been in part transformed -into chemical action. No one who has not -studied the crushed, crumpled, fractured, and altered -condition of many of the sedimentary rocks of the -globe, can form the faintest idea of the enormous -effects of the internal forces which have been in operation -within the earth's crust during earlier geological -periods. And it is only by such studies as these that -we at last learn to regard the earthquake and volcanic -phenomena of our globe, not as the grandest and most -important effects of these forces, but as their secondary -and accidental accompaniments. 'Volcanoes,' it -has been said, 'are the safety-valves of the globe;' -and when we come to realise the real extent and nature -of the internal forces ceaselessly working in the -earth's crust we shall scarcely be disposed to regard -the simile as an overstrained one.</p> - -<div class="sidenote">RELATION TO CONTINENTAL MOVEMENTS.</div> - -<p>The first geologist who attempted to show the exact -relations existing between those subterranean forces -which cause the movements of continental masses of -land, and those more startling displays of energy which -are witnessed in volcanic outbursts, was the late Mr. -Poulett Scrope. At a somewhat later date Mr. Darwin, -in his remarkable paper 'On the Connexion of certain -Volcanic Phenomena in South America, and on the -Formation of Mountain-chains and Volcanoes as the -effect of Continental Elevations,' threw much new and -important light upon the question.</p> - -<p><span class="pagenum" id="Page_290">- 290 -</span></p> - -<p>While, on the one hand, we are led by recent geological -investigations to reject the notions which were -formerly accepted, by which mountain-ranges were supposed -to be suddenly and violently upheaved by volcanic -forces, we are, on the other hand, driven to conclude -that without the action of these subterranean forces, -the irregularities which are exhibited on the earth's -surface could not have had any existence.</p> - -<p>It is true that the actual forms of the mountain-ranges -are due directly to the action of denuding forces, -which have sculptured out from the rude rocky masses -all the varied outlines of peaks and crags, of ravines and -valleys. But it is none the less true that the determining -causes which have directed and controlled all this -earth-sculpture, are found in the relative positions of -hard and soft masses of rock; but these rock-masses -have acquired their hardness and consistency, and have -assumed their present positions, in obedience to the -action of subterranean forces. Hence we see that though -the formation of mountain-ranges is proximately due to -the denuding forces, which have sculptured the earth's -surface, the primary cause for the existence of such -mountain-chains must be sought for in the fact that -subterranean forces have been at work, folding, crumpling, -and hardening the soft sediments, and placing -them in such positions that, by the action of denudation, -the more indurated portions are left standing as -mountain-masses above the general surface.</p> - -<p>The old notion that mountain-chains are due to a -<span class="pagenum" id="Page_291">- 291 -</span> -vertical upthrust from below, finds but little support -when we come to study with due care the positions -of the rock-masses composing the earth's crust. On -the contrary, we find that mountain-ranges are usually -carved out of the crushed and crumpled edges of strata -which have along certain lines been influenced by -great mechanical strains, and subjected to more or less -induration and chemical alteration. When we compare -these folded and contorted portions of the strata -with those parts of the same beds which are not so -affected, we find the effects produced in the former are -not such as would result from an upthrust from below, -but from movements by which a tangential strain -would be brought about. If we imagine certain lines -of weakness to exist in the solid crust of the earth, -then any movements in the portions of the crust between -these lines of weakness would cause crushing -and crumpling of the strata along the latter.</p> - -<div class="sidenote">FORMATION OF MOUNTAIN-CHAINS.</div> - -<p>Recent investigations of Dana and other authors -have thrown much new light upon the question of the -mode of formation of mountain-chains, and the relation -between the movements by which they are produced -and the sudden and violent manifestations of force -witnessed in volcanic outbursts. We cannot, perhaps, -better illustrate this subject than by giving a sketch of -the series of operations to which the great Alpine chains -owe their origin.</p> - -<p>There are good grounds for believing that the great -mountain-axis of Southern Europe, with its continuation -<span class="pagenum" id="Page_292">- 292 -</span> -in Asia, had no existence during the earlier geological -periods. Indeed, it has been proved that all the higher -among the existing mountain-chains of the globe have -been almost entirely formed in Tertiary times. The -reason of this remarkable fact is not far to seek. So -rapid is the work of denudation in the higher regions -of the atmosphere, that the elevated crags and pinnacles -are being broken up by the action of moisture -and frost at an exceedingly rapid rate. This fact is -attested by the existence of those enormous masses of -angular rock-fragments which are found lodged on -every vantage-ground among the mountain-summits, -as well as by the continually descending materials -which are borne by glaciers and mountain-torrents to -the valleys below. Where such a rate of disintegration -as this is maintained, no elevated mountain-crests could -exist through long geological periods. It is true we -find in all parts of the globe relics of many mountain-chains -which were formed before the Tertiary period; -but these have by long-continued denudation been worn -down to 'mere stumps.' Of such worn-down and degraded -mountain-ranges we have examples in the Scandinavian -chains, and some of the low mountain-regions -of Central Europe and North America.</p> - -<p>Let us now proceed to illustrate this subject by -briefly sketching the history of that series of operations -by which the great mountain-chains of the -Alpine system have been formed.</p> - -<p>The first stage of that grand series of operations -<span class="pagenum" id="Page_293">- 293 -</span> -appears from recent geological researches to have consisted -in the opening of a number of fissures running -along a line near to that at which, in a long subsequent -period, the elevation of the mountain-masses -took place. This betrayal of the existence of a line of -weakness in this part of the earth's crust occurred in -the Permian period, and from that time onward a series -of wonderful movements and changes have been going -forward, which have resulted in the production of the -Alpine chains as we now see them.</p> - -<div class="sidenote">VOLCANIC FISSURES OF PERMIAN PERIOD.</div> - -<p>From the great fissures opened in Permian times -along this line of weakness, great quantities of lava, -scoriæ, and tuff were poured out, and these accumulated -to form great volcanic mountains, which we can -now only study at a few isolated spots, as in the Tyrol, -Carinthia, and about Lake Lugano. Everywhere else, -these Permian volcanic rocks appear to be deeply buried -under the later-formed sediments, from which the -Alpine chains have been carved. Few and imperfect, -however, as are the exposures of these ancient rhyolite -and quartz-andesite lavas and agglomerates formed at -the close of the Palæozoic epoch, their greatly denuded -relics form masses which are in places more than 9,000 -feet in thickness. From this fact we are able to form -some slight idea of the scale upon which the volcanic -outbursts in question must have taken place during -Permian times.</p> - -<p>The second stage in the series of operations by -which the Alpine chains have been formed, consisted -<span class="pagenum" id="Page_294">- 294 -</span> -in a general sinking of the surface along that line of -weakness in the earth's crust, the existence of which -had been betrayed by the formation of fissures and -the eruption of volcanic rocks. We have already had -occasion to remark how frequently such subsidences -follow upon the extrusion of volcanic masses at any -part of the earth's surface; and we have referred these -downward movements in part to the removal of support -from below the portion of the crust affected, and in part -to the weight of the materials piled upon its surface by -the volcanic forces.</p> - -<p>The volcanic energy which had been manifested -with such violence during the Permian period, does not -appear to have died out altogether during the succeeding -Triassic period. A number of smaller volcanic vents -were opened from time to time, and from these, lavas, -tuffs, and agglomerates, chiefly of basic composition, -were poured out. The relics of these old Triassic volcanoes -are found at many points along the Alpine -chain, but it is evident that the igneous forces were -gradually becoming exhausted during this period, and -before the close of it they had fallen into a state of -complete extinction.</p> - -<p>But the great subsidence which had commenced in the -Triassic period, along what was to become the future -line of the Alpine chain, was continued almost without -interruption during the Rhætic, the Jurassic, the Tithonian, -the Neocomian, the Cretaceous and the Nummulitic -periods. With respect to the strata formed -<span class="pagenum" id="Page_295">- 295 -</span> -during all these periods, it is found that their thiknesses, -which away from the Alpine axis may be -measured by hundreds of feet, is along that axis increased -to thousands of feet. The united thickness -of sediments accumulated along this great line of subsidence -between the Permian and Nummulitic periods -probably exceeds 60,000 feet, or ten miles. The subsidence -appears to have been very slow and gradual, -but almost uninterrupted, and the deposition of sediments -seems to have kept pace with the sinking of the -sea-bottom, a fact which is proved by the circumstance -that nearly the whole of these sediments were such -as must have been accumulated in comparatively -shallow water.</p> - -<div class="sidenote">FORMATION OF ALPINE GEOSYNCLINAL.</div> - -<p>By the means we have described there was thus -formed a 'geosynclinal,' as geologists have called it, -that is, a trough-like hollow filled with masses of -abnormally thickened sediments, which had been piled -one upon another during the long periods of time in -which almost uninterrupted subsidence was going on -along the Alpine line of weakness in the earth's crust. -In this way was brought together that enormous accumulation -of materials from which the hard masses of -the Alpine chains were subsequently elaborated, and -out of which the mountain-peaks were eventually -carved by denudation.</p> - -<p>The third stage in this grand work of mountain-making -commenced in the Oligocene period. It consisted -of a series of movements affecting the parts of -<span class="pagenum" id="Page_296">- 296 -</span> -the earth's crust on either side of the line of weakness -which had first exhibited itself in Permian times. By -these movements a series of tangential strains were -produced, which resulted in the violent crushing, folding, -and crumpling of the sedimentary materials composing -the geosynclinal.</p> - -<p>One effect of this action was the violent flexure and -frequent fracture of these stratified masses, which are -now found in the Alpine regions assuming the most -abnormal and unexpected positions and relations to one -another. Sometimes the strata are found tortured and -twisted into the most complicated folds and puckerings; -at others they are seen to be completely inverted, so -that the older beds are found lying upon the newer; -and in others, again, great masses of strata have been -traversed by numerous fractures or faults, the rocks on -either side of which are displaced to the extent of thousands -of feet.</p> - -<p>Another effect of the great lateral thrusts by which -the thick sedimentary masses of the geosynclinal were -being so violently disturbed, was the production of a -great amount of induration and chemical change in -these rocks. Masses of soft clay, of the age of that upon -which London is built, were by violent pressure reduced -to the condition of roofing-slate, similar to that of North -Wales. One of the most important discoveries of modern -times is that which has resulted in the recognition of -the fact of the mutual convertibility of different kinds -of energy. We now know that mechanical force may -<span class="pagenum" id="Page_297">- 297 -</span> -be transformed into heat-force or chemical force; and -of such transformations we find abundant illustrations -in the crushed and crumpled rock-masses of the Alpine -chains.</p> - -<p>Under the influence of these several kinds of force, -not only was extreme consolidation and induration -produced among the rock-masses, but chemical affinity -and crystalline action had the fullest play among the -materials of which they were composed. In many -cases we find the originally soft muds, sands, and shell-banks -converted into the most highly crystalline rocks, -which retain their primary chemical composition, but -have entirely lost all their other original features.</p> - -<div class="sidenote">FORMATION OF ALPINE GEANTICLINAL.</div> - -<p>To the mass of folded, crumpled, and altered strata, -formed from a geosynclinal by lateral pressure, geologists -have given the name of a 'geanticlinal.' The -formation of the Alpine geanticlinal was due to movements -which commenced in the Oligocene period, attained -their maximum in the Miocene, and appear to -have declined and almost altogether died out in the -Pliocene period.</p> - -<p>The movements which resulted in the crushing and -crumpling of the thickened mass of sediments along the -Alpine line of weakness, also gave rise to the formation -of a series of fissures from which volcanic action took -place. These fissures were not, however, formed along -the original line of weakness, for this had been -strengthened and repaired by the deposition of ten-miles' -thickness of sediments upon it, but along new -<span class="pagenum" id="Page_298">- 298 -</span> -fissures opened in directions parallel to the original -lines of weakness, and in areas where a much less -considerable amount of deposition had taken place -since Permian times.</p> - -<p>We have abundant evidence that, just at the period -when those great movements were commencing which -resulted in the formation of the great Alpine and -Himalayan geanticlinal, earth-fissures were being -opened upon either side of the latter from which -volcanic outbursts took place. At the period when the -most violent mountain-forming movements occurred, -these fissures were in their most active condition, and -at this time two great volcanic belts stretched east -and west, on either side of, and parallel to, the great -Alpine chain. The Northern volcanic band was formed -by the numerous vents, now all extinct, in Auvergne, -Central Germany, Bohemia, and Hungary, and was -probably continued in the volcanoes of the Thian Shan -and Mantchouria. The Southern volcanic band was -formed by the numerous vents of the Iberian and -Italian peninsulas, and the islands of the Mediterranean, -and were continued to the eastward by those -of Asia Minor, Arabia, and the North Indian Ocean. -As the earth-movements which produced the geanticlinal -died away, the volcanic energy along these -parallel volcanic bands died away at the same time. -In studying the geology of Central and Southern -Europe, no fact comes out more strikingly than that -of the synchronism between the earth-movements by -<span class="pagenum" id="Page_299">- 299 -</span> -which the geanticlinal of the Alps was formed, and the -volcanic manifestations which were exhibited along -lines of fissure parallel to that geanticlinal. The -earth-movements and the volcanic outbursts both commenced -in the Oligocene period, gradually attained -their maximum in the Miocene, and as slowly declined -in the Pliocene.</p> - -<div class="sidenote">SCULPTURING OF ALPS BY DENUDATION.</div> - -<p>The fourth stage in the great work of mountain-building -in the case of the Alps consisted in the operation -of the denuding forces, the disintegrating action -of rain and frost, the transporting action of rivers and -glaciers, by which the Alpine peaks were gradually -sculptured out of the indurated and altered masses constituting -the geanticlinal. The action of this fourth -stage went on to a great extent side by side with that -of the third stage. So soon as the earth-movements -had brought the submerged sedimentary masses of the -geosynclinal under the action of the surface tides and -currents of the ocean, marine denudation would commence; -and, as the work of elevation went on, the -rock-masses would gradually be brought within the -reach of those more silently-working but far more -effective agents which are ever operating in the higher -regions of the atmosphere. It is impossible to say -what would have been the height of the Alpine chain -if the work of denudation had not to a great extent -kept pace with that of elevation. Only the harder and -more crystalline masses have for the most part escaped -destruction, and stand up in high craggy summits; -<span class="pagenum" id="Page_300">- 300 -</span> -while flanking hills, like the well-known Rigi, are Been -to be composed of conglomerates thousands of feet in -thickness, composed of their disintegrated materials. -It is a remarkable fact, as showing how enormous was -the work of elevation daring the formation of the geanticlinal, -that some of the youngest and least consolidated -rocks of the Nummulitic period are still -found at a height of 11,000 feet in the Alps, and of -16,000 feet in the Himalaya.</p> - -<p>From what has been said, it will be seen that -mountain-chains may be regarded as cicatrised wounds -in the earth's solid crust. A line of weakness first -betrays itself at a certain part of the earth's surface by -fissures, from which volcanic outbursts take place; and -thus the position of the future mountain-chain is determined. -Next, subsidence during many millions of -years permits of the accumulation of the raw materials -out of which the mountain-range is to be formed; subsequent -earth-movements cause these raw materials to -be elaborated into the hardest and most crystalline -rock-masses, and place them in elevated and favourable -positions; and lastly, denudation sculptures from these -hardened rock-masses all the varied mountain forms. -Thus the work of mountain-making is not, as was -formerly supposed by geologists, the result of a simple -upheaving force, but is the outcome of a long and complicated -series of operations.</p> - -<div class="sidenote">ORIGIN OF OTHER MOUNTAIN-CHAINS.</div> - -<p>The careful study of other mountain-chains, especially -those of the American continent, has shown that -<span class="pagenum" id="Page_301">- 301 -</span> -the series of actions which we have described as occurring -in the Alps, took place in the same order in -the formation of all mountain-masses. It is doubtful -whether the line of weakness is always betrayed in -the first instance by the formation along its course of -volcanic fissures. But in all cases we have evidence of -the production of a geosynclinal, which is afterwards, -by lateral pressure, converted into a geanticlinal, and -from this the mountain-chains have been carved by -denudation. Professor Dana has shown that the geosynclinal -of the Appalachian chain was made up of -sediments attaining a thickness of 40,000 feet, or eight -miles; while Mr. Clarence King has shown that a part -of the geosynclinal of the Rocky Mountains was built -up of no less than 60,000 feet, or twelve miles of -strata.</p> - -<p>It has thus been established that a very remarkable -relation exists between the forces by which continental -masses of land are raised and depressed, and mountain-ranges -have been developed along lines of weakness -separating such moving continental masses, and those -more sudden and striking manifestations of energy -which give rise to volcanic phenomena. It is in this -relation between the widespread subterranean energies -and the local development of the same forces at volcanic -vents, that we must in all probability seek for -the explanation of those interesting peculiarities of -the distribution of volcanoes upon the face of the globe -which we have described in a former chapter. The -<span class="pagenum" id="Page_302">- 302 -</span> -parallelism of volcanic bands to great mountain-chains -is thus easily accounted for; and in the same way we -may probably explain the position of most volcanoes -with regard to coast-lines. We have already pointed -out the objections to the commonly-received view that -volcanoes depend for their supplies of water on the -proximity of the ocean. This proximity of the ocean -to volcanic vents we are thus inclined to regard, not -as the cause, but as the effect of the subterranean -action. The positions of both volcanoes and coast-lines -are determined by the limits of those great areas of the -earth's crust which are subjected to slow vertical movements, -often in opposite directions.</p> - -<p>Terrible and striking, then, as are the phenomena -connected with volcanic action, such sudden and violent -manifestations of the subterranean energy must -not be regarded as the only, or indeed the chief, effects -which they produce. The internal forces continually -at work within the earth's crust perform a series of -most important functions in connection with the economy -of the globe, and were the action of these forces -to die out, our planet would soon cease to be fit for the -habitation of living beings.</p> - -<p>There is no fact which the geological student is -more constantly called upon to bear in mind than that -of the potency of seemingly insignificant causes which -continue in constant operation through long periods -of time. Indeed these small and almost unnoticed -agencies at work upon the earth's crust are often found, -<span class="pagenum" id="Page_303">- 303 -</span> -in the long ran, to produce far grander effects than -those of which the action is much more striking and -obvious. It is to the silent and imperceptible action -of atmospheric moisture and frost that the disintegration -of the solid rock-masses must be mainly ascribed; -and the noisy cataract and ocean-billow produce effects -which are quite insignificant compared with those -which must be ascribed to the slight and almost unnoticed -forces. Great masses of limestone are built up -of the remains of microscopic organisms, while the -larger and higher life-forms contribute but little to the -great work of rock-building.</p> - -<div class="sidenote">EFFECTS OF SLOW CONTINENTAL MOVEMENTS.</div> - -<p>In the same way it is to the almost unnoticed -action of the subterranean forces in raising some vast -areas of the earth's crust, in depressing others, and in -bringing about the development of mountain-chains -between them, that we must ascribe a far more important -part in the economy of our globe than to the -more conspicuous but less constant action of volcanoes.</p> - -<p>A few simple considerations will serve to convince -us, not only of the beneficial effects of the action of -the subterranean energies within the earth's crust, but -of the absolute necessity of the continued operation of -those energies to the perpetuation of that set of conditions -by which our planet is fitted to be the habitation -of living beings.</p> - -<p>We have already referred to the prodigious effects -which are constantly being produced around us by the -action of the external forces at work upon the globe. -<span class="pagenum" id="Page_304">- 304 -</span> -The source of these external forces is found in the -movements and changes which are ever going on -within the aqueous and atmospheric media in which -the globe is enveloped. The circulation of the air, -influencing the circulation of the waters in the shape -of clouds, rain, snow, rivers, glaciers, and oceans, causes -the breaking up of even the hardest rock-masses, and -the continual removal of their disintegrated fragments -from higher to lower levels. This work goes on with -more or less regularity over every part of the land -raised above the level of the ocean, but the rate of -destruction in the higher regions of the atmosphere is -far more rapid than at lower levels. Hence the circulating -air and water of the globe are found to be -continually acting as levellers of the land-masses of the -earth.</p> - -<p>It is by no means a difficult task to calculate the -approximate rate at which the various continents and -islands are being levelled down, and such calculations -prove that in a very few millions of years the existing -forces operating upon the earth's surface would reduce -the whole of the land-masses to the level of the ocean.</p> - -<p>But a little consideration will convince us that the -circulation of the air and waters of the globe are themselves -dependent upon the existence of those irregularities -of the land-surfaces which they are constantly -tending to destroy. Without elevated mountain ridges -the regular condensation of moisture, and its collection -and distribution in streams and rivers over every part -<span class="pagenum" id="Page_305">- 305 -</span> -of the land surfaces, could not take place. Under these -circumstances the unchecked evaporation of the oceanic -waters would probably go on, till the proportion of water-vapour -increased to such an extent in the atmosphere -as effectually to destroy those nicely-balanced conditions -upon which the continued existence of both vegetable -and animal life depend.</p> - -<p>But the repeated upward and downward movements -which have been shown to be going on in the -great land-masses of the globe, giving rise in turns to -those lateral thrusts and tangential strains to which -mountain-chains owe their formation, afford a perfect -compensation to the action of the external forces ever -operating upon the earth's surface.</p> - -<p>If, however, the uncompensated effect of the external -forces acting on the earth's crust is calculated -to bring about the destruction of those conditions upon -which the existence of life depends, the uncompensated -effect of the internal forces acting on the earth's crust -are fraught with at least equal dangers to those necessary -conditions.</p> - -<div class="sidenote">CONTRAST BETWEEN THE EARTH AND MOON.</div> - -<p>In our nearest neighbour among the planets—the -moon—the telescope has revealed to us the existence -of a globe, in which the internal forces have not been -checked and controlled by the operation of any external -agencies—for the moon appears to be destitute of both -atmosphere and water.</p> - -<p>Under these circumstances we find its surface, as we -might expect, to be composed of rocks which appear -<span class="pagenum" id="Page_306">- 306 -</span> -to be entirely of igneous origin; the mountain-masses, -unworn by rain or frost, river or glacier, being of most -prodigious dimensions as compared with those of our -own globe, while no features at all resembling valleys, -or plains, or alluvial flats are anywhere to be discerned -upon the lunar surface.</p> - -<p>But by the admirable balancing of the external and -internal forces on our own globe, the conditions necessary -to animal and vegetable existence are almost constantly -maintained, and those interruptions of such -conditions, produced by hurricanes and floods, by volcanic -outbursts and earthquakes, may safely be regarded as -the insignificant accidents of what is, on the whole, a -very perfectly working piece of machinery.</p> - -<p>The ancients loved to liken the earth to a living -being—the macrocosm of which man was the puny -representative or microcosm; and when we study the -well-adapted interplay of the forces at work upon the -earth's crust, both from within and without, the analogy -seems a scarcely strained one. In the macrocosm and -the microcosm alike, slight interferences with the regular -functions occasionally take place, and both of them -exhibit the traces of a past evolution and the germs of -an eventual decay.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_307">- 307 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XI">CHAPTER XI.<br /> - -<span class="smaller">WHAT VOLCANOES TEACH US CONCERNING THE NATURE OF -THE EARTH'S INTERIOR.</span></h2> -</div> - - -<p class="p0">In entering upon any speculations or enquiries concerning -the nature of the interior of our globe, it is necessary -before all things that we should clearly realise in -our minds how small and almost infinitesimal is that -part of the earth's mass which can be subjected to -direct examination. The distance from the surface to -the centre of our globe is nearly 4,000 miles, but the -deepest mines do not penetrate to much more than half -a mile from the surface, and the deepest borings fall far -short of a mile in depth. Sometimes, it is true, the -geologist finds means for drawing inferences as to the -nature of the rocks at depths of ten or fifteen miles -below the surface; but the last-named depth must be -regarded as the utmost limit of that portion of our -globe which can be made the object of direct observation -and study. This thin exterior film of the earth's -mass, which the geologist is able to investigate, we call -the 'crust of the globe'; but it must be remembered -that in using this term, it is not intended to imply that -<span class="pagenum" id="Page_308">- 308 -</span> -the outer part of our globe differs in any essential respect -from the interior. The term 'crust of the globe' is -employed by geologists as a convenient way of referring -to that portion of the earth which is accessible to their -observation.</p> - -<p>But if we are unable to make direct investigations -concerning the nature of the internal portions of the -globe, there are nevertheless a number of facts from -which we may draw important inferences upon the -subject. These facts and the inferences based upon -them we shall now proceed to consider.</p> - -<p>First in importance among these we may mention -the results which have been obtained by weighing our -globe. Various methods have been devised for accomplishing -this important object, and the conclusions -arrived at by different methods agree so closely with -one another, that there is no room for doubt as to the -substantial accuracy of those results. It may be taken -as proved beyond the possibility of controversy that -our globe is equal in weight to five and a half globes -of the same size composed of water, or, in other words, -that the average density of the materials composing the -globe is five and a half times as great as that of water.</p> - -<p>Now the density of the materials which compose the -crust of the globe is very much less than this, varying -from about two-and-one-third to three times that of -water. Hence we are compelled to conclude that the -interior portions of the globe are of far greater density -than the exterior portions; that, as a matter of fact, -<span class="pagenum" id="Page_309">- 309 -</span> -the mass of the globe is composed of materials having -twice the density of the rocks exposed at the surface.</p> - -<div class="sidenote">DENSITY OF EARTH'S INTERIOR.</div> - -<p>It has been sometimes argued that as all materials -under intense pressure appear to yield to an appreciable -extent, and to allow their particles to be packed into a -smaller compass, we may find in this fact an explanation -of the great density of the internal parts of the globe. -It has in fact been suggested that under the enormous -pressure which must be exerted by masses of rock -several thousand feet in thickness, the materials of -which our earth is composed may be compelled to pack -themselves into less than one-half the compass which -they occupy at the surface. But the ascription of such -almost unlimited compressibility to solid substances -can be supported neither by experiment nor analogy. -Various considerations point to the probability that -solid bodies yield to pressure up to a certain limit and -no farther, and that when this limit is reached an increase -in pressure is no longer attended with a reduction -in bulk.</p> - -<p>If then we are compelled to reject the idea of the -unlimited compressibility of solid substances, we must -conclude that the interior portions of our globe are -composed of <i>materials of a different kind</i> from those -which occur in its crust. And this conclusion, as we -shall presently see, is borne out by a number of -independent facts.</p> - -<p>The study of the materials ejected from volcanic -vents proves that even at very moderate depths there -<span class="pagenum" id="Page_310">- 310 -</span> -exist substances differing greatly in density, as well -as in chemical composition. The lightest lavas have a -specific gravity of 2·3, the heaviest of over 3. And that -materials of even greater density are sometimes brought -by volcanic action from the earth's interior, we have -now the clearest proofs.</p> - -<div class="sidenote">RELATION BETWEEN EARTH AND OTHER PLANETS.</div> - -<p>But in considering a question of this kind, it will -be well to remember that analogy may furnish us with -hints upon the subject which may prove to be by no -means unimportant. There is no question upon which -modern science has wrought out a more complete revolution -in our ideas, than that of the relation of our -earth to the other bodies of the universe. We know, -as the result of recent research, that our globe is one of -a great family of bodies, moving through space in -similar paths and in obedience to the same laws. A -hundred years ago the primary and secondary planets -of the solar system could be almost numbered upon -the fingers; now we recognise the fact that they -exist in countless millions, presenting every variety -of bulk from masses 1,400 times as large as our -earth down to the merest planetary dust. Between the -orbits of Mars and Jupiter, more than 200 small planets -have been recognised as occurring, and every year -additions are made to the number of these asteroids. -Comets have now been identified with streams of such -planetary bodies, of minute size, moving in regular -orbits through our system. The magnificent showers -of 'shooting-stars' have been proved to be caused by -<span class="pagenum" id="Page_311">- 311 -</span> -the passage of the earth through such bands of travelling -bodies, and 'the zodiacal light' finds its most -probable explanation in the supposition that the sun -is surrounded by a great mass of such minute planets. -Every increase in the power of the telescope reveals to -us the existence of new secondary planets or moons, revolving -about the primaries; and the wonderful system -of the Saturnian rings is now explained by the proved -existence of great streams of such secondary planets -circling around it. The solar system was formerly conceived -of as a vast solitude through which a few gigantic -bodies moved at awful distances from one another. Now -we know that the supposed empty void is traversed by -countless myriads of bodies of the most varied dimensions, -all moving in certain definite paths, in obedience -to the same laws, ever acting and reacting upon each -other, and occasionally coming into collision.</p> - -<p>There are not wanting further facts to prove that -the other planets are like our own in many of their -phenomena and surroundings. In some of them atmospheric -phenomena have been detected, such as the formation -of clouds and the deposition of snow, so that the -external forces at work on our globe act upon them -also. And that internal forces, like those we have been -considering in the case of our earth, are at work in our -neighbours, is proved by the great solar storms and the -condition of the moon's surface.</p> - -<p>But the results of spectrum-analysis in recent years -have furnished new facts in proof of the close relationship -<span class="pagenum" id="Page_312">- 312 -</span> -of our earth to the numerous similar bodies by -which it is surrounded. So far as observation has yet -gone we have reason for believing that not only the -members of the solar system, but the more distant bodies -of the universe, are all composed of the same elementary -substances as those which enter into the composition -of our globe.</p> - -<p>The most satisfactory information concerning the -composition and nature of other planetary bodies is -derived from the study of those small planets which -occasionally come into collision with our globe, and -which have their own proper motion in space thereby -arrested. These meteorites, as such falling planetary -bodies are called, have justly attracted great attention, -and their fragments are treasured as the most valuable -objects in our museums.</p> - -<div class="sidenote">COMPOSITION OF METEORITES.</div> - -<p>The first fact concerning these meteorites, which it -is necessary to notice, is that they are composed of the -same chemical elements as occur in the earth's crust. -No element has yet been found in any meteorite which -was not previously known as existing in the earth, and -of the sixty-five or seventy known terrestrial elements -no less than twenty-two have already been detected in -meteorites.</p> - -<p>There are, however, a dozen elements which occur -in overwhelming proportions in the earth's crust. We -shall probably not be going too far in saying that these -twelve elements—namely, oxygen, silicon, aluminium, -calcium, magnesium, sodium, potassium, iron, carbon, -<span class="pagenum" id="Page_313">- 313 -</span> -hydrogen, sulphur, and chlorine—make up amongst -them not less than 999 out of 1,000 parts of the earth's -crust, and that all the other fifty or sixty elements are -80 comparatively rare that they do not constitute when -taken altogether more than one part in 1,000 of the -rocks of the globe. Now all of these twelve common -terrestrial elements occur in meteorites, and the fact -that the rarer terrestrial elements have not as yet been -found in them will not surprise anyone, who remembers -how small is the bulk of all the specimens of these -meteorites existing in our museums.</p> - -<p>We have hitherto insisted on the points of resemblance -in the chemical composition of meteorites and -that of the rocks of the globe, but we shall now have -to indicate some very important points in which they -differ.</p> - -<p>While in the rocks composing the earth's crust -oxygen forms one-half of their mass, and silicon another -quarter, we find that in the meteorites these elements, -though present, play a much less important part. The -most abundant element in the meteorites is iron; and -nickel, chromium, cobalt, manganese, sulphur, and -phosphorus, are much more abundant in these extra-terrestrial -bodies than they are in the earth's crust.</p> - -<p>We have already referred to the remarkable fact -that in our earth's crust nearly all the other elementary -substances are found combined in the first instance with -oxygen, and that most rocks consist of the oxide of -silicon combined with the oxides of various metals. -<span class="pagenum" id="Page_314">- 314 -</span> -But this is by no means the case with the meteorites. -In them we find metals like iron, nickel, cobalt, &c., in -their uncombined condition, and forming alloys with -one another. The same and other metals also occur -in combination with carbon, phosphorus, chlorine, and -sulphur, and some of the substances thus formed are -quite unknown among terrestrial rocks. Compounds of -the oxide of silicon with the oxides of the metals such -as form the mass of the crust of the globe do occur in -meteorites, but they play a much less important part -than in the case of the terrestrial rocks.</p> - -<p>Among the substances found in meteorites are -several which do not exist among the terrestrial rocks—some, -indeed, which it seems impossible to conceive of -as being formed and preserved under terrestrial conditions. -Among these we may mention the phosphide -of iron and nickel (Schreibersite), the sulphide of -chromium and iron (Daubréelite), the protosulphide of -iron (Troilite), the sulphide of calcium (Oldhamite), -the protochloride of iron (Lawrencite), and a peculiar -form of crystallised silica, called by Professor Maskelyne -'Asmanite.'</p> - -<div class="sidenote">DIFFERENT KINDS OF METEORITES.</div> - -<p>There are other phenomena exhibited by meteorites -which indicate that they must have been formed under -conditions very different to those which prevail upon -the earth's surface. Thus we find that fused iron and -molten slag-like materials have remained entangled with -each other, and have not separated as they would do -if a great body like the earth were near to exercise the -<span class="pagenum" id="Page_315">- 315 -</span> -varying force of gravity upon the two classes of substances. -Again, meteorites are found to have absorbed -many times their bulk of hydrogen gas, and to exhibit -peculiarities in their microscopic structure which can -probably be only accounted for when we remember -that they were formed in the interplanetary spaces, far -away from any great attracting body.</p> - -<p>But in recent years a number of very important -facts have been discovered which may well lead us to -devote a closer attention to the composition and structure -of meteorites. It has been shown, on the one -hand, that some meteorites contain substances precisely -similar to those which are sometimes brought from the -earth's interior during volcanic outbursts; and, on the -other hand, there have been detected, among some of -the ejections of volcanoes, bodies which so closely -resemble meteorites that they were long mistaken for -them. Both kinds of observation seem to point to the -conclusion that the earth's interior is composed of -similar materials to those which we find in the small -planets called meteorites.</p> - -<p>M. Daubrée has proposed a very convenient classification -for meteorites, dividing them into the following -four groups:—</p> - -<p>I. <i>Holosiderites</i>; consisting almost entirely of -metallic iron, or of iron alloyed with nickel, stony -matter being absent; but sulphides, phosphides, and -carbides of several metals are often diffused through -the mass. The polished surfaces of these meteoric -<span class="pagenum" id="Page_316">- 316 -</span> -irons, when etched with acid, often exhibit a remarkable -crystalline structure.</p> - -<p>II. <i>Syssiderites</i>; in which a network of metallic -iron encloses a number of granular masses of stony -materials.</p> - -<p>III. <i>Sporadosiderites</i>; which consist of a mass of -stony materials, through which particles of metallic -iron are disseminated.</p> - -<p>IV. <i>Asiderites</i>; containing no metallic iron, but -consisting entirely of stony materials.</p> - -<p>There are, besides the meteorites belonging to these -principal groups, a few of peculiar and exceptional -composition, which we need not notice further for our -present purpose.</p> - -<p>From the above classification it will be seen that -most meteorites consist of a mixture in varying proportions -of metallic and stony materials. Sometimes the -metallic constituents are present in greater proportions -than the stony, at other times the stony materials predominate, -while occasionally one or other of these -elements may be wholly wanting.</p> - -<p>The stony portions of meteorites, upon careful -examination, prove to be built up of certain minerals, -agreeing in their chemical composition and their crystalline -forms with those which occur in the rocks of -the earth's crust. Among the ordinary terrestrial -minerals occurring in the stony portions of meteorites, -we may especially mention olivine, enstatite, augite, -anorthite, chromite, magnetite, and pyrrhotite.</p> - -<p><span class="pagenum" id="Page_317">- 317 -</span></p> - -<div class="sidenote">METEORITES AND ULTRA-BASIC ROCKS.</div> - -<p>The minerals which occur in meteorites are in -every case such as are found in the more basic volcanic -rocks—quartz, and the acid felspars, with the other -minerals which occur in acid rocks, being entirely -absent in the 'extra-terrestrial' rocks.</p> - -<p>Now, besides the three great classes of lavas which -we have described as being ejected from volcanic vents, -there are some rarer materials occasionally brought -from the earth's interior by the same agency, that -present a most wonderful resemblance to the stony -portions of meteorites. These materials we may call -'ultra-basic rocks.' Their specific gravity is very high, -usually exceeding 3, and they contain a very low percentage -of silica; on the other hand, the proportion -of iron and magnesia is often much greater than in -ordinary terrestrial rocks. But the most remarkable -fact about these ultra-basic rocks is, that they are -almost entirely composed of the minerals which occur -in meteorites; namely, olivine, enstatite, augite, -anorthite, magnetite, and chromite.</p> - -<p>The ultra-basic rocks often occur under very peculiar -conditions. Sometimes they are found forming ordinary -volcanic protrusions through the sedimentary rocks. -The rocks named pikrites, lherzolites, dunites, &c., -are examples of such igneous protrusions composed of -these ultra-basic materials, and probably all the true -serpentines are rocks of the same class which have -absorbed water and undergone great alteration. The -ultra-basic rocks sometimes contain platinum and other -<span class="pagenum" id="Page_318">- 318 -</span> -metals in the free or uncombined state. But not -unfrequently we find among the ordinary ejections of -volcanoes, nodules and fragments of such ultra-basic -materials, which have clearly been carried up with the -other lavas from great depths in the earth's crust. -Thus in Auvergne, the Eifel, Bohemia, Styria, and -many other volcanic districts, the basaltic lavas and -tuffs are found to contain nodules composed of the -minerals which are so highly characteristic of meteorites. -Such nodules, too, often form the centres of -the volcanic bombs which are thrown out of craters -during eruptions.</p> - -<p>We thus see that materials identical in composition -and character with the stony portions of meteorites, -exist within the earth's interior, and are thrown out on -its surface by volcanic action. A still more interesting -discovery has been made in recent years; namely, that -materials similar to the metallic portion of meteorites, -and consisting of nickeliferous iron, also occur in deep-seated -portions of the earth's crust, and are brought to -the surface during periods of igneous activity.</p> - -<p>In the year 1870, Professor Nordenskiöld made a -most important discovery at Ovifak, on the south side -of the Island of Disko, off the Greenland coast. On -the shore of the island a number of blocks of iron were -seen, and the chemical examination of these proved -that, like ordinary metallic meteorites, they consisted -of iron alloyed with nickel and cobalt.</p> - -<p><span class="pagenum" id="Page_319">- 319 -</span></p> - -<div class="sidenote">IRON-MASSES OF OVIFAK.</div> - -<p>Now, when the facts concerning the masses of native -iron of Ovifak were made known, the first and most -natural explanation which presented itself to every -mind was, that these were a number of meteorites -which at some past period had fallen upon the earth's -surface.</p> - -<div class="figcenter" id="fig87" style="width: 426px;"> - <img src="images/fig87.png" width="426" height="263" alt="" /> - -<table summary="MapKey"> -<tr> - <td><img src="images/fig87_met_irn.png" width="59" height="33" alt="" /></td> - <td class="tdl smaller">Metallic iron.</td> -</tr> -<tr> - <td><img src="images/fig87_opq_xls.png" width="56" height="33" alt="" /></td> - <td class="tdl smaller">Opaque crystals of magnetite (black oxide of iron).</td> -</tr> -<tr> - <td><img src="images/fig87_trn_xls.png" width="56" height="32" alt="" /></td> - <td class="tdl smaller">Transparent crystals of felspar, augite, and olivine.</td> -</tr> -</table> - -<div class="figcaption"><span class="smcap">Fig. 87.—Section of basalt from Ovifak, Greenland, with -particles of metallic iron diffused through its mass.</span></div> -</div> - -<p>But a further examination of the locality revealed -a number of facts which, as Professor Steenstrup -pointed out, it is very difficult to reconcile with the -theory that the Ovifak masses of iron are of meteoric -<span class="pagenum" id="Page_320">- 320 -</span> -origin. The district of Western Greenland, where -these masses were discovered, has been the scene of -volcanic outbursts on the grandest scale during the -Miocene period. In close proximity to the great iron -masses, there are seen a number of basaltic dykes; -and, when these dykes are carefully examined, the -basaltic rock of which they are composed is seen to be -full of particles of metallic iron. In <a href="#fig87">fig. 87</a>, we have -a drawing made from a section of the Ovifak basalts -magnified four or five diameters. The rock-mass is -seen to be composed of black, opaque magnetite, and -transparent crystals of augite, labradorite, olivine, &c.; -while, through the whole, particles of metallic iron are -found entangled among the different crystals in the -most remarkable manner.</p> - -<p>It has been suggested that this singular rock might -have been formed by a meteorite falling, in Miocene -times, into a lava-stream in a state of incandescence. -But the relation of the metallic particles to the stony -materials is such as to lend no support whatever to -this rather strained hypothesis.</p> - -<p>A careful study of all the facts of the case by -Lawrence Smith, Daubrée, and others well acquainted -with the phenomena exhibited by meteorites, has led to -the conclusion that the large iron-masses of Ovifak, as -well as the particles of metallic iron diffused through -the surrounding basalts, are all of terrestrial origin, and -have been brought by volcanic action from the earth's -interior. It is probable that, just as we find in many -<span class="pagenum" id="Page_321">- 321 -</span> -basaltic lavas nodules of ultra-basic materials similar to -the stony parts of meteorites, so in these basalts of -Ovifak we have masses of iron alloyed with nickel, -similar to the metallic portions of meteorites. Both -the stony and metallic enclosures in the basalt are in -all probability derived from deeper portions of the -earth's crust. By the weathering away of the basalt -of Ovifak, the larger masses of metallic iron have been -left exposed upon the shore where they were found.</p> - -<p>There are a number of other facts which seem to -support this startling conclusion. Thus it has been -shown by Professor Andrews that certain basalts in our -own islands contain particles of metallic iron of microscopic -dimensions, and it is not improbable that some -of the masses of nickeliferous iron found in various -parts of the earth's surface, which have hitherto been -regarded as meteorites, are, like those of Ovifak, of -terrestrial origin.</p> - -<div class="sidenote">MATERIALS FILLING METALLIC-VEINS.</div> - -<p>Another piece of evidence pointing in the same -direction, is derived from those great fissures communicating -with the interior of our globe which become filled -with metallic minerals, and are known to us as mineral-veins. -In these mineral-veins the native metals, their -alloys, and combinations of these with sulphur, chlorine, -phosphorus, &c., are frequently present. But oxides of -the metals, except as products of subsequent alteration, -occur far less frequently than in the earth's crust generally. -Hence we are led to conclude that the substances -which in the outer part of the earth's crust always exist -<span class="pagenum" id="Page_322">- 322 -</span> -in combination with oxygen, are at greater depths in a -free and uncombined condition.</p> - -<p>Nor is it a circumstance altogether unworthy of -attention that the researches of Mr. Norman Lockyer -and other astronomers, based on the known facts of the -relative densities of the several members of the solar -system, and the ascertained relations of the different -solar envelopes, have led to conclusions closely in accord -with those arrived at by geologists. These researches -appear to warrant the hypothesis that the interior of -our globe consists of metallic substances uncombined -with oxygen, and that among these metallic substances -iron plays an important part. Our globe, as we know, -is a great magnet, and the remarkable phenomena of -terrestrial magnetism may also not improbably find -their explanation in the fact that metallic iron forms -80 large a portion of the earth's interior.</p> - -<p>The interesting facts which we have been considering -may be made clearer by the accompanying diagram -(<a href="#fig88">fig. 88</a>). The materials ejected from volcanic vents -(lavas) are in almost all cases compounds of silicon and -the various metals with oxygen. In the lighter or acid -lavas oxygen constitutes one-half of their weight, and -the proportion of metals of the iron-group is very small. -As we pass to the heavier intermediate and basic lavas, -we find the proportion of oxygen diminishing, and the -metals of the alkaline earths (magnesium and calcium) -with the metals of the iron-group increasing, in quantity. -In the small and interesting group of the ultra-basic -lavas the proportion of oxygen is comparatively -small, and the proportion of magnesium and iron very -high. So much for the terrestrial rocks.</p> - -<div class="figcenter" id="fig88" style="width: 613px;"> - <div class="figcaption"><span class="smcap">Fig. 88.—Diagram illustrating the relation between the - Terrestrial and the Extra-Terrestrial Rock.</span><br /> - <img src="images/fig88.png" width="613" height="424" alt="" /></div> -</div> - -<p><span class="pagenum" id="Page_323">- 323 -</span></p> - -<div class="sidenote">TERRESTRIAL AND EXTRA-TERRESTRIAL ROCKS.</div> - -<p>Now let us turn our attention to the extra-terrestrial -rocks or those found in meteorites. The Asiderites are -quite identical in composition with the ultra-basic lavas -of our globe, but in the Sporadosiderites and the Syssiderites -we find the proportion of oxygen rapidly diminishing, -and that of metallic iron increasing. Finally, -in the Holosiderites the oxygen entirely disappears, -and the whole mass becomes metallic.</p> - -<p>From the Holosiderites at one end of the chain to -the add lavas at the other, we find there is a complete -and continuous series; the rocks of terrestrial origin -overlapping, in their least oxydized representatives, the -most highly oxydized representatives of the extra-terrestrial -rocks. But the discovery at Ovifak of the iron-masses, -and the basalts with iron disseminated, has -afforded another very important link, placing the terrestrial -and extra-terrestrial rocks in closer relations -with one another.</p> - -<p>All these facts appear to point to the conclusion -that the earth's interior consists of metallic substances -either quite uncombined or simply alloyed with one -another, and among these iron is very conspicuous by -its abundance. The outer crust, which is probably of -no great thickness, contains an enormous proportion of -oxygen and silicon combined with the materials which -constitute the interior portions of our globe. It may -<span class="pagenum" id="Page_324">- 324 -</span> -be, as has been suggested by astronomers, that our -earth consisted at one time of a solid metallic mass -surrounded by a vaporous envelope of metalloids, and -that the whole of the latter, with the exception of the -constituents of the atmosphere and ocean, have gradually -entered into combination with the metals of the -nucleus to form the existing crust of the globe. But -of this period the geologist can take no cognisance. -The records which he studies evidently commenced at -a long subsequent period, when the conditions prevailing -at the earth's surface differed but little, if at all, -from those which exist at the present day. Equally little -has the geologist to do with speculations concerning -a far distant future when, as some philosophers have -suggested, the work of combination of the waters and -atmosphere of the earth's surface with the metallic -substances of its interior shall be completed, and our -globe, entirely deprived of its fluid envelopes, reduced to -the condition in which we find our satellite, the moon.</p> - -<hr class="tb" /> - -<div class="sidenote">PHYSICAL CONDITION OF EARTH'S INTERIOR.</div> - -<p>There is another class of enquiries concerning the -earth's interior to which the attention of both geologists -and astronomers has long been directed—that, namely, -which deals with the problem of the <i>physical condition</i> -of the interior of our globe.</p> - -<p>The fact that masses of molten materials are seen -at many points of the earth's surface to issue from -figures in the crust of our globe, seems at first sight -to find a simple explanation if we suppose our planet to -<span class="pagenum" id="Page_325">- 325 -</span> -consist of a fluid central mass surrounded by a solid -crust. Hence we find that among those who first thought -upon this subject, this hypothesis of a liquid centre -and a solid crust was almost universally accepted. This -hypothesis was supposed to find further support in -the fact that, as we penetrate into the earth's crust by -mines or boring operations, the temperature is found -to continually increase. It was imagined, too, that -this condition of our planet would best agree with the -requirements of the nebular hypothesis of Laplace, -which explains the formations and movements of the -bodies of the solar system by the cooling down of -a nebulous mass.</p> - -<p>But a more careful and critical examination of the -question has led many geologists and astronomers to -reject the hypothesis that the earth consists of a great -fluid mass surrounded by a comparatively thin shell -of solid materials.</p> - -<p>Volcanic outbursts and earthquake tremors, though -so terrible and destructive to man and his works, are -but slight and inconsiderable disturbances in a globe of -such vast dimensions as that on which we live. The -condition of the crust of the globe is, in spite of volcanic -and earthquake manifestations, one of general stability; -and this general stability has certainly been maintained -during the vast periods covered by the geological -record. Such a state of things seems quite irreconcilable -with the supposition that, at no great depth -from the surface, the whole mass of the globe is in a -<span class="pagenum" id="Page_326">- 326 -</span> -liquid condition. If, on the other hand, it be supposed -that the solid crust of the globe is several hundreds of -miles in thickness, it is difficult to understand how the -local centres of volcanic activity could be supplied -from such deep-seated sources.</p> - -<p>There are other facts which seem equally irreconcilable -with the hypothesis of a fluid centre and a thin -solid crust in our globe. If all igneous products were -derived from one central reservoir, we might fairly -expect to find a much greater uniformity of character -among those products than really exists. But in some -cases, materials of totally different composition are -ejected at the same time from closely adjoining volcanic -districts. Thus in Hungary and Bohemia, as we -have seen, lavas of totally different character were being -extruded during the Miocene period. In the island of -Hawaii, as Professor Dana has pointed out, igneous -ejections have taken place at a crater 14,000 feet above -the sea-level, while a closely adjoining open vent at a -level 10,000 feet lower exhibited no kind of sympathy -with the disturbance. Whatever may be the cause of -volcanic action, it seems clear that it does not originate -in a universal mass of liquefied material situated at no -great depth from the earth's surface.</p> - -<p>The conclusions arrived at by astronomers and -physicists is one quite in accord with those which -geologists have reached by totally different methods. -It is now very generally admitted that if the earth -were not a rigid mass, its behaviour under the attract -<span class="pagenum" id="Page_327">- 327 -</span> -live influences of the surrounding members of the solar -system would be very different to what is found to be -the case.</p> - -<div class="sidenote">ARGUMENTS AGAINST LIQUID INTERIOR.</div> - -<p>That the earth is in a solid condition to a great -depth from the surface, and possibly quite to the centre, -is a conclusion concerning which there can be little -doubt; and in the next chapter we shall endeavour to -show that such a condition of thirds is by no means -incompatible with those manifestations of internal -energy, the phenomena of which we are considering in -this work. The question, therefore, of the complete -solidity of our globe, or of its consisting of a solid and -a liquid portion, is one of speculative interest only, and -is in no way involved in our investigations concerning -the nature and origin of volcanic activity. We -may conclude this chapter by enumerating the several -hypotheses which have at different times been maintained -concerning the nature of the interior of our globe.</p> - -<p><i>First.</i> It has been suggested that the earth consists -of a fluid or semi-fluid nucleus surrounded and -enclosed in a solid shell. Some have maintained this -shell to be of such insignificant thickness, as compared -with the bulk of the interior liquid mass, that portions -of the latter are able to reach the earth's surface through -movements and fractures of the outer shell, and that in -this manner volcanic manifestations originate. Others, -impressed with the general stability and rigidity of the -globe as a whole, have maintained that the outer solid -shell must have a very considerable thickness, amounting -<span class="pagenum" id="Page_328">- 328 -</span> -probably to not less than several hundreds of miles. -But through a shell of such thickness it is difficult to -conceive of the liquid masses of the interior finding -their way to the surface, and those who have held this -view are driven to suggest some other means by which -local developments of volcanic action might be brought -about.</p> - -<p><i>Secondly.</i> Some physicists have asserted that a globe -of liquid matter radiating its heat into space, would tend -to solidify both at the surface and the centre, at the same -time. The consequence of this action would be the -production of a sphere with a solid external shell and -a solid central nucleus, but with an interposed layer in -a fluid or semi-fluid condition. It has been pointed -out that if we suppose the solidification to have gone -so far, as to have caused the partial union of the interior -nucleus and the external shell, we may conceive -a condition of things in which the stability and -rigidity is sufficient to satisfy both geologists and -astronomers, but that in still unsolidified pockets or -reservoirs, filled with liquefied rock, between the nucleus -and the shell, we should have a competent cause -for the production of the volcanic phenomena of the -globe. In this hypothesis, however, it is assumed -that the cooling at the centre and the surface of the -globe would go on at such rates that the reservoirs of -liquid material would be left at a moderate depth from -the surface, so that easy communication could be -opened between them and volcanic vents.</p> - -<p><span class="pagenum" id="Page_329">- 329 -</span></p> - -<div class="sidenote">REVIEW OF THE SEVERAL HYPOTHESES.</div> - -<p><i>Thirdly.</i> It has been maintained that the earth -may have become perfectly solid from the centre to -the surface. Those who hold this view endeavour to -account for the phenomena of volcanoes in one of two -ways. It may be, they say, that the deep-seated rock-masses, -though actually solid, are in a state of <i>potential</i> -liquidity; that though reduced to a solid state by -the intense pressure of the superincumbent masses, yet -such is the condition of unstable equilibrium in the -whole mass, that the comparatively slight movements -and changes taking place at the earth's surface suffice -to bring about the liquefaction of portions of its crust -and consequent manifestations of volcanic energy. But -It may be, as other supporters of the doctrine of the -earth's complete solidity have maintained, that the -phenomena of volcanoes have no direct connection with -a supposed incandescent condition of our planet at all, -and that there are chemical and mechanical forces at -work within our globe which are quite competent to -produce at the surface all those remarkable phenomena -which we identify with volcanic action.</p> - -<p>From this summary of the speculative views which -have been entertained upon the subject of the physical -condition of the earth's interior, it will be clear that at -present we have not sufficient evidence for arriving at -anything like a definite solution of the problem. The -conditions of temperature and pressure which exist in -the interior of a globe of such vast dimensions as our -earth, are so far removed from those which we can -<span class="pagenum" id="Page_330">- 330 -</span> -imitate in our experimental enquiries, and it is so -unsafe to push the application of laws arrived at by the -latter to the extreme limits required by the former, -that we shall do well to pause before attempting to -dogmatise on such a difficult question.</p> - -<p>In the next chapter we shall endeavour to grapple -with a somewhat more hopeful task, to point out how -far observation and experiment have enabled us to -offer a reasonable explanation of the wonderful series -of phenomena which are displayed during outbursts of -volcanic activity.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_331">- 331 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XII">CHAPTER XII.<br /> - -<span class="smaller">THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN -THE CAUSES OF VOLCANIC ACTION.</span></h2> -</div> - - -<p class="p0">Every completed scientific investigation must consist -of four series of operations. In the first of these an -attempt is made to collect the whole of the facts -bearing on the question, by means of observation and -experiment; the latter being only observation under -conditions determined by ourselves. In the second -stage of the enquiry, the attention is directed to classifying -and grouping the isolated facts, so as to determine -their bearings upon one another, and the general -conclusions to which they appear to point. In the -third stage, it is sought to frame an hypothesis which -shall embrace all the observed facts, and shall be in -harmony with the general conclusions derived from -them. In the fourth stage, this hypothesis is put to -the most rigid test; comparing the results which must -follow, if it be true, with the phenomena actually observed, -and rejecting or amending our hypothesis accordingly. -Every great scientific theory has thus been -<span class="pagenum" id="Page_332">- 332 -</span> -established by these four processes—observation, -generalisation, hypothesis, and verification.</p> - -<p>The enquiry concerning the nature and causes of -volcanic action is far from being a completed one. It -is true that many hypotheses upon the subject have -been framed, but in too many instances these have not -been based on accurate observations and careful generalisations, -and can be regarded as little better than -mere guesses. Indeed, the state of the enquiry at the -present time would seem to be as follows. Although -much remains to be done in the direction both of -observation and experiment, the main facts of the case -have been established upon irrefragable evidence. The -classification and comparison of these facts have led to -the recognition of certain laws, which seem to embrace -all the known facts. To account for these facts and -their demonstrated relations to one another, certain -tentative hypotheses have been suggested; but in no -case can it be truly said that these latter have so far -stood the test of exact enquiry as to deserve to rank -as demonstrated truths. A complete and consistent -theory of volcanic action still remains to be discovered.</p> - -<div class="sidenote">VALUE AND LIMITS OF HYPOTHESES.</div> - -<p>In accordance with the plan which we have sketched -out for ourselves at the commencement of this work, -we shall aim at following what has been the order of -investigation and discovery in our study of volcanic -action; and in this concluding chapter we shall indicate -the different hypotheses by which it has been -proposed to account for the varied phenomena, which -<span class="pagenum" id="Page_333">- 333 -</span> -we have discussed in the preceding pages, and their -remarkable relations to one another. We shall endeavour, -in passing, to indicate how far these several -hypotheses appear to be probable, as satisfying a larger -or smaller number of those conditions of the problem -which have been established by observation, experiment, -and careful reasoning; but we shall at the same -time carefully avoid such advocacy of any particular -views as would tend to a prejudgment of the question. -Hypothesis is, as we have seen, one of the legitimate -and necessary operations in scientific investigation. It -only becomes a dangerous and treacherous weapon -when it is made to precede rather than to follow observation -and experiment, or when being regarded with -paternal indulgence, an attempt is made to shield -it from the relentless logic of facts. Good and bad -hypotheses must be allowed to 'grow together till the -harvest;' such as are unable to accommodate themselves -to the surrounding conditions imposed by -newly-discovered facts and freshly-established laws will -assuredly perish; and in this 'struggle for existence' -the true hypothesis will in the end survive, while the -false ones perish.</p> - -<p>It may well happen, however, that among the -hypotheses which have up to the present time been -framed, none will be found to entirely satisfy all the -conditions of the problem. New discoveries in physics -and chemistry have suggested fresh explanations of -volcanic phenomena in the past, and may continue to -<span class="pagenum" id="Page_334">- 334 -</span> -do so in the future; and the true theory of volcanic -action, when it is at last discovered, may combine -many of the principles which now seem to be peculiar -to different hypotheses.</p> - -<p>Let us, in the first place, enquire what are the facts -which must be accounted for in any theory of volcanic -action. We have already been led to the conclusion -that the phenomena exhibited by volcanoes were entirely -produced by the escape of imprisoned water and -other gases from masses of incandescent and fluid rock. -Our subsequent examination of the problem confirmed -the conclusion that in all cases of volcanic outburst we -have molten rock-materials from which water and other -gases issue with greater or less violence. The two -great facts to be accounted for, then, in any attempted -explanation of volcanic phenomena, are the existence -of this high temperature at certain points within the -earth's crust, and the presence of great quantities of -water and gas, imprisoned in the rocks. We shall -perhaps simplify the enquiry if we examine these two -questions separately, and, in the first place, review those -hypotheses which have been suggested to account for -high temperatures in the subterranean regions, and, in -the second place, examine those which seek to explain -the presence of large quantities of imprisoned water -and gases.</p> - -<div class="sidenote">INCREASE OF TEMPERATURE WITH DEPTH.</div> - -<p>That a high temperature exists in the earth's crust -at some depth from the surface is a £act which does not -admit of any doubt. Every shaft sunk for mining -<span class="pagenum" id="Page_335">- 335 -</span> -operations, and every deep boring made for the purpose -of obtaining water, proves that a more or less regular -increase of temperature takes place as we penetrate -downwards. The average rate of this increase of temperature -has been estimated to be about 1° Fahrenheit -for every 50 or 60 feet of depth.</p> - -<p>Now if it be assumed that this regular increase of -temperature continues to great depths, a simple calculation -proves that at a depth of 9,000 feet a temperature -of 212° Fahrenheit will be found—one sufficient to boil -water at the earth's surface—while at a depth of 28 -miles the temperature will be high enough to melt -cast-iron, and at 34 miles to fuse platinum.</p> - -<p>So marked is this steady increase of temperature as -we go downwards, that it has been seriously proposed -to make very deep borings in order to obtain supplies -of warm water for heating our towns. Arago and Walferdin -suggested this method for warming the Jardin -des Plantes at Paris; and now that such important -improvements have been devised in carrying borings to -enormous depths, the time may not be far distant when -we shall draw extensively upon these supplies of subterranean -heat. At the present time the city of Buda-Pesth -is extensively supplied with hot-water from an -underground source. Should our coal-supply ever fail -it may be well to remember that we have these inexhaustible -supplies of heat everywhere beneath our -feet.</p> - -<p>But although we may conclude that at the moderate -<span class="pagenum" id="Page_336">- 336 -</span> -depths we have indicated such high temperatures exist, -it would not be safe to infer, as some have done, that -at a distance of only 40 or 50 miles from the surface -the materials composing our globe are in a state of -actual fusion. Both theory and experiment indicate -that under increased pressure the fusing point of solid -bodies is raised; and just as in a Papin's digester we -may have water retained by high pressure in a liquid -condition at a temperature far above 212° F., so in the -interior of the earth, masses of rock may exist in a -solid state, at a temperature far above that at which -they would fuse at the earth's surface. We may speak -of such rock-masses, retained in a solid condition by -intense pressure, at a temperature far above their fusing -point at the earth's surface, as being in a 'potentially -liquid condition.' Upon any relief of pressure such -masses would at once assume the liquid state, just -as the superheated water in a Papin's digester immediately -flashes into steam upon the fracture of the -strong vessel by which it is confined. We have already -seen how the action at volcanic vents often appears to -indicate just such a manifestation of elastic forces, as -would be exhibited by the relief of superheated masses -from a state of confinement by pressure.</p> - -<p>In reasoning upon questions of this kind, however, -we must always be upon our guard against giving undue -extension to principles and laws which seem to be -clearly established by experiment at the earth's surface. -It is well to remember how exceedingly limited is our -<span class="pagenum" id="Page_337">- 337 -</span> -command of extreme pressures and high temperatures, -when compared with those which may exist within a -body of the dimensions of our globe.</p> - -<div class="sidenote">EFFECT OF PRESSURE ON FUSION-POINT.</div> - -<p>If we were to imagine a set of intelligent creatures, -who were able to command only a range of temperatures -from 50° to 200° F., engaged upon an investigation of -the properties of water, we shall easily understand how -unsafe it may be to extend generalisations far beyond -the limits covered by actual experiment. Such beings, -from their observation of the regular changes of volume -of water at all the temperatures they could command, -might infer that at still higher and lower temperatures -the same rates of expansion and contraction would be -maintained. Yet, as we well know, such an inference -would be quite wide of the truth; for a little above -200° F. water suddenly expands to 1,700 times its -volume, and not far below 50° F. the contraction is -suddenly changed for expansion.</p> - -<p>It has been argued by the late Mr. David Forbes -and others that, inasmuch as experiment has shown -that—though the fusing points of solids are raised by -pressure, yet that this rise of the fusing points goes -on in a diminishing ratio as compared with the -pressures applied—a limit will probably be reached at -which the most intense pressure will not be sufficient -to retain substances at a high temperature in their -solid state. The fact that gases cannot be retained in -a liquid condition by the most intense pressure at a -temperature above their critical point, may seem by -<span class="pagenum" id="Page_338">- 338 -</span> -analogy to favour the same conclusion. Hence, David -Forbes, Dana, and other authors, have argued in favour -of the existence of a great liquid nucleus in our globe -covered by a comparatively thin, solid crust. And if -we accept the supposed proofs of a constant increase of -temperature from the surface to the centre of the globe, -such a conclusion appears to be at least as well founded -as that which regards the central masses of the earth -as maintained in a solid condition by intense pressure.</p> - -<p>A little consideration will, however, convince us -that the facts which have been relied upon as proving -the intensely heated condition of the central masses of -our globe, are by no means so conclusive as has been -supposed.</p> - -<p>The earth's form, which mathematicians have shown -to be exactly that which would be acquired by a globe -composed of yielding materials rotating on its axis at -the rate which our planet does, has often been adduced -as proving that the latter was not always in a rigid and -unyielding condition. In the same way, all the remarkable -facts and relations of the bodies of the solar -system, which have been shown by astronomers to lend -such support to the nebular hypothesis, have been -thought, at the same time, to favour the view that our -earth is still in a condition of uncompleted solidification.</p> - -<p>But it is quite admissible to accept the nebular -hypothesis and the view that our globe attained its -present form while still in a state of fluidity, and at the -same time to maintain that our earth has long since -<span class="pagenum" id="Page_339">- 339 -</span> -reached its condition of complete solidification. And -there are not a few facts which appear to lend support -to such a conclusion.</p> - -<div class="sidenote">SUPPOSED PROOFS OF LIQUID NUCLEUS.</div> - -<p>If the rapid rate of increase in temperature which -has been demonstrated to occur at so many parts of -the earth's surface be maintained to the centre, then, as -argued by David Forbes and Dana, it is difficult to -conceive of our earth as being in any other condition -than that of a liquid mass covered by a comparatively -thin crust. The objection to this view, both upon geological -and astronomical grounds, we have pointed out -in the previous chapter.</p> - -<p>Before accepting as a demonstrated conclusion this -notion of a constant increase of temperature from the -surface to the centre of our globe, it may be well to -re-examine the facts which are relied upon as proving it.</p> - -<p>That there is a general increase of temperature so -far as we are able to go downwards in the earth's crust, -there can, as we have seen, be no doubt whatever. Yet -it may be well to bear in mind how very limited is the -range of our observation on the subject. The deepest -mines extend to little more than half-a-mile from the -surface, and the deepest borings to little more than -three-quarters of a mile, while the distance from the -earth's surface to its centre is nearly 4,000 miles. -We may well pause before we extend conclusions, derived -from such very limited observations, to such -enormous depths.</p> - -<p>But when we examine critically these observations -<span class="pagenum" id="Page_340">- 340 -</span> -themselves, we shall find equal grounds for caution in -generalising from them. There is the greatest and -most startling divergence in the results of the observations -which have been made at different points at the -earth's surface. Even when every allowance is made -for errors of observation, these discrepancies still remain. -In some places the increase of temperature as we go -downwards is so rapid that it amounts to 1° Fahrenheit -for every 20 feet in depth, while in other cases, in order -to obtain the same increase in temperature of 1° Fahrenheit, -we have to descend as much as 100 feet.</p> - -<p>Now if, as is so often assumed, this increase of -temperature as we go downwards be due to our approach -to incandescent masses forming the interior portions of -the globe, it is difficult to understand why greater -uniformity is not exhibited in the rate of increase in -different areas. No difference in the conducting powers -of the various rock-materials is sufficient to account for -the fact that in some places the rate of increase in -temperature in going downwards is no less than five -times as great as it is in others.</p> - -<div class="sidenote">VARIATIONS IN UNDERGROUND TEMPERATURES.</div> - -<p>Again, there are some remarkable facts concerning -the variation in the rate of increase in temperature -with depth which seem equally irreconcilable with the -theory that the heat in question is directly derived from -a great, central, incandescent mass. M. Walferdin, by a -series of careful observations in two shafts at Creuzot, -proved that down to the depth of 1,800 feet the increase -of temperature amounted to 1° Fahrenheit for every 55 -<span class="pagenum" id="Page_341">- 341 -</span> -feet of descent, but below the depth named, the rate of -increase was as much as 1° Fahrenheit for every 44 feet. -On the other hand, in the great boring of Grenelle at -Paris, the increase in temperature down to the depth of -740 feet amounted to 1° Fahrenheit for every 50 feet of -descent, but from 740 feet down to 1,600 feet, the rate -of increase diminished to 1° for 75 feet of descent. -The same remarkable fact was strikingly shown in the -case of the deepest boring in the world—that of Sperenberg, -near Berlin, which attained the great depth of -4,052 feet. In this case, the rate of increase in temperature -for the first 1,900 feet, was 1° Fahrenheit -for every 55 feet of descent, and for the next 2,000, it -diminished to 1° Fahrenheit for every 62 feet of descent. -In the deep well of Buda-Pesth there was actually found -a decline in temperature below the depth of 3,000 -feet.</p> - -<p>Perhaps the most interesting fact in connection -with this question which has been discovered of late -years, is that in districts which have recently been the -seat of volcanic agencies, the rate of increase in temperature, -as we go downwards in the earth's crust, is -abnormally high. Thus at Monte Massi in Tuscany, -the temperature was found to increase at the rate -of 1° Fahrenheit for every 24 feet of descent. In -Hungary several deep wells and borings have been -made, which prove that a very rapid increase of temperature -occurs. The deep boring at Buda-Pesth penetrates -to a depth of 3,160 feet, and a temperature of -<span class="pagenum" id="Page_342">- 342 -</span> -178° Fahrenheit has been observed near the bottom. -The rate of increase of temperature in this boring was -about 1° for every 23 feet of descent. In the mines -opened in the great Comstock lode, in the western -territories of the United States, an abnormally high -temperature has been met with amounting in some -cases to 157° Fahrenheit. Although this is the richest -mineral-vein in the world, having yielded since 1859, -when it was first discovered, 60,000,000<i>l.</i> worth of gold -and silver, this rapid increase in temperature in going -downwards threatens in the end to entirely baffle the -enterprise of the miner. The rate of increase in temperature -in the case of the Comstock mines has been -estimated at 1° Fahrenheit for every 46 feet of descent, -between 1,000 and 2,000 feet from the surface, but as -much as 1° Fahrenheit for every 25 feet, at depths -below 2,000 feet.</p> - -<p>The facts which we have stated, with others of a -similar kind, have led geologists to look with grave feelings -of doubt upon the old hypothesis which regarded -the increase of temperature found in making excavations -into the earth's crust as a proof that we are approaching -a great incandescent nucleus. They have thus been -led to enquire whether there are any conceivable sources -of high temperatures at moderate depths—temperatures -which would be quite competent to produce locally all -the phenomena of volcanic action.</p> - -<p>There are not wanting other facts which seem to -point to the same conclusion: namely, that volcanic -<span class="pagenum" id="Page_343">- 343 -</span> -action is not due to the existence of a universal reservoir -of incandescent material occupying the central -portion of our globe, but to the local development -of high temperatures at moderate depths from the -surface.</p> - -<div class="sidenote">DEPTHS AT WHICH EARTHQUAKES ORIGINATE.</div> - -<p>The close connection between the phenomena of volcanoes -and earthquakes cannot be doubted. It is true -that some of those vibrations or tremors of the earth's -crust, to which we apply the name of earthquakes, occur -in areas which are not now the seat of volcanic action; -and it is equally true that the stratified rock-masses of -our globe, far away from any volcanic centres, exhibit -proofs of violent movement and fracture, in the production -of which, concussions giving rise to earthquake -vibrations, could scarcely fail to have occurred. But it -is none the less certain that earthquakes as a rule take -place in those areas which are the seats of volcanic action, -and that great earthquake-shocks precede and accompany -volcanic outbursts. Sometimes, too, it has been -noticed that the manifestation of activity at a volcanic -centre is marked by the sudden decline of the earthquake-tremors -of the district around, as though a -safety-valve had been opened at that part of the earth's -surface.</p> - -<p>Mr. Mallet has shown that by the careful study of -the effects produced at the surface by earthquake-vibrations, -we may determine with considerable accuracy the -point at which the shock or concussion occurred which -gave rise to the vibration. Now it is a most remarkable -<span class="pagenum" id="Page_344">- 344 -</span> -fact that such calculations have led to the conclusion -that, so far as is at present known, earthquake shocks -never originate at greater depths than thirty miles from -the surface, and that in some cases the focus from which -the waves of elastic compression producing an earthquake -proceed is only at the depth of seven or eight -miles. As we have already seen, there can be no doubt -that in the great majority of instances the forces -originating earthquake-vibrations and volcanic outbursts -are the same, and independent lines of reasoning have -conducted us to the conclusion that these forces operate -at very moderate distances from the earth's surface.</p> - -<p>Under these circumstances, geologists have been led -to enquire whether there are any means by which we -can conceive of such an amount of heat, as would be -competent to produce volcanic outbursts, being locally -developed at certain points within the earth's crust. -Recent discoveries in physical science which have shown -the close relation to one another of different kinds of -force, and their mutual convertibility, have at least -suggested the possibility of the existence of causes by -which such high temperatures within certain portions -of the earth's crust may be originated.</p> - -<div class="sidenote">DAVY'S CHEMICAL THEORY.</div> - -<p>When, at the commencement of the present century, -Sir Humphry Davy discovered the remarkable metals -of the alkalies and alkaline earths, and at the same time -demonstrated the striking phenomena which are exhibited -if these metals be permitted to unite with -oxygen, he at once perceived that if such metals existed -<span class="pagenum" id="Page_345">- 345 -</span> -in an uncombined condition within the earth's crust, -the access of water and air to the mass might give rise -to the development of such an amount of heat, as would -be competent to produce volcanic phenomena at the -surface. It is true that at a later date Davy recognised -the chemical theory of volcanoes as being beset with -considerable difficulties, and was disposed to abandon -it altogether. It was argued, with considerable show -of reason, that if the heat at volcanic centres were -produced by the access of water to metallic substances, -great quantities of hydrogen would necessarily be -evolved, and this gas ought to be found in prodigious -quantities among the emanations of volcanoes. The -fact that such enormous quantities of hydrogen gas are -not emitted from volcanic vents has been held by many -authors to be fatal to the chemical theory of volcanoes.</p> - -<p>But the later researches of Graham and others have -made known facts which go far towards supplying an -answer to the objections raised against the chemical -theory of volcanoes. Various solids and liquids have -been shown to possess the power of absorbing many -times their volume of certain gases. Among the gases -thus absorbed in large quantities by solids and liquids, -hydrogen is very conspicuous. In some cases gases are -absorbed by metals or other solids in a state of fusion, -and yielded up again by them as they cool.</p> - -<p>It is a very remarkable circumstance that some -meteorites are found to have absorbed large quantities -of hydrogen gas, and this is given off when they are -<span class="pagenum" id="Page_346">- 346 -</span> -heated in vacuo. Thus it has been demonstrated that -certain meteorites have contained as much as forty -seven times their own volume of hydrogen gas.</p> - -<p>We have already pointed out that there are reasons -for believing the internal portions of our globe to be -composed of materials similar to those found in meteorites. -If such be the case, the access of water to these -metallic substances may result in the formation of -oxides, attended with a great local development of heat, -the hydrogen which is liberated being at once absorbed -by the surrounding metallic substances. That this -oxidation of the metallic substances in the interior -of our globe by the access of water and air from the -surface is continually going on, can scarcely be doubted. -We may even look forward to a far-distant period when -the whole of the liquid and gaseous envelopes of the -globe shall have been absorbed into its substance, and -our earth thereby reduced to the condition in which -we now find the moon to be.</p> - -<p>There is a second method by which high temperatures -might be locally developed within the earth's -crust, which has been suggested by Vose, Mallet, and -other authors.</p> - -<p>We have good grounds for believing that the temperature -of our globe is continually diminishing by its -radiation of heat into space. This cooling of our globe -is attended by contraction, which results in movements -of portions of its crust. It may at first sight appear -that such movements would be so small and insignificant -<span class="pagenum" id="Page_347">- 347 -</span> -as to be quite unworthy of notice. But if we -take into account the vast size of our earth it will be -seen that the movements of such enormous masses -may be attended with the most wonderful results.</p> - -<p>It has been shown that if a part of the earth's crust -fifty miles in thickness were to have its temperature -raised 200° Fahrenheit, its surface would be raised to -the extent of 1,000 or 1,500 feet Le Conte has -pointed out that if we conceive the conduction of heat -to take place at slightly different rates along different -radii of our globe, we should at once be able to account -for the existing inequalities of the earth's surface, and -for all those continental movements which can be shown -to have taken place in past geological periods.</p> - -<div class="sidenote">DYNAMICAL THEORIES.</div> - -<p>But if we admit, as we have good grounds for doing, -that the loss of heat from the external portions of our -globe goes on more rapidly than in the case of the -central masses, we have thereby introduced another -powerful agent for the production of high temperatures -within the earth's crust. The external shell of the -globe will tend to contract upon the central mass, and -in so doing a series of tangential strains will result -which will be capable of folding and crumpling the -rocks along any lines of weakness. That such crushing -and crumpling has during all geological periods -taken place along lines of weakness in the earth's crust, -is proved, as we have seen, by the phenomena presented -by mountain-ranges. Now these crushings, -crumplings, and other violent movements of great -<span class="pagenum" id="Page_348">- 348 -</span> -rock-masses must result in the development of a vast -amount of heat, just as the forcing down of a break -upon a moving wheel produces heat. This conclusion -is strikingly confirmed by the well-known geological -fact that nearly all rocks which have undergone great -movement and contortion are found to present evidence -of having been subjected to such chemical and crystalline -actions, as would result from the development of a -high temperature within their mass.</p> - -<div class="sidenote">RECAPITULATION OF SEVERAL THEORIES.</div> - -<p>Let us sum up briefly the various methods which -have been suggested to account for the high temperatures -within certain parts of the earth's crust by which -volcanic phenomena are produced.</p> - -<p>Our globe may be conceived of as an incandescent -liquid mass surrounded by a cooler, solid shell. If we -regard this liquid interior mass as supplying directly -the various volcanic vents of the earth, it must be conceded -that the outer shell is of comparatively slight -thickness. But astronomers are almost universally -agreed that such a thin outer shell and inner liquid -mass are quite incompatible with that rigidity which -our planet exhibits under the attractions of its neighbours. -Geologists are almost equally unanimous in -regarding this hypothesis of a liquid nucleus and thin, -solid shell as contradicted by the stability of the conditions -which have been maintained during such long -past periods, and which exist at the present day. The -extent and character of volcanic action do not indicate -a condition of general instability in our earth, but one -<span class="pagenum" id="Page_349">- 349 -</span> -of stability subject to small and local interferences -The grandest volcanic disturbances appear small and -insignificant, if we take into account the vast dimensions -of the globe upon which they are displayed.</p> - -<p>If, on the other hand, we consider the outer solid -shell to be of great thickness, we are met by the difficulty -of accounting for the upheaval of liquid matter -through such vast thicknesses of a solid shell. The -differences in character of lavas extruded from closely -adjoining volcanic districts seem equally difficult of -explanation on any theory of a central, fluid nucleus -and a solid, outer shell. Nor is the distribution of heat -within the earth's crust so uniform as might be anticipated, -if the source of that heat be a great central -mass of highly heated materials.</p> - -<p>Under these circumstances, geologists and physicists -have enquired whether any other conditions can be -imagined as existing in the earth's interior, which -would better account for the observed phenomena than -does the hypothesis of a liquid nucleus and a solid -outer shell. Two such alternative hypotheses have -been suggested.</p> - -<p>Mr. Hopkins, adopting the theory that the earth -has solidified both at the centre and its outer surface, -endeavoured to explain the occurrence of volcanoes and -earthquakes by supposing that cavities of liquid material -have been left between the solid nucleus and the -solid shell, and these cavities full of liquid material -constitute the sources from which the existing volcanoes -<span class="pagenum" id="Page_350">- 350 -</span> -of the globe draw their supplies. But this hypothesis -is found to be beset with many difficulties when we -attempt to apply it to the explanation of the phenomena -of volcanic action. It entirely fails, among other -things, to account for the remarkable fact that during -past geological periods the scene of volcanic action has -been continually shifting over the surface of the earth, -so that there is probably no considerable area of our -globe which has not at one time or other been invaded -by the volcanic forces.</p> - -<p>By some other theorists, who have felt the full force -of this last objection, an attempt has been made to -explain the phenomena of volcanoes by supposing that -the globe is solid from its surface to its centre, but -that the internal portions of the globe are at such a -high temperature that they are only retained in a solid -condition by the enormous pressure to which they are -subjected. The central masses of the globe are thus -regarded as being in an <i>actually</i> solid, but in a <i>potentially</i> -liquid condition, and any local relief of pressure -is at once followed by the conversion of solid to -liquefied materials, in the district where the relief -takes place, resulting in the manifestation of volcanic -phenomena at the spot. It may be granted that this -hypothesis better accords with the known facts of Vulcanology -than any of those which we have previously -described, but it is impossible to shut our eyes to the -fact that not a few serious difficulties still remain. -Thus it is based upon the assumption that the law of -<span class="pagenum" id="Page_351">- 351 -</span> -the elevation of the point of fusion by pressure is true -at temperatures and pressures almost infinitely above -those at which we are able to conduct observations; -but neither experiment nor analogy warrant this conclusion, -for the former shows that the elevation of the -point of fusion by pressure goes on in a continually -diminishing ratio, and the latter famishes us with the -example of volatile liquids which, above their critical -points, obstinately remain in a gaseous condition under -the highest pressures. Nor is it easy upon this hypothesis -to account for the very irregular distribution of -temperatures within the earth's crust, as demonstrated -by observations in mines, wells, and borings. The hypothesis -further requires the assumption that, at such -very moderate depths as are required for the reservoirs -of volcanoes, the effects of pressure and temperature on -the condition of rock-materials are so nicely balanced -that the smallest changes at the surface lead to a -disturbance of the equilibrium.</p> - -<div class="sidenote">DIFFICULTIES NOT YET EXPLAINED.</div> - -<p>It is the weight of these several objections that has -led geologists in recent years to regard with greater -favour those hypotheses which seek to account for the -production of high temperatures within parts of the -earth's crust, without having recourse to a supposed -incandescent nucleus. If it can be shown that there -are any chemical or mechanical forces at work within -the crust of the globe which are capable of producing -local elevations of temperature, then we may conceive -of a condition of things existing in the earth's interior -<span class="pagenum" id="Page_352">- 352 -</span> -which is free from the objections raised by the astronomer -on the score of the earth's proved rigidity, and -by the geologist on the ground of its general stability, -and which at the same time seems to harmonise better -with the observed facts of the distribution of temperature -within the earth's crust. How far the existence -of such chemical and mechanical agencies capable of -producing high temperatures within the crust of the -globe have been substantiated, we have already endeavoured -to point out.</p> - -<p>It must be admitted, then, that the questions of -the nature of the earth's interior and the cause of the -high temperatures which produce volcanic phenomena, -are still open ones. We have not yet got beyond the -stage of endeavouring to account for the facts observed -by means of tentative hypotheses. Some of these, as -we have seen, agree with the facts, so far as they are -at present known, much better than others; but the -decision between them or the rejection of the whole of -them in favour of some new hypothesis, must depend -on the results of future observation and enquiry.</p> - -<p>It may be well, before leaving this subject, to remark -that they are all equally reconcilable with the -nebular theory of Kant and Laplace. Granting that -the matter composing our globe has passed successively -through the gaseous and liquid conditions, it is open -to us to imagine the earth as now composed of a liquid -nucleus with either a thick or a thin solid shell; of a -solid nucleus and a solid shell with more or less liquid -<span class="pagenum" id="Page_353">- 353 -</span> -matter between them; or, lastly, to conceive of it as -having become perfectly solid from the centre to the -surface.</p> - -<div class="sidenote">CAUSE OF THE PRESENCE OF WATER IN LAVAS.</div> - -<p>But it is not upon the existence of a high temperature -within certain parts of the earth's crust that the -production of volcanic activity alone depends. The presence -of water and other liquid and gaseous substances -in a state of the most intimate admixture with the -fused rock-masses, is, as we have seen, the main cause -of the violent displays of energy exhibited at volcanic -centres. And We shall now proceed to notice the hypotheses -which have been suggested to account for the -presence of these liquid and gaseous bodies in the midst -of the masses of incandescent materials poured out from -volcanic vents.</p> - -<p>There is an explanation of this presence of water -and various gases in the masses of molten rock-materials -within the earth's crust which at once suggests itself, -and which was formerly very generally accepted. Volcanoes, -as we have seen, are usually situated near coast-lines, -and if we imagine fissures to be produced by -which sea-water finds access to masses of incandescent -rock-materials, then we can regard volcanic outbursts -as resulting from this meeting of water with rock-masses -in a highly healed condition. This supposition -has been thought to receive much support from the -fact that many of the gases evolved from volcanic vents -are such as would be produced by the decomposition of -substances present in sea-water.</p> - -<p><span class="pagenum" id="Page_354">- 354 -</span></p> - -<p>But it frequently happens that an explanation which -at first sight appears to be very simple and obvious, -turns out on more critical examination to be quite the -reverse, and this is the case with the supposed origination -of volcanic outbursts by the access of sea-water to -incandescent rock-material by means of earth-fissures. -It is difficult to understand how, by such means, that -wonderfully intimate union between the liquefied rock -and the water, evolved in such quantities during volcanic -outbursts, could be brought about; and moreover, -we can scarcely regard the production of fissures in the -earth's crust as being at the same time both the cause -and the effect of this influx of water to the deep-seated -rock-masses at a high temperature.</p> - -<div class="sidenote">ABSORPTION OF GASES BY LIQUIDS AND SOLIDS.</div> - -<p>During recent years the attention of both geologists -and physicists has been directed to a remarkable property -exhibited by many liquids and solids, as supplying -a possible explanation of the phenomena of volcanic -action. The property to which we refer is that whereby -some liquid and solid substances are able to absorb -many times their volume of certain gases—which gases -under different conditions may be given off again from -the liquids or solids. This power of absorption is a -very remarkable one; it is not attended with chemical -combination, but the amount of condensation which -gases must undergo within the solid or liquid substances -is sometimes enormous. Water may be made -to absorb more than 1,000 times its volume of ammonia, -and more than 500 times its volume of hydrochloric -<span class="pagenum" id="Page_355">- 355 -</span> -acid. Alcohol may absorb more than 300 times its -volume of sulphurous acid. Charcoal may absorb 100 -times its volume of ammonia, 85 times its volume of -hydrochloric acid, 65 times its volume of sulphuretted -hydrogen, 55 times its volume of sulphurous acid, and -35 times its volume of carbonic acid. Platinum-black -absorbs many times its volume of oxygen and other -gases.</p> - -<p>This power of absorption of gases varies in different -solids and liquids according to the conditions to which -they are subjected. Dr. Henry showed it to be a general -law in liquids that, as the pressure is augmented, the -weight of the gas absorbed is proportionately increased.</p> - -<p>Sometimes this absorption of gases takes place only -at high temperatures. Thus silver in a state of fusion -is able to absorb 22 times its volume of oxygen gas. -When the metal is allowed to cool this gas is given -off, and if the cooling takes place suddenly a crust is -formed on the surface, and the phenomenon known as -the 'spitting of silver' is exhibited. Sometimes during -this operation miniature cones and lava-streams are -formed on the surface of the cooling mass, which present -a striking resemblance to those formed on a grand -scale upon the surface of the globe. Similar phenomena -are exhibited by several other metals and by the -oxide of lead.</p> - -<p>The researches of Troost and others have shown -that molten iron and steel possess the property of -absorbing considerable quantities of oxygen, hydrogen, -<span class="pagenum" id="Page_356">- 356 -</span> -carbonic acid, and carbonic oxide, and that these gases -are given off in the operation known as 'seething,' when -either the pressure or the temperature is diminished.</p> - -<p>Hochstetter has shown that in the process of extracting -sulphur from the residues obtained during the -manufacture of soda, some very interesting phenomena -are manifested. The molten sulphur is exposed to a -temperature of 262° Fahrenheit, and a pressure of two -or three atmospheres, in the presence of steam; under -these circumstances it is found that the sulphur absorbs a -considerable quantity of water, which is given off again -with great violence from the mass as it undergoes -solidification. The hardened crust which forms on the -surface of the molten sulphur is agitated and fissured, -miniature cones and lava-streams being formed upon -it, which have a striking resemblance to the grander -phenomena of the same kind exhibited upon the crust -of the globe.</p> - -<p>The observations which we have described prove conclusively -that many liquids and solids in a molten condition -have the power of absorbing many times their -volume of certain gases, and that this action is aided -by heat and pressure.</p> - -<p>That the molten materials which issue from volcanic -vents have absorbed enormous quantities of steam and -other gases, we have the most undisputable evidence. -The volume of such gases given off during volcanic -outbursts, and while the lava-streams are flowing and -consolidating, is enormous, and can only be accounted -<span class="pagenum" id="Page_357">- 357 -</span> -for by supposing that the masses of fluid rock have -absorbed many times their volume of the gases. But -we have another not less convincing proof of the same -fact in the circumstance that volcanic materials which -have consolidated under great pressure—such as -granites, gabbros, porphyries, &c.—exhibit in their -crystals innumerable cavities containing similar gases -in a liquefied state.</p> - -<p>It is to the violent escape of these gases from the -molten rock-masses, as the pressure upon them is relieved, -that nearly all the active phenomena of volcanoes -must be referred; and it was the recognition of this bet -by Spallanzani, while he was watching the phenomena -displayed in the crater of Stromboli, which laid the -foundations of the science of Vulcanology.</p> - -<div class="sidenote">SOURCE OF THE ABSORBED GASES.</div> - -<p>But here another question presents itself to the -investigator of the phenomena of volcanoes: it is this. -At what period did the molten rock-masses issuing from -vents absorb those gaseous materials which are given -off so violently from their midst during eruptions? -Two different answers to this question have been suggested. -It may be that the original materials of which -our globe was composed consisted of metallic substances -in a state of fusion which had absorbed many gases, -and that, in the fluid masses below the solid crust, vast -quantities of vapour and gas are stored up, which are -being gradually added to the atmosphere during volcanic -outbursts. The fact that meteorites, which, as -we have seen, in all probability closely resemble the -<span class="pagenum" id="Page_358">- 358 -</span> -materials forming the earth's interior, sometimes yield -many times their volume of hydrogen and other gases, -may be thought to lend some support to this idea. If -it be the correct one, we must regard our globe as -gradually parting with its pent-up stores of energy, in -those absorbed gases and vapours held in bondage by -the solid and fluid materials of its interior.</p> - -<p>But there is another hypothesis which is, to say -the least, equally probable. Water containing various -gases in solution is continually finding its way downwards -by infiltration into the earth's crust. Much of -this water, after passing through pervious beds, reaches -some impervious stratum and is returned to the surface -in the form of springs. But that some of this percolating -water penetrates to enormous depths is shown by -the fact that the deepest mines and borings encounter -vast underground supplies of water. When we remember -that nearly three-fourths of the earth's surface -is covered by the waters of the ocean, and that the -average depth of these oceanic waters is more than -10,000 feet, we may easily understand how great a -portion of the earth's crust must be penetrated by -infiltrating waters which can find no outlet in springs. -The penetration of the waters of the ocean into the -earth's crust will be aided, too, by the enormous pressure -amounting to not less than several tons to the -square-inch upon the greater part of the ocean-floor. -It might be thought that this downward penetration -of water would be counteracted by the upward current -<span class="pagenum" id="Page_359">- 359 -</span> -of steam that would be produced as these subterranean -waters reach the hotter portions of the earth's crust. -But the experiments of Daubrée have conclusively shown -that the penetration of water through rocks takes place -in opposition to the powerful pressure of steam in the -contrary direction. Hence, we may assume that certain -quantities of water, containing various gases and -solids in solution, are continually finding their way by -capillary infiltration from the surface to the deeply -seated portions of the earth's crust, there to undergo -absorption by the incandescent rock-masses and to produce -oxidation of some of their materials.</p> - -<div class="sidenote">POSITION OF THE ISOGEOTHERMS.</div> - -<p>The deep-sea soundings of the 'Challenger' have -shown that the floor of the ocean is constantly maintained -at a temperature but little above that of the -freezing point of water. This low temperature is probably -produced by the absorption of heat from the -earth's crust by the waters of the ocean, which distribute -it by means of convection currents on the grandest -scale. Hence, the isogeotherms, or lines indicating the -depths at which the same mean temperature is found -within the earth's crust, are probably depressed beneath -the great ocean-floors, and rise towards the land-masses. -It is to this circumstance, combined with that of the -enormous pressure of water on the ocean-beds, that we -must probably ascribe the general absence of volcanoes -in the deep seas and their distribution near coast-lines.</p> - -<p>We have thus briefly reviewed the chief hypotheses -<span class="pagenum" id="Page_360">- 360 -</span> -which have been suggested in order to account for the -two great factors in all volcanic phenomena—namely, -the presence of highly heated rock-masses within the -earth's crust, and the existence of various vapours and -gases in a state of most intimate mechanical, but not -chemical, union with these incandescent materials. It -must be admitted that we do not at present appear to -have the means for framing a complete and consistent -theory of volcanic action, but we may hopefully look -forward to the time when further observation and experiment -shall have removed many of the existing -difficulties which beset the question, and when by the -light of such future researches untenable hypotheses -shall be eliminated and the just ones improved and -established.</p> - -<p>But if we are constrained to admit that a study of -the observed phenomena and established laws of volcanic -action have not as yet enabled us to frame any complete -and satisfactory theory on the subject, we cannot lose -sight of the fact that all modern speculation upon this -question appears to be tending in one definite direction. -It is every day becoming more and more clear that our -earth is bound by ties of the closest resemblance to the -other members of that family of worlds to which it -belongs, and that the materials entering into their constitution, -and the forces operating in all are the same.</p> - -<p>We have had occasion in a previous chapter to point -out that there are the strongest grounds for believing -the interior of our globe to consist of similar materials -<span class="pagenum" id="Page_361">- 361 -</span> -to those found in the small planetary bodies known as -meteorites. That the comets are merely aggregations -of such meteorites, and that the planets differ from -them only in their greater dimensions, may be regarded -as among the demonstrated conclusions of the astronomer. -The materials found most abundantly in meteorites -and in the interior of our globe are precisely -the same as those which are proved to exist in an -incandescent state in our sun. Hence we are led to -conclude that the whole of the bodies of the solar system -are composed of the same chemical elements.</p> - -<div class="sidenote">ERUPTIVE ACTION IN THE SUN.</div> - -<p>That the forces operating in each of these distant -bodies present striking points of analogy is equally -clear. The sun is of far greater dimensions than our -earth, and is still in great part, if not entirely, in a -gaseous condition. The great movements in the outer -envelopes of the sun exhibited in the 'sun-spots' and -'solar prominences,' recall to the mind the phenomena -of volcanic activity upon our globe. But the vast -energy still existing in the intensely heated mass of -the sun, and the wonderful mobility of its gaseous -materials, give rise to appearances beside which all -terrestrial outbursts seem to sink into utter insignificance. -Vast cavities of such dimensions that many -globes of the size of our earth might be swallowed up -in them are formed in the solar envelopes in the -course of a few days or hours. Within these cavities -or sun-spots incandescent vapours are observed, rushing -upwards and downwards with almost inconceivable -velocity.</p> - -<p><span class="pagenum" id="Page_362">- 362 -</span></p> - -<p>The drawings made by Secchi, and reproduced in -figs. <a href="#fig89">89</a> and <a href="#fig90">90</a>, will give some idea of the appearances -presented by these great holes in the solar envelopes.</p> - -<div class="figcenter" id="fig89" style="width: 397px;"> - <img src="images/fig89.png" width="397" height="294" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 89.—A group of Sun-spots.</span> (After Secchi.)</div> -</div> - -<p>In <a href="#fig89">fig. 89</a> a group of sun-spots is represented and, -in their circular outlines and tendency to a linear arrangement, -they can scarcely fail to remind anyone -familiar with volcanic phenomena of terrestrial craters, -though their dimensions are so much greater.</p> - -<p>In <a href="#fig90">fig. 90</a> the sun-spot represented shows the presence -of large floating masses of incandescent materials -rushing upwards and downwards within the yawning -gulf.</p> - -<p><span class="pagenum" id="Page_363">- 363 -</span></p> - -<div class="sidenote">PHENOMENA OF SUN-SPOTS.</div> - -<div class="figcenter" id="fig90" style="width: 404px;"> - <img src="images/fig90.png" width="404" height="319" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 90.—A Sun-spot, showing the great masses of incandescent - vapour rising or falling within it.</span> (After Secchi.)</div> -</div> - -<div class="figright" id="fig91" style="width: 183px;"> - <img src="images/fig91.png" width="183" height="131" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 91.—The edge of a Sun-spot, - showing a portion of the prominent masses of incandescent - gas (A), which detached itself at E and floated into - the midst of the cavity.</span></div> -</div> - -<p>From <a href="#fig91">fig. 91</a>, taken from -a drawing by Mr. Norman -Lockyer, we may understand the movements of -these great protuberances of -incandescent gas which are -seen on the sides of the -sun-spots.</p> - -<p>The so-called solar prominences present even more -striking resemblances to the volcanic outbursts of our -globe.</p> - -<p><span class="pagenum" id="Page_364">- 364 -</span></p> - -<p>Two drawings made by Mr. Norman Lockyer will -serve to give some idea of the vast dimensions of these -solar prominences, and of the rapid changes which take -place in their form.</p> - -<div class="figcenter" id="fig92" style="width: 365px;"> - <img src="images/fig92.png" width="365" height="361" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 92.—Drawing of a Solar prominence, made by Mr. Norman - Lockyer on March 14, 1869, at 11 H. 5 M. A.M.</span></div> -</div> - -<p>The masses of incandescent gas were estimated as -being no less than 27,000 feet in height, yet in ten -minutes they had totally changed their form and appearance, -as shown in <a href="#fig93">fig. 93</a>.</p> - -<p>Even still more striking are the changes recorded -<span class="pagenum" id="Page_365">- 365 -</span> -by Professor Young, of New-Haven, in a solar prominence, -which he observed on September 7, 1871.</p> - -<div class="figcenter" id="fig93" style="width: 360px;"> - <img src="images/fig93.png" width="360" height="358" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 93.—The same object, as seen at 11 H. 15 M. on the same day.</span></div> -</div> - -<div class="sidenote">SOLAR PROMINENCES.</div> - -<p>That astronomer described a mass of incandescent -gas rising from the surface of the sun to the height -of 54,000 miles. In less than twenty-five minutes he -saw the whole mass torn to shreds and blown upwards, -some of the fragments being in ten minutes hurled to -the height of 200,000 miles above the sun's surface. -The masses of incandescent gas thus hurled upwards -were of enormous dimensions, the smallest being estimated -as having a greater area than the whole of the -British Islands, and the force with which they were -urged upwards was so great that they acquired a velocity -of 166 miles per second. The accompanying woodcut -shows the successive appearances presented by this -grand eruptive outburst on the surface of the sun.</p> - -<p><span class="pagenum" id="Page_366">- 366 -</span></p> - -<div class="figcenter" id="fig94" style="width: 667px;"> - <img src="images/fig94.png" width="667" height="319" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 94.—Drawings of a Solar prominence at four - different periods on Sept. 7, 1871.</span> (After Young.)</div> -</div> - -<p><span class="pagenum" id="Page_367">- 367 -</span></p> - -<div class="sidenote">EXTINCT VOLCANOES OF THE MOON.</div> - -<p>The moon, which is of far smaller size than our -earth, exhibits on its surface sufficiently striking -evidences of the action of volcanic forces. Indeed the -dimensions of the craters and fissures which cover the -whole visible lunar surface are such that we cannot but -infer volcanic activity to have been far more violent -on the moon than it is at the present day upon the -earth. This greater violence of the volcanic forces on -the moon is perhaps accounted for by the fact that the -force of gravity on the surface of the moon is only -one-sixth of that at the surface of the earth; and -thus the eruptive energy will have a much less smaller -resistance to overcome in bursting asunder the solid -crust and accumulated heaps of ejected materials on its -surface. But the volcanic action on the moon appears -now to have wholly ceased, and the absence of both -water and atmosphere in our satellite suggests that this -extinction of volcanic energy may have been caused by -the complete absorption of its gaseous envelope. The -appearance presented by a portion of the moon's surface -is shown in <a href="#fig95">fig. 95</a>.</p> - -<p>The sun and the moon appear to exhibit two -widely separated extremes in the condition assumed -during the cooling down from a state of incandescence -<span class="pagenum" id="Page_368">- 368 -</span> -of great globes of vaporised materials. The several -planets, our own among the number, probably exhibit -various intermediate stages of consolidation.</p> - -<div class="figcenter" id="fig95" style="width: 301px;"> - <img src="images/fig95.png" width="301" height="483" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 95.—A group of Lunar craters (Maurolycus, Barocius, etc.), - the largest being more than 60 miles in diameter.</span></div> -</div> - -<div class="sidenote">ERUPTIVE ACTION IN THE SUN, EARTH AND MOON.</div> - -<p>Our earth is, as we have seen, closely allied to the -other bodies of the solar system in its movements, its -relations, and its composition; and a true theory of -<span class="pagenum" id="Page_369">- 369 -</span> -terrestrial vulcanicity, when it is discovered, may be -expected not only to afford an explanation of the phenomena -displayed on our own globe, but to account for -those displays of internal energy which have been -manifested in other members of the same great family -of worlds.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_371">- 371 -</span></p> - -<h2 class="nobreak" id="INDEX">INDEX.</h2> -</div> - -<div class="tdc" style="width: 35em;"> -[ <a href="#A">A</a> ][ <a href="#B">B</a> ][ <a href="#C">C</a> ][ <a href="#D">D</a> ][ <a href="#E">E</a> ][ <a href="#F">F</a> ][ <a href="#G">G</a> ][ <a href="#H">H</a> ]<br /> -[ <a href="#I">I</a> ][ <a href="#J">J</a> ][ <a href="#K">K</a> ][ <a href="#L">L</a> ][ <a href="#M">M</a> ][ <a href="#N">N</a> ][ <a href="#O">O</a> ][ <a href="#P">P</a> ]<br /> -[ <a href="#R">R</a> ][ <a href="#S">S</a> ][ <a href="#T">T</a> ][ <a href="#U">U</a> ][ <a href="#V">V</a> ][ <a href="#W">W</a> ][ <a href="#Y">Y</a> ][ <a href="#Z">Z</a> ] -</div> - -<p>[The subjects illustrated in the engravings are indicated by <i>italics</i>, -the names of authors are in <span class="smcap">Capitals</span>.]</p> - -<p class="p0"> -<a id="A"></a><span class="dropcap">A</span>BICH, cited, <a href="#Page_122">122</a><br /> -— researches of, <a href="#Page_4">4</a><br /> -Absorption of gases by liquids and solids, <a href="#Page_354">354</a>, <a href="#Page_355">355</a><br /> -Acid lavas, <a href="#Page_48">48</a><br /> -Æolian Islands. <i>See</i> <a href="#Lipari">Lipari Islands</a><br /> -Æolus, origin of myth, <a href="#Page_35">35</a><br /> -Africa, volcanoes of, <a href="#Page_227">227</a><br /> -— South, diamonds of, <a href="#Page_147">147</a><br /> -Agates, formation of, <a href="#Page_150">150</a><br /> -<span class="smcap">Allport</span>, Mr., cited, <a href="#Page_259">259</a><br /> -Alps, formation of, <a href="#Page_292">292</a><br /> -Altered lavas, names given to, <a href="#Page_261">261</a><br /> -America, volcanoes of, <a href="#Page_227">227</a><br /> -Amygdaloids, formation of, <a href="#Page_140">140</a>, <a href="#Page_141">141</a><br /> -Andesites, <a href="#Page_50">50</a>, <a href="#Page_59">59</a><br /> -Andesite-volcanoes, <a href="#Page_126">126</a><br /> -<span class="smcap">Andrews</span>, Professor, cited, <a href="#Page_321">321</a><br /> -Anne Boleyn and Etna, <a href="#Page_3">3</a><br /> -<span class="smcap">Armstrong</span>, Sir W., hydro-electric machine, <a href="#Page_29">29</a><br /> -Arthur's Seat, <a href="#Page_275">275</a><br /> -Artificial stone, <a href="#Page_55">55</a><br /> -Asia, volcanoes of, <a href="#Page_227">227</a><br /> -Asiderites, <a href="#Page_316">316</a><br /> -Asmanite, <a href="#Page_314">314</a><br /> -Astroni, crater-ring of, <a href="#Page_170">170</a><br /> -Atlantic, volcanoes in, <a href="#Page_223">223</a><br /> -Auvergne, <i>breached cones of</i>, <a href="#Page_123">123</a>, <a href="#fig40">fig. 40</a><br /> -— <i>denuded cones in</i>, <a href="#Page_124">124</a>, <a href="#fig42">fig. 42</a><br /> -— incrusting springs of, <a href="#Page_184">184</a><br /> -— puys of, <a href="#Page_152">152</a>, <a href="#Page_212">212</a><br /> -— volcanic cones in, <a href="#Page_79">79</a><br /> -<br /> -<a id="B"></a><span class="dropcap">B</span>ALL-AND-SOCKET structure in basaltic columns, <a href="#Page_107">107</a><br /> -Barrancos, formation of, <a href="#Page_209">209</a><br /> -Basalt, controversy concerning origin of, <a href="#Page_249">249</a><br /> -Basalts, <a href="#Page_49">49</a>, <a href="#Page_50">50</a>, <a href="#Page_59">59</a><br /> -Basaltic columns of Bohemia, <a href="#Page_107">107</a><br /> -— — of Central Germany, <a href="#Page_107">107</a><br /> -— — of Monte Albano, <a href="#Page_107">107</a><br /> -— — <i>from the Giant's Causeway</i>, <a href="#Page_107">107</a>, <a href="#fig29">fig. 29</a><br /> -Basic lavas, <a href="#Page_48">48</a><br /> -Bath, hot spring of, <a href="#Page_219">219</a><br /> -Ben Nevis, <a href="#Page_274">274</a><br /> -Bohemia, volcanoes of, <a href="#Page_126">126</a><br /> -— lavas of, <a href="#Page_103">103</a><br /> -Boiling. <i>See</i> <a href="#Ebullition">Ebullition</a><br /> -<span class="smcap">Bonney</span>, Professor, cited, <a href="#Page_69">69</a>, <a href="#Page_109">109</a>, <a href="#Page_259">259</a><br /> -Boracic acid at volcanic vents, <a href="#Page_216">216</a><br /> -<span class="pagenum" id="Page_372">- 372 -</span> -<i>Bourbon, volcano of</i>, <a href="#Page_176">176</a>, figs. <a href="#fig74">74</a>, <a href="#fig75">75</a><br /> -<i>Bracciano, crater-lake of</i>, <a href="#Page_178">178</a>, <a href="#fig77_neg">fig. 77</a><br /> -<i>Breached cones</i>, <a href="#Page_123">123</a>, <a href="#fig40">fig. 40</a><br /> -Babbles of steam, escape from lava, <a href="#Page_21">21</a><br /> -<i>Bubbles, spontaneous movement of, in liquid cavities</i>, <a href="#Page_62">62</a>, <a href="#fig08">fig. 8</a><br /> -— — cause of, <a href="#Page_65">65</a><br /> -<span class="smcap">Buch, Von</span>, researches of, <a href="#Page_4">4</a><br /> -Buda-Pesth, deep well of, <a href="#Page_335">335</a>, <a href="#Page_341">341</a><br /> -Büdos Hegy, Transylvania, <a href="#Page_215">215</a><br /> -<span class="smcap">Bunsen</span>, cited, <a href="#Page_201">201</a><br /> -Burning, does not take place at volcanoes, <a href="#Page_2">2</a><br /> -<br /> -<a id="C"></a><span class="dropcap">C</span>ADER IDRIS, <a href="#Page_274">274</a><br /> -'Calderas,' formation of, <a href="#Page_180">180</a><br /> -Caldera of Palma, <a href="#Page_209">209</a><br /> -Cambro-Silurian volcanoes of British Islands, <a href="#Page_274">274</a><br /> -<i>Campi-Phlegræi, map of</i>, <a href="#fig11">fig. 11</a><br /> -— — volcanoes of, <a href="#Page_79">79</a><br /> -— — tuff-cones of, <a href="#Page_118">118</a><br /> -— — fissures in, <a href="#Page_197">197</a><br /> -Carbonic acid in cavities of crystals, <a href="#Page_63">63</a><br /> -Carboniferous volcanoes of British Islands, <a href="#Page_275">275</a><br /> -Carlsbad, Strudel of, <a href="#Page_218">218</a><br /> -— Strudelstein of, <a href="#Page_184">184</a><br /> -Caspian Sea, mud-volcanoes of, <a href="#Page_182">182</a><br /> -Catacecaumene, volcano cones in, <a href="#Page_79">79</a><br /> -Cause of proximity of volcanoes to sea, <a href="#Page_239">239</a><br /> -Central Asia, volcanoes of, <a href="#Page_236">236</a><br /> -— America, mud-volcanoes, <a href="#Page_182">182</a><br /> -— Pacific, volcanoes of, <a href="#Page_236">236</a><br /> -'Challenger,' H.M.S., voyage of, <a href="#Page_73">73</a><br /> -— — soundings of, <a href="#Page_359">359</a><br /> -<span class="smcap">Chance</span>, Messrs., of Birmingham, <a href="#Page_55">55</a><br /> -Charnwood Forest, ancient volcanic rocks of, <a href="#Page_259">259</a><br /> -Chemical deposits at Vulcano, <a href="#Page_44">44</a><br /> -— — on surfaces of lavas, <a href="#Page_110">110</a><br /> -— elements present in lavas, <a href="#Page_46">46</a><br /> -— theory of volcanoes, <a href="#Page_344">344</a>, <a href="#Page_346">346</a><br /> -Chiaja di Luna, <a href="#Page_108">108</a><br /> -Chimborazo, size of, <a href="#Page_44">44</a><br /> -— <a href="#Page_151">151</a><br /> -Chodi-Berg, Hungary, <a href="#Page_161">161</a><br /> -<i>Citlaltepetl, view of</i>, <a href="#Page_169">169</a>, <a href="#fig69">fig. 69</a><br /> -Coast-lines, proximity of volcanoes to, <a href="#Page_228">228</a><br /> -<span class="smcap">Cole</span>, Mr. <span class="smcap">Grenville</span>, <a href="#Page_110">110</a><br /> -Colours of lavas, <a href="#Page_49">49</a><br /> -Columns in lava, <a href="#Page_105">105</a><br /> -— — dimensions of, <a href="#Page_105">105</a><br /> -— radiating in intrusive masses, <a href="#Page_136">136</a><br /> -Columnar structure in lavas, <a href="#Page_104">104</a><br /> -— — origin of, <a href="#Page_105">105</a><br /> -<i>Columnar lava-stream on the Ardèche</i>, <a href="#Page_107">107</a>, <a href="#fig28">fig. 28</a><br /> -Combustion, does not take place at volcanoes, <a href="#Page_2">2</a><br /> -Composite cones, <a href="#Page_128">128</a>, <a href="#Page_161">161</a><br /> -Comstock mines, temperature of, <a href="#Page_342">342</a><br /> -<i>Concentric jointing in lava</i>, <a href="#Page_108">108</a>, <a href="#fig30">fig. 30</a><br /> -<i>Cones composed of viscid lava</i>, <a href="#Page_129">129</a>, <a href="#fig43">fig. 43</a><br /> -— miniature on lava-streams, <a href="#Page_100">100</a>, <a href="#Page_101">101</a>, figs. <a href="#fig25">25</a>, <a href="#fig26">26</a><br /> -— natural sections of, <a href="#Page_129">129</a><br /> -— shifting of axis in, <a href="#Page_167">167</a><br /> -Coolin Hills, Skye, <a href="#Page_144">144</a><br /> -Cotopaxi, volcanic dust of, <a href="#Page_69">69</a><br /> -— <i>view of</i>, <a href="#Page_168">168</a>, <a href="#fig68">fig. 68</a><br /> -Craters, formation of, <a href="#Page_82">82</a><br /> -— origin of, <a href="#Page_167">167</a><br /> -— position of, <a href="#Page_167">167</a><br /> -— fissuring of sides, <a href="#Page_180">180</a><br /> -Crater of Stromboli, aperture at bottom of, <a href="#Page_15">15</a><br /> -Crater-lakes, formation of, <a href="#Page_171">171</a><br /> -— of Agnano, <a href="#Page_171">171</a><br /> -— of Albano, <a href="#Page_171">171</a> -<span class="pagenum" id="Page_373">- 373 -</span><br /> -— of Avernus, <a href="#Page_171">171</a><br /> -— <i>of Bagno</i>, <a href="#Page_171">171</a>, <a href="#fig71">fig. 71</a><br /> -— of Bolsena, <a href="#Page_171">171</a><br /> -— of Bracciano, <a href="#Page_171">171</a><br /> -— of Frascati, <a href="#Page_173">173</a>, <a href="#Page_175">175</a><br /> -— <i>of Gustavila</i>, <a href="#Page_171">171</a>, <a href="#fig72">fig. 72</a><br /> -— of Laach, <a href="#Page_171">171</a><br /> -— of Nemi, <a href="#Page_171">171</a><br /> -Crater-rings, formation of, <a href="#Page_170">170</a><br /> -<i>Crater-ring of Somma</i>, <a href="#Page_177">177</a>, <a href="#fig76">fig. 76</a><br /> -Crater-ring of Pianura, <a href="#Page_174">174</a><br /> -— — of Piano di Quarto, <a href="#Page_174">174</a><br /> -— — of Vallariccia, <a href="#Page_174">174</a><br /> -Creuzot, shafts at, <a href="#Page_340">340</a><br /> -'Critical point' of liquids, <a href="#Page_63">63</a><br /> -Crust of globe, definition of, <a href="#Page_308">308</a><br /> -Crystals in lavas, <a href="#Page_51">51</a><br /> -— — formed of crystallites, <a href="#Page_54">54-57</a><br /> -— — formed in subterranean reservoirs, <a href="#Page_60">60</a><br /> -— — interruption in growth of, <a href="#Page_60">60</a><br /> -— pressure under which formed, <a href="#Page_65">65</a><br /> -— deposited on surface of lava, <a href="#Page_110">110</a><br /> -— porphyritic, origin of, <a href="#Page_256">256</a><br /> -Crystalline minerals formed beneath volcanoes, <a href="#Page_146">146</a>, <a href="#Page_147">147</a><br /> -— — ejected from volcanoes, <a href="#Page_147">147</a><br /> -Crystallised minerals of volcanoes, <a href="#Page_46">46</a><br /> -<a id="Crystallites"></a>Crystallites, aggregates of, <a href="#Page_54">54</a>, <i>Frontispiece</i><br /> -Crystallites in lavas, <a href="#Page_53">53</a>, <i>Frontispiece</i><br /> -Crypto-crystalline base, <a href="#Page_57">57</a><br /> -'Cupolas,' <a href="#Page_135">135</a><br /> -Corral of Madeira, <a href="#Page_209">209</a><br /> -<br /> -<a id="D"></a><span class="dropcap">D</span>ACITES, <a href="#Page_198">198</a><br /> -<span class="smcap">Dana</span>, Professor, <span class="allsmcap">J. D.</span>, cited, <a href="#Page_100">100</a>, <a href="#Page_159">159</a>, <a href="#Page_291">291</a>, <a href="#Page_301">301</a>, <a href="#Page_327">327</a>, <a href="#Page_338">338</a>, <a href="#Page_339">339</a><br /> -<span class="smcap">Darwin</span>, Mr., cited, <a href="#Page_245">245</a>, <a href="#Page_246">246</a>, <a href="#Page_271">271</a>, <a href="#Page_289">289</a><br /> -<span class="smcap">Daubeny</span>, cited, <a href="#Page_182">182</a><br /> -<span class="smcap">Daubrée</span>, M., cited, <a href="#Page_147">147</a>, <a href="#Page_315">315</a>, <a href="#Page_320">320</a>, <a href="#Page_358">358</a><br /> -Daubréelite, <a href="#Page_314">314</a><br /> -<span class="smcap">Davy</span>, Sir <span class="smcap">Humphry</span>, chemical theory of volcanoes, <a href="#Page_344">344</a>, <a href="#Page_345">345</a><br /> -Deccan of India, <a href="#Page_103">103</a><br /> -Density of the earth, <a href="#Page_306">306</a><br /> -<i>Denuded cones and craters</i>, <a href="#Page_158">158</a>, <a href="#fig59">fig. 59</a><br /> -Denudation, effects of, on volcanoes, <a href="#Page_114">114</a><br /> -Deposits about volcanic fissures, <a href="#Page_42">42</a><br /> -Detonations at Vesuvius, <a href="#Page_26">26</a><br /> -Devonian volcanoes of British Islands, <a href="#Page_274">274</a><br /> -Diorite, <a href="#Page_59">59</a><br /> -<span class="smcap">Dolomieu</span>, cited, <a href="#Page_4">4</a>, <a href="#Page_39">39</a><br /> -<span class="smcap">Durocher</span>, cited, <a href="#Page_201">201</a><br /> -Dykes, formation of, <a href="#Page_116">116</a>, <a href="#Page_117">117</a>, <a href="#Page_209">209</a>, <a href="#Page_210">210</a><br /> -— structure of rock in, <a href="#Page_211">211</a><br /> -— pseudo-, <a href="#Page_119">119</a><br /> -Dynamical theory of volcanoes, <a href="#Page_347">347</a>, <a href="#Page_348">348</a><br /> -<br /> -<a id="E"></a><span class="dropcap">E</span>ARTH'S interior, nature of, <a href="#Page_309">309</a><br /> -— — physical condition of, <a href="#Page_325">325</a><br /> -— — hypothesis concerning, <a href="#Page_328">328-330</a><br /> -— relation to other planets, <a href="#Page_310">310</a>, <a href="#Page_311">311</a><br /> -Earthquakes, depth of origin of, <a href="#Page_343">343</a>, <a href="#Page_344">344</a><br /> -— connection with volcanoes, <a href="#Page_343">343</a><br /> -— accompanying Vesuvian eruption of 1872, <a href="#Page_27">27</a><br /> -<a id="Ebullition"></a>Ebullition, compared to volcanic eruptions, <a href="#Page_19">19</a>, <a href="#Page_20">20</a><br /> -Eifel, volcanic cones of, <a href="#Page_45">45</a><br /> -Ejected blocks, <a href="#Page_45">45</a><br /> -— materials, height to which thrown, <a href="#Page_72">72</a><br /> -— — stratification of, <a href="#Page_117">117-119</a><br /> -Elements, pyroxenic and trachytic, theory of, <a href="#Page_201">201</a> -<span class="pagenum" id="Page_374">- 374 -</span><br /> -Elevation-craters, theory of, <a href="#Page_135">135</a>, <a href="#Page_200">200</a><br /> -Erroneous opinions, sources of, in regard to volcanoes, <a href="#Page_2">2</a><br /> -Eruptions, feeble and violent compared, <a href="#Page_31">31</a><br /> -— prediction of, not possible, <a href="#Page_32">32</a><br /> -— intervals between, <a href="#Page_33">33</a><br /> -— of varying intensity, <a href="#Page_33">33</a><br /> -— and barometric pressure, <a href="#Page_36">36</a><br /> -— effects of repetition of from same fissure, <a href="#Page_80">80</a><br /> -Eruptive action in sun and moon, <a href="#Page_360">360-369</a><br /> -Etna, ideas of ancients concerning, <a href="#Page_3">3</a><br /> -— and Anne Boleyn, <a href="#Page_3">3</a><br /> -— observatory on, <a href="#Page_37">37</a><br /> -— size of, <a href="#Page_44">44</a><br /> -—, <a href="#Page_151">151</a><br /> -— eruptions at summit and on flanks, <a href="#Page_207">207</a><br /> -<i>Etna, dyke and lava-stream in</i>, <a href="#Page_133">133</a>, <a href="#fig54">fig. 54</a><br /> -<i>Etna, views of</i>, <a href="#Page_162">162</a>, <a href="#Page_163">163</a>, figs. <a href="#fig62">62</a>, <a href="#fig63">63</a><br /> -Euganean Hills, <a href="#Page_139">139</a><br /> -— — volcanoes of, <a href="#Page_201">201</a><br /> -Europe, volcanoes of, <a href="#Page_227">227</a><br /> -Extra-terrestrial rocks, <a href="#Page_316">316</a><br /> -<i>Extra-terrestrial rocks, relation to ultra-basic rocks</i>, <a href="#Page_322">322</a>, <a href="#fig88">fig. 88</a><br /> -<br /> -<a id="F"></a><span class="dropcap">F</span>ELSTONES, <a href="#Page_263">263</a><br /> -Ferric-chloride, mistaken for sulphur, <a href="#Page_41">41</a><br /> -<i>Fissure on flanks of Etna</i>, <a href="#Page_194">194</a>, <a href="#fig84">fig. 84</a><br /> -Fissure-eruptions, <a href="#Page_188">188</a><br /> -Fissures, volcanic cones on, <a href="#Page_194">194</a><br /> -— systems of, <a href="#Page_198">198</a><br /> -Fingal's Cave, <a href="#Page_106">106</a><br /> -Flames, phenomena mistaken for, <a href="#Page_2">2</a><br /> -— at volcanoes, feebly luminous, <a href="#Page_17">17</a><br /> -— false appearance of, in volcanoes, <a href="#Page_17">17</a><br /> -Flames at volcanic vents, <a href="#Page_41">41</a><br /> -Flashing lighthouse, compared to Stromboli, <a href="#Page_10">10</a><br /> -Floods, accompanying volcanic outbursts, <a href="#Page_30">30</a><br /> -<span class="smcap">Forbes</span>, Mr. <span class="smcap">David</span>, cited, <a href="#Page_337">337</a>, <a href="#Page_339">339</a><br /> -Fossils, from beneath Vesuvius, <a href="#Page_45">45</a><br /> -— supposed in basalt, <a href="#Page_250">250</a><br /> -<span class="smcap">Fouqué</span>, M., cited, <a href="#Page_110">110</a>, <a href="#Page_213">213</a><br /> -Fumaroles, gases emitted from, <a href="#Page_213">213</a><br /> -Fusiyama, form of, <a href="#Page_90">90</a>, <a href="#Page_166">166</a><br /> -<i>Fusiyama</i>, <a href="#Page_178">178</a>, <a href="#fig77_neg">fig. 77</a><br /> -<br /> -<a id="G"></a><span class="dropcap">G</span>ABBRO, <a href="#Page_59">59</a><br /> -Gardiner's river, travertine terraces of, <a href="#Page_185">185</a><br /> -Gases emitted from volcanoes, <a href="#Page_40">40</a><br /> -— — volcanic vents, <a href="#Page_212">212-216</a><br /> -Geanticlinals, formation of, <a href="#Page_297">297</a><br /> -Gems, formation of, <a href="#Page_147">147</a><br /> -— mode of occurrence, <a href="#Page_148">148</a><br /> -Geological continuity, doctrine of, <a href="#Page_247">247</a><br /> -Geosynclinals, formation of, <a href="#Page_294">294</a><br /> -Geysers, formation of, <a href="#Page_217">217</a><br /> -— intermittent action of, <a href="#Page_218">218</a><br /> -— of Colorado, <a href="#Page_184">184</a>, <a href="#Page_217">217</a><br /> -— of Iceland, <a href="#Page_184">184</a>, <a href="#Page_217">217</a><br /> -Giant's Causeway, <a href="#Page_108">108</a><br /> -<span class="smcap">Gilbert</span>, Mr. <span class="allsmcap">G. K.</span>, cited, <a href="#Page_208">208</a><br /> -Girgenti, mud-volcanoes of, <a href="#Page_182">182</a><br /> -Glass, formed by fusion of lavas, <a href="#Page_52">52</a><br /> -Glasses, composed of certain silicates, <a href="#Page_58">58</a><br /> -Glassy base, <a href="#Page_57">57</a><br /> -<span class="smcap">Goethe</span>, cited, <a href="#Page_112">112</a><br /> -<i>Graham Isle</i>, <a href="#Page_178">178</a>, <a href="#Page_179">179</a>, <a href="#fig78">fig. 78</a><br /> -<span class="smcap">Graham</span>, cited, <a href="#Page_345">345</a><br /> -Grand Sarcoui, Auvergne, <a href="#Page_161">161</a><br /> -Granite, <a href="#Page_59">59</a><br /> -Granite of Secondary and Tertiary ages, <a href="#Page_254">254</a> -<span class="pagenum" id="Page_375">- 375 -</span><br /> -Granitic rocks, position beneath volcanoes, <a href="#Page_145">145</a><br /> -Great earth movements, nature of, <a href="#Page_286">286</a><br /> -Great volcanic bands of the globe, <a href="#Page_232">232-234</a><br /> -Grenelle, boring of, <a href="#Page_341">341</a><br /> -Greystones, <a href="#Page_49">49</a><br /> -Groundmass of lavas, <a href="#Page_52">52</a><br /> -Grotto del Cane, <a href="#Page_215">215</a><br /> -Guevo Upas, Java, <a href="#Page_215">215</a><br /> -<span class="smcap">Guiscardi</span>, Professor, referred to, <a href="#Page_45">45</a><br /> -<i>Gustavila, crater-lake of</i>, <a href="#Page_172">172</a>, <a href="#fig72">fig. 72</a><br /> -<br /> -<a id="H"></a><span class="dropcap">H</span>AMILTON, Sir W., researches of, <a href="#Page_4">4</a>, <a href="#Page_75">75</a>, <a href="#Page_84">84</a><br /> -— — observations on Vesuvius, <a href="#Page_80">80</a><br /> -<span class="smcap">Hannay</span>, Mr., referred to, <a href="#Page_147">147</a><br /> -<span class="smcap">Hartley</span>, Mr. <span class="smcap">Noel</span>, referred to, <a href="#Page_65">65</a><br /> -Hawaii, volcanoes of, <a href="#Page_100">100</a>, <a href="#Page_125">125</a><br /> -— — lava-masses of, <a href="#Page_159">159</a><br /> -— — volcanic eruptions at different levels, <a href="#Page_327">327</a><br /> -Hebrides, volcanoes of, <a href="#Page_271">271</a><br /> -<span class="smcap">Henry</span>, Dr., cited, <a href="#Page_355">355</a><br /> -Henry Mountains, Southern Utah, <a href="#Page_208">208</a><br /> -Hephæstus, forge of, <a href="#Page_3">3</a><br /> -<span class="smcap">Hochstetter</span>, cited, <a href="#Page_135">135</a>, <a href="#Page_356">356</a><br /> -Holosiderites, <a href="#Page_315">315</a><br /> -<span class="smcap">Hopkins</span>, Mr., cited, <a href="#Page_349">349</a><br /> -Hot springs, numbers of, <a href="#Page_219">219</a><br /> -Humboldt, researches of, <a href="#Page_4">4</a><br /> -Hungary, lavas of, <a href="#Page_96">96</a>, <a href="#Page_103">103</a><br /> -— volcanoes of, <a href="#Page_126">126</a>, <a href="#Page_201">201</a><br /> -— deep wells of, <a href="#Page_341">341</a><br /> -<i>Hverfjall, Iceland</i>, <a href="#Page_178">178</a>, <a href="#fig77_neg">fig. 77</a><br /> -Hydro-electric machine of Sir W. Armstrong, <a href="#Page_29">29</a><br /> -Hypothesis, value of, <a href="#Page_331">331-333</a><br /> -<br /> -<a id="I"></a><span class="dropcap">I</span>CE under lava of Vesuvius in 1872, and of Etna, <a href="#Page_110">110</a><br /> -Iceland, volcanic dust of, carried to Norway, <a href="#Page_72">72</a><br /> -Indian Ocean, volcanoes in, <a href="#Page_229">229</a><br /> -<i>Insel Ferdinandez</i>, <a href="#Page_178">178</a>, <a href="#Page_179">179</a>, <a href="#fig78">fig. 78</a><br /> -Intermediate lavas, <a href="#Page_48">48</a><br /> -Intervals between Eruptions, <a href="#Page_33">33</a><br /> -Ireland, north-east of, <a href="#Page_103">103</a><br /> -<i>Iron in Ovifak-basalts</i>, <a href="#Page_319">319</a>, <a href="#fig87">fig. 87</a><br /> -Iron, seething of, <a href="#Page_356">356</a><br /> -— of Ovifak, terrestrial origin of, <a href="#Page_320">320</a><br /> -Ischia, eruption in 1301, <a href="#Page_164">164</a><br /> -— <i>crater-lake of Bagno in</i>, <a href="#Page_172">172</a>, <a href="#fig71">fig. 71</a><br /> -— <i>plan of</i>, <a href="#Page_163">163</a>, <a href="#fig64">fig. 64</a><br /> -— <i>parasitic cones in</i>, <a href="#Page_164">164</a>, <a href="#fig65">fig. 65</a><br /> -Island of Bourbon, <a href="#Page_93">93</a><br /> -<i>Isle Julie</i>, <a href="#Page_178">178</a>, <a href="#Page_179">179</a>, <a href="#fig78">fig. 78</a><br /> -Isogeotherms, <a href="#Page_359">359</a><br /> -<br /> -<a id="J"></a><span class="dropcap">J</span>ANSSEN, referred to, <a href="#Page_42">42</a><br /> -Joint-structures in lava, <a href="#Page_104">104-110</a><br /> -<br /> -<a id="K"></a><i><span class="dropcap">K</span>AMMERBÜHL</i>, <a href="#Page_112">112-114</a>, <a href="#fig33">fig. 33</a><br /> -— <i>section of</i>, <a href="#Page_114">114</a>, <a href="#fig34">fig. 34</a><br /> -— <i>section in side of</i>, <a href="#Page_118">118</a>, <a href="#fig36">fig. 36</a><br /> -<span class="smcap">Kant</span>, nebular hypothesis of, <a href="#Page_352">352</a><br /> -Kilauea, volcano of, <a href="#Page_71">71</a>, <a href="#Page_138">138</a><br /> -— crater of, <a href="#Page_181">181</a><br /> -<span class="smcap">King</span>, Mr. <span class="smcap">Clarence</span>, cited, <a href="#Page_301">301</a><br /> -<br /> -<a id="L"></a><span class="dropcap">L</span>AACHER SEE, minerals ejected at, <a href="#Page_149">149</a><br /> -<i>Lac Paven, Auvergne</i>, <a href="#Page_171">171</a>, <a href="#fig70">fig. 70</a><br /> -'Laccolites,' formation of, <a href="#Page_208">208</a><br /> -Lago di Bolsena, <a href="#Page_173">173</a>, <a href="#Page_175">175</a><br /> -Lago di Bracciano, dimensions of, <a href="#Page_172">172</a>, <a href="#Page_173">173</a><br /> -Lake Avernus, <a href="#Page_215">215</a> -<span class="pagenum" id="Page_376">- 376 -</span><br /> -Lapilli, <a href="#Page_70">70</a><br /> -<span class="smcap">Laplace</span>, nebular hypothesis of, <a href="#Page_325">325</a>, <a href="#Page_352">352</a><br /> -Lavas, action of acid gases on, <a href="#Page_41">41</a><br /> -— resemblance to slags, <a href="#Page_46">46</a><br /> -— chemical elements in, <a href="#Page_46">46</a><br /> -— oxygen in, <a href="#Page_47">47</a><br /> -— silicon in, <a href="#Page_47">47</a><br /> -— proportion of silica and other oxides in, <a href="#Page_47">47</a><br /> -— silicates in, <a href="#Page_47">47</a><br /> -— acid, intermediate, basic, <a href="#Page_48">48</a><br /> -— specific gravities of, <a href="#Page_49">49</a><br /> -— colours of, <a href="#Page_49">49</a><br /> -— microscopic study of, <a href="#Page_50">50</a><br /> -— fusibility of, <a href="#Page_51">51</a><br /> -— minerals in, <a href="#Page_51">51</a><br /> -— artificially fused, <a href="#Page_51">51</a><br /> -— crystals in, <a href="#Page_51">51</a>, <a href="#Page_93">93</a><br /> -— ground mass of, <a href="#Page_52">52</a><br /> -— crystalline forms of, <a href="#Page_59">59</a><br /> -— of Bohemia, <a href="#Page_103">103</a><br /> -— of Hungary, <a href="#Page_96">96</a>, <a href="#Page_103">103</a><br /> -— of Kilauea, <a href="#Page_95">95</a><br /> -— of Lipari, <a href="#Page_96">96</a><br /> -— of Niedermendig, <a href="#Page_103">103</a><br /> -— of Vesuvius, <a href="#Page_104">104</a><br /> -— of Volvic, <a href="#Page_95">95</a><br /> -— of Volcano, <a href="#Page_95">95</a><br /> -— presence of water In, <a href="#Page_102">102</a><br /> -— chemical deposits on, <a href="#Page_110">110</a><br /> -— different fluidity of, <a href="#Page_204">204</a><br /> -— augite and hornblende in, <a href="#Page_267">267</a><br /> -<i>Lava, cascade of</i>, <a href="#Page_93">93</a>, <a href="#fig18">fig. 18</a><br /> -Lava-cones, composed of liquid lava, <a href="#Page_125">125</a><br /> -— — of viscid lava, <a href="#Page_126">126</a>, <a href="#Page_127">127</a><br /> -— characters of, of liquid lava, <a href="#Page_159">159</a><br /> -— — of viscid lava, <a href="#Page_160">160</a><br /> -<i>Lava-cones, outlines of</i>, <a href="#Page_160">160</a>, <a href="#fig60_neg">fig. 60</a><br /> -Lava, in deep-seated reservoirs, <a href="#Page_138">138</a><br /> -— consolidation of, at great depths, <a href="#Page_139">139</a><br /> -Lava-fountains, <a href="#Page_94">94</a><br /> -<i>Lava-sheets, intrusive</i>, <a href="#Page_136">136</a>, <a href="#Page_137">137</a>, <a href="#fig56">fig. 56</a><br /> -'Lava' ornaments of Naples, <a href="#Page_45">45</a><br /> -'Lava,' slow-cooling of, <a href="#Page_110">110</a><br /> -— a bad conductor of heat, <a href="#Page_110">110</a><br /> -— ice under, <a href="#Page_110">110</a><br /> -Lava-streams, nature of movements, <a href="#Page_92">92</a><br /> -— difference in liquidity of, <a href="#Page_92">92</a><br /> -— miniature cones on, <a href="#Page_100">100</a>, <a href="#Page_101">101</a><br /> -— vast dimensions of, <a href="#Page_102">102</a><br /> -— structure of, <a href="#Page_103">103</a><br /> -— position of columns in, <a href="#Page_106">106</a><br /> -— sinking of surface of, <a href="#Page_111">111</a><br /> -'Lave di fango,' <a href="#Page_30">30</a><br /> -'Lave di fuoco,' <a href="#Page_30">30</a><br /> -Lawrencite, <a href="#Page_314">314</a><br /> -Laws of volcanic action, <a href="#Page_38">38</a><br /> -<span class="smcap">Le Conte</span>, cited, <a href="#Page_347">347</a><br /> -Leucite, absence from ancient lavas, <a href="#Page_268">268</a><br /> -Lightning, accompanying volcanic outbursts, <a href="#Page_28">28</a><br /> -Linear arrangement of volcanic vents, <a href="#Page_191">191</a><br /> -— — of volcanoes, <a href="#Page_231">231</a><br /> -<a id="Lipari"></a>Lipari Islands, <a href="#Page_3">3</a>, <a href="#Page_39">39</a><br /> -— — fissures in, <a href="#Page_197">197</a><br /> -— — pumice-cones in, <a href="#Page_154">154</a><br /> -— — order of appearance of lavas in, <a href="#Page_200">200</a><br /> -— — <i>breached pumice-cones in</i>, <a href="#Page_124">124</a>, <a href="#fig41">fig. 41</a><br /> -— — <i>map of</i>, <a href="#Page_192">192</a>, <a href="#fig81">fig. 81</a><br /> -— — <i>lavas of</i>, <a href="#Page_96">96</a>, figs. <a href="#fig20">20</a>, <a href="#fig21">21</a><br /> -Liquids in cavities of crystals, <a href="#Page_63">63</a><br /> -<i>Liquid cavities in lavas</i>, <a href="#Page_60">60</a>, <a href="#fig07">fig. 7</a><br /> -— — <i>spontaneous movement of bubbles in</i>, <a href="#Page_62">62</a>, <a href="#fig08">fig. 8</a><br /> -— — spontaneous movement of bubbles in, cause of, <a href="#Page_65">65</a><br /> -<span class="smcap">Lockyer</span>, Mr. <span class="smcap">Norman</span>, cited, <a href="#Page_322">322</a>, <a href="#Page_363">363</a>, <a href="#Page_364">364</a><br /> -<i>Lunar craters</i>, <a href="#Page_368">368</a>, <a href="#fig95">fig. 95</a><br /> -<span class="smcap">Lyell</span>, Sir <span class="smcap">Charles</span>, cited, <a href="#Page_135">135</a>, <a href="#Page_167">167</a>, <a href="#Page_197">197</a><br /> -<br /> -<a id="M"></a><span class="dropcap">M</span>ACCULLOCH, cited, <a href="#Page_207">207</a>, <a href="#Page_208">208</a><br /> -<i>Madeira, cliff-section in</i>, <a href="#Page_128">128</a>, <a href="#fig47">fig. 47</a> -<span class="pagenum" id="Page_377">- 377 -</span><br /> -Magmas, theory of, <a href="#Page_201">201</a><br /> -— objections to, <a href="#Page_202">202</a>, <a href="#Page_203">203</a><br /> -<span class="smcap">Mallet</span>, Mr., cited, <a href="#Page_269">269</a>, <a href="#Page_343">343</a>, <a href="#Page_346">346</a><br /> -<i>Mamelons of Bourbon</i>, <a href="#Page_126">126</a>, <a href="#Page_127">127</a>, figs. <a href="#fig45">45</a>, <a href="#fig46">46</a><br /> -<span class="smcap">Maskelyne</span>, Professor, cited, <a href="#Page_314">314</a><br /> -Massa di Somma, destruction of, <a href="#Page_26">26</a><br /> -Mauna Loa, <a href="#Page_138">138</a><br /> -Metamorphism around volcanic vents, <a href="#Page_145">145</a><br /> -Meteorites, nature of, <a href="#Page_312">312</a><br /> -— composition of, <a href="#Page_313">313</a><br /> -— minerals of, <a href="#Page_314">314</a><br /> -— classification of, <a href="#Page_315">315</a><br /> -Melaphyres, <a href="#Page_262">262</a><br /> -Miascite, <a href="#Page_59">59</a><br /> -<span class="smcap">Michel Lévy</span>, M., <a href="#Page_110">110</a><br /> -Micro-crystalline base, <a href="#Page_58">58</a><br /> -Microliths. <i>See</i> <a href="#Crystallites">Crystallites</a><br /> -Microscopic study of lavas, <a href="#Page_50">50</a><br /> -Minerals in lavas, <a href="#Page_51">51</a><br /> -— of Vesuvius, <a href="#Page_46">46</a><br /> -Mineral-veins, formation of, <a href="#Page_149">149</a><br /> -— connection with volcanoes, <a href="#Page_220">220</a><br /> -— nature of materials in, <a href="#Page_321">321</a><br /> -<i>Misenum, Cape of, section of tuff-cone of</i>, <a href="#Page_121">121</a>, <a href="#fig38">fig. 38</a><br /> -Modena, mud-volcanoes of, <a href="#Page_182">182</a><br /> -<i>Mont Dore, section at</i>, <a href="#Page_130">130</a>, <a href="#fig48">fig. 48</a><br /> -Monte Cerboli, Tuscany, <a href="#Page_216">216</a><br /> -Monte Massi, Tuscany, well at, <a href="#Page_341">341</a><br /> -Monte Nuovo, history of formation of, <a href="#Page_76">76</a><br /> -— — <i>description of</i>, <a href="#Page_77">77</a>, <a href="#Page_78">78</a>, <a href="#fig10">fig. 10</a><br /> -— — <a href="#Page_152">152</a><br /> -— — crater of, <a href="#Page_168">168</a><br /> -— — production of fissure at, <a href="#Page_190">190</a><br /> -Monte Rotondo, Tuscany, <a href="#Page_216">216</a><br /> -Moon, effect of internal forces on, <a href="#Page_305">305</a><br /> -Mountains, all volcanoes not, <a href="#Page_2">2</a><br /> -Mountain-chains, formation of, <a href="#Page_291">291</a><br /> -— — all of recent date, <a href="#Page_292">292</a><br /> -Mud-streams at volcanoes, <a href="#Page_30">30</a><br /> -Mud-volcanoes, formation of, <a href="#Page_181">181</a>, <a href="#Page_182">182</a><br /> -<i>Mull, dissected volcano of</i>, <a href="#Page_142">142-4</a>, figs. <a href="#fig57">57</a>, <a href="#fig58">58</a><br /> -Muscovite, absence of, from modern lavas, <a href="#Page_268">268</a><br /> -<br /> -<a id="N"></a><span class="dropcap">N</span>EBULAR hypothesis of Laplace, <a href="#Page_325">325</a>, <a href="#Page_352">352</a><br /> -— — of Kant, <a href="#Page_352">352</a><br /> -New Zealand, geysers of, <a href="#Page_217">217</a><br /> -— — volcanoes of, <a href="#Page_135">135</a><br /> -— — volcanic cones in, <a href="#Page_79">79</a><br /> -Niedermendig, lava of, <a href="#Page_103">103</a><br /> -<span class="smcap">Nordenskiöld</span>, Professor, cited, <a href="#Page_318">318</a><br /> -<br /> -<a id="O"></a><span class="dropcap">O</span>BSERVATORY on Vesuvius, <a href="#Page_24">24</a>, <a href="#Page_37">37</a><br /> -— on Etna, <a href="#Page_37">37</a><br /> -Obsidian, <a href="#Page_59">59</a><br /> -Oceans, depth of, in volcanic areas, <a href="#Page_242">242</a><br /> -Oceanic islands, volcanoes in, <a href="#Page_228">228</a><br /> -<span class="smcap">Oliver</span>, Capt. <span class="allsmcap">S. P.</span>, <a href="#Page_92">92</a><br /> -Oldhamite, <a href="#Page_314">314</a><br /> -<i>Outlines of Vesuvius</i>, <a href="#Page_87">87</a>, <a href="#fig17">fig. 17</a><br /> -Ovifak, iron-masses of, <a href="#Page_319">319</a><br /> -Oxidation of materials of globe, <a href="#Page_324">324</a><br /> -Oxygen, proportion in lavas, <a href="#Page_47">47</a><br /> -<br /> -<a id="P"></a><span class="dropcap">P</span>ACIFIC, volcanoes in, <a href="#Page_229">229</a><br /> -<span class="smcap">Palmieri</span>, Professor, cited, <a href="#Page_25">25</a>, <a href="#Page_37">37</a><br /> -Papandayang, eruption of, <a href="#Page_169">169</a><br /> -Papin's digester, nature of action in, <a href="#Page_22">22</a><br /> -<i>Parasitic cones, formation of</i>, <a href="#Page_161">161</a>, <a href="#Page_162">162</a>, <a href="#fig61">fig. 61</a><br /> -Pele's Hair, <a href="#Page_71">71</a><br /> -Perlitic structure, <a href="#Page_109">109</a><br /> -<span class="smcap">Phillips</span>, Mr. <span class="allsmcap">J. A.</span>, cited, <a href="#Page_220">220</a><br /> -Phonolites, <a href="#Page_50">50</a>, <a href="#Page_59">59</a> -<span class="pagenum" id="Page_378">- 378 -</span><br /> -Phonolite-volcanoes, <a href="#Page_126">126</a><br /> -<i>Photograph of Vesuvius eruption</i>, <a href="#Page_24">24</a>, <a href="#fig05">fig. 5</a><br /> -'Pine-tree, appendage of Vesuvius, <a href="#Page_29">29</a><br /> -Pitchstones, porphyritic, <a href="#Page_60">60</a><br /> -Plateaux formed of lava-sheets, <a href="#Page_270">270</a><br /> -Pliny, Elder, death of, <a href="#Page_7">7</a><br /> -Plombières, hot springs of, <a href="#Page_147">147</a><br /> -Plutonic rocks, <a href="#Page_61">61</a><br /> -Pompeii, nature of materials covering, <a href="#Page_117">117</a><br /> -Ponza Islands, <a href="#Page_39">39</a><br /> -<i>Ponza, sections in</i>, <a href="#Page_131">131</a>, <a href="#Page_132">132</a>, figs. <a href="#fig51">51</a>, <a href="#fig52">52</a><br /> -Porphyrites, <a href="#Page_263">263</a><br /> -Porphyritic pitchstones, <a href="#Page_60">60</a><br /> -Potentially liquid rock, <a href="#Page_250">250</a><br /> -Pre-Cambrian volcanoes of British Islands, <a href="#Page_274">274</a><br /> -Presence of water in lavas, <a href="#Page_353">353</a><br /> -Pressure under which crystals were formed, <a href="#Page_65">65</a><br /> -<i>Predazzo, ancient volcano of</i>, <a href="#Page_165">165</a>, <a href="#fig67">fig. 67</a><br /> -Propylites, <a href="#Page_199">199</a><br /> -Pseudo-dykes, <a href="#Page_119">119</a><br /> -Pumice, how formed, <a href="#Page_68">68</a><br /> -— cause of white colour of, <a href="#Page_71">71</a><br /> -— floating on ocean, <a href="#Page_73">73</a><br /> -— on ocean-beds, <a href="#Page_73">73</a><br /> -Pumice-cones, <a href="#Page_154">154</a><br /> -<i>Puy de Pariou, Auvergne</i>, <a href="#Page_193">193</a>, <a href="#Page_194">194</a>, figs. <a href="#fig82">82</a>, <a href="#fig83">83</a><br /> -Puzzolana, <a href="#Page_89">89</a><br /> -<br /> -<a id="R"></a><span class="dropcap">R</span>AIN, accompanying volcanic outbursts, <a href="#Page_30">30</a><br /> -Rate of movement of lava-streams, <a href="#Page_97">97</a><br /> -<span class="smcap">Rath</span>, Professor <span class="smcap">Vom</span>, <a href="#Page_72">72</a><br /> -Red clay of ocean-beds, <a href="#Page_74">74</a><br /> -Red Mountains, Skye, <a href="#Page_144">144</a><br /> -Reservoirs beneath volcanoes, <a href="#Page_145">145</a><br /> -<span class="smcap">Reyer</span>, Dr. <span class="smcap">Ed.</span>, experiments of, <a href="#Page_125">125</a>, <a href="#Page_160">160</a><br /> -Reykjanes, eruption of, in 1783, <a href="#Page_102">102</a><br /> -Rhyolites, <a href="#Page_50">50</a>, <a href="#Page_59">59</a><br /> -<span class="smcap">Richthofen, Von</span>, cited, <a href="#Page_196">196</a>, <a href="#Page_199">199</a>, <a href="#Page_200">200</a>, <a href="#Page_205">205</a><br /> -<i>Rocca-Monfina</i>, <a href="#Page_178">178</a>, <a href="#fig77_neg">fig. 77</a><br /> -— —, <a href="#Page_204">204</a><br /> -Rock-masses, movements of, <a href="#Page_288">288</a><br /> -Rocky Mountains, <a href="#Page_103">103</a><br /> -— — volcanoes of, <a href="#Page_201">201</a><br /> -Rotomahana, sinter-terraces of, <a href="#Page_185">185</a><br /> -<i>Ropy-lavas</i>, <a href="#Page_98">98</a>, <a href="#fig24">fig. 24</a><br /> -<br /> -<a id="S"></a><i><span class="dropcap">S</span>ALINA, section in</i>, <a href="#Page_132">132</a>, <a href="#fig53">fig. 53</a><br /> -Sandwich Islands, lavas of, <a href="#Page_125">125</a><br /> -San Sebastiano, destruction of, <a href="#Page_26">26</a><br /> -<i>San Stephano, section in</i>, <a href="#Page_131">131</a>, <a href="#fig50">fig. 50</a><br /> -Santorin, <a href="#Page_42">42</a><br /> -<i>Sarcoui, Grand Puy of</i>, <a href="#Page_126">126</a>, <a href="#fig44">fig. 44</a><br /> -Sciarra del fuoco, <a href="#Page_13">13</a><br /> -Scoria, how formed, <a href="#Page_68">68</a>, <a href="#Page_70">70</a><br /> -Scoria-cones, altered by acid gas, <a href="#Page_155">155</a><br /> -— breached, <a href="#Page_156">156</a><br /> -— characters of, <a href="#Page_153">153</a><br /> -— preservation of, <a href="#Page_155">155</a><br /> -— red colour of, <a href="#Page_154">154</a><br /> -<i>Scoria-cone in Vesuvius</i>, <a href="#Page_122">122</a>, <a href="#fig39">fig. 39</a><br /> -<i>Scoria-cone near Auckland, N. Z.</i>, <a href="#Page_165">165</a>, <a href="#fig66">fig. 66</a><br /> -<span class="smcap">Schmidt</span>, referred to, <a href="#Page_153">153</a><br /> -Schreibersite, <a href="#Page_314">314</a><br /> -<span class="smcap">Scrope</span>, Mr. <span class="smcap">Poulett</span>, cited, <a href="#Page_5">5</a>, <a href="#Page_69">69</a>, <a href="#Page_106">106</a>, <a href="#Page_135">135</a>, <a href="#Page_198">198</a>, <a href="#Page_205">205</a>, <a href="#Page_212">212</a>, <a href="#Page_238">238</a>, <a href="#Page_289">289</a><br /> -Sea of Azof, mud-volcanoes of, <a href="#Page_182">182</a><br /> -<span class="smcap">Secchi</span>, Father, cited, <a href="#Page_362">362</a><br /> -Shiant Isles, <a href="#Page_105">105</a><br /> -Silica, presence in lavas, <a href="#Page_47">47</a><br /> -Silicates in lavas, <a href="#Page_47">47</a><br /> -Silicon, proportion in lavas, <a href="#Page_47">47</a> -<span class="pagenum" id="Page_379">- 379 -</span><br /> -Siliceous sinter, deposits of, <a href="#Page_220">220</a><br /> -Silver, spitting of, <a href="#Page_355">355</a><br /> -<span class="smcap">Silvestri</span>, Professor, cited, <a href="#Page_230">230</a><br /> -Similarity of lavas of different ages, <a href="#Page_260">260</a><br /> -<i>Sinter-cones, forms of</i>, <a href="#Page_183">183</a>, <a href="#fig79_neg">fig. 79</a><br /> -Skye, dissected volcano of, <a href="#Page_144">144</a><br /> -Slags, compared with lavas, <a href="#Page_46">46</a><br /> -<span class="smcap">Smith, Lawrence</span>, cited, <a href="#Page_320">320</a><br /> -Smoke, appearance of, due to steam, <a href="#Page_2">2</a><br /> -Snowdon, <a href="#Page_274">274</a><br /> -<i>Solar prominences</i>, <a href="#Page_364">364-366</a>, figs. <a href="#fig92">92</a>, <a href="#fig93">93</a>, <a href="#fig94">94</a><br /> -Solfatara of Naples, <a href="#Page_214">214</a><br /> -Solfatara-stage of volcanoes, <a href="#Page_215">215</a><br /> -Somma, <a href="#Page_133">133</a><br /> -— crater-ring of, <a href="#Page_83">83</a><br /> -<span class="smcap">Sorby</span>, Mr. <span class="allsmcap">H. C.</span>, referred to, <a href="#Page_59">59</a>, <a href="#Page_252">252</a><br /> -<span class="smcap">Spallanzani</span>, early researches of, <a href="#Page_4">4</a><br /> -— observations on Stromboli, <a href="#Page_8">8</a><br /> -— cited, <a href="#Page_39">39</a>, <a href="#Page_367">367</a><br /> -Specific gravities of lavas, <a href="#Page_49">49</a><br /> -— — of glassy and crystalline rocks, <a href="#Page_59">59</a><br /> -Spectroscope in vulcanology, <a href="#Page_41">41</a><br /> -Spectrum-analysis, results of, <a href="#Page_311">311</a><br /> -Specular-iron, deposited on lava-streams, <a href="#Page_110">110</a><br /> -Sperenberg, boring of, <a href="#Page_341">341</a><br /> -<i>Sphærulites</i>, <a href="#Page_54">54</a>, <i>Frontispiece</i><br /> -Sporadosiderites, <a href="#Page_316">316</a><br /> -Stability of crust of globe, <a href="#Page_326">326</a><br /> -Staffa, Isle of, <a href="#Page_106">106</a><br /> -<span class="smcap">Steenstrup</span>, cited, <a href="#Page_319">319</a><br /> -Steam-engine compared to volcano, <a href="#Page_8">8</a><br /> -Steam, emitted by lava of Vesuvius, <a href="#Page_27">27</a><br /> -<span class="smcap">Sternberg</span>, referred to, <a href="#Page_113">113</a><br /> -St. Kilda, <a href="#Page_181">181</a><br /> -<span class="smcap">Stokes</span>, Professor, <a href="#Page_65">65</a><br /> -St. Paul, Island of, <a href="#Page_180">180</a><br /> -Stromboli, <a href="#Page_42">42</a>, <a href="#Page_158">158</a><br /> -— apertures at bottom of crater, <a href="#Page_15">15</a><br /> -— appearances in crater of, <a href="#Page_16">16</a><br /> -— — at night, <a href="#Page_10">10</a><br /> -— compared with Vesuvius, <a href="#Page_23">23</a><br /> -— crater of, <a href="#Page_13">13</a><br /> -— dependence of eruptions on atmospheric conditions, <a href="#Page_34">34</a><br /> -— eruption of, <a href="#Page_14">14</a>, <a href="#fig04">fig. 4</a><br /> -— general features of, <a href="#Page_11">11</a><br /> -— map of, <a href="#Page_11">11</a>, <a href="#fig02">fig. 2</a><br /> -— observations by Spallanzani, <a href="#Page_8">8</a><br /> -— resemblance to flashing light, <a href="#Page_10">10</a><br /> -— <i>section of</i>, <a href="#Page_13">13</a>, <a href="#fig08">fig. 8</a><br /> -— soundings around, <a href="#Page_12">12</a><br /> -— vapour-cloud above, <a href="#Page_9">9</a><br /> -— violent eruptions of, <a href="#Page_23">23</a><br /> -Strombolian stage, <a href="#Page_23">23</a><br /> -Stufas, nature of, <a href="#Page_217">217</a><br /> -Submarine volcanoes, <a href="#Page_179">179</a><br /> -Subterranean forces, beneficial effects of, <a href="#Page_303">303</a><br /> -Subsidence in centre of volcanoes, <a href="#Page_165">165</a><br /> -Sulphur, absorption of water by molten, <a href="#Page_356">356</a><br /> -— deposited on lava-streams, <a href="#Page_110">110</a><br /> -— how formed at volcanoes, <a href="#Page_18">18</a><br /> -— not the cause of volcanic outbursts, <a href="#Page_18">18</a><br /> -<i>Surfaces of lava-streams</i>, <a href="#Page_97">97-99</a>, figs. <a href="#fig22">22</a>, <a href="#fig23">23</a><br /> -<i>Sun-spots</i>, <a href="#Page_361">361-363</a>, figs. <a href="#fig89">89</a>, <a href="#Page_90">90</a>, <a href="#Page_91">91</a><br /> -Syenite, <a href="#Page_59">59</a><br /> -Syssiderites, <a href="#Page_316">316</a><br /> -<span class="smcap">Szabo</span>, Professor, cited, <a href="#Page_199">199</a><br /> -<br /> -<a id="T"></a><span class="dropcap">T</span>ACHYLYTE, <a href="#Page_59">59</a><br /> -Tertiary volcanoes of British Islands, <a href="#Page_276">276</a><br /> -<i>Terraces, sinter- and travertine-formation of</i>, <a href="#Page_185">185</a>, <a href="#fig80_neg">fig. 80</a><br /> -Temperature, increase in deeper parts of earth's crust, <a href="#Page_335">335</a><br /> -— rate of increase in different areas, <a href="#Page_340">340</a><br /> -Teneriffe, <a href="#Page_44">44</a>, <a href="#Page_151">151</a> -<span class="pagenum" id="Page_380">- 380 -</span><br /> -<i>Teneriffe</i>, <a href="#Page_178">178</a>, <a href="#fig77_neg">fig. 77</a><br /> -— <i>peak of</i>, <a href="#Page_175">175</a>, <a href="#fig73">fig. 73</a><br /> -Tenon-and-mortise structure in basaltic columns, <a href="#Page_107">107</a><br /> -Theodosius and Vulcano, <a href="#Page_3">3</a><br /> -Thunder, accompanying volcanic outbursts, <a href="#Page_28">28</a><br /> -Trachytes, <a href="#Page_49">49</a>, <a href="#Page_50">50</a>, <a href="#Page_69">69</a><br /> -Trap-rocks, origin of, <a href="#Page_241">241</a><br /> -Trass, <a href="#Page_90">90</a><br /> -Travertine or Tibur-stone, <a href="#Page_184">184</a><br /> -— deposits of, <a href="#Page_220">220</a><br /> -Triassic volcanoes of British Islands, <a href="#Page_275">275</a><br /> -Tridymite deposited on lava-streams, <a href="#Page_110">110</a><br /> -Troilite, <a href="#Page_314">314</a><br /> -<span class="smcap">Troost</span>, cited, <a href="#Page_355">355</a><br /> -Tufa, or tuff, <a href="#Page_90">90</a><br /> -Tuff-cones, character of, <a href="#Page_157">157</a><br /> -— denudation of, <a href="#Page_157">157</a>, <a href="#Page_158">158</a><br /> -Typhon, fable of, <a href="#Page_3">3</a><br /> -<br /> -<a id="U"></a><span class="dropcap">U</span>LTRA-BASIC lavas, <a href="#Page_50">50</a>, <a href="#Page_66">66</a><br /> -— rocks, <a href="#Page_317">317</a><br /> -<br /> -<a id="V"></a><span class="dropcap">V</span>AL DEL BOVE, Etna, <a href="#Page_133">133</a>, <a href="#Page_180">180</a>, <a href="#Page_209">209</a><br /> -— — <i>dykes in</i>, <a href="#Page_134">134</a>, <a href="#fig55">fig. 55</a><br /> -Vapour-cloud over Vesuvius, <a href="#Page_26">26</a>, <a href="#Page_29">29</a><br /> -— — Stromboli, <a href="#Page_9">9</a><br /> -Ventotienne, Island of, section at, <a href="#Page_130">130</a>, <a href="#fig49">fig. 49</a><br /> -Vesuvius, <a href="#Page_37">37</a><br /> -— changes in form of, <a href="#Page_81">81</a><br /> -— compared with Stromboli, <a href="#Page_23">23</a><br /> -— <i>crater of in 1756</i>, <a href="#Page_84">84</a>, <a href="#fig14">fig. 14</a><br /> -— — <i>of in 1767</i>, <a href="#Page_85">85</a>, <a href="#fig16">fig. 16</a><br /> -— — <i>of in 1822</i>, <a href="#Page_82">82</a>, <a href="#fig13">fig. 13</a><br /> -— — <i>of in 1843</i>, <a href="#Page_86">86</a>, <a href="#fig16">fig. 16</a><br /> -— detonations at, <a href="#Page_26">26</a><br /> -— early history of, <a href="#Page_83">83</a><br /> -— eruption of year 79, <a href="#Page_84">84</a><br /> -— — of 1822, <a href="#Page_69">69</a><br /> -— — of April 1872, <a href="#Page_24">24</a><br /> -— — of October 1822, <a href="#Page_24">24</a><br /> -— ejected blocks of, <a href="#Page_45">45</a><br /> -— first eruption of, <a href="#Page_7">7</a><br /> -— form of, <a href="#Page_166">166</a><br /> -— fossils of, <a href="#Page_45">45</a><br /> -— growth of cone of, <a href="#Page_80">80</a><br /> -— history of, <a href="#Page_204">204</a><br /> -— last eruption of, <a href="#Page_7">7</a><br /> -— lava-stream of 1855, <a href="#Page_101">101</a><br /> -— lava-streams of 1858, 1872, <a href="#Page_97">97</a><br /> -— lavas of, <a href="#Page_104">104</a><br /> -— minerals of, <a href="#Page_46">46</a><br /> -— — ejected at, <a href="#Page_149">149</a><br /> -— observatory on, <a href="#Page_24">24</a>, <a href="#Page_37">37</a><br /> -— outlines of, <a href="#Page_87">87</a><br /> -— pine-tree appendage of, <a href="#Page_29">29</a><br /> -— scoria-cones in lava, <a href="#Page_122">122</a><br /> -— — on lava of 1855, <a href="#Page_153">153</a><br /> -— steam emitted by lava of, <a href="#Page_27">27</a><br /> -— vapour-cloud over, <a href="#Page_26">26</a>, <a href="#Page_29">29</a><br /> -Vesuvian stage, <a href="#Page_23">23</a><br /> -— <i>eruption, photograph of</i>, <a href="#Page_24">24</a>, <a href="#fig05">fig. 5</a><br /> -Viscid lavas of Lipari Islands, <a href="#Page_94">94-96</a><br /> -Vitreous lavas, devitrification of, <a href="#Page_259">259</a><br /> -Volcanic action, laws of, <a href="#Page_32">32</a><br /> -— bombs, <a href="#Page_70">70</a>, <a href="#Page_71">71</a><br /> -— cycles, nature of, <a href="#Page_221">221</a>, <a href="#Page_222">222</a><br /> -— — duration of, <a href="#Page_223">223</a><br /> -— cones, internal structure of, <a href="#Page_115">115-122</a><br /> -— — <i>experimental illustration of formation of</i>, <a href="#Page_120">120</a>, <a href="#fig37">fig. 37</a><br /> -— — limits to height of, <a href="#Page_166">166</a><br /> -— — form of, <a href="#Page_152">152</a><br /> -— — dimensions of, <a href="#Page_152">152</a><br /> -— — irregular development of, <a href="#Page_90">90</a><br /> -— — slopes of sides of, <a href="#Page_91">91</a><br /> -— — composed of ejected rock-fragments, <a href="#Page_156">156</a><br /> -— — curved slopes of, <a href="#Page_156">156</a><br /> -— <i>débris</i> on sea-bottom, <a href="#Page_240">240</a><br /> -— dust, fineness of, <a href="#Page_69">69</a><br /> -— districts, areas of upheaval, <a href="#Page_245">245</a><br /> -— ejections, alteration of, <a href="#Page_258">258</a> -<span class="pagenum" id="Page_381">- 381 -</span><br /> -— eruptions, compared to ebullition, <a href="#Page_19">19</a>, <a href="#Page_20">20</a><br /> -— forces, compensate for denudation, <a href="#Page_283">283</a><br /> -— — intensity at former periods, <a href="#Page_278">278</a><br /> -— — necessity for action of, <a href="#Page_285">285</a><br /> -— — shifting of from one area to another, <a href="#Page_277">277</a><br /> -— mountains, origin of conical forms, <a href="#Page_89">89</a><br /> -— — mode of growth, <a href="#Page_89">89</a><br /> -— phenomena of the past similar to those at present, <a href="#Page_273">273</a><br /> -— products, order of appearance of, <a href="#Page_198">198</a>, <a href="#Page_199">199</a><br /> -Volcanic rocks, <a href="#Page_61">61</a><br /> -— — similarity of ancient and modern, <a href="#Page_253">253</a><br /> -<a id="Volcano"></a>Volcano, origin of name, <a href="#Page_3">3</a><br /> -— craters of, <a href="#Page_167">167</a><br /> -— Island of. <i>See</i> <a href="#Volcano">Vulcano</a>.<br /> -— compared to steam-engine, <a href="#Page_8">8</a><br /> -Volcanoes, blocks, ejected from, <a href="#Page_45">45</a><br /> -— built up of ejected fragments, <a href="#Page_74">74</a><br /> -— destruction caused by, <a href="#Page_281">281</a><br /> -— dissected by denudation, <a href="#Page_115">115</a>, <a href="#Page_139">139</a><br /> -— erroneous ideas concerning, <a href="#Page_1">1</a><br /> -— ejection of different materials from, <a href="#Page_205">205</a><br /> -— known to ancients, <a href="#Page_3">3</a><br /> -— life-history of, <a href="#Page_186">186</a><br /> -— number of, <a href="#Page_224">224</a>, <a href="#Page_225">225</a><br /> -— of Africa, <a href="#Page_227">227</a><br /> -— of America, <a href="#Page_236">236</a><br /> -— of Asia, <a href="#Page_236">236</a><br /> -— of Bohemia, <a href="#Page_126">126</a><br /> -— of Central Asia, <a href="#Page_236">236</a><br /> -— of Central Pacific, <a href="#Page_236">236</a><br /> -— of Europe, <a href="#Page_227">227</a><br /> -— of Hungary, <a href="#Page_126">126</a><br /> -— position in relation to mountain chains, <a href="#Page_243">243</a><br /> -— popular ideas concerning, <a href="#Page_1">1</a><br /> -— reservoirs beneath, <a href="#Page_145">145</a><br /> -Volvic lava of, <a href="#Page_103">103</a><br /> -<span class="smcap">Vose</span>, cited, <a href="#Page_346">346</a><br /> -Vulcan, forge of, <a href="#Page_3">3</a><br /> -Vulcano, island of, <a href="#Page_3">3</a>, <a href="#Page_158">158</a><br /> -Vulcano and Theodosius, <a href="#Page_3">3</a><br /> -<i>Vulcano</i>, <a href="#Page_178">178</a>, <a href="#fig77_neg">fig. 77</a><br /> -— <i>and Vulcanello, view of</i>, <a href="#Page_43">43</a>, <a href="#fig06">fig. 6</a><br /> -— chemical deposits at, <a href="#Page_44">44</a><br /> -— eruption in 1786, <a href="#Page_43">43</a><br /> -— — in 1873, <a href="#Page_43">43</a><br /> -— — <i>lava-stream in</i>, <a href="#Page_95">95</a>, <a href="#fig19">fig. 19</a><br /> -— —, <a href="#Page_103">103</a>, <a href="#fig27">fig. 27</a><br /> -— <i>plan of</i>, <a href="#Page_195">195</a>, <a href="#fig85">fig. 85</a><br /> -— <i>section of volcanic cone in</i>, <a href="#Page_116">116</a>, <a href="#fig35">fig. 35</a><br /> -— section in, <a href="#Page_129">129</a><br /> -— shifting of centre of eruption in, <a href="#Page_196">196</a><br /> -<i>Vulcanello, craters of</i>, <a href="#Page_197">197</a>, <a href="#fig86">fig. 86</a><br /> -Vulcanology, origin of the science, <a href="#Page_4">4</a><br /> -— earliest treatise on, <a href="#Page_5">5</a><br /> -<br /> -<a id="W"></a><span class="dropcap">W</span>ALFERDIN, M., cited, <a href="#Page_340">340</a><br /> -Water in lavas, <a href="#Page_353">353</a><br /> -— penetration through rocks, <a href="#Page_358">358</a><br /> -— presence of in lavas, <a href="#Page_102">102</a><br /> -— and saline solutions in cavities of crystals, <a href="#Page_63">63</a><br /> -<span class="smcap">Werner</span>, cited, <a href="#Page_201">201</a><br /> -Western Isles of Scotland, <a href="#Page_103">103</a>, <a href="#Page_139">139</a>, <a href="#Page_142">142</a><br /> -— — volcanoes of, <a href="#Page_212">212</a><br /> -<span class="smcap">Whymper</span>, Mr., <a href="#Page_69">69</a><br /> -<span class="smcap">Woodward</span>, Mr., experiments of, <a href="#Page_119">119</a><br /> -Wrekin, ancient volcanic rocks of, <a href="#Page_259">259</a><br /> -<br /> -<a id="Y"></a><span class="dropcap">Y</span>OUNG, Professor, cited, <a href="#Page_365">365</a><br /> -<br /> -<a id="Z"></a><span class="dropcap">Z</span>EOLITES, formation of, <a href="#Page_150">150</a><br /> -</p> 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M.A., F.L.S., -F.G.S., &c., author of 'The Origin of Floral Structures,' &c.</p> - -<p>LXXVIII. ICE-WORK PRESENT and PAST. By Rev. <span class="smcap">T. G. Bonney</span>, -D.Sc., LL.D., F.R.S., &c., Professor of Geology at University -College, London; Fellow of St. John's College, Cambridge.</p> - -<p>LXXIX. A CONTRIBUTION to our KNOWLEDGE of SEEDLINGS. By Rt. Hon. -Sir <span class="smcap">John Lubbock</span>, Bart., M.P., F.R.S.</p> - -<p>LXXX. The ART of MUSIC. By Sir <span class="smcap">C. Hubert H. Parry</span>, Mus. -Doc.</p> - -<p>LXXXI. The POLAR AURORA. By <span class="smcap">Alfred Angot</span>. Illustrated.</p> - -<p>LXXXII. WHAT is ELECTRICITY? By <span class="smcap">J. Trowbridge</span>. Illustrated.</p> - -<p>LXXXIII. MEMORY. By <span class="smcap">F. W. Edridge-Green</span>, M.D. With -Frontispiece.</p> - -<p>LXXXIV. The ELEMENTS of HYPNOTISM. By <span class="smcap">B. Harry Vincent</span>. -With Diagrams. Second Edition.</p> - -<p>LXXXV. SEISMOLOGY. 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