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+The Project Gutenberg eBook, Volcanoes: Past and Present, by Edward Hull
+
+
+This eBook is for the use of anyone anywhere 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
+
+
+
+
+
+Title: Volcanoes: Past and Present
+
+
+Author: Edward Hull
+
+
+
+Release Date: March 13, 2010 [eBook #31627]
+
+Language: English
+
+Character set encoding: ISO-646-US (US-ASCII)
+
+
+***START OF THE PROJECT GUTENBERG EBOOK VOLCANOES: PAST AND PRESENT***
+
+
+E-text prepared by Steven Gibbs, Stephen H. Sentoff, and the Project
+Gutenberg Online Distributed Proofreading Team (http://www.pgdp.net)
+
+
+
+Note: Project Gutenberg also has an HTML version of this
+ file which includes the original illustrations.
+ See 31627-h.htm or 31627-h.zip:
+ (http://www.gutenberg.org/files/31627/31627-h/31627-h.htm)
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+ (http://www.gutenberg.org/files/31627/31627-h.zip)
+
+
+
+
+
+The Contemporary Science Series.
+
+Edited by Havelock Ellis.
+
+VOLCANOES: PAST AND PRESENT.
+
+
+[Illustration: Fig. 1.--Eruption of Vesuvius, 1872-1873]
+
+
+VOLCANOES: PAST AND PRESENT.
+
+by
+
+EDWARD HULL, M.A., LL.D., F.R.S.
+Examiner in Geology to the University of London.
+
+With 41 Illustrations and 4 Plates of Rock-Sections.
+
+
+
+
+
+
+
+London:
+Walter Scott, Limited,
+24, Warwick Lane, Paternoster Row.
+1892.
+
+
+
+
+_BY THE SAME AUTHOR._
+
+ The Coal-fields of Great Britain: their History, Structure, and
+ Resources. 4th edit. (1881.) E. Stanford.
+
+ The Physical History of the British Isles. With a Dissertation on
+ the Origin of Western Europe and of the Atlantic Ocean. (1882.) E.
+ Stanford.
+
+ The Physical Geology and Geography of Ireland. 2nd edit. (1891.)
+ E. Stanford.
+
+ Treatise on the Building and Ornamental Stones of Great Britain
+ and Foreign Countries. (1872.) Macmillan and Co.
+
+ Memoir on the Physical Geology and Geography of Arabia-Petraea,
+ Palestine, and adjoining Districts. (1886.) Committee of the
+ Palestine Exploration Fund.
+
+ Mount Seir, Sinai, and Western Palestine. Being a Narrative of a
+ Scientific Expedition, 1883-84. (1885.) Committee of the Palestine
+ Exploration Fund.
+
+ Text-book of Physiography. (1888.) C. W. Deacon and Co.
+
+ Sketch of Geological History. (1887.) C. W. Deacon and Co.
+
+
+
+
+PREFACE.
+
+
+It has not been my object to present in the following pages even an
+approximately complete description of the volcanic and seismic phenomena
+of the globe; such an undertaking would involve an amount of labour
+which few would be bold enough to attempt; nor would it be compatible
+with the aims of the _Contemporary Science Series_.
+
+I have rather chosen to illustrate the most recent conclusions regarding
+the phenomena and origin of volcanic action, by the selection of
+examples drawn from the districts where these phenomena have been most
+carefully observed and recorded under the light of modern geological
+science. I have also endeavoured to show, by illustrations carried back
+into later geological epochs, how the volcanic phenomena of the present
+day do not differ in kind, though they may in degree, from those of the
+past history of our globe. For not only do the modes of eruption of
+volcanic materials in past geological times resemble those of the
+present or human epoch, but the materials themselves are so similar in
+character that it is only in consequence of alterations in structure or
+composition which the original materials have undergone, since their
+extrusion, that any important distinctions can be recognised between the
+volcanic products of recent times and those of earlier periods.
+
+I have, finally, endeavoured to find an answer to two interesting and
+important questions: (1) Are we now living in an epoch of extraordinary
+volcanic energy?--a question which such terrible outbursts as we have
+recently witnessed in Japan, the Malay Archipelago, and even in Italy,
+naturally suggest; and (2) What is the ultimate cause of volcanic
+action? On this latter point I am gratified to find that my conclusions
+are in accordance with those expounded by one who has been appropriately
+designated "the Nestor of Modern Geology," Professor Prestwich.
+
+Within the last few years the study of the structure and composition of
+volcanic rocks, by means of the microscope brought to bear on their
+translucent sections, has added wonderfully to our knowledge of such
+rocks, and has become a special branch of petrological investigation.
+Commenced by Sorby, and carried on by Allport, Zirkel, Rosenbusch, Von
+Lasaulx, Teall, and many more enthusiastic students, it has thrown a
+flood of light upon our knowledge of the mutual relations of the
+component minerals of igneous masses, the alteration these minerals have
+undergone in some cases, and the conditions under which they have been
+erupted and consolidated. But nothing that has been observed has tended
+materially to alter conclusions arrived at by other processes of
+reasoning regarding volcanic phenomena, and for these we have to fall
+back upon observations conducted in the field on a more or less large
+scale, and carried on before, during, and after eruptions. Macroscopic
+and microscopic observations have to go hand in hand in the study of
+volcanic phenomena.
+
+ E. H.
+
+
+
+
+CONTENTS.
+
+
+PART I.
+
+_INTRODUCTION._
+
+ PAGE
+
+Chap. I. Historic Notices of Volcanic Action 1-9
+
+ " II. Form, Structure, and Composition of Volcanic
+ Mountains 10-19
+
+ " III. Lines and Groups of Active Volcanic Vents 20-29
+
+ " IV. Mid-ocean Volcanic Islands 30-40
+
+
+PART II.
+
+_EUROPEAN VOLCANOES._
+
+Chap. I. Vesuvius 41-60
+
+ " II. Etna 61-68
+
+ " III. The Lipari Islands, Stromboli 69-75
+
+ " IV. The Santorin Group 76-83
+
+ " V. European Extinct or Dormant Volcanoes 84-91
+
+ " VI. Extinct Volcanoes of Central France 92-112
+
+ " VII. The Volcanic District of the Rhine Valley 13-125
+
+
+PART III.
+
+_DORMANT OR MORIBUND VOLCANOES OF OTHER PARTS OF THE WORLD._
+
+Chap. I. Dormant Volcanoes of Palestine and Arabia 126-135
+
+ " II. The Volcanic Regions of North America 136-145
+
+ " III. Volcanoes of New Zealand 146-153
+
+
+PART IV.
+
+_TERTIARY VOLCANIC DISTRICTS OF THE BRITISH ISLES._
+
+Chap. I. Antrim 154-159
+
+ " II. Succession of Volcanic Eruptions 160-171
+
+ " III. Island of Mull and Adjoining Coast 172-176
+
+ " IV. Isle of Skye 177-179
+
+ " V. The Scuir of Eigg 180-184
+
+ " VI. Isle of Staffa 185-186
+
+
+PART V.
+
+_PRE-TERTIARY VOLCANIC ROCKS._
+
+Chap. I. The Deccan Trap-series of India 187-189
+
+ " II. Abyssinian Table-lands 190-193
+
+ " III. Cape Colony 194-195
+
+ " IV. Volcanic Rocks of Past Geological
+ Periods of the British Isles 196-199
+
+
+PART VI.
+
+_SPECIAL VOLCANIC AND SEISMIC PHENOMENA._
+
+Chap. I. The Eruption of Krakatoa in 1883 201-216
+
+ " II. Earthquakes 217-224
+
+
+PART VII.
+
+_VOLCANIC AND SEISMIC PROBLEMS._
+
+Chap. I. The Ultimate Cause of Volcanic Action 225-235
+
+ " II. Lunar Volcanoes 236-252
+
+ " III. Are we Living in an Epoch of Special
+ Volcanic Activity? 253-257
+
+
+APPENDIX.
+
+A Brief Account of the Principal Varieties of
+ Volcanic Rocks 259-265
+
+Index 268
+
+
+
+
+ILLUSTRATIONS.
+
+
+Fig. 1. Eruption of Vesuvius, 1872-73 _Frontispiece_
+
+ " 2. Cotopaxi _Page_ 16
+
+ " 3. Volcanic Cone of Orizaba " 21
+
+ Map of the World, showing Active and
+ Extinct Volcanoes " 23
+
+ " 4. Teneriffe, seen from the Ocean " 31
+
+ " 5. View of the Summit of Teneriffe " 35
+
+ " 6. Probable Aspect of Vesuvius at Beginning
+ of Christian Era " 43
+
+ " 7. View of Vesuvius before 1767 " 50
+
+ " 8. Map of District bordering Bay of Naples " 52
+
+ " 9. View of Vesuvius in 1872 " 53
+
+ " 10. Ideal Section through Etna " 63
+
+ " 11. Map of the Lipari Islands " 70
+
+ " 12. The Island of Vulcano in Eruption " 71
+
+ " 13. Ideal Section through Gulf of Santorin " 76
+
+ " 14. Bird's-eye View of Gulf of Santorin " 79
+
+ " 15. Ground Plan of Rocca Monfina " 80
+
+ " 16. Geological Section of Tiber Valley at
+ Rome " 88
+
+ " 17. Generalised Section Through the Vale
+ of Clermont " 93
+
+Fig. 18. View of Puy de Dome and Neighbouring
+ Volcanoes _Page_ 95
+
+ " 19. Mont Demise, seen from the S.E. " 103
+
+ " 20. Sketch Map of Rhenish Area in the
+ Miocene Epoch " 114
+
+ " 21. The Volcanic Range of the Siebengebirge " 117
+
+ " 22. Section of Extinct Crater of the Roderberg " 120
+
+ " 23. Plan and Section of the Laacher See " 122
+
+ " 24. Extinct Craters in the Jaulan " 130
+
+ " 25. Mount Shasta " 139
+
+ " 26. Forms of Volcanic Tuff-Cones, Auckland " 148
+
+ " 27. "The White Rocks," Portrush, Co.
+ Antrim " 157
+
+ " 28. Section across the Volcanic Plateau
+ of Antrim " 159
+
+ " 29. Section at Templepatrick " 161
+
+ " 30. Cliff above the Giant's Causeway " 163
+
+ " 31. The Giant's Causeway, Co. Antrim " 165
+
+ " 32. "The Chimneys," North Coast of
+ Antrim " 166
+
+ " 33. Section at Alt na Searmoin, Mull " 175
+
+ " 34. View of the Scuir of Eigg from the East " 181
+
+ Map of Volcanic Band of the Moluccas " 200
+
+ " 35. Map of the Krakatoa Group of Islands " 203
+
+ " 36. Section from Verlaten Island through
+ Krakatoa " 204
+
+Fig. 37. Isoseismals of the Charleston Earthquake _Page_ 223
+
+ " 38. Photograph of the Moon's Surface " 241
+
+ " 39. Portion of the Moon's Surface " 243
+
+
+_PLATES._
+
+I. & II. Magnified Sections of Vesuvian Lavas.
+
+III. & IV. Magnified Sections of Volcanic Rocks.
+
+
+
+
+Volcanoes: Past and Present.
+
+
+
+
+PART I.
+
+INTRODUCTION.
+
+
+
+
+CHAPTER I.
+
+HISTORIC NOTICES OF VOLCANIC ACTION.
+
+
+There are no manifestations of the forces of Nature more calculated to
+inspire us with feelings of awe and admiration than volcanic eruptions
+preceded or accompanied, as they generally are, by earthquake shocks.
+Few agents have been so destructive in their effects; and to the real
+dangers which follow such terrestrial convulsions are to be added the
+feelings of uncertainty and revulsion which arise from the fact that the
+earth upon which we tread, and which we have been accustomed to regard
+as the emblem of stability, may become at any moment the agent of our
+destruction. It is, therefore, not surprising that the ancient Greeks,
+who, as well as the Romans, were close observers of the phenomena of
+Nature, should have investigated the causes of terrestrial disturbances,
+and should have come to some conclusions upon them in accordance with
+the light they possessed. These terrible forces presented to the Greeks,
+who clothed all the operations of Nature in poetic imagery and deified
+her forces, their poetical and mystical side; and as there was a deity
+for every natural force, so there was one for earthquakes and volcanoes.
+Vulcan, the deformed son of Juno (whose name bears so strange a
+resemblance to that of "the first artificer in iron" of the Bible, Tubal
+Cain), is condemned to pass his days under Mount Etna, fabricating the
+thunderbolts of Jove, and arms for the gods and great heroes of
+antiquity.
+
+The Pythagoreans appear to have held the doctrine of a central fire
+(meson pyr) as the source of volcanic phenomena; and in the Dialogues of
+Plato allusion is made to a subterranean reservoir of lava, which,
+according to Simplicius, was in accordance with the doctrine of the
+Pythagoreans which Plato was recounting.[1] Thucydides clearly describes
+the effect of earthquakes upon coast-lines of the Grecian Archipelago,
+similar to that which took place in the case of the earthquake of
+Lisbon, the sea first retiring and afterwards inundating the shore.
+Pliny supposed that it was by earthquake avulsion that islands were
+naturally formed. Thus Sicily was torn from Italy, Cyprus from Syria,
+Euboea from Boeotia, and the rest; but this view was previously
+enunciated by Aristotle in his "Peri kosmou," where he states that
+earthquakes have torn to pieces many parts of the earth, while lands
+have been converted into sea, and that tracts once covered by the sea
+have been converted into dry land.
+
+But the most philosophical views regarding terrestrial phenomena are
+those given by Ovid as having been held by Pythagoras (about B.C. 580).
+In the _Metamorphoses_ his views regarding the interchange of land and
+sea, the effects of running water in eroding valleys, the growth of
+deltas, the effect of earthquakes in burying cities and diverting
+streams from their sources, are remarkable anticipations of doctrines
+now generally held.[2] But what most concerns us at present are his
+views regarding the changes which have come over volcanic mountains. In
+his day Vesuvius was dormant, but Etna was active; so his illustrations
+are drawn from the latter mountain; and in this connection he observes
+that volcanic vents shift their position. There was a time, he says,
+when Etna was not a burning mountain, and the time will come when it
+will cease to burn; whether it be that some caverns become closed up by
+the movements of the earth, or others opened, or whether the fuel is
+finally exhausted.[3] Strabo may be regarded as having originated the
+view, now generally held, that active volcanoes are safety-valves to the
+regions in which they are situated. Referring to the tradition recorded
+by Pliny, that Sicily was torn from Italy by an earthquake, he observes
+that the land near the sea in those parts was rarely shaken by
+earthquakes, since there are now orifices whereby fire and ignited
+matters and waters escape; but formerly, when the volcanoes of Etna, the
+Lipari Islands, Ischia, and others were closed up, the imprisoned fire
+and wind might have produced far more violent movements.[4]
+
+The account of the first recorded eruption of Vesuvius has been
+graphically related by the younger Pliny in his two letters to Tacitus,
+to which I shall have occasion to refer further on.[5] These bring down
+the references to volcanic phenomena amongst ancient authors to the
+commencement of the Christian era; from all of which we may infer that
+the more enlightened philosophers of antiquity had a general idea that
+eruptions had their origin in a central fire within the interior of the
+earth, that volcanic mountains were liable to become dormant for long
+periods, and afterwards to break out into renewed activity, that there
+existed a connection between volcanic action and earthquakes, and that
+volcanoes are safety-valves for the regions around.
+
+It is unnecessary that I should pursue the historical sketch further.
+Those who wish to know the views of writers of the Middle Ages will find
+them recorded by Sir Charles Lyell.[6] The long controversy carried on
+during the latter part of the eighteenth century between "Neptunists,"
+led by Werner on the one side, and "Vulcanists," led by Hutton and
+Playfair on the other, regarding the origin of such rocks as granite and
+basalt, was finally brought to a close by the triumph of the
+"Vulcanists," who demonstrated that such rocks are the result of igneous
+fusion; and that in the cases of basalt and its congeners, they are
+being extruded from volcanic vents at the present day. The general
+principles for the classification of rocks as recognised in modern
+science may be regarded as having been finally established by James
+Hutton, of Edinburgh, in his _Theory of the Earth_,[7] while they were
+illustrated and defended by Professor Playfair in his work entitled,
+_Illustrations of the Huttonian Theory of the Earth_,[8] although other
+observers, such as Desmarest, Collini, and Guettard, had in other
+countries come to very clear views on this subject.
+
+The following are some of the more important works on the phenomena of
+volcanoes and earthquakes published during the present century:--[9]
+
+1. Poulett Scrope, F.R.S., _Considerations on Volcanoes_ (1825). This
+work is dedicated to Lyell, his fellow-worker in the same department of
+science, and was undertaken, as he says, "in order to help to dispel
+that signal delusion as to the mode of action of the subtelluric forces
+with which the Elevation-Crater theory had mystified the geological
+world." The second edition was published in 1872.
+
+2. This was followed by the admirable work, _On the Extinct Volcanoes
+of Central France_, published in 1826 (2nd edition, 1858), and is one of
+the most complete monographs on a special volcanic district ever
+written.
+
+3. Dr. Samuel Hibbert, _History of the Extinct Volcanoes of the Basin of
+Neuwied on the Lower Rhine_ (1832). Dr. Hibbert's work is one of
+remarkable merit, if we consider the time at which it was written. For
+not only does it give a clear and detailed account of the volcanic
+phenomena of the Eifel and the Lower Rhine, but it anticipates the
+principles upon which modern writers account for the formation of river
+valleys and other physical features; and in working out the physical
+history of the Rhine valley below Mainz, and its connection with the
+extinct volcanoes which are found on both banks of that river, he has
+taken very much the same line of reasoning which was some years
+afterwards adopted by Sir A. Ramsay when dealing with the same subject.
+It does not appear that the latter writer was aware of Dr. Hibbert's
+treatise.
+
+4. Leopold von Buch, _Description Physique des Iles Canaries_ (1825),
+translated from the original by C. Boulanger (1836); _Geognostische
+Reise_ (Berlin, 1809), 2 vols.; and _Reise durch Italien_ (1809). From a
+large number of writings on volcanoes by this distinguished traveller,
+whom Alexander von Humboldt calls "dem geistreichen Forscher der Natur,"
+the above are selected as being the most important. That on the Canaries
+is accompanied by a large atlas, in which the volcanoes of Teneriffe,
+Palma, and Lancerote, with some others, are elaborately represented, and
+are considered to bear out the author's views regarding the formation
+of volcanic cones by elevation or upheaval. The works dealing with the
+volcanic phenomena of Central and Southern Italy are also written with
+the object, in part at least, of illustrating and supporting the same
+theoretical views; with these we have to deal in the next chapter.
+
+5. Dr. Charles Daubeny, F.R.S., _Description of Active and Extinct
+Volcanoes, of Earthquakes, and of Thermal Springs, with remarks on the
+causes of these phenomena, the character of their respective products,
+and their influence on the past and present condition of the globe_ (2nd
+edition, 1848). In this work the author gives detailed descriptions of
+almost all the known volcanic districts of the globe, and defends what
+is called "the chemical theory of volcanic action"--a theory at one time
+held by Sir Humphrey Davy.
+
+6. Wolfgang Sartorius von Waltershausen, _Der Aetna_. This work possesses
+a melancholy interest from the fact that its distinguished author did
+not live to see its publication. Von Waltershausen, having spent several
+years in making an elaborate survey of Etna, produced an atlas
+containing numerous detailed maps, views, and drawings of this mountain
+and its surroundings, which were published at Weimar by Engelmann in
+1858. A description in MS. to accompany the atlas was also prepared, but
+before it was printed, the author died, on the 16th October 1876. The
+MS. having been put into the hands of the late Professor Arnold von
+Lasaulx by the publisher of the atlas, it was subsequently brought out
+under the care of this distinguished petrologist, who was so fully
+fitted for an undertaking of this kind.
+
+7. Sir Charles Lyell in his _Principles of Geology_[10] devotes several
+chapters to the consideration of volcanic phenomena, in which, being in
+harmony with the views of his friend, Poulett Scrope, he combats the
+"elevation theory" of Von Buch, as applied to the formation of volcanic
+mountains, holding that they are built up of ashes, stones, and scoriae
+blown out of the throat of the volcano and piled around the orifice in a
+conical form. Together with these materials are sheets of lava extruded
+in a molten condition from the sides or throat of the crater itself.
+
+8. Professor J. W. Judd, F.R.S., in his able work entitled, _Volcanoes:
+What they are, and what they teach_,[11] has furnished the student of
+vulcanicity with a very complete manual of a general character on the
+subject. The author, having extensive personal acquaintance with the
+volcanoes of the south of Europe and the volcanic rocks of the British
+Isles, was well equipped for undertaking a work of the kind; and in it
+he supports the views of Lyell and Scrope regarding the mode of
+formation of volcanic mountains.
+
+9. Sir Archibald Geikie, F.R.S., in his elaborate monograph[12] on the
+Tertiary Volcanic Rocks of the British Isles, has recorded his views
+regarding the origin and succession of the plateau basalts and
+associated rocks over the region extending from the north of Ireland to
+the Inner Hebrides; and in dealing with these districts in the following
+pages I have made extensive use of his observations and conclusions.
+
+10. _Report published by the Royal Society on the Eruption of
+Krakatoa_--drawn up by several authors (1885)--and the work on the same
+subject by Chev. Verbeek, and published by the Government of the
+Netherlands (1886). In these works all the phenomena connected with the
+extraordinary eruptions of Krakatoa in 1883 are carefully noted and
+scientifically discussed, and illustrated by maps and drawings.
+
+11. _The Charleston Earthquake of August 31, 1886_, by Captain Clarence
+Edward Dutton, U.S. Ordnance Corps. Ninth Annual Report of the United
+States Geological Survey, 1887-88, with maps and illustrations.
+
+12. Amongst other works which may be consulted with advantage is that of
+Mr. T. Mellard Reade on _The Origin of Mountain Ranges_; the Rev. Osmond
+Fisher's _Physics of the Earth_; Professor G. H. Darwin and Mr. C.
+Davison on "The Internal Tension of the Earth's Crust," _Philosophical
+Transactions of the Royal Society_, vol. 178; Mr. R. Mallet, "On the
+Dynamics of Earthquakes," _Trans. Roy. Irish Academy_, vol. xxi.;
+Professor O'Reilly's "Catalogues of Earthquakes," _Trans. Roy. Irish
+Academy_, vol. xxviii. (1884 and 1888); and Mr. A. Ent. Gooch _On the
+Causes of Volcanic Action_ (London, 1890). These and other authorities
+will be referred to in the text.
+
+[1] See Julius Schwarez _On the Failure of Geological Attempts made by
+the Greeks_. (Edition 1888.)
+
+[2]
+ "Vidi ego, quod fuerat quondam solidissima tellus,
+ Esse fretum. Vidi factas ex aequore terras:
+ Et procul a pelago conchae jacuere marinae;
+ Et vetus inventa est in montibus anchora sumnis.
+ Quodque fuit campus, vallem de cursus aquarum
+ Fecit; et eluvie mons est deductus in aequor:
+ Eque paludosa siccis humus aret arenis;
+ Quaeque sitim tulerant, stagnata paludibus hument.
+ Hic fontes Natura novos emissit, at illuc
+ Clausit: et antiquis concussa tremoribus orbis
+ Fulmina prosiliunt...."
+ --Lib. xv. 262.
+
+[3]
+ "Nec, quae sulfureis ardet fornacibus, Aetne
+ Ignea semper erit; neque enim fuit ignea semper.
+ Nam, sive est animal tellus, et vivit, habetque
+ Spiramenta locis flammam exhalantia multis;
+ Spirandi mutare vias, quotiesque movetur,
+ Has finire potest, illas aperire cavernas:
+ Sive leves imis venti cohibentur in antris;
+ Saxaque cum saxis...."
+ --_Ibid._, 340.
+
+[4] Strabo, lib. vi.
+
+[5] Tacitus, lib. vi. 16, 20.
+
+[6] _Principles of Geology_, 11th edition, vol. i., ch. 3.
+
+[7] 2 vols., Edin. (1795).
+
+[8] Edin. (1802).
+
+[9] A more extended list of early works will be found in Daubeny's
+_Volcanoes_ (1848).
+
+[10] 11th edition (1872).
+
+[11] 4th edition (1888).
+
+[12] "The History of Volcanic Action during the Tertiary Period in the
+British Isles," _Trans. Roy. Soc., Edin._ Vol. xxxv, (1888).
+
+
+
+
+CHAPTER II.
+
+FORM, STRUCTURE, AND COMPOSITION OF VOLCANIC MOUNTAINS.
+
+
+The conical form of a volcanic mountain is so generally recognised, that
+many persons who have no intelligent acquaintance with geological
+phenomena are in the habit of attributing to all mountains having a
+conical form, and especially if accompanied by a truncated apex, a
+volcanic origin. Yet this is very far from being the fact, as some
+varieties of rock, such as quartzite, not unfrequently assume this
+shape. Of such we have an example in the case of Errigal, a quartzite
+mountain in Donegal, nearly 3000 feet high, which bears a very near
+approach in form to a perfect cone or pyramid, and yet is in no way
+connected, as regards its origin or structure, with volcanic phenomena.
+Another remarkable instance is that of Schehallion in Scotland, also
+composed of quartz-rock; and others may be found amongst the ranges of
+Islay and Jura, described by Sir A. Geikie.[1]
+
+Notwithstanding, however, such exceptions, which might be greatly
+multiplied, the majority of cone-shaped mountains over the globe have a
+volcanic origin.[2] The origin of this form in each case is entirely
+distinct. In the case of quartzite mountains, the conical form is due to
+atmospheric influences acting on a rock of uniform composition,
+traversed by numerous joints and fissures crossing each other at obtuse
+angles, along which the rock breaks up and falls away, so that the sides
+are always covered by angular shingle forming slopes corresponding to
+the angle of friction of the rock in question. In the case of a volcanic
+mountain, however, the same form is due either to accumulation of
+fragmental material piled around the cup-shaped hollow, or crater, which
+is usually placed at the apex of the cone, and owing to which it is
+bluntly terminated, or else to the welling up from beneath of viscous
+matter in the manner presently to be described.
+
+_Views of Sir Humphrey Davy and L. von Buch._--The question how a
+volcanic cone came to be formed was not settled without a long
+controversy carried on by several naturalists of eminence. Some of the
+earlier writers of modern times on the subject of vulcanicity--such as
+Sir Humphrey Davy and Leopold von Buch--maintained that the conical form
+was due to upheaval by a force acting from below at a central focus,
+whereby the materials of which the mountain is formed were forced to
+assume a _qua-qua versal_ position--that is, a position in which the
+materials dip away from the central focus in every direction. But this
+view, originally contested by Scrope and Lyell, has now been generally
+abandoned. It will be seen on reflection that if a series of strata of
+ashes, tuff, and lava, originally horizontal, or nearly so, were to be
+forced upwards into a conical form by a central force, the result would
+be the formation of a series of radiating fissures ever widening from
+the circumference towards the focus. In the case of a large mountain
+such fissures, whether filled with lava or otherwise, would be of great
+breadth towards the focus, or central crater, and could not fail to make
+manifest beyond dispute their mechanical origin. But no fissures of the
+kind here referred to are, as a matter of fact, to be observed. Those
+which do exist are too insignificant and too irregular in direction to
+be ascribed to such an origin; so that the views of Von Buch and Davy
+must be dismissed, as being unsupported by observation, and as untenable
+on dynamical grounds. As a matter of fact, the "elevatory theory," or
+the "elevation-crater theory," as it is called by Scrope, has been
+almost universally abandoned by writers on vulcanicity.
+
+_Principal Varieties of Volcanic Mountains as regards Form._--But whilst
+rejecting the "elevatory theory," it is necessary to bear in mind that
+volcanic cones and dome-shaped elevations have been formed in several
+distinct ways, giving rise to varieties of structure essentially
+different. Two of the more general of these varieties of form, the
+crater-cone and the dome, are found in some districts, as in Auvergne,
+side by side. The crater-cone consists of beds or sheets of ashes,
+lapilli, and slag piled up in a conical form, with a central crater (or
+cup) containing the principal pipe through which these materials have
+been erupted; the dome, of a variety of trachytic lava, which has been
+extruded in a molten, or viscous, condition from a central pipe, and in
+such cases there is no distinct crater. There are other forms of
+volcanic mountains, such as those built up of basaltic matter, of which
+I shall have to speak hereafter, but the two former varieties are the
+most prevalent; and we may now proceed to consider the conditions under
+which the crater-cone volcanoes have been formed.
+
+_Crateriform Volcanic Cones._--Of this class nearly all the active
+volcanoes of the Mediterranean region--Etna, Vesuvius, Stromboli, and
+the Lipari Islands--may be considered as representatives. They consist
+essentially of masses of fragmental material, which have from time to
+time been blown out of an orifice and piled up around with more or less
+regularity (according to the force exerted, and direction of the
+prevalent winds), alternating with sheets of lava. In this way mountains
+several thousand feet in height and of vast horizontal extent are
+formed. The fragmental materials thus accumulated are of all sizes, from
+the finest dust up to blocks many tons in weight, the latter being
+naturally piled around nearest to the orifice. The fine dust, blown high
+into the air by the explosive force of the gases and vapours, is often
+carried to great distances by the prevalent winds. Thus during the
+eruption of Vesuvius in A.D. 472 showers of ashes, carried high into the
+air by the westerly wind, fell over Constantinople at a distance of 750
+miles.[3]
+
+These loose, or partially consolidated, fragmental materials are rudely
+stratified, and slope downwards and outwards from the edge of the
+crater, so as to present the appearance of what is known as "the dip" of
+stratified deposits which have been upraised from the horizontal
+position by terrestrial forces. It was this excentrical arrangement
+which gave rise to the supposition that such volcanic ash-beds had been
+tilted up by a force acting in the direction of the volcanic throat, or
+orifice of eruption. The interior wall of Monte di Somma, the original
+crater of Vesuvius, presents a good illustration of such fragmental
+beds. I shall have occasion further on to describe more fully the
+structure of this remarkable mountain; so that it will suffice to say
+here that this old prehistoric crater, the walls of which enclose the
+modern cone of Vesuvius, is seen to be formed of irregular beds of ash,
+scoriae, and fragmental masses, traversed by numerous dykes of lava, and
+sloping away outwards towards the surrounding plains.
+
+Of similar materials are the flanks of Etna composed, even at great
+distances from the central crater; the beds of ash and agglomerate
+sometimes alternating with sheets of solidified lava and traversed by
+dykes of similar material of later date, injected from below through
+fissures formed during periods of eruptive energy. Numerous similar
+examples are to be observed in the Auvergne region of Central France and
+the Eifel. And here we find remarkable cases of "breached cones," or
+craters, which will require some special description. Standing on the
+summit of the Puy de Dome, and looking northwards or southwards, the eye
+wanders over a tract formed of dome-shaped hills and of extinct
+crater-cones rising from a granitic platform. But what is most peculiar
+in the scene is the ruptured condition of a large number of the cones
+with craters. In such cases the wall of the crater has been broken down
+on one side, and we observe that a stream of lava has been poured out
+through the breach and overflowed the plain below. The cause of this
+breached form is sufficiently obvious. In such cases there has been an
+explosion of ashes, stones, and scoriae from the volcanic throat, by
+which a cone-shaped hill with a crater has been built up. This has been
+followed by molten lava welling up through the throat, and gradually
+filling the crater. But, as the lava is much more dense than the
+material of which the crater wall is composed, the pressure of the lava
+outwards has become too great for the resistance of the wall, which
+consequently has given way at its weakest part and, a breach being
+formed, the molten matter has flowed out in a stream which has inundated
+the country lying at the base of the cone. In one instance mentioned by
+Scrope, the original upper limit of the lake of molten lava has left its
+mark in the form of a ring of slag on the inside of the breached
+crater.[4]
+
+_Craterless Domes._--These differ essentially both in form and
+composition from those just described, and have their typical
+representatives in the Auvergne district, though not without their
+analogues elsewhere, as in the case of Chimborazo, in South America, one
+of the loftiest volcanic mountains in the world.
+
+[Illustration: Fig. 2.--Cotopaxi, a volcano of the Cordilleras of
+Quito, still active, and covered by snow down to a level of 14,800 feet.
+Below this is a zone of naked rock, succeeded by another of forest
+vegetation. Owing to the continuous extrusion of lava from the crater,
+the cone is being gradually built up of fresh material, and the crater
+is comparatively small in consequence.--(A diagrammatic view after A.
+von Humboldt.)]
+
+Taking the Puy de Dome, Petit Suchet, Cliersou, Grand Sarcoui in
+Auvergne, and the Mamelon in the Isle of Bourbon as illustrations, we
+have in all these cases a group of volcanic hills, dome-shaped and
+destitute of craters, the summits being rounded or slightly flattened.
+We also observe that the flanks rise more abruptly from their bases, and
+contrast in outline with the graceful curve of the crater cones. The
+dome-shaped volcanoes are generally composed of felsitic matter, whether
+domite, trachyte, or andesite, which has been extruded in a molten or
+viscous condition from some orifice or fissure in the earth's crust, and
+being piled up and spreading outwards, necessarily assumes such a form
+as that of a dome, as has been shown by experiment on a small scale by
+Dr. E. Reyer, of Graetz.[5] The contrast between the two forms (those of
+the dome and the crater-cone) is exemplified in the case of the Grand
+Sarcoui and its neighbours. The former is composed of a species of
+trachyte; the latter of ashes and fragmental matter which have been
+blown out of their respective vents of eruption into the air, and piled
+up and around in a crateriform manner with sides of gradually
+diminishing slope outwards, thus giving rise to the characteristic
+volcanic curve. The two varieties here referred to, contrasting in form,
+composition, and colour of material, can be clearly recognised from the
+summit of the Puy de Dome, which rises by a head and shoulders above its
+fellows, and thus affords an advantageous standpoint from which to
+compare the various forms of this remarkable group of volcanic
+mountains.
+
+Cotopaxi (Fig. 2) has been generally supposed to be a dome; but Whymper,
+who ascended the mountain in 1880, shows that it is a cone with a
+crater, 2,300 feet in largest diameter. He determined the height to be
+19,613 feet above the ocean. Its real elevation above the sea is
+somewhat masked, owing to the fact that it rises from the high plain of
+Tapia, which is itself 8,900 feet above the sea surface. The smaller
+peak on the right (Fig. 2) is that of Carihuairazo, which reaches an
+elevation of over 16,000 feet.
+
+Chimborazo, in Columbia, province of Quito, is one of the loftiest of
+the chain of the Andes, and is situated in lat. 1 deg. 30' S., long. 78 deg. 58'
+W. Though not in a state of activity, it is wholly composed of volcanic
+material, and reaches an elevation of over 20,000 feet above the ocean;
+its sides being covered by a sheet of permanent snow to a level of 2,600
+feet below the summit.[6] Seen from the shores of the Pacific, after the
+long rains of winter, it presents a magnificent spectacle, "when the
+transparency of the air is increased, and its enormous circular summit
+is seen projected upon the deep azure blue of the equatorial sky. The
+great rarity of the air through which the tops of the Andes are seen
+adds much to the splendour of the snow, and aids the magical effect of
+its reflection."
+
+Chimborazo was ascended by Humboldt and Bonpland in 1802 almost to the
+summit; but at a height of 19,300 feet by barometrical measurement,
+their further ascent was arrested by a wide chasm. Boussingault, in
+company with Colonel Hall, accomplished the ascent as far as the foot of
+the mass of columnar "trachyte," the upper surface of which, covered by
+a dome of snow, forms the summit of the mountain. The whole mass of the
+mountain consists of volcanic rock, varieties of andesite; there is no
+trace of a crater, nor of any fragmental materials, such as are usually
+ejected from a volcanic vent of eruption.[7]
+
+_Lava Crater-Cones._--A third form of volcanic mountain is that which
+has been built up by successive eruptions of basic lava, such as basalt
+or dolerite, when in a molten condition. These are very rare, and the
+slope of the sides depends on the amount of original viscosity. Where
+the lava is highly fused its slope will be slight, but if in a viscous
+condition, successive outpourings from the orifice, unable to reach the
+base of the mountain, will tend to form a cone with increasing slope
+upwards. Mauna Loa and Kilauea, in the Hawaiian Group, according to
+Professor J. D. Dana, are basalt volcanoes in a normal state. They have
+distinct craters, and the material of which the mountain is formed is
+basalt or dolerite. The volcano of Rangitoto in Auckland, New Zealand,
+appears to belong to this class.
+
+Basalt is the most fusible of volcanic rocks, owing to the augite and
+magnetite it contains, so that it spreads out with a very slight slope
+when highly fused. Trachyte, on the other hand, is the least fusible
+owing to the presence of orthoclase felspar, or quartz; so that the
+volcanic domes formed of this material stand at a higher angle from the
+horizon than those of basaltic cones.
+
+[1] _Scenery and Geology of Scotland_ (1865), p. 214.
+
+[2] Humboldt says: "The form of isolated conical mountains, as those of
+Vesuvius, Etna, the Peak of Teneriffe, Tunguagua, and Cotopaxi, is
+certainly the shape most commonly observed in volcanoes all over the
+globe."--_Views of Nature_, translated by E. C. Otte and H. G. Bohn
+(1850).
+
+[3] It is supposed that after the disastrous explosion of Krakatoa in
+1883 the fine dust carried into the higher regions of the atmosphere was
+carried round almost the entire globe, and remained suspended for a
+lengthened period, as described in a future page.
+
+[4] Another remarkable case is mentioned and figured by Judd, where one
+of the Lipari Isles, composed of pumice and rising out of the
+Mediterranean, has been breached by a lava-stream of obsidian.--_Loc.
+cit._, p. 123.
+
+[5] Reyer has produced such dome-shaped masses by forcing a quantity of
+plaster of Paris in a pasty condition up through an orifice in a board;
+referred to by Judd, _loc. cit._, p. 125.
+
+[6] Whymper determined the height to be 20,498 feet; Reiss and Stuebel
+make it 20,703 feet. Whymper thinks there may be a crater concealed
+beneath the dome of snow.--_Travels amongst the Great Andes of the
+Equator_, by Edward Whymper (1892).
+
+[7] Whymper states that there is a prevalent idea that Cotopaxi and a
+volcano called Sangai act as safety-valves to each other. Sangai reaches
+an elevation (according to Reiss and Stuebel) of 17,464 feet, and sends
+intermittent jets of steam high into the air, spreading out into vast
+cumulus clouds, which float away southwards, and ultimately
+disappear.--_Ibid._, p. 73.
+
+
+
+
+CHAPTER III.
+
+LINES AND GROUPS OF ACTIVE VOLCANIC VENTS.
+
+
+The globe is girdled by a chain of volcanic mountains in a state of
+greater or less activity, which may perhaps be considered a girdle of
+safety for the whole world, through which the masses of molten matter in
+a state of high pressure beneath the crust find a way of escape; and
+thus the structure of the globe is preserved from even greater
+convulsions than those which from time to time take place at various
+points on its surface. This girdle is partly terrestrial, partly
+submarine; and commencing at Mount Erebus, near the Antarctic Pole,
+ranging through South Shetland Isle, Cape Horn, the Andes of South
+America, the Isthmus of Panama, then through Central America and Mexico,
+and the Rocky Mountains to Kamtschatka, the Aleutian Islands, the
+Kuriles, the Japanese, the Philippines, New Guinea, and New Zealand,
+reaches the Antarctic Circle by the Balleny Islands. This girdle sends
+off branches at several points. (See Map, p. 23.)
+
+[Illustration: Fig. 3.--Volcanic cone of Orizaba (Cittaltepeth), in
+Mexico, now extinct; the upper part snow-clad, and at its base forest
+vegetation; it reaches a height of 16,302 Parisian feet above the
+sea.--(After A. von Humboldt.)]
+
+(_a._) The linear arrangement of active or dormant volcanic vents has
+been pointed out by Humboldt, Von Buch, Daubeny, and other writers. The
+great range of burning mountains of the Andes of Chili, Peru, Bolivia,
+and Mexico, that of the Aleutian Islands, of Kamtschatka and the Kurile
+Islands, extending southwards into the Philippines, and the branching
+range of the Sunda Islands are well-known examples. That of the West
+Indian Islands, ranging from Grenada through St. Vincent, St. Lucia,
+Martinique, Dominica, Guadeloupe, Montserrat, Nevis, and St. Eustace,[1]
+is also a remarkable example of the linear arrangement of volcanic
+mountains. On tracing these ranges on a map of the world[2] (Map, p.
+23), it will be observed that they are either strings of islands, or lie
+in proximity to the ocean; and hence the view was naturally entertained
+by some writers that oceanic water, or at any rate that of a large lake
+or sea, was a necessary agent in the production of volcanic eruptions.
+This view seems to receive further corroboration from the fact that the
+interior portions of the continents and large islands such as Australia
+are destitute of volcanoes in action, with the remarkable exceptions of
+Mounts Kenia and Kilimanjaro in Central Africa, and a few others. It is
+also very significant in this connection that many of the volcanoes now
+extinct, or at least dormant, both in Europe and Asia, appear to have
+been in proximity to sheets of water during the period of activity. Thus
+the old volcanoes of the Hauran, east of the Jordan, appear to have been
+active at the period when the present Jordan valley was filled with
+water to such an extent as to constitute a lake two hundred miles in
+length, but which has now shrunk back to within the present limits of
+the Dead Sea.[3] Again, at the period when the extinct volcanoes of
+Central France were in active operation, an extensive lake overspread
+the tract lying to the east of the granitic plateau on which the craters
+and domes are planted, now constituting the rich and fertile plain of
+Clermont.
+
+[Illustration: Map Of The World Showing Active And Extinct Volcanoes
+(Large Dots)]
+
+Such instances are too significant to allow us to doubt that water in
+some form is very generally connected with volcanic operations; but it
+does not follow that it was necessary to the original formation of
+volcanic vents, whether linear or sporadic. If this were so, the extinct
+volcanoes of the British Isles would still be active, as they are close
+to the sea-margin, and no volcano would now be active which is not near
+to some large sheet of water. But Jorullo, one of the great active
+volcanoes of Mexico, lies no less than 120 miles from the ocean, and
+Cotopaxi, in Ecuador, is nearly equally distant. Kilimanjaro, 18,881
+feet high, and Kenia, in the equatorial regions of Central Africa, are
+about 150 miles from the Victoria Nyanza, and a still greater distance
+from the ocean; and Mount Demavend, in Persia, which rises to an
+elevation of 18,464 feet near the southern shore of the Caspian Sea, a
+volcanic mountain of the first magnitude, is now extinct or dormant.[4]
+Such facts as these all tend to show that although water may be an
+accessory of volcanic eruptions, it is not in all cases essential; and
+we are obliged, therefore, to have recourse to some other theory of
+volcanic action differing from that which would attribute it to the
+access of water to highly heated or molten matter within the crust of
+the earth.
+
+(_b._) _Leopold von Buch on Rents and Fissures in the Earth's
+Crust._--The view of Leopold von Buch, who considered that the great
+lines of volcanic mountains above referred to rise along the borders of
+rents, or fissures, in the earth's crust, is one which is inherently
+probable, and is in keeping with observation. That the crust of the
+globe is to a remarkable extent fissured and torn in all directions is a
+phenomenon familiar to all field geologists. Such rents and fissures are
+often accompanied by displacement of the strata, owing to which the
+crust has been vertically elevated on one side or lowered on the other,
+and such displacements (or "faults") sometimes amount to thousands of
+feet. It is only occasionally, however, that such fractures are
+accompanied by the extrusion of molten matter; and in the North of
+England and Scotland dykes of igneous rock, such as basalt, which run
+across the country for many miles in nearly straight lines, often cut
+across the faults, and are only rarely coincident with them.
+Nevertheless, it can scarcely be a question that the grand chain of
+volcanic mountains which stretches almost continuously along the Andes
+of South America, and northwards through Mexico, has been piled up along
+the line of a system of fissures in the fundamental rocks parallel to
+the coast, though not actually coincident therewith.
+
+(_c._) _The Cordilleras of Quito._--The structure and arrangement of the
+Cordilleras of Quito, for example, are eminently suggestive of
+arrangement along lines of fissure. As shown by Alexander von
+Humboldt,[5] the volcanic mountains are disposed in two parallel chains,
+which run side by side for a distance of over 500 miles northwards into
+the State of Columbia, and enclose between them the high plains of Quito
+and Lacunga. Along the eastern chain are the great cones of El Altar,
+rising to an elevation of 16,383 feet above the ocean, and having an
+enormous crater apparently dormant or extinct, and covered with snow;
+then Cotopaxi (Fig. 2), its sides covered with snow, and sending forth
+from its crater several columns of smoke; then Guamani and Cayambe
+(19,000 feet), huge truncated cones apparently extinct; these constitute
+the eastern chain of volcanic heights. The western chain contains even
+loftier mountains. Here we find the gigantic Chimborazo, an extinct
+volcano whose summit is white with snow; Carihuairazo[6] and Illiniza, a
+lofty pointed peak like the Matterhorn; Corazon, a snow-clad dome,
+reaching a height of 15,871 feet; Atacazo and Pichincha, the latter an
+extinct volcano reaching an elevation of 15,920 feet; such is the
+western chain, remarkable for its straightness, the volcanic cones being
+planted in one grand procession from south to north. This rectilinear
+arrangement of the western chain, only a little less conspicuous in the
+eastern, is very suggestive of a line of fracture in the crust beneath.
+And when we contemplate the prodigious quantity of matter included
+within the limits of these colossal domes and their environments, all of
+which has been extruded from the internal reservoirs, we gain some idea
+of the manner in which the contracting crust disposes of the matter it
+can no longer contain.[7]
+
+Between the volcanoes of Quito and those of Peru there is an intervening
+space of fourteen degrees of latitude. This is occupied by the Andes,
+regarding the structure of which we have not much information except
+that at this part of its course it is not volcanic. But from Arequipa in
+Peru (lat. 16 deg. S.), an active volcano, we find a new series of volcanic
+mountains continued southwards through Tacora (19,740 feet), then
+further south the more or less active vents of Sajama (22,915 feet),
+Coquina, Tutupaca, Calama, Atacama, Toconado, and others, forming an
+almost continuous range with that part of the desert of Atacama
+pertaining to Chili. Through this country we find the volcanic range
+appearing at intervals; and still more to the southwards it is doubtless
+connected with the volcanoes of Patagonia, north of the Magellan
+Straits, and of Tierra del Fuego. Mr. David Forbes considers that this
+great range of volcanic mountains, lying nearly north and south,
+corresponds to a line of fracture lying somewhat to the east of the
+range.[8]
+
+(_d._) _Other Volcanic Chains._--A similar statement in all probability
+applies to the systems of volcanic mountains of the Aleutian Isles,
+Kamtschatka, the Kuriles, the Philippines, and Sunda Isles. Nor can
+it be reasonably doubted that the western American coast-line has
+to a great extent been determined, or marked out, by such lines of
+displacement; for, as Darwin has shown, the whole western coast of South
+America, for a distance of between 2000 and 3000 miles south of the
+Equator, has undergone an upward movement in very recent times--that is,
+within the period of living marine shells--during which period the
+volcanoes have been in activity.[9]
+
+(_e._) _The Kurile Islands._--This chain may also be cited in evidence
+of volcanic action along fissure lines. It connects the volcanoes of
+Kamtschatka with those of Japan, and the linear arrangement is apparent.
+In the former peninsula Erman counted no fewer than thirteen active
+volcanic mountains rising to heights of 12,000 to 15,000 feet above the
+sea.[10] In the chain of the Kuriles Professor John Milne counted
+fifty-two well-defined volcanoes, of which nine, perhaps more, are
+certainly active.[11] They are not so high as those of Kamtschatka; but,
+on the other hand, they rise from very deep oceanic waters, and have
+been probably built up from the sea bottom by successive eruptions of
+tuff, lava, and ash. According to the view of Professor Milne, the
+volcanoes of the Kurile chain are fast becoming extinct.
+
+(_f._) _Volcanic Groups._--Besides the volcanic vents arranged in lines,
+of which we have treated above, there are a large number, both active
+and extinct, which appear to be disposed in groups, or sporadically
+distributed, over various portions of the earth's surface. I say _appear
+to be_, because this sporadic distribution may really be resolvable (at
+least in some cases) into linear distribution for short distances. Thus
+the Neapolitan Group, which might at first sight seem to be arranged
+round Vesuvius as a centre, really resolves itself into a line of active
+and extinct vents of eruption, ranging across Italy from the Tyrrhenian
+Sea to the Adriatic, through Ischia, Procida, Monte Nuovo and the
+Phlegraean Fields, Vesuvius, and Mount Vultur.[12] Again, the extinct
+volcanoes of Central France, which appear to form an isolated group,
+indicate, when viewed in detail, a linear arrangement ranging from north
+to south.[13] Another region over which extinct craters are distributed
+lies along the banks of the Rhine, above Bonn and the Moselle; a fourth
+in Hungary; a fifth in Asia Minor and Northern Palestine; and a sixth in
+Central Asia around Lake Balkash. These are all continental, and the
+linear distribution is not apparent.
+
+[1] For an interesting account of this range of volcanic islands see
+Kingsley's _At Last_. The grandest volcanic peak is that of Guadeloupe,
+rising to a height of 5000 feet above the ocean, amidst a group of
+fourteen extinct craters. But the most active vent of the range is the
+Souffriere of St. Vincent. In the eruption of 1812 this mountain sent
+forth clouds of pumice, scoriae and ashes, some of which were carried by
+an upper counter current to Barbados, one hundred miles to the eastward,
+covering the surface with volcanic dust to a depth of several inches.
+
+[2] An excellent, and perhaps the most recent, map of this kind is that
+given by Professor Prestwich in his _Geology_, vol. i. p. 216. One on a
+larger scale is that by Keith Johnston in his _Physical Atlas_.
+
+[3] _Memoir on the Physical Geology and Geography of Arabia Petraea,
+Palestine_, etc., published for the Committee of the Palestine
+Exploration Fund (1886), p. 113, etc.
+
+[4] This mountain was ascended in 1837 by Mr. Taylor Thomson, who found
+the summit covered with sulphur, and from a cone fumes at a high
+temperature issued forth, but there was no eruption.--_Journ. Roy.
+Geographical Soc._, vol. viii. p. 109.
+
+[5] Humboldt, _Atlas der Kleineren Schriften_ (1853).
+
+[6] Ascended by Whymper June 29, 1880. He found the elevation to be
+16,515 feet.
+
+[7] The arrangement of the volcanoes of Peru and Bolivia is also
+suggestive of a double line of fissure, while those of Chili suggest one
+single line. The volcanoes of Arequipa, in the southern part of Peru,
+are dealt with by Dr. F. H. Hatch, in his inaugural dissertation, _Ueber
+die Gesteine der Vulcan-Gruppe von Arequipa_ (Wien, 1886). The volcanoes
+rise to great elevations, having their summits capped by snow. The
+volcano of Charchani, lying to the north of Arequipa, reaches an
+elevation of 18,382 Parisian feet. That of Pichupichu reaches a height
+of 17,355 Par. feet. The central cone of Misti has been variously
+estimated to range from 17,240 to 19,000 Par. feet. The rocks of which
+the mountains are composed consist of varieties of andesite.
+
+[8] D. Forbes, "On the Geology of Bolivia and Southern Peru," _Quarterly
+Journal of the Geological Society_, vol. xvii. p. 22 (1861).
+
+[9] Darwin, _Structure and Distribution of Coral Reefs_, second edition,
+p. 186.
+
+[10] Erman, _Reise um die Welt_.
+
+[11] Milne, "Cruise amongst the Kurile Islands," _Geol. Mag._, New Ser.
+(August 1879).
+
+[12] See Daubeny, _Volcanoes_, Map I.
+
+[13] Sir A. Geikie has connected as a line of volcanic vents those of
+Sicily, Italy, Central France, the N. E. of Ireland, the Inner Hebrides
+and Iceland, of which the central vents are extinct or dormant, the
+extremities active.
+
+
+
+
+CHAPTER IV.
+
+MID-OCEAN VOLCANIC ISLANDS.
+
+
+_Oceanic Islands._--By far the most extensive regions with sporadically
+distributed volcanic vents, both active and extinct, are those which are
+overspread by the waters of the ocean, where the vents emerge in the
+form of islands. These are to be found in all the great oceans, the
+Atlantic, the Pacific, and the Indian; but are especially numerous over
+the central Pacific region. As Kotzebue and subsequently Darwin have
+pointed out, all the islands of the Pacific are either coral-reefs or of
+volcanic origin; and many of these rise from great depths; that is to
+say, from depths of 1000 to 2000 fathoms. It is unnecessary here to
+attempt to enumerate all these islands which rise in solitary grandeur
+from the surface of the ocean, and are the scenes of volcanic
+operations; a few may, however, be enumerated.
+
+[Illustration: Fig. 4.--The Peak of Teneriffe (Pic de Teyde) as seen
+from the ocean.--(From a photograph.)]
+
+(_a._) _Iceland._--In the Atlantic, Iceland first claims notice, owing
+to the magnitude and number of its active vents and the variety of the
+accompanying phenomena, especially the geysers. As Lyell has
+observed,[1] with the exception of Etna and Vesuvius, the most complete
+chronological records of a series of eruptions in existence are those
+of Iceland, which come down from the ninth century of our era, and which
+go to show that since the twelfth century there has never been an
+interval of more than forty years without either an eruption or a great
+earthquake. So intense is the volcanic energy in this island that some
+of the eruptions of Hecla have lasted six years without cessation.
+Earthquakes have often shaken the whole island at once, causing great
+changes in the interior, such as the sinking down of hills, the rending
+of mountains, the desertion by rivers of their channels, and the
+appearance of new lakes. New islands have often been thrown up near the
+coast, while others have disappeared. In the intervals between the
+eruptions, innumerable hot springs afford vent to the subterranean heat,
+and solfataras discharge copious streams of inflammable matter. The
+volcanoes in different parts of the island are observed, like those of
+the Phlegraean Fields, to be in activity by turns, one vent serving for a
+time as a safety-valve for the others. The most memorable eruption of
+recent years was that of Skaptar Jokul in 1783, when a new island was
+thrown up, and two torrents of lava issued forth, one 45 and the other
+50 miles in length, and which, according to the estimate of Professor
+Bischoff, contained matter surpassing in magnitude the bulk of Mont
+Blanc. One of these streams filled up a large lake, and, entering the
+channel of the Skapta, completely dried up the river. The volcanoes of
+Iceland may be considered as safety-valves to the region in which lie
+the British Isles.
+
+(_b._) _The Azores, Canary, and Cape de Verde Groups._--This group of
+volcanic isles rises from deep Atlantic waters north of the Equator,
+and the vents of eruption are partially active, partially dormant, or
+extinct. It must be supposed, however, that at a former period volcanic
+action was vastly more energetic than at present; for, except at the
+Grand Canary, Gomera, Forta Ventura, and Lancerote, where various
+non-volcanic rocks are found, these islands appear to have been built up
+from their foundations of eruptive materials. The highest point in the
+Azores is the Peak of Pico, which rises to a height of 7016 feet above
+the ocean. But this great elevation is surpassed by that of the Peak of
+Teneriffe (or Pic de Teyde) in the Canaries, which attains to an
+elevation of 12,225 feet, as determined by Professor Piazzi Smyth.[2]
+
+This great volcanic cone, rising from the ocean, its summit shrouded in
+snow, and often protruding above the clouds, must be an object of
+uncommon beauty and interest when seen from the deck of a ship. (Fig.
+4.) The central cone, formed of trachyte, pumice, obsidian, and ashes,
+rises out of a vast caldron of older basaltic rocks with precipitous
+inner walls--much as the cone of Vesuvius rises from within the
+partially encircling walls of Somma. (Fig. 5.) From the summit issue
+forth sulphurous vapours, but no flame.
+
+Piazzi Smyth, who during a prolonged visit to this mountain in 1856 made
+a careful survey of its form and structure, shows that the great cone is
+surrounded by an outer ring of basalt enclosing two _foci_ of eruption,
+the lavas from which have broken through the ring of the outer crater
+on the western side, and have poured down the mountain. At the top of
+the peak its once active crater is filled up, and we find a convex
+surface ("The Plain of Rambleta") surmounted towards its eastern end by
+a diminutive cone, 500 feet high, called "Humboldt's Ash Cone." The
+slope of the great cone of Teneriffe ranges from 28 deg. to 38 deg.; and below a
+level of 7000 feet the general slope of the whole mountain down to the
+water's edge varies from 10 deg. to 12 deg. from the horizontal. The great cone
+is penetrated by numerous basaltic dykes.
+
+The Cape de Verde Islands, which contain beds of limestone along with
+volcanic matter, possess in the island of Fuego an active volcano,
+rising to a height of 7000 feet above the surface of the ocean. The
+central cone, like that of Teneriffe, rises from within an outer crater,
+formed of basalt alternating with beds of agglomerate, and traversed by
+numerous dykes of lava. This has been broken down on one side like that
+of Somma; and over its flanks are scattered numerous cones of scoriae,
+the most recent dating from the years 1785 and 1799.[3]
+
+[Illustration: Fig. 5.--View of the summit of the Peak of Teneriffe
+(12,225 feet) and of the secondary crater, or outer ring of basaltic
+sheets which surrounds its base; seen from the east.--(After Leopold von
+Buch.)]
+
+The volcanoes of Lancerote have a remarkably linear arrangement from
+west to east across the island. They are not yet extinct; for an
+eruption in 1730 destroyed a large number of villages, and covered with
+lava the most fertile tracts in the island, which at the time of Leopold
+von Buch's visit lay waste and destitute of herbage.[4] In the island of
+Palma there is one large central crater, the Caldera de Palma, three
+leagues in diameter, the walls of which conform closely to the margin
+of the coast. Von Buch calls this crater "une merveille de la nature,"
+for it distinguishes this isle from all the others, and renders it one
+of the most interesting and remarkable amongst the volcanic islands of
+the ocean. The outer walls are formed of basaltic sheets, and towards
+the south this great natural theatre is connected with the ocean by a
+long straight valley, called the "Barranco de los Dolores," along whose
+sides the structure of the mountain is deeply laid open to view. The
+outer flanks of the crater are furrowed by a great number of smaller
+barrancos radiating outward from the rim of the caldera. Von Buch
+regards the barrancos as having been formed during the upheaval of the
+island, according to his theory of the formation of such mountains (the
+elevation-theory); but unfortunately for his views, these ravines widen
+outwards from the centre, or at least do not become narrower in that
+direction, as would be the case were the elevation-theory sound. The
+maps which accompany Von Buch's work are remarkably good, and were
+partly constructed by himself.
+
+(_c._) _Volcanic Islands in the Atlantic south of the Equator._--The
+island of Ascension, formed entirely of volcanic matter, rises from a
+depth of 2000 fathoms in the very centre of the Atlantic. As described
+by Darwin, the central and more elevated portions are formed of
+trachytic matter, with obsidian and laminated ash beds. Amongst these
+are found ejected masses of unchanged granite, fragments of which have
+been torn from the central pipe during periods of activity, and would
+seem to indicate a granitic floor, or at least an original floor upon
+which more recent deposits may have been superimposed. In St. Helena we
+seem, according to Daubeny, to have the mere wreck of one great crater,
+no one stream of lava being traceable to its source, while dykes of lava
+are scattered in profusion throughout the whole substance of the
+basaltic masses which compose the island. Tristan da Cunha, in the
+centre of the South Atlantic, rises abruptly from a depth of 12,150
+feet, at a distance of 1500 miles from any land; and one of its summits
+reaches an elevation of 7000 feet, being a truncated cone composed of
+alternating strata of tuff and augitic lava, surrounding a crater in
+which is a lake of pure water. The volcano is extinct or dormant.
+
+Were the waters of the ocean to be drawn off, these volcanic islands
+would appear like stupendous conical mountains, far loftier, and with
+sides more precipitous, than any to be found on our continental lands,
+all of which rise from platforms of considerable elevation. The enormous
+pressure of the water on their sides enables these mid-oceanic islands
+to stand with slopes varying from the perpendicular to a smaller extent
+than if they were sub-aerial; and it is on this account that we find them
+rising with such extraordinary abruptness from the "vasty deep."
+
+(_d._) _Volcanic Islands of the Pacific._--The volcanic islands of this
+great ocean are scattered over a wide tract on both sides of the
+equator. Those to the north of this line include the Sandwich Islands,
+the Mariana or Ladrone Islands, South Island, and Bonin Sima; south of
+the equator, the Galapagos, New Britain, Salomon, Santa Cruz, New
+Hebrides, the Friendly and Society Isles. While the coral reefs and
+islands of the Pacific may be recognised by their slight elevation
+above the surface of the waters, those of volcanic origin and containing
+active or extinct craters of eruption generally rise into lofty
+elevations, so that the two kinds are called the _Low_ Islands and
+_High_ Islands respectively. Amongst the group are trachytic domes such
+as the Mountain of Tobreonu in the Society Islands, rising to a height
+probably not inferior to that of Etna, with extremely steep sides, and
+holding a lake on its summit.[5] The linear arrangement of some of the
+volcanic islands of the Pacific is illustrated by those of the Tonga, or
+Friendly, Group, lying to the north of New Zealand. They consist of
+three divisions--(1) the volcanic; (2) those formed of stratified
+volcanic tuff, sometimes entirely or partially covered by coralline
+limestone; and (3) those which are purely coralline. The first form a
+chain of lofty cones and craters, lying in a E.N.E. and W.S.W.
+direction, and rising from depths of over 1000 fathoms. Mr. J. J.
+Lister, who has described the physical characters of these islands, has
+shown very clearly that they lie along a line--probably that of a great
+fissure--stretching from the volcanic island of Amargura on the north
+(lat. 18 deg. S.), through Lette, Metis, Kao (3030 feet), Tofua, Falcon,
+Honga Tonga, and the Kermadec Group into the New Zealand chain on the
+south. Some of these volcanoes are in a state of intermittent activity,
+as in the case of Tofua (lat. 20 deg. 30' S.), Metis Island, and Amargura;
+the others are dormant or extinct. The whole group appears to have been
+elevated at a recent period, as some of the beds of coral have been
+raised 1272 feet and upward above the sea-level, as in the case of Eua
+Island.[6] The greater number of the Pacific volcanoes appear to be
+basaltic; such as those of the Hawaiian Group, which have been so fully
+described by Professor J. D. Dana.[7] Here fifteen volcanoes of the
+first class have been in brilliant action; all of which, except three,
+are now extinct, and these are in Hawaii the largest and most eastern of
+the group. This island contains five volcanic mountains, of which Kea,
+13,805 feet, is the highest; next to that, Loa, 13,675 feet; after
+these, Hualalai, rising 8273 feet; Kilauea, 4158 feet; and Kohala, 5505
+feet above the sea; this last is largely buried beneath the lavas of
+Mauna Kea. The group contains a double line of volcanoes, one lying to
+the north and west of the other; and as the highest of the Hawaiian
+Group rises from a depth in the ocean of over 2000 fathoms, the total
+elevation of this mountain from its base on the bed of the ocean is not
+far from 26,000 feet, an elevation about that of the Himalayas. Mauna
+Kea has long been extinct, Hualalai has been dormant since 1801; but
+Mauna Loa is terribly active, there having been several eruptions,
+accompanied by earthquakes, within recent years, the most memorable
+being those of 1852 and 1868. In the former case the lava rose from the
+deep crater into "a lofty mountain," as described by Mr. Coan,[8] and
+then flowed away eastward for a distance of twenty miles. The interior
+of the crater consists of a vast caldron, surrounded by a precipice 200
+to 400 feet in depth, with a circumference of about fifteen miles, and
+containing within it a second crater, bounded by a black ledge with a
+steep wall of basalt--a crater within a crater; and from the floor of
+the inner crater, formed of molten basalt, in a seething and boiling
+state, arise a large number of small cones and pyramids of lava, some
+emitting columns of grey smoke, others brilliant flames and streams of
+molten lava, presenting a wonderful spectacle, the effect of which is
+heightened by the constant roaring of the vast furnaces below.[9]
+
+[1] _Principles of Geology_, 11th edition, vol. ii. p. 48.
+
+[2] Smyth, _Report on the Teneriffe Astronomical Experiment of 1856_.
+Humboldt makes the elevation 12,090 feet. A beautiful model of the Peak
+was constructed by Mr. J. Nasmyth from Piazzi Smyth's plans, of which
+photographs are given by the latter.
+
+[3] Daubeny, _loc. cit._, p. 460.
+
+[4] _Iles Canaries_, p. 37.
+
+[5] Daubeny, _loc. cit._, p. 426.
+
+[6] Lister, "Notes on the Geology of the Tonga Islands," _Quart. Jour.
+Geol. Soc._, No. 188, p. 590 (1891).
+
+[7] Dana, _Characteristics of Volcanoes, with Contributions of Facts and
+Principles from the Hawaiian Islands_. London, 1890.--Also, _Geology of
+the American Exploring Expedition--Volcanoes of the Sandwich Islands_.
+
+[8] Coan, _Amer. Jour. of Science_, 1853.
+
+[9] W. Ellis, the missionary, has given a vivid description of this
+volcano in his _Tour of Hawaii_. London, 1826.--Plans of the crater will
+be found in Professor Dana's work above quoted.
+
+
+
+
+PART II.
+
+EUROPEAN VOLCANOES.
+
+
+
+
+CHAPTER I.
+
+VESUVIUS.
+
+
+Having now dealt in a necessarily cursory manner with volcanoes of
+distant parts of the globe, we may proceed to the consideration of the
+group of active volcanoes which still survive in Europe, as they possess
+a special interest, not only from their proximity and facility of
+access, at least to residents in Europe and the British Isles, but from
+their historic incidents; and in this respect Vesuvius, though not by
+any means the largest of the group, stands the first, and demands more
+special notice. The whole group rises from the shores of the
+Mediterranean, and consists of Vesuvius, Etna, Stromboli, one of the
+Lipari Islands, and Vulcano, a mountain which has given the name to all
+mountains of similar origin with itself.[1] Along with these are
+innumerable cones and craters of extinct or dormant volcanoes, of which
+a large number have been thrown out on the flanks of Etna.
+
+(_a._) _Prehistoric Ideas regarding the Nature of this Mountain._--Down
+to the commencement of the Christian era this mountain had given no
+ostensible indication that it contained within itself a powerful focus
+of volcanic energy. True, that some vague tradition that the mountain
+once gave forth fire hovered around its borders; and several ancient
+writers, amongst them Diodorus Siculus and Strabo, inferred from the
+appearances of the higher parts of the mountain and the character of the
+rocks, which were "cindery and as if eaten by fire," that the country
+was once in a burning state, "being full of fiery abysses, though now
+extinct from want of fuel." Seneca (B.C. 1 to A.D. 64) had detected the
+true character of Vesuvius, as "having been a channel for the internal
+fire, but not its food;" nevertheless, at this period the flanks of the
+mountain were covered by fields and vineyards, while the summit,
+partially enclosed with precipitous walls of the long extinct volcano,
+Somma, was formed of slaggy and scoriaceous material, with probably a
+covering of scrub. Here it was that the gladiator Spartacus (B.C. 72),
+stung by the intolerable evils of the Roman Government, retreated to the
+very summit of the mountain with some trusty followers. Clodius the
+Praetor, according to the narration of Plutarch, with a party of three
+thousand men, was sent against them, and besieged them in a mountain
+(meaning Vesuvius or Somma) having but one narrow and difficult passage,
+which Clodius kept guarded; all the rest was encompassed with broken and
+slippery precipices, but upon the top grew a great many wild vines; the
+besieged cut down as many as they had need of, and twisted them into
+ladders long enough to reach from thence to the bottom, by which,
+without any danger, all got down except one, who stayed behind to throw
+them their arms, after which he saved himself with the rest.[2] "On the
+top" must (as Professor Phillips observes) be interpreted the summit of
+the exterior slope or crater edge, which would appear from the narrative
+to have broken down on one side, affording an entrance and mode of
+egress by which Spartacus fell upon, and surprised, the negligent
+Clodius Glabrus.
+
+[Illustration: Fig. 6.--Probable aspect of Vesuvius as it appeared at
+the beginning of the Christian era; seen from the Bay of Naples.]
+
+In fancied security, villas, temples, and cities had been erected on
+the slopes of the mountain. Herculaneum, Pompeii, and Stabiae, the abodes
+of art, luxury, and vice, had sprung up in happy ignorance that they
+"stood on a volcano," and that their prosperity was to have a sudden and
+disastrous close.[3]
+
+(_b._) _Premonitory Earthquake Shocks._--The first monitions of the
+impending catastrophe occurred in the 63rd year after Christ, when the
+whole Campagna was shaken by an earthquake, which did much damage to the
+towns and villas surrounding the mountain even beyond Naples. This was
+followed by other shocks; and in Pompeii the temple of Isis was so much
+damaged as to require reconstruction, which was undertaken and carried
+out by a citizen at his own expense.[4] These earthquake shakings
+continued for sixteen years. At length, on the night of August 24th,
+A.D. 79, they became so violent that the whole region seemed to reel and
+totter, and all things appeared to be threatened with destruction. The
+next day, about one in the afternoon, there was seen to rise in the
+direction of Vesuvius a dense cloud, which, after ascending from the
+summit of the mountain into the air for a certain height in one narrow,
+vertical trunk, spread itself out laterally in such a form that the
+upper part might be compared to the cluster of branches, and the lower
+to the stem of the pine which forms so common a feature in the Italian
+landscape.[5]
+
+(_c._) _Pliny's Letters to Tacitus._--For an account of what followed we
+are indebted to the admirable letters of the younger Pliny, addressed to
+the historian Tacitus, recounting the events which caused, or
+accompanied, the death of his uncle, the elder Pliny, who at the time of
+this first eruption of Vesuvius was in command of the Roman fleet at the
+entrance to the Bay of Naples. These letters, which are models of style
+and of accurate description, are too long to be inserted here; but he
+recounts how the dense cloud which hung over the mountain spread over
+the whole surrounding region, sometimes illuminated by flashes of light
+more vivid than lightning; how showers of cinders, stones, and ashes
+fell in such quantity that his uncle had to flee from Stabiae, and that
+even at so great a distance as Misenum they encumbered the surface of
+the ground; how the ground heaved and the bed of the sea was upraised;
+how the cloud descended on Misenum, and even the island of Capreae was
+concealed from view; and finally, how, urged by a friend who had
+arrived from Spain, he, with filial affection, supported the steps of
+his mother in flying from the city of destruction. Such being the
+condition of the atmosphere and the effects of the eruption at a point
+so distant as Cape Misenum, some sixteen geographical miles from the
+focus of eruption, it is only to be expected that places not half the
+distance, such as Herculaneum, Pompeii, and even Stabiae, with many
+villages and dwellings, should have shared a worse fate. The first of
+these cities, situated on the coast of the Bay of Naples, appears to
+have been overwhelmed by volcanic mud; Pompeii was buried in ashes and
+lapilli, and Stabiae probably shared a similar fate.[6]
+
+(_d._) _Appearance of the Mountain at the Commencement of the Christian
+Era._--At the time of the first recorded eruption Vesuvius appears to
+have consisted of only a single cone with a crater, now known as Monte
+di Somma, the central cone of eruption which now rises from within this
+outer ruptured casing not having been formed. (Fig. 6.) The first effect
+of the eruption of the year 79 was to blow out the solidified covering
+of slag and scoriae forming the floor of the caldron. Doubtless at the
+close of the eruption a cone of fragmental matter and lava of some
+slight elevation was built up, and, if so, was subsequently destroyed;
+for, as we shall presently see by the testimony of the Abate Guilio
+Cesare Braccini, who examined the mountain not long before the great
+eruption of A.D. 1631, there was no central cone to the mountain at that
+time; and the mountain had assumed pretty much the appearance it had at
+the time that Spartacus took refuge within the walls of the great
+crater.
+
+(_e._) _Destruction of Pompeii._--Pompeii was overwhelmed with dry ashes
+and lapilli. Sir W. Hamilton found some of the stones to weigh eight
+pounds. At the time of the author's visit, early in April 1872, the
+excavations had laid open a section about ten feet deep, chiefly
+composed of alternating layers of small pumice stones (lapilli) and
+volcanic dust. It was during the sinking of a well in 1713 upon the
+theatre containing the statues of Hercules and Cleopatra that the
+existence of the ancient city was accidentally discovered.
+
+(_f._) _More recent eruptions._--Since the first recorded eruption in
+A.D. 79 down to the present day, Vesuvius has been the scene of numerous
+intermittent eruptions, of which some have been recorded; but many,
+doubtless, are forgotten.
+
+In A.D. 203, during the reign of Severus, an eruption of extraordinary
+violence took place, which is related by Dion Cassius, from whose
+narrative we may gather that at this time there was only one large
+crater, and that the central cone of Vesuvius had not as yet been
+upraised. In A.D. 472 an eruption occurred of such magnitude as to cover
+all Europe with fine dust, and spread alarm even at Constantinople.
+
+(_g._) _Eruption of 1631._--In December 1631 occurred the great
+convulsion whose memorials are written widely on the western face of
+Vesuvius in ruined villages. This eruption left layers of ashes over
+hundreds of miles of country, or heaps of mud swept down by hot water
+floods from the crater; the crater itself having been dissipated in the
+convulsion. Braccini, who examined the mountain not long before this
+eruption, found apparently no cone (or mount) like that of the modern
+Vesuvius. He states that the crater was five miles in circumference,
+about a thousand paces deep (or in sloping descent), and its sides
+covered with forest trees and brushwood, while at the bottom there was a
+plain on which cattle grazed.[7] It would seem that the mountain had at
+this time enjoyed a long interval of rest, and that it had reverted to
+very much the same state in which it was at the period of the first
+eruption, when the flanks were peopled by inhabitants living in fancied
+security. But six months of violent earthquakes, which grew more violent
+towards the close of 1631, heralded the eruption which took place in
+December, accompanied by terrific noises from within the interior of the
+mountain. The inhabitants of the coast were thus warned of the
+approaching danger, and had several days to arrange for their safety;
+but in the end, a great part of Torre del Greco was destroyed, and a
+like fate overtook Resina and Granatello, with a loss of life reported
+at 18,000 persons. During the eruption clouds condensed into tempests of
+rain, and hot water from the mountain, forming deluges of mud, swept
+down the sides, and reached even to Nola and the Apennines. Nor was the
+sea unmoved. It retired during the violent earthquakes, and then
+returned full thirty paces beyond its former limits.
+
+Not indeed until near the close of the seventeenth century is there any
+evidence that the central cone of Vesuvius was in existence; but in
+October 1685 an eruption occurred which is recorded by Sorrentino,
+during which was erected "a new mountain within, and higher than the old
+one, and visible from Naples," a statement evidently referable to the
+existing cone--so that it is little more than two centuries since this
+famous volcanic mountain assumed its present form.
+
+(_h._) _Eruptions between the years 1500 and 1800._--Since A.D. 1500
+there have been fifty-six recorded eruptions of Vesuvius; one of these
+in 1767 was of terrific violence and destructiveness, and is represented
+by Sir William Hamilton in views taken both before and during the
+eruption. A pen-and-ink drawing of the appearance of the crater before
+the eruption is here reproduced from Hamilton's picture, from which it
+will be seen that the central crater contained within itself a second
+crater-cone, from whence steam, lava, and stones were being erupted
+(Fig. 7). Thus it will be seen that Vesuvius at this epoch consisted of
+three crater-cones within each other. The first, Monte di Somma; the
+second, the cone of Vesuvius; and the third, the little crater-cone
+within the second. During this eruption, vast lava-sheets invaded the
+fields and vineyards on the flanks of the mountain. A vivid account of
+this eruption, as witnessed by Padre Torre, is given by Professor
+Phillips.[8] We shall pass over others without further reference until
+we come down to our own times, in which Vesuvius has resumed its old
+character, and in one grand exhibition of volcanic energy, which took
+place in 1872, has evinced to the world that it still contains within
+its deep-seated laboratory all the elements of destructive force which
+it exhibited at the commencement of our era.
+
+[Illustration: Fig. 7.--View of the crater of Vesuvius before the
+eruption of 1767, showing an interior crater-cone rising from the centre
+of the exterior crater.--(After Sir W. Hamilton.)]
+
+(_i._) _Structure of the Neapolitan Campagna._--But before giving a
+description of this terrific outburst of volcanic energy, it may be
+desirable to give some account of the physical position and structure of
+this mountain, by which the phenomena of the eruption will be better
+understood. Vesuvius and the Neapolitan Campagna are formed of volcanic
+materials bounded on the west by the Gulf of Naples, and on the east and
+south by ranges of Jurassic limestone, a prolongation of the Apennines,
+which send out a spur bounding the bay on the south, and forming the
+promontory of Sorrento. The little island of Capri is also formed of
+limestone, and is dissevered from the promontory by a narrow channel.
+The northern side of the bay is, however, formed of volcanic materials;
+it includes the Phlegraean Fields (Campi Phlegraei), and terminates in the
+promontory of Miseno. Lying in the same direction are the islands of
+Procida and Ischia, also volcanic. Hence it will be seen that the two
+horns of the bay are formed of entirely different materials, that of
+Miseno on the north being volcanic, that of Sorrento on the south being
+composed of Jurassic limestone, of an age vastly more ancient than the
+volcanic rocks on the opposite shore. (Map, p. 52.)
+
+The general composition of the Neapolitan Campagna, from which the
+mountain rises, has been revealed by means of the Artesian well sunk to
+a depth of about 500 metres (1640 feet) at the Royal Palace of Naples,
+and may be generalised as follows:--
+
+ { Recent beds of volcanic tuff
+(1) From surface to depth of { with marine shells, and containing
+ 715 feet { fragments of trachytic
+ { lava, etc. (_Volcanic Beds_).
+
+ { Bituminous sands and marls
+(2) From 715 to 1420 { with marine shells of recent
+ { species(?) (_Pre-Volcanic Beds_).
+
+(3) From 1420 to 1574 { EOCENE BEDS. Micaceous sandstone
+ { and marl (_Macigno_).
+
+(4) From 1574 to bottom { JURASSIC BEDS. Apennine
+ { Limestone.
+
+[Illustration: Fig. 8.--Map of the district bordering the Bay of
+Naples, with the islands of Capri, Ischia, and Procida.]
+
+From the above section, for which we are indebted to Mr. Johnston-Lavis,
+the most recent writer on Vesuvius, it would appear that the first
+volcanic explosions by which the mountain was ultimately to be built up
+took place after the deposition of the sands and marls (No. 2), while
+the whole Campagna was submerged under the waters of the Mediterranean.
+By the accumulation of the stratified tuff (No. 1), the sea-bed was
+gradually filled up during a period of volcanic activity, and afterwards
+elevated into dry land.[9]
+
+[Illustration: Fig. 9.--View of Vesuvius from the Harbour of Naples at
+the commencement of the eruption of 1872.--(From a sketch by the
+author.)]
+
+(_j._) _Present Form and Structure of Vesuvius and Somma._--The outer
+cone of Vesuvius, or Monte di Somma, rises from a circular platform by a
+moderately gentle ascent, and along the north and east terminates in a
+craggy crest, with a precipitous cliff descending into the Atria del
+Cavallo, forming the wall of the ancient crater throughout half its
+circumference; this wall is formed of scoriae, ashes, and lapilli, and is
+traversed by numerous dykes of lava. Along the west and south this old
+crater has been broken down; but near the centre there remains a
+round-backed ridge of similar materials, once doubtless a part of the
+original crater of Somma, rising above the slopes of lava on either
+hand. On this has been erected the Royal Observatory, under the
+superintendence of Professor Luigi Palmieri, where continuous
+observations are being made, by means of delicate seismometers, of the
+earth-tremors which precede or accompany eruptions; for it is only
+justice to say that Vesuvius gives fair warning of impending mischief,
+and the instruments are quick to notify any premonitory symptoms of a
+coming catastrophe. The elevation of the Observatory is 2080 feet above
+the sea.
+
+On either side of the Observatory ridge are wide channels filled to a
+certain height with lavas of the nineteenth and preceding centuries, the
+most recent presenting an aspect which can only be compared to a
+confused multitude of black serpents and pachyderms writhing and
+interlocked in some frightful death-struggle. Some of this lava, ten
+years old, as we cross its rugged and black surface presents gaping
+fissures, showing the mass to be red-hot a few feet from the surface, so
+slow is the process of cooling. These lava-streams--some of them
+reaching to the sea-coast--have issued forth from the Atria at
+successive periods of eruption.
+
+From the midst of the Atria rises the central cone, formed of cinders,
+scoriae, and lava-streams, and fissured along lines radiating from the
+axis. This cone is very steep, the angle being about 40 deg.-45 deg. from the
+horizontal, and is formed of loose cindery matter which gives way at
+every step, and is rather difficult to climb. But on reaching the summit
+we look down into the crater, displaying a scene of ever-varying
+characters, rather oval in form, and about 1100 yards in diameter. From
+the map of Professor Guiscardi, published in 1855, there are seen two
+minor craters within the central one, formed in 1850, and an outflow of
+lava from the N.W. down the cone. At the time of the author's visit the
+crater was giving indications, by the great quantity of sulphurous gas
+and vapour rising from its surface, and small jets of molten lava
+beginning to flow down the outer side, of the grand outburst of internal
+forces which was presently to follow.
+
+(_k._) _Eruption of 1872._--The grand eruption of 1872, of which a
+detailed account is given by Professor Palmieri,[10] commenced with a
+slight discharge of incandescent projectiles from the crater; and on the
+13th January an aperture appeared on the upper edge of the cone from
+which at first a little lava issued forth, followed by the uprising of a
+cone which threw out projectiles accompanied by smoke, whilst the
+central crater continued to detonate more loudly and frequently. This
+little cone ultimately increased in size, until in April it filled the
+whole crater and rose four or five metres above the brim. At this time
+abundant lavas poured down from the base of the cone into the Atria del
+Cavallo, thence turned into the Fossa della Vetraria in the direction of
+the Observatory and towards the Crocella, where they accumulated to
+such an extent as to cover the hillside for a distance of about 300
+metres; then turning below the Canteroni, formed a hillock without
+spreading much farther.
+
+In October another small crater was formed by the falling in of the
+lava, which after a few days gave vent to smoke and several jets of
+lava; and towards the end of October the detonations increased, the
+smoke from the central crater issued forth more densely mixed with
+ashes, and the seismographical apparatus was much disturbed. On the 3rd
+and 4th November copious and splendid lava-streams coursed down the
+principal cone on its western side, but were soon exhausted; and in the
+beginning of 1872 the little cone, regaining vigour, began to discharge
+lava from the summit instead of the base as heretofore.
+
+In the month of March 1873, with the full moon, the cone opened on the
+north-west side--the cleavage being indicated by a line of
+fumaroles--and lava issued from the base and poured down into the Atria
+as far as the precipices of Monte di Somma. On the 23rd April (another
+full moon) the activity of the craters increased, and on the evening of
+the 24th splendid lava-streams descended the cone in various directions,
+attracting on the same night the visits of a great many strangers. A
+lamentable event followed on the 26th. A party of visitors, accompanied
+by inexperienced guides, and contrary to the advice of Professor
+Palmieri, insisted on ascending to the place from which the lava issued.
+At half-past three on the morning of the 26th they were in the Atria del
+Cavallo, when the Vesuvian cone was rent in a north-west direction and a
+copious torrent of lava issued forth. Two large craters formed at the
+summit of the mountain, discharging incandescent projectiles and ashes.
+A cloud of smoke enveloped the unhappy visitors, who were under a
+hail-storm of burning projectiles. Eight were buried beneath it, or in
+the lava, while eleven were grievously injured.[11] The lava-stream,
+flowing over that of 1871 in the Atria, divided into two branches, the
+smaller one flowing towards Resina, but stopping before reaching the
+town; the larger precipitated itself into the Fossa della Vetraria,
+occupying the whole width of 800 metres, and traversing the entire
+length of 1300 metres in three hours. It dashed into the Fossa di
+Farone, and reached the villages of Massa and St. Sebastiano, covering a
+portion of the houses, and, continuing its course through an artificial
+foss, or trench, invaded cultivated ground and several villages. If it
+had not greatly slackened after midnight, from failure of supply at its
+source, it would have reached Naples by Ponticelli and flowed into the
+sea. The eruption towards the end of April had reached its height. The
+Observatory ridge was bounded on either side by two fiery streams, which
+rendered the heat intolerable. Simultaneously with the opening of the
+great fissure two large craters opened at the summit, discharging with a
+dreadful noise an immense cloud of smoke and ashes, with bombs which
+rose to a height of 1300 metres above the brim of the volcano.[12] The
+torrents of fire which threatened Resina, Bosco, and Torre Annunziata,
+and which devastated the fertile country of Novelle, Massa, St.
+Sebastiano, and Cerole, and two partially buried cities, the continual
+thunderings and growling of the craters, caused such terror, that
+numbers abandoned their dwellings, flying for refuge into Naples, while
+many Neapolitans went to Rome or other places. Fortunately, the paroxysm
+had now passed, the lava-streams stopped in their course, and the great
+torrent which passed the shoulders of the Observatory through the Fossa
+della Vetraria lowered the level of its surface below that of its sides,
+which appeared like two parallel ramparts above it. Had these streams
+continued to flow on the 27th of April as they had done on the previous
+night, they would have reached the sea, bringing destruction to the very
+walls of Naples. During this eruption Torre del Greco was upraised to
+the extent of two metres, and nearly all the houses were knocked down.
+
+The igneous period of eruption having terminated, the ashes, lapilli,
+and projectiles became more abundant, accompanied by thunder and
+lightning. On the 28th they darkened the air, and the terrific noise of
+the mountain continuing or increasing, the terror at Resina, Portici,
+and Naples became universal. It seemed as though the tragic calamities
+of the eruption of A.D. 79 were about to be repeated. But gradually the
+force of the explosions decreased, and the noise from the crater became
+discontinuous, so that on the 30th the detonations were very few, and by
+the 1st May the eruption was completely over.
+
+Such is a condensed account of one of the most formidable eruptions of
+our era. In the frontispiece of this volume a representation, taken (by
+permission) from a photograph by Negretti & Zambra, is given, showing
+the appearance of Vesuvius during the final stage of the eruption, when
+prodigious masses of smoke, steam, and illuminated gas issued forth from
+the summit and overspread the whole country around with a canopy which
+the light of the sun could scarcely penetrate.
+
+It will be noticed in the above account that, concurrently with the full
+moon, there were two distinct and special outbreaks of activity; one
+occurring in March, the other in the month following. That the
+conditions of lunar and solar attraction should have a marked effect on
+a part of the earth's crust, while under the tension of eruptive forces,
+is only what might be expected. At full moon the earth is between the
+sun and the moon, and at new moon the moon is between the sun and the
+earth; under these conditions (the two bodies acting in concert) we have
+spring tides in the ocean, and a maximum of attraction on the mass of
+the earth. Hence the crust, which at the time referred to was under
+tremendous strain, only required the addition of that caused by the
+lunar and solar attractions to produce rupture in both cases, giving
+rise to increased activity, and the extrusion of lava and volatile
+matter. It may, in general, be safely affirmed that low barometric
+pressure on the one hand, and the occurrence of the syzygies (when the
+attractions of the sun and moon are in the same line) on the other, have
+had great influence in determining the crises of eruptions of volcanic
+mountains when in a state of unrest.
+
+_Contrast between the Northern and Southern Slopes._--Before leaving
+Vesuvius it may be observed that throughout all the eruptions of modern
+times the northern side of the mountain, that is the old crater and
+flank of Somma, has been secure from the lava-flows, and has enjoyed an
+immunity which does not belong to the southern and western side. If we
+look at a map of the mountain showing the direction of the streams
+during the last three centuries,[13] we observe that all the streams of
+that period flowed down on the side overlooking the Bay of Naples; on
+the opposite side the wall of Monte di Somma presents an unbroken front
+to the lava-streams. From this it may be inferred that one side, the
+west, is weaker than the other; and consequently, when the lava and
+vapours are being forced upwards, under enormous pressure from beneath,
+the western side gives way under the strain, as in the case of the
+fissure of 1872, and the lava and vapours find means of escape. From
+what has happened in the past it is clear that no place on the western
+side of the mountain is entirely safe from devastation by floods of
+lava; while the prevalent winds tend to carry the ashes and lapilli,
+which are hurled into the air, in the same westerly direction.
+
+[1] For an excellent view of this remarkable volcanic group see Judd's
+_Volcanoes_, 4th edition, p. 43.
+
+[2] Plutarch, _Life of Cassius_; _ed. Reiske_, vol. iii. p. 240.
+
+[3] Strabo gives the following account of the appearance and condition
+of Vesuvius in his day:--"Supra haec loca situs est Vesuvius mons, agris
+cinctus optimis; dempto vertice, qui magna sui parte planus, totus
+sterilis est, adspectu sinereus, cavernasque ostendens fistularum plenas
+et lapidum colore fuliginoso, utpote ab igni exesorum. Ut conjectarum
+facere possis, ista loca quondam arsisse et crateras ignis habuisse,
+deinde materia deficiente restricta fuisse."--_Rer. Geog._, lib. v.
+
+[4] A tablet over the entrance records this act of pious liberality, and
+is given by Phillips, _loc. cit._, p. 12.
+
+[5] The stone pine, _Pinus pinea_, which Turner knew how to use with so
+much effect in his Italian landscapes.
+
+[6] Bulwer Lytton's _Last Days of Pompeii_ presents to the reader a
+graphic picture of the terrible event here referred to:--"The eyes of
+the crowd followed the gesture of the Egyptian, and beheld with
+ineffable dismay a vast vapour shooting from the summit of Vesuvius, in
+the form of a gigantic pine tree; the trunk--blackness, the
+branches--fire! A fire that shifted and wavered in its hues with every
+moment--now fiercely luminous, now of a dull and dying red that again
+blazed terrifically forth with intolerable glare!... Then there arose on
+high the shrieks of women; the men stared at each other, but were
+speechless. At that moment they felt the earth shake beneath their feet;
+the walls of the theatre trembled; and beyond, in the distance, they
+heard the crash of falling roofs; an instant more and the mountain-cloud
+seemed to roll towards them, dark and rapid; at the same time it cast
+forth from its bosom a shower of ashes mixed with vast fragments of
+burning stone. Over the crushing vines--over the desolate streets--over
+the amphitheatre itself--far and wide, with many a mighty splash in the
+agitated sea, fell that awful shower." A visit to the disinterred city
+will probably produce on the mind a still more lasting and vivid
+impression of the swift destruction which overtook this city.
+
+[7] Quoted by Phillips, _loc. cit._, p. 45.
+
+[8] _Vesuvius_, p. 72 _et seq._
+
+[9] Johnston-Lavis, "On the Geology of Monti Somma and Vesuvius,"
+_Quart. Jour. Geol. Soc._, vol. 40 (1884).
+
+[10] Palmieri, _Eruption of Vesuvius in 1872_, with notes, etc., by
+Robert Mallet, F.R.S. London, 1873.
+
+[11] Those who lost their lives were medical students, and an Assistant
+Professor in the University, Antonio Giannone by name.
+
+[12] Involving, as Mr. Mallet calculates, an initial velocity of
+projection of above 600 feet per second.
+
+[13] Such as that given by Professor Phillips in his _Vesuvius_.
+
+
+
+
+CHAPTER II.
+
+ETNA.
+
+
+(_a._) _Structure of the Mountain._--Etna, unlike Vesuvius, has ever
+been a burning mountain; hence it was well known as such to classic
+writers before the Christian era. The structure and features of this
+magnificent mountain have been abundantly illustrated by Elie de
+Beaumont,[1] Daubeny,[2] Baron von Waltershausen,[3] and Lyell,[4] of
+whose writings I shall freely avail myself in the following account, not
+having had the advantage of a personal examination of this region.
+
+_Structure of Etna._--So large is Etna that it would enclose within its
+ample skirts several cones of the size of Vesuvius. It rises to a height
+of nearly 11,000 feet above the waters of the Mediterranean,[5] and is
+planted on a floor consisting of stratified marine volcanic matter, with
+clays, sands, and limestones of newer Pliocene age. Its base is nearly
+circular, and has a circumference of 87 English miles. In ascending its
+flanks we pass successively over three well-defined physical zones: the
+lowest, or fertile zone, comprising the tract around the skirts of the
+mountain up to a level of about 2500 feet, being well cultivated and
+covered by dwellings surrounded by olive groves, fields, vineyards, and
+fruit-trees; the second, or forest zone, extending to a level of about
+6270 feet, clothed with chestnut, oak, beech, and cork trees, giving
+place to pines; and the third, extending to the summit and called "the
+desert region," a waste of black lava and scoriae with mighty crags and
+precipices, terminating in a snow-clad tableland, from which rises the
+central cone, 1100 feet high, emitting continually steam and sulphurous
+vapours, and in the course of almost every century sending forth streams
+of molten lava.
+
+The forest zone is remarkable for the great number of minor craters
+which rise up from the midst of the foliage, and are themselves clothed
+with trees. Sartorius von Waltershausen has laid down on his map of Etna
+about 200 of these cones and craters, some of which, like those of
+Auvergne, have been broken down on one side. Many of these volcanoes of
+second or third magnitude lie outside the forest zone, both above and
+below it; such as the double hill of Monti Rossi, near Nicolosi, formed
+in 1659, which is 450 feet in height, and two miles in circumference at
+its base. Sir C. Lyell observes that these minor crater-cones present us
+with one of the most delightful and characteristic scenes in Europe.
+They occur of every variety of height and size, and are arranged in
+picturesque groups. However uniform they may appear when seen from the
+sea or the plains below, nothing can be more diversified than their
+shape when we look from above into their ruptured craters. The cones
+situated in the higher parts of the forest zone are chiefly clothed with
+lofty pines; while those at a lower elevation are adorned with
+chestnuts, oaks, and beech trees. These cones have from time to time
+been buried amidst fresh lava-streams descending from the great crater,
+and thus often become obliterated.
+
+[Illustration: Fig. 10.--Ideal Section through Etna. (After Lyell.)--A.
+Axis of present cone of eruption; B. Axis of extinct cone of eruption;
+_a._ Older lavas, chiefly trachytic; _b._ Newer lavas, erupted (with
+_a_) before origin of the Val del Bove; _c._ Scoria and lava of recent
+age; T. Tertiary strata forming the foundation to the volcanic rocks.
+The position of the Val del Bove before its formation is shown by the
+lightly-shaded portion above B.]
+
+(_b._) _Val del Bove._--The most wonderful feature of Mount Etna is the
+celebrated Val del Bove (Valle del Bue), of which S. von Waltershausen
+has furnished a very beautiful plate[6]--a vast amphitheatre hewn out of
+the eastern flank of the mountain, just below the snow-mantled platform.
+It is a physical feature somewhat after the fashion of Monte Somma in
+Vesuvius, but exceeds it in magnitude as Etna exceeds Vesuvius. The Val
+del Bove is about five miles in diameter, bounded throughout
+three-fourths of its circumference by precipitous walls of ashes,
+scoriae, and lava, traversed by innumerable dykes, and rising inwards to
+a height of between 3000 and 4000 feet. Towards the east the cliffs
+gradually fall to a height of about 500 feet, and at this side the vast
+chasm opens out upon the slope of the mountain. At the head of the Val
+del Bove rises the platform, surmounted by the great cone and crater. It
+will thus be seen that by means of this hollow we have access almost to
+the very heart of the mountain.
+
+What is very remarkable about the structure of this valley is that the
+beds exhibit "the _qua-qua_ versal dip"--in other words, they dip away
+on all sides from the centre--which has led to the conclusion that in
+the centre is a focus of eruption which had become closed up
+antecedently to the formation of the valley itself. Lyell has explained
+this point very clearly by showing that this focus had ceased to eject
+matter at some distant period, and that the existing crater at the
+summit of the mountain had poured out its lavas over those of the
+extinct orifice. This was prior to the formation of the Val del Bove
+itself; and the question remains for consideration how this vast natural
+amphitheatre came to be hollowed out; for its structure shows
+unquestionably that it owes its form to some process of excavation.
+
+In the first place, it is certainly not the work of running water, as in
+the case of the canyons of Colorado; the porous matter of which the
+mountain is formed is quite incapable of originating and supporting a
+stream of sufficient volume to excavate and carry away such enormous
+masses of matter within the period required for the purpose. We must
+therefore have recourse to some other agency. Numerous illustrations are
+to be found of the explosive action of volcanoes in blowing off either
+the summits of mountains, or portions of their sides. For example, there
+is reason for believing that the first result of the renewed energy of
+Vesuvius was to blow into the air the upper surface of the mountain.
+Again, so late as 1822, during a violent earthquake in Java, a country
+which has been repeatedly devastated by earthquakes and volcanic
+eruptions, the mountain of Galongoon, which was covered by a dense
+forest, and situated in a fertile and thickly-peopled region, and had
+never within the period of tradition been in activity, was thus ruptured
+by internal forces. In the month of July 1822, after a terrible
+earthquake, an explosion was heard, and immense columns of boiling
+water, mixed with mud and stones, were projected from the mountain like
+a water-spout, and in falling filled up the valleys, and covered the
+country with a thick deposit for many miles, burying villages and their
+inhabitants. During a subsequent eruption great blocks of basalt were
+thrown to a distance of seven miles; the result of all being that an
+enormous semicircular gulf was formed between the summit and the plain,
+bounded by steep cliffs, and bearing considerable resemblance to the Val
+del Bove. Other examples of the power of volcanic explosions might be
+cited; but the above are sufficient to show that great hollows may thus
+be formed either on the summits or flanks of volcanic mountains. Chasms
+may also be formed by the falling in of the solidified crust, owing to
+the extrusion of molten matter from some neighbouring vent of eruption;
+and it is conceivable that by one or other of these processes the vast
+chasm of the Val del Bove on the flanks of Etna may have been produced.
+
+(_c._) _The Physical History of Etna._--The physical history of Etna
+seems to be somewhat as follows:--
+
+_First Stage._--Somewhere towards the close of the Tertiary
+period--perhaps early Pliocene or late Miocene--a vent of eruption
+opened on the floor of the Mediterranean Sea, from which sheets of lava
+were poured forth, and ashes mingled with clays and sands, brought down
+from the neighbouring lands, were strewn over the sea-bed. During a
+pause in volcanic activity, beds of limestone with marine shells were
+deposited.
+
+_Second Stage._--This sea-bed was gradually upraised into the air, while
+fresh sheets of lava and other _ejecta_ were accumulated round the vents
+of eruption, of which there were two principal ones--the older under the
+present Val del Bove, the newer under the summit of the principal cone.
+Thus was the mountain gradually piled up.
+
+_Third Stage._--The vent under the Val del Bove ceased to extrude more
+matter, and became extinct. Meanwhile the second vent continued active,
+and, piling up more and more matter round the central crater, surmounted
+the former vent, and covered its _ejecta_ with newer sheets of lava,
+ashes, and lapilli, while numerous smaller vents, scattered all over the
+sides of the mountain, gave rise to smaller cones and craters.
+
+_Fourth Stage._--This stage is signalised by the formation of the Val
+del Bove through some grand explosion, or series of explosions, by which
+this vast chasm was opened in the side of the mountain, as already
+explained.
+
+_Fifth Stage._--This represents the present condition of the mountain,
+whose height above the sea is due, not only to accumulation of volcanic
+materials round the central cone, but to elevation of the whole island,
+as evinced by numerous raised beaches of gravel and sand, containing
+shells and other forms of marine species now living in the waters of the
+Mediterranean.[7] Since then the condition and form of the mountain has
+remained very much the same, varied only by the results of occasional
+eruptions.
+
+(_d._) _Dissimilarity in the Constitution of the Lavas of Etna and
+Vesuvius._--Before leaving the subject we have been considering, it is
+necessary that I should mention one remarkable fact connected with the
+origin of the lavas of Etna and Vesuvius respectively; I refer to their
+essential differences in mineral composition. It might at first sight
+have been supposed that the lavas of these two volcanic
+mountains--situated at such a short distance from each other, and
+evidently along the same line of fracture in the crust--would be of the
+same general composition; but such is not the case. In the lava of
+Vesuvius leucite is an essential, and perhaps the most abundant mineral.
+It is called by Zirkel _Sanidin-Leucitgestein_. (See Plate IV.) But in
+that of Etna this mineral is (as far as I am aware) altogether absent.
+We have fortunately abundant means of comparison, as the lavas of these
+two mountains have been submitted to close examination by petrologists.
+In the case of the Vesuvian lavas, an elaborate series of chemical
+analyses and microscopical observations have been made by the Rev.
+Professor Haughton, of Dublin University, and the author,[8] from
+specimens collected by Professor Guiscardi from the lava-flows extending
+from 1631 to 1868, in every one of which leucite occurs, generally as
+the most abundant mineral, always as an essential constituent. On the
+other hand, the composition of the lavas of Etna, determined by
+Professor A. von Lasaulx, from specimens taken from the oldest
+(voraetnaeischen) sheets of lava down to those of the present day,
+indicates a rock of remarkable uniformity of composition, in which the
+components are plagioclase felspar, augite, olivine, magnetite, and
+sometimes apatite; but of leucite we have no trace.[9] In fact, the
+lavas of Etna are very much the same in composition as the ordinary
+basalts of the British Isles, while those of Vesuvius are of a different
+type. This seems to suggest an origin of the two sets of lavas from a
+different deep-seated magma; the presence of leucite in such large
+quantity requiring a magma in which soda is in excess, as compared with
+that from which the lavas of Etna have been derived.[10]
+
+[1] _Memoires pour Servir_, etc., vol. ii.
+
+[2] Daubeny, _Volcanoes_, p. 270.
+
+[3] Von Waltershausen, _Der Aetna_, edited by A. von Lasaulx.
+
+[4] Lyell, _Principles of Geology_, vol. ii., edition 1872.
+
+[5] Its height, as determined by Captain Smyth in 1875
+trigonometrically, was 10,874 feet, and afterwards by Sir J. Herschel
+barometrically, 10,872 feet.
+
+[6] _Atlas des Aetna_ (Weimar, 1858), in which the different lava-streams
+of 1688, 1802, 1809, 1811, 1819, 1824, and 1838 are delineated.
+
+[7] Sir William Hamilton observes that history is silent regarding the
+first eruptions of Etna. It was in activity before the Trojan War, and
+even before the arrival of the "Sizilien" settlers. Diodorus and
+Thucydides notice the earliest recorded eruptions, those from 772 to 388
+B.C., during which time the mountain was thrice in eruption. Later
+eruptions took place in the year 140, 135, 125, 122 B.C. In the year 44
+B.C., in the reign of Julius Caesar, there was a very violent outburst of
+volcanic activity.--_Neuere Beobachtungen ueber die Vulkane Italiens und
+am Rhein_, p. 173, Frankfurt (1784).
+
+[8] "Report on the Chemical and Mineralogical Characters of the Lavas of
+Vesuvius from 1631 to 1868," _Transactions of the Royal Irish Academy_,
+vol. xxvi. (1876). In the lava of 1848 leucite was found to reach 44.9
+per cent. of the whole mass. In that of Granatello, 1631, it reaches its
+lowest proportion--viz., 3.37 per cent.
+
+[9] A. von Lasaulx, in Von Waltershausen's _Der Aetna_, Book II., x. 423.
+
+[10] The view of Professor Judd, that leucite easily changes into
+felspar, and that some ancient igneous rocks which now contain felspar
+were originally leucitic, does not seem to be borne out by the above
+facts. In such cases the felspar crystals ought to retain the forms of
+leucite. See _Volcanoes_, 4th edition, p. 268.
+
+
+
+
+CHAPTER III.
+
+THE LIPARI ISLANDS, STROMBOLI.
+
+
+(_a._) A brief account of this remarkable group of volcanic islands must
+here be given, inasmuch as they seem to be representatives of a stage of
+volcanic action in which the igneous forces are gradually losing their
+energy. According to Daubeny, the volcanic action in these islands seems
+to be developed along two lines, nearly at right angles to each other,
+one parallel to that of the Apennines, beginning with Stromboli,
+intersecting Panaria, Lipari, and Vulcano; the other extending from
+Panaria to Salina, Alicudi, and Felicudi, and again visible in the
+volcanic products which make their appearance at Ustica. (See Map, Fig.
+11.) The islands lie between the north coast of Sicily and that of
+Italy, and from their position seem to connect Etna with Vesuvius; but
+this is very problematical, as would appear from the difference of their
+lavas. The principal islands are those of Stromboli, Panaria, Lipari,
+Vulcano, Salina, Felicudi, and Alicudi. These three last are extinct or
+dormant, but Salina contains a crater, rising, according to Daubeny, not
+less than 3500 feet above the sea.[1] Vulcano (referred to by Strabo
+under the name of Hiera) consists of a crater which constantly emits
+large quantities of sulphurous vapours, but was in a state of activity
+in the year 1786, when, after frequent earthquake shocks and
+subterranean noises, it vomited forth during fifteen days showers of
+sand, together with clouds of smoke and flame, altering materially the
+shape of the crater from which they proceeded.
+
+[Illustration: LIPARI ISLANDS.
+Fig. 11.--Map to show the position of these islands, showing the
+branching lines of volcanic action--one parallel to that of the
+Apennines, the other stretching westwards at right angles thereto.]
+
+The islands of Lipari are formed of beds of tuff, penetrated by numerous
+dykes of lava, from which uprise two or three craters, formed of pumice
+and obsidian passing into trachyte. Volcanic operations might have here
+been said to be extinct, were it not that their continuance is
+manifested by the existence of hot springs and "stufes," or vapour
+baths, at St. Calogero, about four miles from the town of Lipari.
+Daubeny considers it not improbable that this island may have had an
+active volcano even within the historical period, a view which is borne
+out by the statement of Strabo.[2]
+
+[Illustration: Fig. 12.--Island of Vulcano, one of the Lipari Group, in
+eruption.--(After Sir W. Hamilton.)]
+
+(_b._) But by far the most remarkable island of the group, as regards
+its present volcanic condition, is Stromboli, which has ever been in
+active eruption from the commencement of history down to the present
+day. Professor Judd, who visited this island in 1874, and has produced a
+striking representation of its aspect,[3] gives an account of which I
+shall here avail myself.[4] The island is of rudely circular outline,
+and rises into a cone, the summit of which is 3090 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
+summit; 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 of Professor Judd's visit, 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 is a product of a distinct outburst of volcanic forces.
+Viewed at night-time, Stromboli presents a far more striking and
+singular spectacle. When watched from the deck of a vessel, a glow of
+red light is seen to make its appearance from time to time above the
+summit of the mountain; it 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 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."
+
+The mountain is built up of ashes, slag, and scoriae, to a height of (as
+already stated) over 3000 feet above the surface of the sea; but, as
+Professor Judd observes, this by no means gives a just idea of its vast
+bulk. Soundings in the sea surrounding the island show that the bottom
+gradually shelves around the shores to a depth of nearly 600 fathoms, so
+that Stromboli is a great conical mass of cinders and slaggy materials,
+having a height above its floor of about 6600 feet, and a base the
+diameter of which exceeds four miles.
+
+The crater of Stromboli is situated, not at the apex of the cone, but at
+a distance of 1000 feet below it. The explosions of steam, accompanied
+by the roaring as of a smelting furnace, or of a railway engine when
+blowing off its steam, are said by Judd to take place at very irregular
+intervals of time, "varying from less than one minute to twenty minutes,
+or even more." On the other hand, Hoffmann describes them as occurring
+at "perfectly regular intervals," so that, perhaps, some variation has
+taken place within the interval of about forty years between each
+observation. Both observers agree in stating that lava is to be seen
+welling up from some of the apertures within the crater, and pouring
+down the slope towards the sea, which it seldom or never reaches.[5] The
+intermittent character of these eruptions appears to be due, as Mr.
+Scrope has suggested, to the exact proportion between the expansive and
+repressive forces; the expansive force arising from the generation of a
+certain amount of aqueous vapour and of elastic gas; the repressive,
+from the pressure of the atmosphere and from the weight of the
+superincumbent volcanic products. Steam is here, as in a steam-engine,
+not the originating agent in the phenomena recorded; but the result of
+water coming in contact with molten lava constantly welling up from the
+interior, by which it is converted into steam, which from time to time
+acquires sufficient elastic force to produce the eruptions; the water
+being obviously derived from the surrounding sea, which finds its way by
+filtration through fissures, or through the porous mass of which the
+mountain is formed. Were it not for the access of water this volcano
+would probably appear as a fissure-cone extruding a small and continuous
+stream of molten lava. The adventitious access of the sea water gives
+rise to the phenomena of intermittent explosions. The vitality of the
+volcano is therefore due, not to the presence of water, but to the
+welling up of matter from the internal reservoir through the throat of
+the volcano.
+
+_Pantelleria._--This island, lying between the coast of Sicily and Cape
+Bon in Africa, is wholly volcanic. It has a circumference of thirty
+miles, and from its centre rises an extinct crater-cone to a height of
+about 3000 feet. The flanks of this volcano are diversified by several
+fresh craters and lava-streams, while hot springs burst out with a
+hissing noise on its southern flank, showing that molten matter lies
+below at no very great depth.
+
+This island probably lies along the dividing line between the
+non-volcanic and volcanic region of the Mediterranean, and is
+consequently liable to intermittent eruptions. It was at a short
+distance from this island that the remarkable submarine outburst of
+volcanic forces took place on October 17th, 1891, for an account of
+which we are indebted to Colonel J. C. Mackowen.[6] On that day, after a
+succession of earthquake shocks, the inhabitants were startled by
+observing a column of "smoke" rising out of the sea at a distance of
+three miles, in a north-westerly direction. The Governor, Francesco
+Valenza, having manned a boat, rowed out towards the fiery column, and
+on arriving found it to consist of black scoriaceous bombs, which were
+being hurled into the air to a height of nearly thirty yards; some of
+them burst in the air, others, discharging steam, ran hissing over the
+water; many of them were very hot, some even red-hot. One of these
+bombs, measuring two feet in diameter, was captured and brought to
+shore. It was observed that after the eruption the earthquake shocks
+ceased. A vast amount of material was cast out of the submarine crater,
+forming an island 500 yards in length and rising up to nine feet above
+the surface, but after a few days it was broken up and dispersed over
+the sea-bed by the action of the waves.
+
+[1] _Volcanoes_, p. 262. These islands are described by Hoffmann,
+_Poggendorf Annal._, vol. xxvi. (1832); also by Lyell, _Principles of
+Geology_, vol. ii., and by Judd, who personally visited them, and gives
+a very vivid account of their appearance and structure.
+
+[2] Strabo, lib. vi.
+
+[3] Judd, _Volcanoes_, p. 8.
+
+[4] Stromboli has also been described by Spallanzani, Hoffmann, Daubeny,
+and others. The account of Judd is the most recent. Of this island
+Strabo says, "Strongyle a rotundate figurae sic dicta, ignita ipsa
+quoque, violentia flammarum minor, fulgore excellens; ibi habitasse
+Aecolum ajunt."--Lib. vi.
+
+[5] _Poggend. Annal._, vol. xxvi., quoted by Daubeny.
+
+[6] Communicated by Captain Petrie to the Victoria Institute, 1st
+February 1892. See also a detailed and illustrated account of the
+eruption communicated by A. Ricco to the _Annali dell' Ufficio centrale
+Meteorologico e Geodonamico_, Ser. ii., Parte 3, vol. xi. Summarised by
+Mr. Butler in _Nature_, April 21, 1892.
+
+
+
+
+CHAPTER IV.
+
+THE SANTORIN GROUP.
+
+
+[Illustration: Fig. 13.--Ideal Section through the Gulf of Santorin, to
+show the structure of the submerged volcano.--_a._ Island of Aspronisi;
+_b._ Island of Thera; 1. Old Kaimeni Island; 2. New Kaimeni Island; 3.
+Little Kaimeni Island.]
+
+(_a._) Before leaving the subject of European active volcanoes, it is
+necessary to give some account of the remarkable volcanic island of
+Santorin, in the Grecian archipelago. This island for 2000 years has
+been the scene of active volcanic operations, and in its outline and
+configuration, both below and above the surface of the Mediterranean,
+presents the aspect of a partially submerged volcanic mountain. (See
+Section, Fig. 13.) If, for example, we can imagine the waters of the sea
+to rise around the flanks of Vesuvius until they have entered and
+overflowed to some depth the interior caldron of Somma, thus converting
+the old crater into a crescent-shaped island, and the cone of Vesuvius
+into an island--or group of islands--within the caldron, then we shall
+form some idea of the appearance and structure of the Santorin group.
+
+_Form of the Group._--The principal island, Thera, has somewhat the
+shape of a crescent, breaking off in a precipitous cliff on the inner
+side, but on the outer side sloping at an angle of about fifteen degrees
+into deep water. Continuing the curvature of the crescent, but separated
+by a channel, is the island of Therasia; and between this and the
+southern promontory of Thera is another island called Aspronisi. All
+these islands, if united, would form the rim of a crater, in which the
+volcanic matter slopes outward into deep water, descending at a short
+distance to a depth of 200 fathoms and upwards. In the centre of the
+gulf thus formed rise three islands, called the Old, New, and Little
+Kaimenis. These may be regarded as cones of eruption, which history
+records as having been thrown up at successive intervals. According to
+Pliny, the year 186 B.C. gave birth to Old Kaimeni, also called Hiera,
+or the Sacred Isle; and in the first year of our era Thera (the Divine)
+made its appearance above the water, and was soon joined to the older
+island by subsequent eruptions. Old Kaimeni also increased in size by
+the eruptions of 726 and 1427. A century and a half later, in 1573,
+another eruption produced the cone and crater called Micra-Kaimeni. Thus
+were formed, or rather were rendered visible above the water, the
+central craters of eruption; and between these and the inner cliff of
+Thera and Therasia is a ring of deep water, descending to a depth of
+over 200 fathoms. So that, were these islands raised out of the sea, we
+should have presented to our view a magnificent circular crater about
+six miles in diameter, bounded by nearly vertical walls of rock from
+1000 to 1500 feet in height, and ruptured at one point, from the centre
+of which would rise two volcanic cones--namely, the Kaimenis--one with a
+double crater, still foci of eruption, and from time to time bursting
+forth in paroxysms of volcanic energy, of which those of 1650, 1707, and
+1866 were the most violent and destructive.[1] Of this last I give a
+bird's-eye view (Fig. 14).
+
+The only rock of non-volcanic origin in these islands consists of
+granular limestone and clay slate forming the ridge of Mount St. Elias,
+which rises to a height of 1887 feet at the south-eastern side of the
+island of Thera, crossing the island from its outer margin nearly to the
+interior cliff, so that the volcanic materials have been piled up along
+its sides. The rocks of St. Elias are much more ancient than any of the
+volcanic materials around; and, as Bory St. Vincent has shown, have been
+subjected to the same flexures, dip and strike, as those sedimentary
+rocks which go to form the non-volcanic islands of the Grecian
+archipelago.
+
+[Illustration: Fig. 14.--Bird's-eye View of the Gulf of Santorin during
+the volcanic eruption of February 1866.--(After Lyell.)]
+
+[Illustration: _Ground Plan of Rocca Monfina_
+Fig. 15.--Rocca Monfina, in Southern Italy, showing a crater-ring of
+trachytic tuffs, from the midst of which, according to Judd, an andesite
+lava-cone has been built up. Compare with the Santorin Group.]
+
+(_b._) _Origin of the Santorin Group._--In reference to the origin of
+the Santorin group, Lyell regards it as a remnant of a great volcanic
+mountain which possessed a focus of eruption rising in the position of
+the present foci, but afterwards partially destroyed and the whole
+submerged to a depth of over 1000 feet. But another explanation is open
+to us, and one not inconsistent with what we now know of the physical
+changes to which the Mediterranean has been subjected since early
+Tertiary times. To my mind it is difficult to conceive how such a
+volcanic mountain as that of Santorin could have been formed under
+water; while, on the other hand, its physical structure and contour bear
+so striking a resemblance (as already observed) to those of Vesuvius and
+Rocca Monfina that we are much tempted to infer that it had a somewhat
+similar origin. Now we know that Vesuvius was built up by means of
+successive eruptions taking place under the air; and the question arises
+whether it could be possible that Santorin had a similar origin owing to
+the waters of the Mediterranean having been temporally lowered at a
+later Tertiary epoch. It has been stated by M. Fouque that the age of
+the more ancient volcanic beds of Santorin belong, as shown by the
+included fossils, to the newer Pliocene epoch. These are of course the
+unsubmerged, and therefore more recent strata, and may have been
+recently upheaved during one or more of the outbursts of volcanic
+energy. But it seems an impossibility that the Gulf of Santorin, with
+its precipitous walls and deep circular interior channel, as shown by
+the Ideal Section (Fig. 13), could have been formed otherwise than under
+the air. We are led, therefore, to inquire whether there was a time in
+the history of the Mediterranean, since the Eocene period, when the
+waters were lower than at present. That this was the case we have clear
+evidence. The remains of elephants, hippopotami, and other animals,
+which have been discovered in great numbers in the Maltese caves, show
+that this island was united to Sicily, and this again to Europe, during
+the later Pliocene epoch, so as to have become the abode of an
+Europasian fauna. According to Dr. Wallace, a causeway of dry land
+existed, stretching from Italy to Tunis in North Africa through the
+Maltese Islands--an inference involving the lowering of the waters of
+the Mediterranean by several hundred feet.[2] There is every reason for
+supposing that the old volcano of Santorin was in active eruption at
+this period; and its history may be considered to be similar to that of
+Vesuvius until, at the rising of the waters during the Pluvial (or
+Post-Pliocene) epoch, during which they rose higher than at present,
+Santorin was converted into a group of islands, slightly differing in
+form from those of the present day. This view seems to meet the
+difficulties regarding the origin of this group, difficulties which
+Lyell had long since clearly recognised.
+
+(_c._) _Limit of the Mediterranean Volcanic Region._--With the Santorin
+group we conclude our account of the active European volcanoes. It may
+be observed, however, that from some cause not ascertained the volcanic
+districts of the Mediterranean and its shores are confined to the north
+side of that great inland sea; so that as regards vulcanicity the
+African coast presents a striking contrast to that of the opposite side.
+If we draw a line from the shores of the Levant to the Straits of
+Gibraltar, by Candia, Malta, and to the south of Pantelleria and
+Sardinia, we shall find that the volcanic islands and districts of the
+mainland lie to the north of it.[3] This has doubtless some connection
+with the internal geological structure. The immunity of the Libyan
+desert from volcanic irruptions is in keeping with the remarkably
+undisturbed condition of the Secondary strata, which seldom depart much
+from the horizontal position; while the igneous rocks of the Atlas
+mountains are probably of great geological antiquity. On the other hand,
+the Secondary and Tertiary formations of the northern shores and islands
+of the Mediterranean are generally characterised by the highly-inclined,
+flexured, and folded position of the strata. Hence we may suppose that
+the crust over the region lying to the north of the volcanic line, owing
+to its broken and ruptured condition, was less able to resist the
+pressure of the internal forces of eruption than that lying to the south
+of it; and that, in consequence, vents and fissures of eruption were
+established over the former of these regions, while they are absent in
+the latter.
+
+[1] Fuller details will be found in Daubeny's _Volcanoes_, chap. xviii.,
+and Lyell's _Principles of Geology_, vol. ii. p. 65 (edition 1872). The
+bird's-eye view is taken from this latter work by kind permission of the
+publisher, Mr. J. Murray, as also the accompanying Ideal Section, Fig.
+13.
+
+[2] Wallace, _Geographical Distribution of Animals_ (1876). The author's
+_Sketch of Geological History_, p. 130 (Deacon & Co., 1887).
+
+[3] The _volcanic area_ lying to the north of this line will include
+Sardinia, Sicily, Pantelleria, the Grecian Archipelago, Asia Minor, and
+Syria; the _non-volcanic area_ lying to the south of this line will
+include the African coast, Malta, Isles of Crete and Cyprus. The Isle of
+Pantelleria is apparently just on the line, which, continued eastward,
+probably follows the north coast of Cyprus, parallel to the strike of
+the strata and of the central axis of that island.--See "Carte
+Geologique de l'ile de Chypre, par MM. Albert Gaudry et Amedee Damour"
+(1860).
+
+
+
+
+CHAPTER V.
+
+EUROPEAN EXTINCT OR DORMANT VOLCANOES.
+
+
+We are naturally led on from a consideration of the active volcanoes of
+Europe to that of volcanoes which are either dormant or extinct in the
+same region. Such are to be found in Italy, Central France, both banks
+of the Rhine and Moselle, the Westerwald, Vogelsgebirge, and other
+districts of Germany; in Hungary, Styria, and the borders of the Grecian
+archipelago. But the subject is too large to be treated here in detail;
+and I propose to confine my observations to some selected cases which
+are to be found in Southern Italy, Central France, and the Rhenish
+districts, where the volcanic features are of so recent an age as to
+preserve their outward form and structure almost intact.
+
+(_a._) _Southern Italy._--Extinct volcanoes and volcanic rocks occupy
+considerable tracts between the western flanks of the Apennines and the
+Mediterranean coast in the Neapolitan and Roman States, forming the
+remarkable group of the Phlegraean fields (Campi Phlegraei), with the
+adjoining islands of Ischia, Procida, Nisida, Vandolena, Ponza, and
+Palmarola; at Melfi and Avellino. All the region around Rome extending
+along the western slopes of the Apennines from Velletri to Orvieto,
+together with Mount Annato in Tuscany, is formed of volcanic material,
+and the same may be said of a large part of the island of Sardinia. From
+these districts I shall select some points which seem to be of special
+interest.
+
+_Monte Nuovo and the Phlegraean Fields._--The tract of which this
+celebrated district forms a part lies as it were in a bay of the
+Apennine limestone of Jurassic age. The floor of this bay is composed of
+puzzolana, a name given to beds of volcanic tuff of great thickness, and
+rising into considerable hills in the vicinity of the city of Naples,
+such as that of St. Elmo. Its composition is peculiar, as it is chiefly
+formed of small pieces of pumice, obsidian, and trachyte, in beds
+alternating with loam, ferriferous sand, and fragments of limestone. It
+is evidently of marine formation, as Sir William Hamilton, Professor
+Pilla, and others have detected sea-shells therein, of the genera
+_Ostraea_, _Cardium_, _Pecten_ and _Pectunculus_, _Buccinum_, etc. It is
+generally of a greyish colour, and sometimes sufficiently firm to be
+used as a building stone. The Roman Campagna is largely formed of
+similar materials, which were deposited at a time when the districts in
+question were submerged, and matter was being erupted from volcanic
+vents at various points around, and spread over the sea-bed.
+
+Such is the character of the general floor on which the more recent
+crater-cones of this district have been built. These are numerous, and
+all extinct with the exception of the Solfatara, near Puzzuoli, from
+which gases mixed with aqueous vapour are continually being exhaled. The
+gases consist of sulphuretted hydrogen mixed with a minute quantity of
+muriatic acid.[1] This district is also remarkable for containing
+several lakes occupying the interiors of extinct craters; amongst
+others, Lake Avernus, which, owing to its surface having been darkened
+by forests, and in consequence of the effluvia arising from its stagnant
+waters, has had imparted to it a character of gloom and terror, so that
+Homer in the _Odyssey_ makes it the entrance to hell, and describes the
+visit of Ulysses to it. Virgil follows in his steps. Another lake of
+similar origin is Lake Agnano. Here also is the Grotto del Cane, a
+cavern from which are constantly issuing volumes of carbonic acid gas
+combined with much aqueous vapour, which is condensed by the coldness of
+the external air, thus proving the high temperature of the ground from
+which the gaseous vapour issues. This whole volcanic region, so replete
+with objects of interest,[2] may be considered, as regards its volcanic
+character, in a moribund condition; but that it is still capable of
+spasmodic movement is evinced by the origin of Monte Nuovo, the most
+recent of the crater-cones of the district. This mountain, rising from
+the shore of the Bay of Baiae, was suddenly formed in September 29th,
+1538, and rises to a height of 440 feet above the sea-level. It is a
+crater-cone, and the depth of the crater has been determined by the
+Italian mineralogist Pini to be 421 English feet; its bottom is thus
+only 19 feet above the sea-level. A portion of the base of the cone is
+considered partly to occupy the site of the Lucrine Lake, which was
+itself nothing more than the crater of a pre-existent volcano, and was
+almost entirely filled up during the explosion of 1538. Monte Nuovo is
+composed of ashes, lapilli, and pumice-stones; and its sudden formation,
+heralded by earthquakes, and accompanied by the ejection of volcanic
+matter mixed with fire and water, is recorded by Falconi, who vividly
+depicts the terror and consternation of the inhabitants of the
+surrounding country produced by this sudden and terrible outburst of
+volcanic forces.[3]
+
+(_b._) _Central Italy and the Roman States._--The tract bordering the
+western slopes of the Apennines northward from Naples into Tuscany, and
+including the Roman States, is characterised by volcanic rocks and
+physical features of remarkable interest and variety. These occur in the
+form of extinct craters, sometimes filled with water, and thus converted
+into circular lakes; or of extensive sheets and conical hills of tuff;
+or, finally, of old necks and masses of trachyte and basalt, sometimes
+exhibiting the columnar structure. The Eternal City itself is built on
+hills of volcanic material which some observers have supposed to be the
+crater of a great volcano; but Ponzi, Brocchi, and Daubeny all concur in
+the opinion that this is not the case, as will clearly appear from the
+following account.
+
+The geological structure of the valley of the Tiber at Rome is very
+clearly described by Professor Ponzi in a memoir published in 1850, from
+which the accompanying section is taken.[4] (Fig. 16.) From this it will
+be seen that "the Seven-hilled City" is built upon promontories of
+stratified volcanic tuff, of which the Campagna is formed, breaking off
+along the banks of the Tiber, the hills being the result of the erosion,
+or denudation, of the strata along the side of the river valley. As the
+strata dip from west to east across the course of the river, it follows
+that those on the western banks are below those on the opposite side;
+and thus the marine sands and marls which underlie the volcanic tuff,
+and are concealed by it along the eastern side of the valley, emerge on
+the west, and form the range of hills on that side. Such being the
+structure of the formations under Rome, it is evident that it is not
+"built on a volcano."
+
+[Illustration: Fig. 16.--Geological Section across the Valley of the
+Tiber at Rome. 1. Alluvium of the Tiber; 2. Diluvium; 3. Volcanic tuff
+(recent deposits); 4. Sands, etc.; 5. Blue marl (sub-Apennine
+deposits).]
+
+The tuff contains fragments of lava and pebbles of Apennine limestone,
+and was deposited under the waters of an extensive lake at a time when
+volcanic action was rife amidst the Alban Hills. This lacustrine
+formation rests in turn on deposits of marine origin, containing
+oysters, patellae, and other sea-shells, of which the chain of hills on
+the right bank of the Tiber is chiefly formed.
+
+The district around Albano lying to the south of Rome is of peculiar
+interest from the assemblage of old crater-lakes which it contains; as,
+for instance, those of Albano, Vallariccia, Nemi, Juturna, and the lake
+of Gabii. The lake of Albano, one of the most beautiful sheets of water
+in the world, is about six miles in circumference, and surrounded by
+beds of peperino, a variety of tuff presenting a bright, undecomposed
+aspect when newly broken. The level of this lake was lowered by the
+Romans during the siege of Veii by means of a tunnel, so that the waters
+are 200 feet lower than the level at which they originally stood. In the
+same district is the lake of Nemi, very regular in its circular outline;
+that of Juturna lying near the foot of the Alban Hills, and that of
+Ariccia lying in a deep hollow eight miles in circumference;--all may be
+supposed to have been the craters of extinct volcanoes, both by reason
+of their shape and of the materials of which they are formed. All these
+old craters are, however, according to Daubeny, "only the dependencies
+and offshoots, as it were, of the great extinct volcano, the traces of
+which still remain upon the summit of the Alban Hills, and which is
+comparable in its form to that of Vesuvius, as it is surrounded by an
+outer circle of volcanic rock comparable to that of Somma."[5]
+
+To the north of the city of Rome are several crateriform lakes, some of
+which are of great size, such as that of Bolsena, over twenty miles in
+circumference, and the Lago di Bracciano, almost as large, and lying
+about twelve miles from the city. These extensive sheets of water are
+surrounded by banks of tuff and volcanic sand, in which fragments of
+augite, leucite, and crystals of titanite are distributed. The town of
+Viterbo is built up at the foot of a steep hill called Monte Cimini, the
+lower part of which is composed of trachyte; this is surmounted by
+tuff, which appears to have been ejected from an extinct crater
+occupying the summit of the mountain, and now converted into a lake
+called the Lake of Vico. This crater is perfectly circular, and from its
+centre rises a little conical hill covered by trees.
+
+(_c._) _Physical History._--Space does not permit of a fuller
+description of the remarkable volcanic features of the tract lying along
+the western slope of the Apennines; but from what has been stated it
+will be clear that volcanic forces have been in operation at one time on
+a grand scale in the Roman States and the South of Tuscany, over a tract
+extending from Mount Annato to Velletri and Segni.
+
+This tract was separated from that of the Neapolitan volcanic region by
+a range of limestone hills of Jurassic age between Segni and Gaeta, a
+protrusion of the Alban Hills westward; but the general structure and
+physical history of both regions are probably very similar, with the
+exception that the igneous forces still retain their vitality in the
+more southerly region. In the case of the Roman volcanic district, a bay
+seems to have been formed about the close of the Miocene period, bounded
+on all sides but the west by hills of limestone, over whose bed strata
+of marl, sandstone, and conglomerate were deposited. This tract was
+converted by subsequent movements into a fresh-water lake, and
+contemporaneously volcanic operations commenced over the whole region,
+and beds of tuff, often containing blocks of rock ejected from
+neighbouring craters, were deposited over those of marine origin.
+Meanwhile numerous crater-cones were thrown up; and, as the land
+gradually rose, the waters of the lake were drained off, leaving dry
+the Campagna and plain of the Tiber. Ultimately the volcanic fires
+smouldered down and died out, whether within the historic epoch or not
+is uncertain; lakes were formed within the now dormant craters, and the
+face of nature gradually assumed a more placid and less forbidding
+aspect over this memorable region, destined to be the site of Rome, the
+Mistress of the World.
+
+[1] As determined by Daubeny in 1825.
+
+[2] Including the ruins of the Temple of Serapis, whose pillars are
+perforated by marine boring shells up to a height of about 16 feet from
+their base; indicating that the land had sunk down beneath the sea, and
+afterwards been elevated to its present level.
+
+[3] The account of Falconi, and another by Pietro Giacomo di Toledo, are
+given by Sir W. Hamilton, _op. cit._, p. 198, and also reproduced by Sir
+C. Lyell, _Principles_, vol. i. p. 608.
+
+[4] Guiseppe Ponzi, "Sulla storia fisica del Bacino di Roma," _Annali di
+Scienze Fisiche_ (Roma, 1850).
+
+[5] Daubeny, _Volcanoes_, p. 171.
+
+
+
+
+CHAPTER VI.
+
+EXTINCT VOLCANOES OF CENTRAL FRANCE.
+
+
+(_a._) _General Structure of the Auvergne District._--From a granitic
+and gneissose platform situated near the centre of France, and separated
+from the western spurs of the Alps by the wide valley of the Rhone,
+there rises a group of volcanic mountains surpassing in variety of form
+and structure any similar mountain group in Europe, and belonging to an
+epoch ranging from the Middle Tertiary down almost to the present day.
+This volcanic group of mountains gives rise to several important rivers,
+such as the Loire, the Allier, the Soule (a branch of the Loire), the
+Creuse, the Dordogne, and the Lot; and in the Plomb du Cantal attains an
+elevation of 6130 feet above the sea. Its southern section, that of Mont
+Dore, the Cantal, and the Haute Loire, is characterised by magnificent
+valleys, traversing plateaux of volcanic lava, and exhibiting the
+results of river erosion on a grand scale; while its northern section,
+that of the Puy de Dome, presents to us a varied succession of volcanic
+crater-cones and domes, with their extruded lava-streams, almost as
+fresh and unchanged in form as if they had only yesterday become
+extinct. A somewhat similar, but less important, chain of extinct
+volcanoes also occurs in the Velay and Vivarais, between the upper
+waters of the Loire and the Allier, in the vicinity of the town of Le
+Puy.[1] The principal city in this region is Clermont-Ferrand, lying
+near the base of the Puy de Dome, and ever memorable as the birthplace
+of Blaise Pascal.[2]
+
+[Illustration: Fig. 17.--Generalised Section through the Puy de Dome and
+Vale of Clermont, distance about ten miles. The general floor formed of
+granite and gneiss (G); D. Domite-lava of the Puy de Dome; Sc. Cones of
+ashes and scoriae; L. Lava-sheets; A. Alluvium of the Vale of Clermont
+and Lake deposits.]
+
+The physical structure of this region is on the whole very simple. The
+fundamental rocks consist of granite and gneiss passing into schist, all
+of extreme geological antiquity, forming a vast platform gradually
+rising in a southerly direction towards the head waters of the Loire
+and the Allier in the Departments of Haute Loire, Lozere, and Ardeche.
+On this platform are planted the whole of the volcanic mountains. (See
+Fig. 17.)
+
+The granitic plateau is bounded on the east, throughout a distance of
+about 50 miles, by the wide and fertile plain of Clermont, watered by
+the Allier and its numerous branches descending from the volcanic
+mountains, and is about 25 miles in width from east to west in the
+parallel of Clermont, but gradually narrowing in a southerly direction,
+till at Brioude it becomes an ordinary mountain ravine. The eastern
+margin of the plain is formed by another granitic ridge expanding into a
+plateau towards the south, and joining in with that already described;
+but towards the north and directly east of Clermont it forms a high
+ridge traversed by the railway to St. Etienne and Lyons, and descending
+towards the east into the valley of the Loire. No more impressive view
+is to be obtained of the volcanic region than that from the summit of
+this second ridge, on arriving there towards evening from the city of
+Lyons. At your feet lies the richly-cultivated plain of Clermont, dotted
+with towns, villages, and hamlets, and decorated with pastures,
+orchards, vineyards, and numerous trees; while beyond rises the granitic
+plateau, breaking off abruptly along the margin of the plain, and deeply
+indented by the valleys and gorges along which the streams descend to
+join the Allier. But the chief point of interest is the chain of
+volcanic crater-cones and dome-shaped eminences which rise from the
+plateau, amongst which the Puy de Dome towers supreme. Their individual
+forms stand out in clear and sharp relief against the western sky, and
+gradually fade away towards the south into the serried masses of Mont
+Dore and Cantal, around whose summits the evening mists are gathering.
+Except the first view of the Mont Blanc range from the crest of the
+Jura, there is no scene perhaps which is calculated to impress itself
+more vividly on the memory than that here faintly described.[3]
+
+[Illustration: Fig. 18.--Transverse view of the Puy de Dome and
+neighbouring volcanoes from the Puy de Chopine.--(After Scrope.)]
+
+(_b._) _The Vale of Clermont._--The plain upon which we look down was
+once the floor of an extensive lake, for it is composed of various
+strata of sand, clay, marl, and limestone, containing various genera and
+species of fresh-water shells. These strata are of great thickness,
+perhaps a thousand feet in some places; and along with such shells as
+_Paludina_, _Planorbis_, and _Limnaea_ are also found remains of various
+other animals, such as fish, serpents, batrachians, crocodiles,
+ruminants, and those of huge pachyderms, as _Rhinoceros_, _Dinotherium_,
+and _Caenotherium_. This great lake, occupying a hollow in the old
+granitic platform of Central France, must have been in existence for an
+extensive period, which MM. Pomel, Aymard, and Lyell all unite in
+referring to that of the Lower Miocene. But what is to us of special
+interest is the fact that, in the deposits of this lake of the Haute
+Loire, with the exception of the very latest, there is no intermixture
+of volcanic products such as might have been expected to occur if the
+neighbouring volcanoes had been in activity during its existence. Hence
+it may be supposed that, as Scrope suggested, the waters of the lake
+were drained off owing to the disturbance in the levels of the country
+caused by the first explosions of the Auvergne volcanoes.[4] If this be
+so, then we possess a key by which to determine the period of the first
+formation of volcanoes in Central France; for, as the animal remains
+enclosed in the lacustrine deposits of the Vale of Clermont belong to
+the early Miocene stage, and the earliest traces of contemporaneous
+volcanic _ejecta_ are found only in the uppermost deposits, we may
+conclude that the first outburst of volcanic action occurred towards the
+close of the Miocene period--a period remarkable for similar exhibitions
+of internal igneous action in other parts of the world.
+
+(_c._) _Successive Stages of Volcanic Action in Auvergne._--The volcanic
+region here described, which has an area of about one hundred square
+miles, does not appear to have been at one and the same period of time
+the theatre of volcanic action over its whole extent. On the contrary,
+this action appears to have commenced at the southern border of the
+region in the Cantal, and travelling northwards, to have broken out in
+the Mont Dore region; finally terminating its outward manifestations
+among the craters and domes of the Puy de Dome. In a similar manner the
+volcanic eruptions of the Haute Loire and Ardeche, lying to the
+eastward, and separated from those of the Cantal by the granitoid ridge
+of the Montagnes de Margeride, belong to two successive periods
+referable very closely to those of the Mont Dore and the Puy de Dome
+groups.[5] The evidence in support of this view is very clear and
+conclusive; for, while the volcanic craters formed of ash, lapilli, and
+scoriae, together with the rounded domes of trachytic rock of which the
+Puy de Dome group is composed, preserve the form and surface indications
+of recently extinguished volcanoes, those which we may assume to have
+been piled up in the region of Mont Dore and Cantal have been entirely
+swept away by prolonged rain and river action, and the sites of the
+ancient craters and cones of eruption are only to be determined by
+tracing the great sheets of lava up the sides of the valleys to their
+sources, generally situated at the culminating points of their
+respective groups. Other points of evidence of the great antiquity of
+the latter groups might be adduced from the extent of the erosion which
+has taken place in the sheets of lava having their sources in the vents
+of the Plomb du Cantal and of Mont Dore, owing to which, magnificent
+valleys, many miles in length and hundreds of feet in depth, have been
+cut out of these sheets of lava and their supporting rocks, whether
+granitic or lacustrine, and the materials carried away by the streams
+which flow along their beds. These points will be better understood when
+I come to give an account of the several groups; and in doing so I will
+commence with that of the Cantal.[6]
+
+(_d._) _The Volcanoes of the Cantal._--The original crater-cones of
+this group have entirely disappeared throughout the long ages which have
+elapsed since the lava-streams issued forth from their internal
+reservoirs. The general figure of this group of volcanic mountains is
+that of a depressed cone, whose sides slope away in all directions from
+the central heights, which are deeply eroded by streams rising near the
+apex and flowing downwards in all directions towards the circumference
+of the mountain, where they enter the Lot, the Dordogne, and the Allier.
+The orifice of eruption was situated at the Plomb du Cantal, formed of
+solid masses of trachyte, which, owing, as Mr. Scrope supposes, to a
+high degree of fluidity, were able to extend to great distances in
+extensive sheets, and were afterwards covered by repeated and
+widely-spread flows of basalt; so that the trachyte towards the margin
+of the volcanic area becomes less conspicuous than the basalt by which
+it is more or less concealed from view, or overlapped. Extensive beds of
+tuff and breccia accompany the trachytic masses.
+
+Magnificent sections of the rocks are laid open to view along the sides
+of the valleys, which are steep and rock-bound. Except towards the
+south-west, about Aurillac, where lacustrine strata overlie the granite,
+the platform from which rises the volcanic dome is composed of granitic
+or gneissose rocks. Accompanying the lava-streams are great beds of
+volcanic agglomerate, which Mr. Scrope considers to have been formed
+contemporaneously with the lava which they envelop, and to be due to
+torrents of water tumultuously descending the sides of the volcano at
+periods of eruption, and bearing down immense volumes of its fragmental
+_ejecta_ in company with its lava-streams.[7] Nowhere throughout this
+region do beds of trachyte and basalt alternate with one another; in all
+cases the basalt is the newer of the two varieties of rock, and this is
+generally the case throughout the region here described.
+
+(_e._) _Volcanoes of Mont Dore._--This mountain lies to the north of
+that of Cantal, and somewhat resembles it in general structure and
+configuration. Like Cantal, it is destitute of any distinct crater; all
+that is left of the central focus of eruption being the solidified
+matter which filled the throat of the original volcano, and which forms
+a rocky mass of lava, rising in its highest point, the Pic de Saucy, to
+an elevation (as given by Ramond) of 6258 feet above the level of the
+sea, thus exceeding that of the Plomb du Cantal by 128 feet. Its figure
+will be best understood by supposing seven or eight rocky summits
+grouped together within a circle of about a mile in diameter, from
+whence, as from the apex of an irregular and flattened cone, all the
+sides slope more or less rapidly downwards, until their inclination is
+gradually lost in the plain around. This dome-shaped mass has been
+deeply eroded on opposite sides by the valleys of the Dordogne and
+Chambon; while it is further furrowed by numerous minor streams.[8]
+
+The great beds of volcanic rock, disposed as above stated, consist of
+prodigious layers of scoriae, pumice-stones, and detritus, alternating
+with beds of trachyte and basalt, which often descend in uninterrupted
+currents till they reach the granite platform, and then spread
+themselves for miles around. The sheets of basalt are found to stretch
+to greater distances than those of trachyte, and have flowed as far as
+15 or 20 miles from their orifices of eruption; while in some cases, on
+the east and north sides, they have extended as far as 25 or 30 miles
+from the central height. On the other hand, a radius of about ten miles
+from the centre would probably include all the streams of trachyte;--so
+much greater has been the viscosity of the basalt over the latter rock.
+Some portions of these great sheets of lava, cut off by river valleys or
+eroded areas from the main mass of which they once formed a part, are
+found forming isolated terraces and plateaux either on the granitic
+platform, or resting on the fresh-water strata of the valley of the
+Allier, while in a northern direction they overspread a large portion of
+the granitic plateau from which rise the Puy de Dome and associated
+volcanic mountains. Still more remarkable are the cases in which these
+lava-streams have descended into the old river channels which drained
+the granitic plateau. Thus the current which took its origin in the Puy
+Gros descended into the valley of the Dordogne, while another stream
+invaded the gorge of Champeix on the eastern side.
+
+The more ancient lava-streams just described are invaded by currents and
+surmounted by cones of eruption of more recent date, similar to those of
+the Puy de Dome group lying to the northward. Such cones and currents,
+amongst which are the Puy de Tartaret and that of Montenard, present
+exactly the same characters as those of this group, to which we shall
+return further on.
+
+(_f._) _Volcanoes of the Haute Loire and Ardeche._--Separated by the
+valley of the Allier and the granitic ridge of La Margeride from the
+volcanic regions of Cantal and Mont Dore is another volcanic region of
+great extent, which reaches its highest elevation in the central points
+of Mont Mezen, attaining an elevation (according to Cordier) of 5820
+feet, and formed of "clinkstone." The volcanic products of Mezen have
+been erupted from one central orifice of vast size, and consist mainly
+of extensive sheets of "clinkstone," a variety of trachytic lava, which
+have taken courses mainly towards the north-west and south-east. These
+great sheets, one of which appears to have covered a space more than 26
+miles in length with an average breadth of 6 miles, thus overspreading
+an estimated area of 156 square miles, has been deeply eroded by streams
+draining into the Loire, along whose banks the rocks tower in lofty
+cliffs; while it has also suffered enormous denudation, by which
+outlying fragments are disconnected from the main mass, and form
+flat-topped hills and plateaux as far distant as Roche en Reigner and
+Beauzac, at the extreme distance (as stated above) of 26 miles from the
+source of eruption.
+
+But even more remarkable than the above are the vast basaltic sheets
+which stretch away for a distance of 30 miles by Privas almost to the
+banks of the Rhone, opposite Montlimart. These have their origin amongst
+the clinkstone heights of Mont Mezen, and taking their course along the
+granitic plateau in a south-easterly direction, ultimately pass over on
+to the Jurassic and Cretaceous formations composing the plateau of the
+Coiron, which break off in vertical cliffs from 300 to 400 feet in
+height, surmounting the slopes that rise from the banks of the Ardeche
+and Escourtais rivers near Villeneuve de Bere. This is probably one of
+the most extensive sheets of basalt with which we are acquainted in the
+European area, and it is only a remnant of a vastly greater original
+sheet.[9]
+
+[Illustration: Fig. 19.--Mont Demise, near Le Puy, seen from the S.E.
+(After Scrope.)--1. Building standing on old breccia, rocks of the Col;
+2. Road to Brioude; 3. Croix de la Paille; 4. Orgue d'Expailly (basalt);
+5. Spot where human bones were found.]
+
+(_g._) _Newer Volcanoes of the Haute Loire (the Velay and
+Vivarais)._--Subsequently to the formation of the lava-streams above
+described, and probably after the lapse of a lengthened period, the
+region of the Haute Loire and Ardeche became the scene of a fresh
+outburst of volcanic action, during which the surface of the older
+lavas, or of the fundamental granite, was covered by numerous
+crater-cones and lava-streams strewn along the banks of the Allier and
+of the Loire for many miles. These cones and craters are not quite so
+fresh as those of the Mont Dome group; those of the Haute Loire being
+slightly earlier in point of time, and, as Daubeny shows, belonging to a
+different system. So numerous are these more recent cones and craters
+that Scrope counted more than 150 of them, and probably omitted many.
+
+The volcanic phenomena now described have a special interest as bearing
+on the question whether man was an inhabitant of this region at the time
+of these later eruptions. The question seems to be answered in the
+affirmative by the discovery of a human skull and several bones in the
+volcanic breccia of Mont Demise, in company with remains of the elephant
+(_E. primigenius_), rhinoceros (_R. tichorhinus_), stag, and other large
+mammifers. The discovery of these remains was made in the year 1844, and
+the circumstances were fully investigated and reported upon by M.
+Aymard, and afterwards by Mr. Poulett Scrope, upon whose mind no
+possible doubt of the fact remained. From what we now know of the
+occurrence of human remains and works of art in other parts of France
+and Europe, no surprise need be felt at the occurrence of human remains
+in company with some extinct mammalia in these volcanic tuffs, which
+belong to the Post-Pliocene or superficial alluvia antecedent to the
+historic period.[10]
+
+(_h._) _Mont Dome Chain._--We now come to the consideration of the most
+recent of all the volcanic mountain groups of the region of Central
+France, that of the Puy de Dome, lying to the north of Mont Dore and
+Cantal. We have seen that there is almost conclusive evidence that man
+was a witness to the later volcanic outbursts of the Vivarais, and as
+these craters seem to be of somewhat earlier date than those of the Puy
+de Dome group, we cannot doubt that they were in active eruption when
+human beings inhabited the country, and not improbably within what is
+known as the _Historic Period_. No mention, however, is made either by
+Caesar, Pliny, or other Roman writers of the existence of active
+volcanoes in this region. Caesar, who was a close observer, and who
+carried the Roman arms into Auvergne, makes no mention of such; nor yet
+does the elder Pliny, who enumerated the known burning mountains of his
+day all over the Roman Empire. It is not till we come down to the fifth
+century of our era that we find any notices which might lead us to infer
+the existence of volcanic action in Central France. This is the
+well-known letter written by Sidonius Apollonarius, bishop of Auvergne,
+to Alcinus Avitus, bishop of Vienne, in which the former refers to
+certain terrific terrestrial manifestations which had occurred in the
+diocese of the latter. But, as Dr. Daubeny observes, this is no evidence
+of volcanic action in Auvergne, where Sidonius himself resided; the
+terrestrial disturbances above referred to may have been earthquake
+shocks of extreme severity.[11]
+
+But although we have no reliably historical record of volcanic action
+amongst the mountains of the Mont Dome group, the fact that these are,
+comparatively, extremely recent will be evident to an observer visiting
+this district, and this conclusion is based on three principal grounds:
+first, because of the well-preserved forms of the original craters,
+though generally composed of very loose material, such as ashes,
+lapilli, and slag; secondly, because of the freshness of the
+lava-streams over whose rugged surfaces even a scanty herbage has in
+some places scarcely found a footing;[12] and thirdly, because the lava
+from the crater-cones has invaded channels previously occupied by the
+earlier lavas, or those which had been eroded since the overflow of the
+great basaltic sheets of Mont Dore. Still, as in the case of the valleys
+of Lake Aidot, of Channonat, and of Royat, these streams are
+sufficiently ancient to have given time for the existing rivers to have
+worn out in them channels of some depth, but bearing no comparison to
+the great valleys which had been eroded out of the more ancient lavas,
+such as those of the Coiron, of the Ardeche, and of the Dordogne and
+Chambon in the district of Mont Dore.
+
+(_i._) _Dome-shaped Volcanic Hills._--I have previously (page 15)
+referred to the two classes of volcanic eminences to be found in the
+chain of the Puy de Dome; one indicated by the name itself, formed of a
+variety of trachytic lava called "domite," and of the form of a dome;
+the other, composed of fragmental matter piled up in the form of a
+crater or cup, often ruptured on one side by a stream of lava which has
+burst through the side, owing to its superior density. Of the former
+class the Puy de Dome and the Grand Sarcoui (see Fig. 18) are the most
+striking examples out of the five enumerated by Scrope, while there is a
+large number, altogether sixty-one, belonging to the latter class. These
+domes and crater-cones, as already stated, rise from a platform of
+granite, either directly or from one formed of the lava-sheets of the
+Mont Dore region, which in turn overlies the granitic platform. Of the
+nearly perfect craters there are the Petit Puy de Dome, lying partially
+against the northern flank of the greater eminence; the Puy de Cone,
+remarkable for the symmetry of its conical form, rising to a height of
+900 feet from the plain; and the Puys de Chaumont and Thiolet lying to
+the north of the Puy de Dome. Of those to the south of this mount, two
+out of the three craters of the Puy de Barme and the Puy de Vichatel are
+perfect; but most of the crater-cones south of the Puy de Dome are
+breached. Some of the lava streams by which these craters were broken
+down flowed for long distances. That the lava followed the showers of
+ashes and lapilli forming the walls of the craters is rendered very
+evident in the case of the Puy de la Vache, whose lava-stream coalescing
+with those from the Puy de la Solas and Puy Noir, deluged the
+surrounding tracts and flowed down the Channonat Valley as far as La
+Roche Blanc in the Vale of Clermont. In the interior of the upper part
+of the crater still remaining may be seen the level (so to speak) to
+which the molten lava rose before it burst its barrier. This level is
+marked by a projecting platform of reddish or yellow material, rich in
+specular iron, apparently part of the frothy scum which formed on the
+surface of the lava and adhered to the side of the basin at the moment
+of its being emptied.
+
+Space does not permit a fuller description of this remarkable assemblage
+of extinct volcanoes, and the reader must be referred for further
+details to the work of Mr. Scrope. I shall content myself with some
+further reference to the central figure in this grand chain, the Puy de
+Dome itself.
+
+_Ascent of the Puy de Dome._--On ascending by the winding path up the
+steep side of the mount, and on reaching the somewhat flattened summit,
+the first objects which strike the eye are the massive foundations of
+the Roman temple of Mercury; they are hewn out of solid grey lava,
+altogether different from the rock of the Puy de Dome itself, which must
+have been obtained from one of the lava-sheets of the Mont Dore group.
+To have carried these large blocks to their present resting-place must
+have cost no little labour and effort. The temple is supposed to have
+been surmounted by a colossal statue of the winged deity, visible from
+all parts of the surrounding country which was dedicated to his honour,
+and the foundations were only discovered a few years ago when excavating
+for the foundation of the observatory, which stands a little further on
+under the charge of Professor Janssen. On proceeding to the northern
+crest of the platform a wonderful view of the extinct craters and
+domes--about forty in number, and terminating in the Puy de Beauny, the
+most northerly member of the chain--is presented to the spectator. To
+the right is the Vale of Clermont and the rich valley of the Allier
+merging into the great plain of Central France. On the south side of the
+platform a no less remarkable spectacle meets the eye. The chain of Puys
+and broken craters stretches away southwards for a distance of nearly
+ten miles, while the horizon is bounded in that direction by the lofty
+masses of the Mont Dore, Cantal, and Le Puy ranges. Nor does it require
+much effort of the imagination to restore the character of the region
+when these now dormant volcanoes were in full activity, projecting
+showers of ashes and stones high into the air amidst flames of fire and
+vast clouds of incandescent gas and steam.
+
+The material of which the Puy de Dome is formed consists of a light
+grey, nearly white, soft felsitic lava, containing crystals of mica,
+hornblende, and specular iron-ore. It is highly vesicular, and was
+probably extruded in a pasty condition from a throat piercing the
+granitic plateau and the overlying sheet of ancient lava of Mont Dore.
+It has been suggested that such highly felsitic and acid lavas as that
+of which the Puy de Dome, the Grand Sarcoui, and Cliersou are composed,
+may have had their origin in the granite itself, melted and rendered
+viscous by intense heat. Dr. E. Gordon Hull has suggested that the
+domite hills (owing to their low specific gravity) may have filled up
+pre-existing craters of ashes and scoriae without rupturing them, as in
+the case of the heavier basaltic lavas, and then still continuing to be
+extruded, may have entirely enveloped them in its mass; so that each
+domite hill encloses within its interior a crater formed of ashes,
+stones, and scoriae. In the case of the Puy de Dome there is some
+evidence that the domite matter rests on a basis of ashes and scoriae,
+which may be seen in a few places around the base of the cone. It is
+difficult without some such theory as this to explain how a viscous mass
+was able to raise mountains some 2000 or 3000 feet above the surrounding
+plain.[13]
+
+(_j._) _Sketch of the Volcanic History of Central France._--It now only
+remains to give a brief _resume_ of the volcanic history of this region
+as it may be gathered from the relations of the rocks and strata to the
+volcanic products, and of these latter to each other.
+
+_1st Stage._--It would appear that at the close of the Eocene period
+great terrestrial changes occurred. The bed of the sea was converted
+into dry land, the strata were flexured and denuded, and a depression
+was formed in the granitic floor of Central France, which, in the
+succeeding Miocene period, was converted into an extensive lake peopled
+by molluscs, fishes, reptiles, and pachyderms of the period.
+
+_2nd Stage._--Towards the close of the Miocene epoch volcanic eruptions
+commenced on a grand scale over the granitic platform in the districts
+now called Mont Dore, Cantal, and the Vivarais. Vast sheets of trachytic
+and basaltic lavas successively invaded the tracts surrounding the
+central orifices of eruption, now constituting the more ancient of the
+lava-sheets of the Auvergne region, and, invading the waters of the
+neighbouring lake, overspread the lacustrine deposits which were being
+accumulated therein. These volcanic eruptions probably continued
+throughout the Pliocene period, interrupted by occasional intervals of
+inactivity, and ultimately altogether ceased.
+
+_3rd Stage._--Towards the close of the Pliocene period terrestrial
+movements took place, owing to which the waters of the lake began to
+fall away, and the sheets of lava were subjected to great denudation.
+This process, probably accelerated by excessive rainfall during the
+succeeding Post-Pliocene and Pluvial periods, was continued until plains
+and extensive river-valleys were eroded out of the sheets of lava and
+their supporting granitic rocks and the adjoining lacustrine strata.
+
+_4th Stage._--A new outburst of volcanic forces marks this stage, during
+which the chain of the Puy de Dome was thrown up on the west, and that
+of the newer cones of the Vivarais on the south-east of the lacustrine
+tract. The waters of the lake were now completely drained away through
+the channel of the Allier, and denudation, extending down to the present
+day, began over the area now forming the Vale of Clermont and adjoining
+districts. The volcanic action ultimately spent its force; and somewhere
+about the time of the appearance of man, the mammoth, rhinoceros, stag,
+and reindeer on the scene, eruptions entirely ceased, and gradually the
+region assumed those conditions of repose by which it is now physically
+characterised.
+
+[1] The literature referring to this region is very extensive. Guettard
+in 1775, afterwards Faujas, published descriptions of the rocks of the
+Vivarais and Velay; and Desmarest's geological map, published in 1779,
+is a work of great merit. The district was afterwards described by
+Daubeny, Lyell, Von Buch, and others; but by far the most complete work
+is that of Scrope, entitled _Volcanoes of Central France_, containing
+maps and numerous illustrations, published in 1826, and republished in a
+more extended form in 1858; to this I am largely indebted.
+
+[2] A monument to Pascal, erected by the citizens, occupies the centre
+of the square in Clermont. It will be remembered that Pascal verified
+the conclusions arrived at by Torricelli regarding the pressure of the
+atmosphere, by carrying a Torricellian tube to the summit of the Puy de
+Dome, and recording how the mercury continually fell during the ascent,
+and rose as he descended. This experiment was made in 1645.
+
+[3] In this visit to Auvergne in the summer of 1880, the author was
+accompanied by his son, Dr. E. Gordon Hull, and Sir Robert S. Ball. On
+reaching the station at the summit of the ridge it seemed as if the
+volcanic fires had again been lighted, for the whole sky was aglow with
+the rays of the western sun.
+
+[4] On the other hand, certain beds of ash and other volcanic _ejecta_
+occur in _the uppermost_ strata of lake deposits of Limagne, so that
+these may indicate the commencement of the period of eruption, as
+suggested further on.
+
+[5] Only very closely; for Mr. Scrope considers that the crater-cones of
+the chain of the Haute Loire give evidence of a somewhat earlier epoch
+of activity than those of the Puy de Dome, as they have undergone a
+greater amount of subaerial erosion.
+
+[6] The extent of this river erosion has been clearly brought out by
+Scrope, and is admirably illustrated by several of his panoramic views,
+such as that in Plate IX. of his work.
+
+[7] Scrope, _loc. cit._, p. 147.
+
+[8] Scrope, _loc. cit._, p. 144.
+
+[9] Scrope gives a view of these remarkable basaltic cliffs in Plate
+XII. of his work, from which the above account is taken. At one spot
+near the village of Le Gua there is a break in the continuity of the
+sheet.
+
+[10] See Scrope, _loc. cit._, p. 181; also Appendix, p. 228. While there
+is no _prima facie_ reason for questioning the origin of the Demise
+skull, yet from what Lyell states in his _Antiquity of Man_, p. 196, it
+will be seen that he found it impossible to identify its position, or to
+determine beyond question that its interment was due to natural causes.
+But assuming this to be the case, he shows how the individual to whom it
+belonged might have been enveloped in volcanic tuff or mud showered down
+during the final eruption of the volcano of Demise. MM. Hebert and
+Lartet, on visiting the locality, also failed to find _in situ_ any
+exact counterpart of the stone now in the museum of Le Puy.
+
+[11] See Daubeny, _Volcanoes_, p. 31.
+
+[12] That is to say, the surfaces of the lava-streams are not at all, or
+only slightly, decomposed into soil suitable for the growth of plants,
+except in rare instances.
+
+[13] E. G. Hull, "On the Domite Mountains of Central France," _Scien.
+Proc. Roy. Dublin Society_, July 1881, p. 145. Dr. Hull determined the
+density of the domite of the Puy de Dome to be 2.5, while that of lava
+is about 3.0.
+
+
+
+
+CHAPTER VII.
+
+THE VOLCANIC DISTRICT OF THE RHINE VALLEY.
+
+
+The region bordering the Rhine along both its banks above Bonn, and
+extending thence along the valley of the Moselle and into the Eifel, has
+been the theatre of active volcanic phenomena down into recent times,
+but at the present day the volcanoes are dormant or extinct.
+
+(_a._) _Geological Structure._--The fundamental rocks of this region
+belong to the Silurian, Devonian, and Carboniferous systems, consisting
+of schists, grits, and limestones, with occasional horizontal beds of
+Miocene sandstone and shale with lignite, resting on the upturned edges
+of the older rocks. Scattered over the greater part of the district here
+referred to are a number of conical eminences, often with craters, the
+bottoms of which are usually sunk much below the present level of the
+country, and thus receiving the surface drainage, have been converted
+into little lakes called "maars," differing from ordinary lakes by their
+circular form and the absence of any _apparent_ outlet for their
+waters.[1]
+
+But before entering into details, it may be desirable to present the
+reader with a short outline of the physical history of the region (which
+has been ably done by Dr. Hibbert in his treatise, to which I have
+already referred), so as to enable him better to understand the
+succession of physical events in its volcanic history.
+
+[Illustration: Fig. 20.--Sketch Map to show the physical condition of
+the Rhenish area in the Miocene epoch.--(After Hibbert.)]
+
+(_b._) _Physical History._--From the wide distribution of stratified
+deposits of sand and clay at high levels on both banks of the Rhine
+north of the Moselle, it would appear that an extensive fresh-water
+basin, which Dr. Hibbert calls "The Basin of Neuwied," occupied a
+considerable tract on both banks, in the centre of which the present
+city of Neuwied stands. This basin was bounded towards the south by the
+slopes of the Huendsruck and Taunus, which at the time here referred to
+formed a continuous chain of mountains. (Fig. 20.) To the south of this
+chain lay the Tertiary basin of Mayence, which was connected at an early
+period--that of the Miocene--with the waters of the ocean, as shown by
+the fact that the lower strata contain marine shells; these afterwards
+gave place to fresh-water conditions. The basin of Neuwied was bounded
+towards the north by a ridge of Devonian strata which extended across
+the present gorge of the Rhine between Andernach and Linz, and to the
+north of this barrier lay another more extensive fresh-water basin, that
+of Cologne. From this it will be seen that the Rhine, as we now find it,
+had then only an infantile existence; in fact, its waters to the south
+of the Huendsruck ridge drained away towards the south. But towards the
+commencement of the Pliocene period the barriers of the Huendsruck and
+Taunus, as also that of the Linz, were broken through, and the course of
+the waters was changed; and thus gradually, as the river deepened its
+bed, the waters were drained off from the great lakes.[2] This rupture
+of the barriers may have been due, in the first instance, to the
+terrestrial disturbances accompanying the volcanic eruptions of the
+Eifel and Siebengebirge, though the erosion of the gorges at Bingen and
+at Linz to their present depth and dimensions is of course due to
+prolonged river action. It was about the epoch we have now arrived
+at--viz., the close of the Miocene--that volcanic action burst forth in
+the region of the Lower Rhine. It is probable that this action commenced
+in the district of the Siebengebirge, and afterwards extended into that
+of the Moselle and the Eifel, the volcanoes of which bear evidence of
+recent date. Layers of trachytic tuff are interstratified with the
+deposits of sand, clay, and lignite of the formation known as that of
+the Brown Coal--of Miocene age--which underlies nearly the whole of the
+volcanic district on both sides of the Rhine near Bonn,[3] thus showing
+that volcanic action had already commenced in that part to some extent;
+but it does not appear from Dr. Hibbert's statement that any such
+fragments of eruptive rock are to be found in the strata which were
+deposited over the floor of the Neuwied basin.[4] It will be recollected
+that the epoch assigned for the earliest volcanic eruptions of Auvergne
+was that here inferred for those of the Lower Rhine--viz., the close of
+the Miocene stage--and from evidence subsequently to be adduced from
+other European districts, it will be found that there was a very widely
+spread outburst of volcanic action at this epoch.
+
+(_c._) _The Range of the Siebengebirge._--This range of hills--formed of
+the older volcanic rocks of the Lower Rhine--rises along the right bank
+of this noble river opposite Bonn, where it leaves the narrow gorge
+which it traverses all the way from Bingen, and opens out on the broad
+plain of Northern Germany. The range consists of a succession of conical
+hills sometimes flat-topped--as in the case of Petersberg; and at the
+Drachenfels, near the centre of the range it presents to the river a
+bold front of precipitous cliffs of trachyte porphyry. The sketch (Fig.
+21) here presented was taken by the author in 1857 from the old extinct
+volcano of Roderberg, and will convey, perhaps, a better idea of the
+character of this picturesque range than a description. The
+Siebengebirge, although appearing as an isolated group of hills, is in
+reality an offshoot from the range of the Westerwald, which is connected
+with another volcanic district of Central Germany known as the
+Vogelsgebirge. The highest point in the range is attained in the
+Lohrberg, which rises 1355 feet above the sea; the next, the Great
+Traenkeberg, 1330 feet; and the next, Great Oelberg, 1296 feet.
+
+[Illustration: Fig. 21.--The Volcanic Range of the Siebengebirge, seen
+from the left bank of the Rhine, above Bonn.--(Original.)]
+
+The range consists mainly of trachytic rocks--namely,
+trachyte-conglomerate, and solid trachyte, of which H. von Dechen makes
+two varieties--that of the Drachenfels, and that of the Wolkenburg. But
+associated with these highly-silicated varieties of lava--and generally,
+if not always, of later date--are basaltic rocks which cap the hills of
+Petersberg, Nonnenstrom, Gr. and Ll. Oelberg, Gr. Weilberg, and Ober
+Dollendorfer Hardt. The question whether there is a transition from the
+one variety of volcanic rock into the other, or whether each belongs to
+a distinct and separate epoch of eruption, does not seem to be very
+clearly determined. Mr. Leonard Horner states that it would be easy to
+form a suite of specimens showing a gradation from a white trachyte to a
+black basalt;[5] but we must recollect that when Mr. Horner wrote, the
+microscopic examination of rocks by means of thin sections was not known
+or practised, and an examination by this process might have proved that
+this apparent transition is unreal. According to H. von Dechen, there
+are sheets of basalt older than the greater mass of the brown coal
+formation, and others newer than the trachyte;[6] while dykes of basalt
+traversing the trachytic lavas are not uncommon.[7]
+
+The trachyte-conglomerate--which seems to be associated with the upper
+beds of the brown coal strata--is traversed by dykes of trachyte of
+later date; and though it is difficult to trace the line between the two
+varieties of this rock on the ground, Dr. von Rath has recognised the
+general distinction between them, which consists in the greater
+abundance of hornblende and mica in the trachyte of the Wolkenburg than
+in that of the Drachenfels.
+
+The trachyte of the Drachenfels was probably the neck of a volcano which
+burst through the fundamental schists of the Devonian period. It is
+remarkable for the large crystals of sanidine (glassy felspar) which it
+contains, and has a rude columnar structure.
+
+The absence of any clearly-defined craters of eruption, such as are to
+be found in the Eifel district and on the left bank of the Rhine--as,
+for example, in the case of the Roderberg--may be regarded as sufficient
+evidence that this range is of comparatively high antiquity. It seems to
+bear the same relation to the more modern craters of the Eifel and
+Moselle that the Mont Dore and Cantal volcanoes do to those of the Puy
+de Dome. In both cases, denudation carried on throughout perhaps the
+Pliocene and Post-Pliocene periods down to the present day has had the
+effect of demolishing the original craters; so that what we now observe
+as forming these ranges are the consolidated columns of original molten
+matter which filled the throats of the old volcanoes, or the sheets of
+lava which were extruded from them, but are now probably much reduced in
+size and extent.
+
+Having thus given a description of the older volcanic range on the right
+bank of the Rhine, we shall cross the river in search of some details
+regarding the more recent group of Rhenish volcanoes, commencing with
+that of the Roderberg, a remarkable hill a few miles south of Bonn, from
+which the view of the Seven Mountains was taken.
+
+[Illustration: Fig. 22.--Section of the extinct crater of the Roderberg
+on the bank of the Rhine, above Bonn.--(Original.)]
+
+(_d._) _The Roderberg._--This crater, which was visited by the author in
+1857, is about one-fourth of a mile in diameter, and is in the form of a
+cup with gentle slopes on all sides. In its centre is a farmhouse
+surrounded by corn-fields. The general section through the hill is
+represented above (Fig. 22).
+
+The flanks on the north side are composed of loose quartzose gravel
+(gerolle), a remnant of the deposits formed around the margin of the
+"Basin of Neuwied" described above (p. 114). This gravel is found
+covering the terraces of the brown coal formation several hundred feet
+above the Rhine. Besides quartz-pebbles, the deposit contains others of
+slate, grit, and volcanic rock. On reaching the edge of the crater we
+find the gravel covered over by black and purple scoria or slag the
+superposition of the scoria on the gravel being visible in several
+places, showing that the former is of more recent origin. On the
+opposite side of the crater, overlooking the Rhine, we find the cliff of
+Rolandsec composed of hard vesicular lava, rudely prismatic, and
+extending from the summit of the hill to its base, about 250 feet below.
+This is the most northerly of the group of the Eifel volcanoes.
+
+(_e._) _District of the Rivers Bruehl and Nette._--The volcanic region of
+the Lower Eifel, drained by these two principal streams which flow into
+the Rhine, will amply repay exploration by the student of volcanic
+phenomena, owing to the variety of forms and conditions under which
+these present themselves within a small space. The fundamental rock is
+slate or grit of Devonian age, furrowed by numerous valleys, often
+richly wooded, and diversified by conical hills of trachyte; or by
+crater-cones, formed of basalt or ashes, sometimes ruptured on one side,
+and occasionally sending forth streams of lava, as in the cases of the
+Perlinkopf, the Bausenberg, and the Engelerkopf. The district attains
+its greatest altitude in the High Acht (Der Hohe Acht), an isolated cone
+of slate capped by basalt with olivine, and reaching a level of 2434
+Rhenish feet.[8]
+
+(_f._) _The Laacher See._--It would be impossible in a work of this kind
+to attempt a detailed description of the Eifel volcanoes, often of a
+very complex character and obscure physical history, as in the case of
+the basin of Rieden, where tufaceous deposits, trachytic and basaltic
+lavas and crater-cones, are confusedly intermingled, so that I shall
+confine my remarks to the deservedly famous district of the Laacher
+See, which I had an opportunity of personally visiting some years
+since.[9]
+
+[Illustration: Fig. 23.--Plan and Section of the Laacher See, a lake on
+the borders of the Eifel, occupying the crater of an old volcano.--G.
+Gravel and volcanic sand forming banks of the lake and rim of old
+crater; L. Sheet of trachytic lava with columnar structure; B. Basaltic
+dyke; S. Devonian slate, etc.]
+
+The Laacher See is a lake of an oval form, over an English mile in the
+shorter diameter, and surrounded by high banks of volcanic sand, gravel,
+and scoriae, except on the east side, where cliffs of clay-slate, in a
+nearly vertical position, and striking nearly E.W., may be observed. Its
+depth from the surface of the water is 214 feet.[10] The ashes of the
+encircling banks contain blocks of slate and lava which have been torn
+from the sides of the orifice or neck of the volcano and blown into the
+air; and there can be no doubt that the ashes and volcanic gravel is the
+result of very recent eruptions.
+
+At the east side of the lake we find a stream of scoriaceous lava of a
+purple or reddish colour, highly vesicular, and containing crystals of
+mica; but the most important lava-stream is that which has taken a
+southerly direction from the crater of the Laacher See towards Nieder
+Mendig and Mayen, for a distance of about six miles. This great stream
+is covered throughout half its distance by beds of volcanic ash and
+lapilli, but emerges into the air at a distance of about two miles from
+the edge of the crater (see Fig. 23), and was formerly extensively
+quarried in underground caverns for millstones. Here the rock is a
+vesicular trachyte, of a greyish colour, solidified in vertical columns
+of hexagonal form, about four feet in diameter, and traversed by
+transverse joint planes. These quarries have been worked from the time
+of the Roman occupation of the country; and, before the introduction of
+iron or steel rollers for grinding corn, millstones were exported to all
+parts of Europe and the British Isles from this quarry.[11]
+
+The district around the Laacher See is covered by laminated _ejecta_ of
+the old volcano, probably of subaerial origin, through which bosses of
+the fundamental slate peer up at intervals, while the surface is
+diversified by several truncated cones.
+
+(_g._) _Trass of the Bruehl Valley._--The Bruehl Valley, which unites
+with that of the Rhine at the town of that name, and drains the northern
+side of the volcanic region, has always been regarded with much interest
+by travellers for the presence of a deposit of "trass" with which it is
+partially filled. The origin of this valley was pre-volcanic, as it is
+hewn out of the slaty rocks of the district. But at a later period it
+became filled with volcanic mud (tuffstein), out of which the stream has
+made for itself a fresh channel. The source of this mud is considered by
+Hibbert[12] to have been the old volcano of the Lummerfeld, which, after
+becoming dormant, was filled with water, and thus became a lake. At a
+subsequent period, however, a fresh eruption took place near the edge of
+the lake, resulting in the remarkable ruptured crater known as the
+Kunkskoepfe, which rises about four miles to the north of the Laacher
+See. The eruptions of this volcano appear to have displaced the mud of
+the Lummerfeld, causing it to flow down into the deep gorge of the
+Bruehl, which it completely filled, as stated above.
+
+On walking down the valley one may sometimes see the junction of the
+tuff with the slate-rock which enfolds it. The tuff consists of white
+felspathic mud, with fragments of slate and lava, reaching a depth in
+some places of 150 feet. After it has been quarried it is ground in
+mills, and used for cement stone under the name of _trass_. It is said
+to resemble the volcanic mud by which Herculaneum was overwhelmed during
+the first eruption of Vesuvius, and which was produced by the torrents
+of rain mixing with the ashes as they were blown out of the volcano.
+
+Sufficient has probably now been written regarding the dormant, or
+recently extinct, volcanic districts of Europe to give the reader a
+clear idea regarding their nature and physical structure. Other
+districts might be added, such as those of Central Germany, Hungary,
+Transylvania, and Styria; but to do so would be to exceed the proposed
+limits of this work; and we may therefore pass on to the consideration
+of the volcanic region of Syria and Palestine, which adjoins the
+Mediterranean district we have considered in a former page.
+
+[1] Daubeny, _loc. cit._, p. 71. The geology of this region has had many
+investigators, of whom the chief are Steininger, _Erloschenen Vulkane in
+der Eifel_ (1820); Hibbert, _Extinct Volcanoes of the Basin of Neuwied_,
+1832; Noeggerath, _Das Gebirge im Rheinland_, etc., 4 vols.; Horner, "On
+the Geology of Bonn," _Transactions of the Geological Society, London_,
+vol. iv.
+
+[2] The views of Dr. Hibbert are not inconsistent with those of the late
+Sir A. Ramsay, on "The Physical History of the Valley of the Rhine,"
+_Quart. Jour. Geol. Soc._, vol. xxx. (1874).
+
+[3] Von Dechen, _Geog. Beschreib. des Siebengebirges am Rhein_ (Bonn,
+1852).
+
+[4] Hibbert, _loc. cit._, p. 18.
+
+[5] Horner, "Geology of Environs of Bonn," _Transactions of the
+Geological Society_, vol. iv., new series.
+
+[6] H. von Dechen, _Geog. Fuehrer in das Siebengebirge am Rhein_ (Bonn,
+1861).
+
+[7] _Ibid._, p. 191.
+
+[8] Dr. Hibbert's work is illustrated by very carefully drawn and
+accurate views of some of the old cones and craters of this district,
+accompanied by detailed descriptions.
+
+[9] The lava of Schorenberg, near Rieden, is interesting from the fact,
+stated by Zirkel, that it contains leucite, nosean, and nephelin.--_Die
+Mikros. Beschaf. d. Miner. u. Gesteine_, p. 154 (1873).
+
+[10] Hibbert, _loc. cit._, p. 23.
+
+[11] At the time of the author's visit the underground caverns, which
+are deliciously cool in summer, were used for the storage of the
+celebrated beer brewed by the Moravians of Neuwied.
+
+[12] Hibbert, _loc. cit._, p. 129.
+
+
+
+
+PART III.
+
+DORMANT OR MORIBUND VOLCANOES OF OTHER PARTS OF THE WORLD.
+
+
+
+
+CHAPTER I.
+
+DORMANT VOLCANOES OF PALESTINE AND ARABIA.
+
+
+(_a._) _Region east of the Jordan and Dead Sea._--The remarkable line of
+country lying along the valley of the Jordan, and extending into the
+great Arabian Desert, has been the seat of extensive volcanic action in
+prehistoric times. The specially volcanic region seems to be bounded by
+the depression of the Jordan, the Dead Sea, and the Arabah as far south
+as the Gulf of Akabah; for, although Safed, lying at the head of the Sea
+of Galilee on the west of the Jordan valley, is built on a basaltic
+sheet, and is in proximity to an extinct crater, its position is
+exceptional to the general arrangement of the volcanic products which
+may be traced at intervals from the base of Hermon into Central Arabia,
+a distance of about 1000 miles.[1]
+
+The tract referred to has been described at intervals by several
+authors, of whom G. Schumacher,[2] L. Lartet,[3] Canon Tristram,[4] M.
+Niebuhr,[5] and C. M. Doughty[6] may be specially mentioned in this
+connection.
+
+The most extensive manifestations of volcanic energy throughout this
+long tract of country appear to be concentrated at its extreme limits.
+At the northern extremity the generally wild and rugged tract of the
+Jaulan and Hauran, called in the Bible _Trachonitis_, and still farther
+to the eastward the plateau of the Lejah, with its row of volcanic peaks
+sloping down to the vast level of Bashan, is covered throughout nearly
+its whole extent by great sheets of basaltic lava, above which rise at
+intervals, and in very perfect form, the old crater-cones of eruption. A
+similar group of extinct craters with lava-flows has been described and
+figured by a recent traveller, Mr. C. M. Doughty, in parts of Central
+Arabia. The general resemblance of these Arabian volcanoes to those of
+the Jaulan is unquestionable; and as they are connected with each other
+by sheets of basaltic lava at intervals throughout the land of Moab, it
+is tolerably certain that the volcanoes lying at either end of the chain
+belong to one system, and were contemporaneously in a state of activity.
+
+(_b._) _Geological Conditions._--Before entering any further into
+particulars regarding the volcanic phenomena of this region, it may be
+desirable to give a short account of its geological structure, and the
+physical conditions amongst which the igneous eruptions were developed.
+
+Down to the close of the Eocene period the whole region now under
+consideration was occupied by the waters of the ocean. The mountains of
+Sinai were islands in this ocean, which had a very wide range over parts
+of Asia, Africa, and Europe. But at the commencement of the succeeding
+Miocene stage the crust was subjected to lateral contraction, owing to
+which the ocean bed was upraised. The strata were flexured, folded, and
+often faulted and fissured along lines ranging north and south, the
+great fault of the Jordan-Arabah valley being the most important. At
+this period the mountains of the Lebanon, the table-lands of Judaea and
+of Arabia, formed of limestone, previously constituting the bed of the
+ocean during the Eocene and Cretaceous periods, were converted into land
+surfaces. Along with this upheaval of the sea-bed there was extensive
+denudation and erosion of the strata, so that valleys were eroded over
+the subaerial tracts, and the Jordan-Arabah valley received its primary
+form and outline.
+
+Up to this time there does not appear to have been any outbreak of
+volcanic forces; but with the succeeding Pliocene period these came into
+play, and eruptions of basaltic lava took place along rents and fissures
+in the strata, while craters and cones of slag, scoriae, and ashes were
+thrown up over the region lying to the east of the Sea of Galilee and
+the sources of the Jordan on the one hand, and the central parts of the
+great Arabian Desert on the other. These eruptions, probably
+intermittent, continued into the succeeding Glacial or Pluvial period,
+and only died out about the time that the earliest inhabitants appeared
+on the scene.
+
+(_c._) _The Jaulan and Hauran._--This tract is bounded by the valley of
+the Jordan and the Sea of Galilee on the west, from which it rises by
+steep and rocky declivities into an elevated table-land, drained by the
+Yarmuk (Hieromax), the Nahr er Rukkad, and other streams, which flow
+westwards into the Jordan along deep channels in which the basaltic
+sheets and underlying limestone strata are well laid open to view.
+
+On consideration it seems improbable that the great sheets of augitic
+lava, such as cover the surface of the land of Bashan, are altogether
+the product of the volcanic mountains which appear to be confined to
+special districts in this wide area. Some of the craters do indeed send
+forth visible lava-streams, but they are insignificant as compared with
+the general mass of the plateau-basalts; and the crater-cones themselves
+appear in some cases to be posterior to the platforms of basalt from
+which they rise. It is very probable, therefore, that the lavas of this
+region have, in the main, been extruded from fissures of eruption at an
+early period, and spread over the surface of the country in the same
+manner as those of the Snake River region, and the borders of the
+Pacific Ocean of North America, and possibly of the Antrim Plateau in
+Ireland, afterwards to be described.
+
+The volcanic hills which rise above the plateau are described in detail
+by Schumacher. Of these, Tell Abu Nedir is the largest in the Jaulan. It
+reaches an elevation of 4132 feet above the Mediterranean Sea, and 1710
+feet above the plain from which it rises; the circumference of its base
+is three miles, and the rim of the crater itself, which is oval in form,
+is 1331 yards in its larger diameter. The interior is cultivated by
+Circassians, and is very fruitful; the walls descend at an angle of
+about 30 deg. on the inside, the exterior slope of the mountain being about
+22 deg.. The cone seems to be formed chiefly of scoriae, and the lava-stream,
+which issues forth from the interior, forms a frightfully stony and
+lacerated district.[7]
+
+[Illustration: Fig. 24.--Extinct Craters in the Jaulan, north-east from
+the Sea of Galilee, called Tell Abu en Neda and Tell el Uram, with a
+central cone.--(After Schumacher.)]
+
+Another remarkable volcano is the Tell Abu en Neda (Fig. 24). This is a
+double crater, with a cone (probably of cinders) rising from the
+interior of one of them. The highest point of the rim of one of the
+craters reaches a level of 4042 feet above the sea. A lava-stream issues
+forth from Abu en Neda, and unites with another from a neighbouring
+volcano.
+
+Tell el Ahmar is a ruptured crater of imposing aspect, reaching an
+elevation of 4060 feet, and sending forth a lava-current, which falls in
+regular terraces from the outlet towards the west and north.
+
+The ruptured crater of Tell el Akkasheh, which reaches a height of 3400
+feet, has a less forbidding aspect than the greater number of the
+extinct volcanoes of this region, owing to the fact that its sides are
+covered by oaks, which attain to magnificent proportions along the
+summit. Numerous other volcanic hills occur in this district, but the
+most remarkable is that called Tell el Farras (the Hill of the Horse).
+It is an isolated mountain, visible from afar, and reaches an elevation
+of 3110 feet, or nearly 800 feet above the surrounding plain. The oval
+crater of this volcano opens towards the north, and has a depth of 108
+feet below the edge, with moderately steep sloping sides (17 deg.-32 deg.),
+while the slope of the exterior, at first steep, gradually lessens to
+20 deg.-21 deg.. These slopes are covered with reddish or yellowish slag. The
+above examples will probably suffice to afford the reader a general idea
+of the size and form of the volcanoes in this little known region.
+
+It has been stated above that the great lava-floods have probably been
+poured forth intermittently. The statement receives confirmation from
+the observations of Canon Tristram, made in the valley of the Yarmuk.[8]
+This impetuous torrent rushes down a gorge, sometimes having limestone
+on one side and a wall of basalt on the other. This is due to the fact
+that the river channel had been eroded before the volcanic eruptions had
+commenced; but on the lava-stream reaching the channel, it naturally
+descended towards the valley of the Jordan along its bed, displacing the
+river, or converting it into clouds of steam. Subsequently the river
+again hewed out its channel, sometimes in the lava, sometimes between
+this rock and the chalky limestone. But, in addition to this, it has
+been observed that there is a bed of river gravel interposed between two
+sheets of basalt in the Yarmuk ravine; showing that after the first flow
+of that molten rock the river reoccupied its channel, which was
+afterwards invaded by another molten lava-stream, into which the waters
+have again furrowed the channel which they now occupy. The basaltic
+sheets descend under the waters of the Sea of Galilee on the east side,
+and were probably connected with those of Safed, crossing the Jordan
+valley north of that lake; owing to this the waters of the Lake of Merom
+(Huleh) were pent up, and formerly covered an extensive tract, now
+formed of alluvial deposits.
+
+(_d._) _Land of Moab._--Proceeding southwards into the Land of Moab, the
+volcanic phenomena are here of great interest. Extensive sheets of
+basaltic lava, described as far back as 1807 by Seetzen, and more
+recently by Lartet and Tristram, are found at intervals between the
+Wadies Mojib (Arnon) and Haidan. On either side of the Mojib, cliffs of
+columnar basalt are seen capping the beds of white Cretaceous limestone,
+while a large mass has descended into the W. Haidan between cliffs of
+limestone and marl on either hand.
+
+Around Jebel Attarus--a dome-shaped hill of limestone--a sheet of
+basaltic lava has been poured, and has descended the deep gorge of the
+Zerka Main, which enters the Dead Sea some 2000 feet below. This gorge
+had been eroded before the basaltic eruption, so that the stream of
+molten lava took its course down the bed of this stream to the water's
+edge, and grand sections have been laid bare by subsequent erosion along
+the banks. Pentagonal columns of black basalt form perpendicular walls,
+first on one side, then on the other; while considerable masses of
+scoriae, peperino, and breccia appear at the head of the glen, probably
+marking the orifice of eruption. Other eruptions of basalt occur, one at
+Mountar ez Zara, to the south of Zerka Main, and another at Wady
+Ghuweir, near the north-eastern end of the Dead Sea. There are no
+lava-streams on the western side of the Ghor, or of the Dead Sea.[9]
+
+The outburst of the celebrated thermal springs of Callirrhoe, together
+with nine or ten others, along the channel of the Zerka Main, is a
+circumstance which cannot be dissociated from the occurrence of basaltic
+lava at this spot. In a reach of three miles, according to Tristram,
+there are ten principal springs, of which the fifth in descent is the
+largest; but the seventh and eighth, about half a mile lower down, are
+the most remarkable, giving forth large supplies of sulphurous water.
+The tenth and last is the hottest of all, indicating a temperature of
+143 deg. Fahr. Thus it would appear that the heat increases with the depth
+from the upper surface of the table-land; a result which might be
+expected, supposing the heated volcanic rocks to be themselves the
+source of the high temperature. To a similar cause may be attributed the
+hot-springs of Hammath, near Tiberias, and those of the Yarmuk near its
+confluence with the Jordan. Some of these and other springs break out
+along, or near, the line of the great Jordan-Arabah fault which ranges
+throughout the whole extent of this depression, from the base of Hermon
+to the Gulf of Akabah, generally keeping close to the eastern margin of
+the valley.
+
+(_e._) _The Arabian Desert._--The basaltic lava-floods occupy a very
+large extent of the Arabian Desert, from El Hisma (lat. 27 deg. 35' N.) to
+the neighbourhood of Mecca on the south, a distance of about 440 miles,
+with occasional intervals. The lava-sheets are called "Harras" (or
+"Harrat"), one of which, Harrat Sfeina, terminates about ten miles north
+of Mecca. The lava-sheets rest sometimes on the red sandstone, at other
+times, on the granite and other crystalline rocks of great geological
+antiquity. In addition to the sheets of basalt, numerous crater-cones
+rise from the basaltic platform at a level of 5000 feet above the sea,
+and two volcanic mountains, rising far to the west of the principal
+range, called respectively Harrat Jeheyma and H. Rodwa, almost overlook
+the coast of the Red Sea.[10]
+
+(_f._) _Age of the Volcanic Eruptions._--It is very clear that the first
+eruptions, producing the great basaltic sheets of Moab and Arabia,
+occurred after the principal features of the country had been developed.
+The depression of the Jordan-Arabah valley, the elevation of the eastern
+side of this valley along the great fault line, and the channels of the
+principal tributary streams, such as those of the Yarmuk and Zerka Main,
+all these had been eroded out before they were invaded by the molten
+streams of lava. Now, as these physical features were developed and
+sculptured out during the Miocene period, as I have elsewhere shown to
+be the case,[11] we may with great probability refer the volcanic
+eruptions to the geological epoch following--namely, the Pliocene. How
+far downwards towards the historic period the eruptions continued is not
+so certain. Dr. Daubeny, quoting several passages from the Old Testament
+prophets,[12] says it might be inferred that volcanoes were in activity
+even so late as to admit of their being included within the limits of
+authentic history. The poetic language and imagery used in these
+passages by the prophets certainly lends a probability to this view, but
+nothing more. On the other hand, these regions have suffered through
+many centuries from the secondary effects of seismic action and
+subterranean forces, and earthquake shocks have laid in ruins the great
+temples and palaces of Palmyra, Baalbec, and other cities of antiquity.
+The same uncertainty regarding the time at which volcanic action died
+out, with reference to the appearance of man on the scene, hangs over
+the region of Arabia and Syria, as we have seen to be the case in
+reference to the extinct volcanoes of Auvergne, the Eifel, and the Lower
+Rhine. In all these cases the commencement and close of eruptive action
+appear to have been very much about the same period--namely, the Miocene
+period on the one hand, and that at which man entered upon the scene on
+the other; but in the case of Syria and Western Palestine, the close of
+the volcanic period may have been somewhat more than 2000 B.C.
+
+[1] Lake Phiala, near the Lake of Huleh, is also situated to the west of
+the Jordan valley. Its origin, according to Tristram, is volcanic.
+
+[2] Schumacher, "The Jaulan," _Quarterly Statement of the Palestine
+Exploration Fund_, 1886 and 1888; and _Across the Jordan_, London, 1886.
+
+[3] Lartet, _Voyage d'Exploration de la mer Morte_ (Geologie), Paris,
+1880.
+
+[4] Tristram, _Land of Moab_, London, 1873; and _Land of Israel_, 1866.
+
+[5] Niebuhr, _Beschreibung von Arabien_, 1773.
+
+[6] C. M. Doughty, _Arabia Deserta_, 2 vols., 1888. A generalised
+account of this volcanic region by the author will be found in the
+"Memoir on the Physical Geology of Arabia Petraea, and Palestine,"
+_Palestine Exploration Fund_, 1887.
+
+[7] Schumacher, _loc. cit._, p. 248.
+
+[8] _Land of Israel_, p. 461.
+
+[9] "Geology of Arabia Petraea, and Palestine," _Memoirs of the Palestine
+Exploration Fund_, p. 95.
+
+[10] Doughty, _loc. cit._, vol. i., plate vi., p. 416. An excellent
+geological sketch map accompanies this work.
+
+[11] "Memoir of the Geology of Arabia Petraea, and Palestine," chap. vi.
+p. 67.
+
+[12] Nahum, i. 5, 6; Micah, i. 3, 4; Isaiah, lxiv. 1-3; Jeremiah, l. 25.
+
+
+
+
+CHAPTER II.
+
+THE VOLCANIC REGIONS OF NORTH AMERICA.
+
+
+(_a._) _Contrast between the Eastern and Western Regions._--In no point
+is there a more remarkable contrast between the physical structure of
+Eastern and Western America than in the absence of volcanic phenomena in
+the former and their prodigious development in the latter. The great
+valley of the Mississippi and its tributaries forms the dividing
+territory between the volcanic and non-volcanic areas; so that on
+crossing the high ridges in which the western tributaries of America's
+greatest river have their sources, and to which the name of the "Rocky
+Mountains" more properly belongs, we find ourselves in a region which,
+throughout the later Tertiary times down almost to the present day, has
+been the scene of volcanic operations on the grandest scale; where
+lava-floods have been poured over the country through thousands of
+square miles, and where volcanic cones, vying in magnitude with those of
+Etna, Vesuvius, or Hecla, have established themselves. This region,
+generally known as "The Great Basin," is bounded on the west by the
+"Pacific Range" of mountains, and includes portions of New Mexico,
+Arizona, California, Nevada, Utah, Colorado, Idaho, Oregon, Wyoming,
+Montana, and Washington. To the south it passes into the mountainous
+region of Mexico, also highly volcanic; and thence into the ridge of
+Panama and the Andes. It cannot be questioned but that the volcanic
+nature of the Great Basin is due to the same causes which have
+originated the volcanic outbursts of the Andes; but, from whatever
+cause, the volcanic forces have here entered upon their secondary or
+moribund stage. In the Yellowstone Valley, geysers, hot springs, and
+fumaroles give evidence of this condition. In other districts the
+lava-streams are so fresh and unweathered as to suggest that they had
+been erupted only a few hundred years ago; but no active vent or crater
+is to be found over the whole of this wide region. A few special
+districts only can here be selected by way of illustration of its
+special features in connection with its volcanic history.
+
+(_b._) _The Plateau Country of Utah and Arizona._--This tract, which is
+drained by the Colorado River and its tributaries, is bounded on the
+north by the Wahsatch range, and extends eastwards to the base of the
+Sierra Nevada. Round its margin extensive volcanic tracts are to be
+found, with numerous peaks and truncated cones--the ancient craters of
+eruption--of which Mount San Francisco is the culminating eminence.
+South of the Wahsatch, and occupying the high plateaux of Utah, enormous
+masses of volcanic products have been spread over an area of 9000 square
+miles, attaining a thickness of between 3000 and 4000 feet. The earlier
+of these great lava-floods appear to have been trachytic, but the later
+basaltic; and in the opinion of Captain Dutton, who has described them,
+they range in point of time from the Middle Tertiary (Miocene) down to
+comparatively recent times.
+
+(_c._) _The Grand Canyon._--To the south of the high plateaux of Utah
+are many minor volcanic mountains, now extinct; and as we descend
+towards the Grand Canyon of Colorado we find numerous cinder-cones
+scattered about at intervals near the cliffs.[1] Extensive lava-fields,
+surmounted by cinder-cones, occupy the plateau on the western side of
+the Grand Canyon; and, according to Dutton, the great sheets of basaltic
+lava, of very recent age, which occupy many hundred square miles of
+desert, have had their sources in these cones of eruption.[2] Crossing
+to the east of the Grand Canyon, we find other lava-floods poured over
+the country at intervals, surmounted by San Francisco--a volcanic
+mountain of the first magnitude--which reaches an elevation, according
+to Wheeler, of 12,562 feet above the ocean. It has long been extinct,
+and its summit and flanks are covered with snow-fields and glaciers.
+Other parts of Arizona are overspread by sheets of basaltic lava,
+through which old "necks" of eruption, formed of more solid lava than
+the sheets, rise occasionally above the surface, and are prominent
+features in the landscape.
+
+Further to the eastward in New Mexico, and near the margin of the
+volcanic region, is another volcanic mountain little less lofty than San
+Francisco, called Mount Taylor, which, according to Dutton, rises to an
+elevation of 11,390 feet above the ocean, and 8200 feet above the
+general level of the surrounding plateau of lava. This mountain forms
+the culminating point of a wide volcanic tract, over which are
+distributed numberless vents of eruption. Scores of such
+vents--generally cinder-cones--are visible in every part of the plateau,
+and always in a more or less dilapidated condition.[3] Mount Taylor is a
+volcano, with a central pipe terminating in a large crater, the wall of
+which was broken down on the east side in the later stage of its
+history.
+
+[Illustration: Fig. 25.--Mount Shasta (14,511 feet), a snow-clad
+volcanic cone in California, with Mount Shastina, a secondary crater, on
+the right; the valley between is filled with glacier-ice.--(After
+Dutton).]
+
+(_d._) _California._--Proceeding westwards into California, we are
+again confronted with volcanic phenomena on a stupendous scale. The
+coast range of mountains, which branches off from the Sierra Nevada at
+Mount Pinos, on the south, is terminated near the northern extremity of
+the State by a very lofty mountain of volcanic origin, called Mount
+Shasta, which attains an elevation of 14,511 feet (see Fig. 25). This
+mountain was first ascended by Clarence King in 1870,[4] and although
+forming, as it were, a portion of the Pacific Coast Range, it really
+rises from the plain in solitary grandeur, its summit covered by snow,
+and originating several fine glaciers.
+
+The summit of Mount Shasta is a nearly perfect cone, but from its
+north-west side there juts out a large crater-cone just below the
+snow-line, between which and the main mass of the mountain there exists
+a deep depression filled with glacier ice. This secondary crater-cone
+has been named Mount Shastina, and round its inner side the stream of
+glacier ice winds itself, sometimes surmounting the rim of the crater,
+and shooting down masses of ice into the great caldron. The length of
+this glacier is about three miles, and its breadth about 4000 feet.
+Another very lofty volcanic mountain is Mount Rainier, in the Washington
+territory, consisting of three peaks of which the eastern possesses a
+crater very perfect throughout its entire circumference. This mountain
+appears to be formed mainly of trachytic matter. Proceeding further
+north into British territory, several volcanic mountains near the
+Pacific Coast are said to exhibit evidence of activity. Of these may be
+mentioned Mount Edgecombe, in lat. 57 deg..3; Mount Fairweather, lat. 57 deg..20
+which rises to a height of 14,932 feet; and Mount St. Elias, lat. 60 deg..5,
+just within the divisional line between British and Russian territory,
+and reaching an altitude of 16,860 feet. This, the loftiest of all the
+volcanoes of the North American continent, except those of Mexico, may
+be considered as the connecting link in the volcanic chain between the
+continent and the Aleutian Islands.[5]
+
+(_e._) _Lake Bonneville._--Returning to Utah we are brought into contact
+with phenomena of special interest, owing to the inter-relations of
+volcanic and lacustrine conditions which once prevailed over large
+tracts of that territory. The present Great Salt Lake, and the smaller
+neighbouring lakes, those called Utah and Sevier, are but remnants of an
+originally far greater expanse of inland water, the boundaries of which
+have been traced out by Mr. C. K. Gilbert, and described under the name
+of Lake Bonneville.[6] The waters of this lake appear to have reached
+their highest level at the period of maximum cold of the Post-Pliocene
+period, when the glaciers descended to its margin, and large streams of
+glacier water were poured into it. Eruptions of basaltic lava from
+successive craters appear to have gone on before, during, and after the
+lacustrine epochs; and the drying up of the waters over the greater
+extent of their original area, now converted into the Sevier Desert, and
+their concentration into their present comparatively narrow basins,
+appears to have proceeded _pari passu_ with the gradual extinction of
+the volcanic outbursts. Two successive epochs of eruption of basalt
+appear to have been clearly established--an earlier one of the "Provo
+Age," when the lava was extruded from the Tabernacle craters, and a
+later epoch, when the eruptions took place from the Ice Spring craters.
+The oldest volcanic rock appears to be rhyolite, which peers up in two
+small hills almost smothered beneath the lake deposits. Its eruption was
+long anterior to the lake period. On the other hand, the cessation of
+the eruptions of the later basaltic sheets is evidently an event of such
+recent date that Mr. Gilbert is led to look forward to their resumption
+at some future, but not distant, epoch. As he truly observes, we are not
+to infer that, because the outward manifestations of volcanic action
+have ceased, the internal causes of those manifestations have passed
+away. These are still in operation, and must make themselves felt when
+the internal forces have recovered their exhausted energies; but perhaps
+not to the same extent as before.
+
+(_f._) _Region of the Snake River._--The tract of country bordering the
+Snake River in Idaho and Washington is remarkable for the vast sheets of
+plateau-basalt with which it is overspread, extending sometimes in one
+great flood farther than the eye can reach, and what is still more
+remarkable, they are often unaccompanied by any visible craters or vents
+of eruption. In Oregon the plateau-basalt is at least 2,000 feet in
+thickness, and where traversed by the Columbia River it reaches a
+thickness of about 3,000 feet. The Snake and Columbia rivers are lined
+by walls of volcanic rock, basaltic above, trachytic below, for a
+distance of, in the former, one hundred, in the latter, two hundred,
+miles. Captain Dutton, in describing the High Plateau of Utah, observes
+that the lavas appear to have welled up in mighty floods without any of
+that explosive violence generally characteristic of volcanic action.
+This extravasated matter has spread over wide fields, deluging the
+surrounding country like a tide in a bay, and overflowing all
+inequalities. Here also we have evidence of older volcanic cones buried
+beneath seas of lava subsequently extruded.
+
+(_g._) _Fissures of Eruption._--The absence, or rarity, of volcanic
+craters or cones of eruption in the neighbourhood of these great sheets
+has led American geologists to the conclusion that the lavas were in
+many cases extruded from fissures in the earth's crust rather than from
+ordinary craters.[7] This view is also urged by Sir A. Geikie, who
+visited the Utah region of the Snake River in 1880, and has vividly
+described the impression produced by the sight of these vast fields of
+basaltic lava. He says, "We found that the older trachytic lavas of the
+hills had been deeply trenched by the lateral valleys, and that all
+these valleys had a floor of black basalt that had been poured out as
+the last of the molten materials from the now extinct volcanoes. There
+were no visible cones or vents from which these floods of basalt could
+have proceeded. We rode for hours by the margin of a vast plain of
+basalt stretching southward and westward as far as the eye could
+reach.... I realised the truth of an assertion made first by
+Richthofen,[8] that our modern volcanoes, such as Vesuvius and Etna,
+present us with by no means the grandest type of volcanic action, but
+rather belong to a time of failing activity. There have been periods of
+tremendous volcanic energy, when instead of escaping from a local vent,
+like a Vesuvian cone, the lava has found its way to the surface by
+innumerable fissures opened for it in the solid crust of the globe over
+thousands of square miles."[9]
+
+(_h._) _Volcanic History of Western America._--The general succession of
+volcanic events throughout the region of Western America appears to have
+been somewhat as follows:--[10]
+
+The earliest volcanic eruptions occurred in the later Eocene epoch and
+were continued into the succeeding Miocene stage. These consisted of
+rocks moderately rich in silica, and are grouped under the heads of
+propylite and andesite. To these succeeded during the Pliocene epoch
+still more highly silicated rocks of trachytic type, consisting of
+sanidine and oligoclase trachytes. Then came eruptions of rhyolite
+during the later Pliocene and Pleistocene epochs; and lastly, after a
+period of cessation, during which the rocks just described were greatly
+eroded, came the great eruptions of basaltic lava, deluging the plains,
+winding round the cones or plateaux of the older lavas, descending into
+the river valleys and flooding the lake beds, issuing forth from both
+vents and fissures, and continuing intermittently down almost into the
+present day--certainly into the period of man's appearance on the scene.
+Thus the volcanic history of Western America corresponds remarkably to
+that of the European regions with which we have previously dealt, both
+as regards the succession of the various lavas and the epochs of their
+eruption.
+
+(_i._) _The Yellowstone Park._--The geysers and hot springs of the
+Yellowstone Park, like those in Iceland and New Zealand, are special
+manifestations of volcanic action, generally in its secondary or
+moribund stage. The geysers of the Yellowstone occur on a grand scale;
+the eruptions are frequent, and the water is projected into the air to a
+height of over 200 feet. Most of these are intermittent, like the
+remarkable one known as Old Faithful, the Castle Geyser, and the
+Giantess Geyser described by Dr. Hayden, which ejects the water to a
+height of 250 feet. The geyser-waters hold large quantities of silica
+and sulphur in solution, owing to their high temperature under great
+pressure, and these minerals are precipitated upon the cooling of the
+waters in the air, and form circular basins, often gorgeously tinted
+with red and yellow colours.[11]
+
+[1] J. W. Powell, _Exploration of the Canyons of the Colorado_, pp. 114,
+196. Major Powell describes a fault or fissure through which floods of
+lava have been forced up from beneath and have been poured over the
+surface. Many cinder-cones are planted along the line of this fissure.
+
+[2] Capt. C. E. Dutton. _Sixth Ann. Rep. U.S. Geol. Survey_, 1884-85.
+
+[3] Dutton, _loc. cit._, chap. iv. p. 165.
+
+[4] _Amer. Jour. Science_, vol. 3., ser. (1871). A beautiful map of this
+mountain is given in the _Fifth Annual Report, U.S. Geol. Survey_,
+1883-84. Plate 44.
+
+[5] Daubeny, _loc. cit._, p. 474.
+
+[6] Gilbert, _Monograph U.S. Geol. Survey_, vol. i. (1890).
+
+[7] Powell, _Exploration of the Colorado River_, p. 177, etc. (1875).
+Hayden, _Rep. U.S. Geol. Survey of the Colorado, etc._ (1871-80).
+
+[8] Richthofen, _Natural System of Volcanic Rocks_, Mem. California
+Acad. Sciences, vol. i. (1868).
+
+[9] Geikie, _Geological Sketches at Home and Abroad_, p. 271 (1882).
+
+[10] Prestwich, _Geology_, vol. i. p. 370, quoting from Richthofen.
+
+[11] The origin of geysers is variously explained; see Prestwich,
+_Geology_, vol. i. p. 170. They are probably due to heated waters
+suddenly converted into steam by contact with rock at a high
+temperature.
+
+
+
+
+CHAPTER III.
+
+VOLCANOES OF NEW ZEALAND.
+
+
+One other region of volcanic action remains to be noticed before passing
+on to the consideration of those of less recent age. New Zealand is an
+island wherein seem to be concentrated all the phenomena of volcanic
+action of past and present time. Though it is doubtful if the term
+"active," in its full sense, can be applied to any of the existing
+craters (with two or three exceptions, such as Tongariro and Whakari
+Island), we find craters and cones in great numbers in perfectly fresh
+condition, extensive sheets of trachytic and basaltic lavas, ashes, and
+agglomerates; lava-floods descending from the ruptured craters of ashes
+and scoriae; old crater-basins converted into lakes; geysers, hot springs
+and fumaroles which may be counted by hundreds, and cataracts breaking
+over barriers of siliceous sinter; and, lastly, lofty volcanic mountains
+vying in magnitude with Vesuvius and Etna. All these wonderful
+exhibitions of moribund volcanic action seem to be concentrated in the
+northern island of Auckland. The southern island, which is the larger,
+also has its natural attractions, but they are of a different kind;
+chief of all is the grand range of mountains called, not
+inappropriately, the "Southern Alps," vying with its European
+representative in the loftiness of its peaks and the splendour of its
+snowfields and glaciers, but formed of more ancient and solid rocks than
+those of the northern island.
+
+(_a._) _Auckland District._--We are indebted to several naturalists for
+our knowledge of the volcanic regions of New Zealand, but chiefly to
+Ferdinand von Hochstetter, whose beautiful maps and graphic descriptions
+leave nothing to be desired.[1] In this work Hochstetter was assisted by
+Julius Haast and Sir J. Hector. From their account we learn that the
+Isthmus of Auckland is one of the most remarkable volcanic districts in
+the world. It is characterised by a large number of extinct
+cinder-cones, in a greater or less perfect state of preservation, and
+giving origin to lava-streams which have poured down the sides of the
+hills on to the plains. Besides these are others formed of stratified
+tuff, with interior craters, surrounding in mural cliffs eruptive cones
+of scoriae, ashes, and lapilli; these cones are scattered over the
+isthmus and shores of Waitemata and Manukau. The tuff cones and craters
+rise from a floor of Tertiary sandstone and shale, the horizontal strata
+of which are laid open in the precipitous bluffs of Waitemata and
+Manukau harbours; they sometimes contain fossil shells of the genera
+_Pecten_, _Nucula_, _Cardium_, _Turbo_, and _Neritae_. As the volcanic
+tuff-beds are intermingled with the Upper Tertiary strata, it is
+inferred that the first outbursts of volcanic forces occurred when the
+region was still beneath the waters of the ocean. Cross-sections show
+that the different layers slope both outwards (parallel to the sides)
+and inwards towards the bottom of the craters. Sometimes these craters
+have been converted into lakes, as in the case of those of the Eifel;
+but generally they are dry or have a floor of morass. Of the
+crater-lakes, those of Kohuora, five in number, are perhaps the most
+remarkable; and in the case of two of these the central cones of slag
+appear as islets rising from the surface of the waters. The fresh-water
+lake Pupuka has a depth of twenty-eight fathoms. To the north of
+Auckland Harbour rises out of the waters of the Hauraki Gulf the cone of
+Rangitoto, 920 feet high, the flanks formed of rugged streams of basalt,
+and the summit crowned by a circular crater of slag and ash, out of the
+centre of which rises a second cone with the vent of eruption. This is
+the largest and newest of the Auckland volcanoes, and appears to have
+been built up by successive outpourings of basaltic lava from the
+central orifice, after the general elevation of the island.
+
+[Illustration: Fig. 26.--Forms of volcanic tuff cones, with their
+cross-sections, in the Province of Auckland.--No. 1. Simple tuff cone
+with central crater; No. 2. Outer tuff cone with interior cinder cone
+and crater; No. 3. The same with lava-stream issuing from the interior
+cone.--(After Hochstetter.)]
+
+Before leaving the description of the tuff-cones, which are a peculiar
+feature in the volcanic phenomena of New Zealand, and are of many forms
+and varieties, we must refer to that of Mount Wellington (Maunga Rei).
+This is a compound volcano, in which the oldest and smallest of the
+group is a tuff-crater-cone, exhibiting very beautifully the outward
+slope of its beds. Within this crater arise two cones of cinders, each
+with small craters. It would appear that after a long interval the
+larger of the two principal cones, formed of cinders and known as Mount
+Wellington, burst forth from the southern margin of the older tuff-cone,
+and, being built up to a height of 850 feet, gradually overspread the
+sides of its older neighbour. Mount Wellington itself has three craters,
+and from these large streams of basaltic lava have issued forth in a
+westerly direction, while a branch entered and partially filled the old
+tuff-crater to the northwards.
+
+Southwards from Manukau Harbour, and extending a short distance from
+the coast-line to Taranaki Point, there occurs a plateau of
+basalt-conglomerate (_Basaltkonglomerat_), with sheets of basaltic lava
+overspreading the Tertiary strata. These plateau-basalts are intersected
+by eruptive masses in the form of dykes, but still there are no craters
+or cones of eruption to be seen; so that we may infer that the sheets,
+at least, were extruded from fissures in the manner of those of the
+Colorado or Idaho regions of America. Proceeding still further south
+into the interior of the island, we here find a lofty plateau of an
+average elevation of 2,000 feet, interposed between the Tertiary beds of
+the Upper and Middle Waikato, and formed of trachytic and pitch-stone
+tuff, amongst which arise old extinct volcanic cones, such as those of
+Karioi, Pirongia, Kakepuku, Maunga Tautari, Aroha, and many others.
+These trachytic lavas would seem to be more ancient than the basaltic,
+previously described.
+
+(_b._) _Taupo Lake, and surrounding district._--But of all these
+volcanic districts, none is more remarkable than that surrounding the
+Taupo Lake, which lies amidst the Tertiary strata of the Upper Waikato
+Basin. The surface of this lake is 1,250 feet above that of the ocean,
+and its margin is enclosed within a border of rhyolite and
+pitchstone--rising into a mass of the same material 1,800 feet high on
+the eastern side. The form of the lake does not suggest that it is
+itself the crater of a volcano, but rather that it was originated by
+subsidence. On all sides, however, trachytic cones arise, of which the
+most remarkable group lies to the south of the lake, just in front of
+the two giant trachytic cones, the loftiest in New Zealand, one called
+Tongariro, rising about 6,500 feet, and the other Ruapahu, which attains
+an elevation of over 9,000 feet, with the summit capped by snow. These
+two lofty cones, standing side by side, are supposed by the Maoris to be
+the husband and wife to whom were born the group of smaller cones above
+referred to as occupying the southern shore of Taupo Lake. The volcano
+of Tongariro may still be considered as in a state of activity, as its
+two craters (Ngauruhoe and Ketetahi) constantly emit steam, and several
+solfataras break out on its flanks.[2]
+
+(_c._) _Roto Mahana._--In a northerly direction from Tongariro, and
+distant from the coast by a few miles, lies in the Bay of Plenty the
+second of the active volcanoes of New Zealand, the volcanic island of
+Whakari (White Island), from the crater of which are constantly erupted
+vast masses of steam clouds. The distance between these two active
+craters is 120 nautical miles; and along the tract joining them
+steam-jets and geysers issue forth from the deep fissures through which
+the lava sheets have formerly been extruded. Numerous lakes also occupy
+the larger cavities in the ground; and hot-springs, steam-fumaroles and
+solfataras burst out in great numbers along the banks of the Roto Mahana
+Lake and the Kaiwaka River by which it is drained. Amongst such
+eruptions of hot-water and steam we might expect the formation of
+siliceous sinter, and the deposition of sulphur and other minerals; nor
+will our expectations be disappointed. For here we have the wonderful
+terraces of siliceous sinter deposited by the waters entering Roto
+Mahana as they descend from the numerous hot-springs or pools near its
+margin. All travellers concur in describing these terraces as the most
+wonderful of all the wonders of the Lake district of New Zealand--so
+great is their extent, and so rich and varied is their colouring.
+
+The beautiful map of Roto Mahana on an enlarged scale by Hochstetter
+shows no fewer than ten large sinter terraces descending towards the
+margin of this lake, besides several mud-springs, fumaroles, and
+solfataras. But the largest and most celebrated of all the sinter
+terraces has within the last few years been buried from view beneath a
+flood of volcanic trass, or mud, an event which was as unexpected as it
+was unwelcome. In May, 1887, the mountain of Tarawera, which rises to
+the north-east of Roto Mahana, and on the line of eruption above
+described, suddenly burst forth into violent activity, covering the
+country for miles around with clouds of ashes, and, pouring down
+torrents of mud, completely enveloped the beautiful terrace of sinter
+which had previously been one of the wonders of New Zealand. By the same
+eruption several human beings were entombed, and their residences
+destroyed.
+
+The waters of Roto Mahana, together with the hot-springs and fountains
+are fed from rain, and from the waters of Taupo Lake, which, sinking
+through fissures in the ground, come in contact with the interior heated
+matter, and thus steam at high temperature and pressure is generated.[3]
+
+(_d._) _Moribund condition of New Zealand Volcanoes._--From what has
+been said, it will be inferred that in the case of New Zealand, as in
+those of Auvergne, the Eifel and Lower Rhine, Arabia, and Western
+America, we have an example of a region wherein the volcanic forces are
+well-nigh spent, but in which they were in a state of extraordinary
+activity throughout the later Tertiary, down to the commencement of the
+present epoch. In most of these cases the secondary phenomena of
+vulcanicity are abundantly manifest; but the great exhibitions of
+igneous action, when the plains were devastated by sheets of lava, and
+cones and craters were piled up through hundreds and thousands of feet,
+have for the present, at least, passed away.
+
+[1] _Geol.-topographischer Atlas von Neu-Seeland_, von Dr. Ferd. von
+Hochstetter und Dr. A. Petermann. Gotha: Justus Perthes (1863). Also
+_New Zealand_, trans. by E. Sauter, Stuttgart (1867).
+
+[2] Tongariro was visited in 1851 by Mr. H. Dyson, who describes the
+eruption of steam.
+
+[3] Mr. Froude figures and describes the two terraces, the "White" and
+"Pink," in _Oceana_, 2nd edition, pp. 285-291.
+
+
+
+
+PART IV.
+
+TERTIARY VOLCANIC DISTRICTS OF THE BRITISH ISLES.
+
+
+
+
+CHAPTER I.
+
+ANTRIM.
+
+
+It is an easy transition to pass from the consideration of European and
+other dormant, or extinct, volcanic regions to those of the British
+Isles, though the volcanic forces may have become in this latter
+instance quiescent for a somewhat longer period. In all the cases we
+have been considering, whether those of Central Italy, of the Rhine and
+Moselle, of Auvergne, or of Syria and Arabia, the cones and craters of
+eruption are generally present entire, or but slightly modified in form
+and size by the effects of time. But in the case of the Tertiary
+volcanic districts of the British Isles this is not so. On the contrary,
+these more prominent features of vulcanicity over the surface of the
+ground have been removed by the agents of denudation, and our
+observations are confined to the phenomena presented by extensive sheets
+of lava and beds of ash, or the stumps and necks of former vents of
+eruption, together with dykes of trap by which the plateau-lavas are
+everywhere traversed or intersected.
+
+The volcanic region of the British Isles extends at intervals from the
+North-east of Ireland through the Island of Mull and adjoining districts
+on the mainland of Morvern and Ardnamurchan into the Isle of Skye, and
+comprises several smaller islets; the whole being included in the
+general name of the Inner Hebrides. It is doubtful if the volcanic lavas
+of Co. Antrim were ever physically connected with those of the west of
+Scotland, though they may be considered as contemporary with them; and
+in all cases the existing tracts of volcanic rock are mere fragments of
+those originally formed by the extrusion of lavas from vents of
+eruption. In addition to these, there are large areas of volcanic rock
+overspread by the waters of the ocean.
+
+(_a._) _Geological Age._--The British volcanic eruptions now under
+consideration are all later than the Cretaceous period. Throughout
+Antrim, and in parts of Mull, the lavas are found resting on highly
+eroded faces either of the Upper Chalk (Fig. 27), or, where it has been
+altogether denuded away, on still older Mesozoic strata. From the
+relations of the basaltic sheets of Antrim to the Upper Chalk, it is
+clear that the latter formation, after its deposition beneath the waters
+of the Cretaceous seas, was elevated into dry land and exposed to a long
+period of subaerial erosion before the first sheets of lava invaded the
+surface of the ground. We are, therefore, tolerably safe in considering
+the first eruptions to belong to the Tertiary period; but the evidence,
+derived as it is exclusively from plant remains, is somewhat conflicting
+as to the precise epoch to which the lavas and beds of tuff containing
+the plant-remains are to be referred. The probabilities appear to be
+that they are of Miocene age; and if so, the trachytic lavas, which in
+Antrim are older than those containing plants, may be referred to a
+still earlier epoch--namely, that of the Eocene.[1] As plant remains are
+not very distinctive, the question regarding the exact time of the first
+volcanic eruptions will probably remain for ever undecided; but we are
+not likely to be much in error if we consider the entire volcanic period
+to range from the close of the Eocene to that of the Miocene; by far the
+greater mass of the volcanic rocks being referable to the latter epoch.
+
+In describing the British volcanic districts it will be most convenient
+to deal with them in three divisions--viz., those of Antrim, Mull, and
+Skye, commencing with Antrim.[2]
+
+(_b._) _Volcanic Area._--The great sheets of basalt and other volcanic
+products of the North-east of Ireland overspread almost the whole of the
+County Antrim, and adjoining districts of Londonderry and Tyrone,
+breaking off in a fine mural escarpment along the northern shore of
+Belfast Lough and the sea coast throughout the whole of its range from
+Larne Harbour to Lough Foyle; the only direction in which these features
+subside into the general level of the country being around the shores of
+Lough Neagh. Several outliers of the volcanic sheets are to be found at
+intervals around the great central plateau; such as those of Rathlin
+Island, Island Magee, and Scrabo Hill in Co. Down. The area of the
+basaltic plateau may be roughly estimated at 2,000 square miles.
+
+[Illustration: Fig. 27.--"The White Rocks," Portrush, Co. Antrim,
+showing the plateau-basalt resting on an eroded surface of the Upper
+Chalk, with bands of flint.--(From a photograph.)]
+
+The truncated edges of this marginal escarpment rising to levels of
+1,000 to 1,260 feet, as in the case of Benevenagh in Co. Derry, and
+1,825 feet at Mullaghmore, attest an originally greatly more extended
+range of the basaltic sheets; and it is not improbable that at the close
+of the Miocene epoch they extended right across the present estuary of
+Lough Foyle to the flanks of the mountains of Inishowen in Donegal in
+one direction, and to those of Slieve Croob in the other. In the
+direction of Scotland the promontories of Kintyre and Islay doubtless
+formed a part of the original margin. Throughout this vast area the
+volcanic lavas rest on an exceedingly varied rocky floor, both as
+regards composition and geological age. (See Fig. 28.) Throughout the
+central, southern, eastern, and northern parts of their extent, the
+Chalk formation may be considered to form this floor; but in the
+direction of Armagh and Tyrone, towards the southwestern margin, the
+basaltic sheets are found resting indiscriminately on Silurian,
+Carboniferous, and Triassic strata. The general relations of the
+plateau-basalts to the underlying formations show, that at the close of
+the Cretaceous period there had been considerable terrestrial
+disturbances and great subaerial denudation, resulting in some cases in
+the complete destruction of the whole of the Cretaceous strata, before
+the lava floods were poured out; owing to which, these latter are found
+resting on formations of older date than the Cretaceous.[3]
+
+[Illustration: Fig. 28.--Section across the volcanic plateau of Antrim,
+from the Highlands of Inishowen, Co. Donegal, on the N.W., to Belfast
+Lough on the S.E., to show the relations of the volcanic rocks to the
+older formations.--B. Basaltic sheets breaking off in high escarpments;
+T. Trachyte porphyry of Tardree mountain rising from below the newer
+plateau-basalts; C. Upper Chalk with flints; N.R. New Red marl and
+sandstone (Trias); M. Metamorphic beds of quartzite, various schists and
+crystalline limestone; F. Large fault.]
+
+[1] Mr. J. Starkie Gardner, from a recent comparison of the
+plant-remains of Antrim and Mull, concludes that "that they might belong
+to any age between the beginning and the end of the warmer Eocene
+period; and that they cannot be of earlier, and are unlikely to be of
+later, date."--_Trans. Palaeont. Soc._, vol. xxxvii. (1883).
+
+[2] Having dealt with this district rather fully in _The Physical
+Geology and Geography of Ireland_ (Edit. 1891, p. 81), and also in my
+Presidential Address (Section C.) at the meeting of the British
+Association, 1874, a brief review of the subject will be sufficient
+here, the reader being referred to the former treatises for fuller
+details. The following should also be consulted: Gen. Portlock, _Geology
+of Londonderry and Tyrone_ (1843); Sir A. Geikie, "History of Volcanic
+Action during the Tertiary Period in the British Isles," _Trans. Roy.
+Soc. Edinburgh_, 1888; and the _Descriptive Memoirs_ of the Geological
+Survey relating to this tract of country.
+
+[3] Owing to the superposition of the basaltic masses on beds of chalk
+throughout a long line of coast, we are presented with the curious
+spectacle of the whitest rocks in nature overlain by the blackest, as
+may be seen in the cliffs at Larne, Glenarm, Kinbane and Portrush. (See
+Fig. 27.)
+
+
+
+
+CHAPTER II.
+
+SUCCESSION OF VOLCANIC ERUPTIONS.
+
+
+(_c._) _First Stage._--The earliest eruptions of lava in the North-east
+of Ireland belonged to the highly acid varieties, consisting of
+quartz-trachyte with tridymite.[1] This rock rises to the surface at
+Tardree and Brown Dod hills and Templepatrick. It consists of a
+light-greyish felsitic paste enclosing grains of smoke-quartz, crystals
+of sanidine, plagioclase and biotite, with a little magnetite and
+apatite. It is a rock of peculiar interest from the fact that it is
+almost unique in the British Islands, and has its petrological
+counterpart rather amongst the volcanic hills of the Siebengebirge than
+elsewhere. It is generally consolidated with the columnar structure.
+
+[Illustration: Fig. 29.--Part of the section shown in the quarry at
+Templepatrick, showing the superposition of the basalt (_d_) to the
+trachyte (_b_), with the intervening bed of flint gravel (_c_). All
+these rocks are seen to rest upon an eroded surface of the Chalk
+formation (_a_).]
+
+The trachyte appears to have been extruded from one or more vents in a
+viscous condition, the principal vent being probably situated under
+Tardree mountain, where the rock occurs in greatest mass, and it
+probably arose as a dome-shaped mass, with a somewhat extended margin,
+above the floor of Chalk which formed the surface of the ground.[2]
+(Fig. 27.) At Templepatrick the columnar trachyte may be observed
+resting on the Chalk, or upon a layer of flint gravel interposed between
+the two rocks, and which has been thrust out of position by a later
+intrusion of basalt coming in from the side.[3] It is to be observed,
+however, that the trachytic lavas nowhere appear cropping out along with
+the sheets of basalt around the escarpments overlooking the sea, or
+inland; showing that they did not spread very far from their vents of
+eruption; a fact illustrating the lower viscosity, or fluidity, of the
+acid lavas as compared with those of the basic type.
+
+(_d._) _Second Stage._--After an interval, probably of long duration, a
+second eruption of volcanic matter took place over the entire area; but
+now the acid lavas of the first stage are replaced by basic lavas. Now,
+for the first time, vast masses of basalt and dolerite are extruded both
+from vents of eruption and fissures; and, owing to their extreme
+viscosity, spread themselves far and wide until they reach the margin of
+some uprising ground of old Palaeozoic or Metamorphic rocks by which the
+volcanic plain is almost surrounded. The great lava sheets thus produced
+are generally more or less amorphous, vesicular and amygdaloidal, often
+exhibiting the globular concentric structure, and weathering rapidly to
+a kind of ferruginous sand or clay under the influence of the
+atmosphere. Successive extrusions of these lavas produce successive
+beds, which are piled one over the other in some places to a depth of
+600 feet; and at the close of the stage, when the volcanic forces had
+for the time exhausted themselves, the whole of the North-east of
+Ireland must have presented an aspect not unlike that of one of those
+great tracts of similar lava in the region of Idaho and the Snake River
+in Western America, described in a previous chapter.
+
+(_e._) _Third Stage (Inter-volcanic)._--The third stage may be described
+as inter-volcanic. Owing to the formation of a basin, probably not deep,
+and with gently sloping sides, a large lake was formed over the centre
+of the area above described. Its floor was basalt, and the streams from
+the surrounding uplands carried down leaves and stems of trees, strewing
+them over its bed. Occasionally eruptions of ash took place from small
+vents, forming the ash-beds with plants found at Ballypallidy, Glenarm,
+and along the coast as at Carrick-a-raide. The streams also brought down
+sand and gravel from the uprising domes of trachyte, and deposited them
+over the lake-bed along with the erupted ashes.[4] The epoch we are now
+referring to was one of economic importance; as, towards its close,
+there was an extensive deposition of pisolitic iron-ore over the floor
+of the lake, sometimes to the depth of two or three feet. This ore has
+been extensively worked in recent years.
+
+[Illustration: Fig. 30.--Cliff section above the Giant's Causeway, coast
+of Co. Antrim, showing successive tiers of basaltic lava, with
+intervening bands of bole.]
+
+(_f._) _Fourth Stage (Volcanic)._--The last stage described was brought
+to a termination by a second outburst of basic lavas on a scale probably
+even grander than the preceding. These lavas consisting of basalt and
+dolerite, with their varieties, and extruded from vents and fissures,
+spread themselves in all directions over the pre-existing lake deposits
+or the older sheets of augitic lava, and probably entirely buried the
+trachytic hills. These later sheets solidified into more solid masses
+than those of the second stage. They form successive terraces with
+columnar structure, each terrace differing from that above and below it
+in the size and length of the columns, and separated by thin bands of
+"bole" (decomposed lava), often reddish in colour, clearly defining the
+limits of the successive lava-flows. Nowhere throughout the entire
+volcanic area are these successive terraces so finely laid open to view
+as along the north coast of Antrim, where the lofty mural cliffs, worn
+back into successive bays with intervening headlands by the
+irresistible force of the Atlantic waves, present to the spectator a
+vertical section from 300 to 400 feet in height, in which the successive
+tiers of columnar basalt, separated by thin bands of bole, are seen to
+rise one above the other from the water's edge to the summit of the
+cliff, as shown in Fig. 30. Here, also, at the western extremity of the
+line of cliffs we find that remarkable group of vertical basaltic
+columns, stretching from the base of the cliff into the Atlantic, and
+known far and wide by the name of "The Giant's Causeway," the upper ends
+of the columns forming a tolerably level surface, gently sloping
+seawards, and having very much the aspect of an artificial tesselated
+pavement on a huge scale. A portion of the Causeway, with the cliff in
+the background, is shown in the figure (Fig. 31). The columns are
+remarkable for their symmetry, being generally hexagonal, though
+occasionally they are pentagons, and each column is horizontally
+traversed by joints of the ball-and-socket form, thus dividing them into
+distinct courses of natural masonry. These are very well shown in the
+accompanying view of the remarkable basaltic pillars known as "The
+Chimneys," which stand up from the margin of the headland adjoining the
+Causeway, monuments of past denudation, as they originally formed
+individuals amongst the group belonging to one of the terraces in the
+adjoining coast.[5] (Fig. 32).
+
+[Illustration: Fig. 31.--The Giant's Causeway, formed of basaltic
+columns in a vertical position, and of pentagonal or hexagonal section;
+above the Causeway is seen a portion of the cliff composed of tiers of
+lava with intervening bands of bole, etc.--(From a photograph.)]
+
+[Illustration: Fig. 32.--"The Chimneys," columns of basalt on slope of
+cliff overlooking the Atlantic, north coast of Co. Antrim. The
+horizontal segments, or cup-and-ball joints, of the columns are well
+shown in this figure. (From a photograph.)]
+
+(_g._) _Original Thickness of the Antrim Lavas._--It is impossible to
+determine with certainty what may have been the original thickness of
+the accumulated sheets of basic lavas with their associated beds of ash
+and bole. The greatest known thickness of the lower zone of lavas is, as
+I have already stated, about 600 feet. The intermediate beds of ash and
+bole sometimes attain a thickness of 40 feet, and the upper group of
+basalt about 400 feet; these together would constitute a series of over
+1,000 feet in thickness. But this amount, great as it is, is undoubtedly
+below the original maximum, as the uppermost sheets have been removed by
+denuding agencies, we know not to what extent. Nor is it of any great
+importance. Sufficient remains to enable us to form a just conception of
+the magnitude both as regards thickness and extent of the erupted matter
+of the Miocene period over the North-east of Ireland and adjoining
+submerged tracts, and of the magnitude of the volcanic operations
+necessary for the production of such masses.
+
+(_h._) _Volcanic Necks._--As already remarked, no craters of eruption
+survive throughout the volcanic region of the North-east of Ireland,
+owing to the enormous extent of the denudation which this region has
+undergone since the Miocene Epoch; but the old "necks" of such
+craters--in other words, the pipes filled with either solid basalt, or
+basalt and ashes--are still to be found at intervals over the whole
+area. Owing to the greater solidity of the lava which filled up these
+"necks" over the plateau-basaltic sheets which surround them, they
+appear as bosses or hills rising above the general level of the ground.
+One of these bosses of highly columnar basalt occurs between Portrush
+and Bushmills, not far from Dunluce Castle, another at Scawt Hill, near
+Glenarm, and a third at Carmoney Hill above Belfast Lough. But by far
+the most prominent of these old solidified vents of eruption is that of
+Sleamish, a conspicuous mountain which rises above the general level of
+the plateau near Ballymena, and attains an elevation of 1,437 feet above
+the sea. Seen from the west, the mountain has the appearance of a
+round-topped cone; but on examination it is found to be in reality a
+huge dyke, breaking off abruptly towards the north-west, in which
+direction it reaches its greatest height, then sloping downwards towards
+the east. This form suggests that Sleamish is in reality one of the
+fissure-vents of eruption rather than the neck of an old volcano. The
+rock of which it is formed consists of exceedingly massive,
+coarsely-crystalline dolerite, rich in olivine, and divided into large
+quadrangular blocks by parallel joint planes. Its junction with the
+plateau-basalt from which it rises can nowhere be seen; but at the
+nearest point where the two rocks are traceable the plateau-basalt
+appears to be somewhat indurated; breaking with a splintery fracture and
+a sharp ring under the hammer, suggesting that the lava of Sleamish had
+been extruded through the horizontal sheets, and had considerably
+indurated the portions in contact with, or in proximity to, it.[6]
+Amongst the vents filled with ash and agglomerate, the most remarkable
+is that of Carrick-a-raide, near Ballycastle. It forms this rocky island
+and a portion of the adjoining coast, where the beds of ash are finely
+displayed; consisting of fragments and bombs of basalt, with pieces of
+chalk, flint, and peperino, which is irregularly bedded. These ash-beds
+attain a thickness of about 120 feet just below the road to Ballycastle,
+but rapidly tail out in both directions from the locality of the vent.
+Just below the ash-beds, the white chalk with flints may be seen
+extending down into the sea-bed. Nowhere in Antrim is there such a
+display of volcanic ash and agglomerate as at this spot.[7]
+
+(_i._) _Dykes: Conditions under which they were Erupted._--No one can
+visit the geological sections in Co. Antrim and the adjoining districts
+of Down, Armagh, Derry, and Tyrone, without being struck by the great
+number and variety of the igneous dykes by which the rocks are
+traversed. The great majority of these dykes are basaltic, and they are
+found traversing all the formations, including the Cretaceous and
+Tertiary basaltic sheets. The Carlingford and Mourne Mountains are
+seamed with such dykes, and they are splendidly laid open to view along
+the coast south of Newcastle in Co. Down, as also along the Antrim coast
+from Belfast to Larne. The fine old castle of Carrickfergus has its
+foundations on one of those dyke-like intrusions, but one of greater
+size than ordinary. All the dykes here referred to are not, however, of
+the same age, as is conclusively proved by sections amongst the Mourne
+Mountains where cliffs of Lower Silurian strata, superimposed on the
+intrusive granite of the district, exhibit two sets of basaltic
+dykes--one (the older) abruptly terminated at the granite margin, the
+other and newer penetrating the granite and Silurian rocks alike. It is
+not improbable that the older dykes belong to the Carboniferous or
+Permian age, while the newer are with equal probability of Tertiary age.
+Sir A. Geikie has shown that the Tertiary dykes of the North of Ireland
+are representatives of others occurring at intervals over the North of
+England, and Central and Western Scotland, all pointing towards the
+central region of volcanic activity; or in a parallel direction thereto,
+approximating to the N.W. in Ireland, the Island of Islay, and East
+Argyleshire, but in the centre of Scotland generally ranging from east
+to west.[8] The area affected by the dykes of undoubted Tertiary age
+Geikie estimates at no less than 40,000 square miles--a territory
+greater than either Scotland or Ireland, and equal to more than a third
+of the total land-surface of the British Isles;[9] and he regards them
+as posterior "to the rest of the geological structures of the regions
+which they traverse." It is clear that the dykes referred to belong to
+one great system of eruption or intrusion; and they may be regarded as
+the manifestation of the final effort of internal forces over this
+region of the British Isles. They testify to the existence of a
+continuous _magma_ (or shell) of augitic lava beneath the crust; and as
+the aggregate horizontal extent of all these dykes, or of the fissures
+which they fill, must be very considerable, it is clear that the crust
+through which they have been extruded has received an accession of
+horizontal space, and has been fissured by forces acting from beneath,
+as the late Mr. Hopkins, of Cambridge, had explained on mechanical
+grounds in his elaborate essay many years ago.[10] This view occurred to
+myself when examining the region of the North-east of Ireland, but I was
+not then aware that it had been dealt with on mathematical principles by
+so eminent a mathematician. The bulging of the crust is a necessary
+consequence of the absence of plication of the strata due to the
+extrusion of this enormous quantity of molten lava; and the intrusion of
+thousands of dykes over the North-east of Ireland, unaccompanied by
+foldings of the strata, must have added a horizontal space of several
+thousand feet to that region.[11]
+
+[1] A peculiar form of crystalline quartz first recognized in this rock
+by a distinguished German petrologist, the late Prof. A. von Lasaulx,
+who visited the district in 1876.
+
+[2] Sir A. Geikie has disputed the correctness of the view, which I
+advocated as far back as 1874, that the trachytic lavas of Antrim are
+the earliest products of volcanic action; but at the time he wrote his
+paper on the volcanic history of these islands, it was not known that
+pebbles of this trachyte are largely distributed amongst the ash-beds
+which occur in the very midst of the overlying basaltic sheets, as I
+shall have to explain later on. This discovery puts the question at rest
+as regards the relations of the two sets of rocks.
+
+[3] This remarkable section at the chalk quarries of Templepatrick the
+author has figured and described in the _Physical Geology and Geography
+of Ireland_, p. 99, 2nd edit. (1891), where the reader will find the
+subject discussed more fully than can be done here.
+
+[4] These pebbles were first noticed by Mr. McHenry, of the Irish
+Geological Survey, in 1890.
+
+[5] The vertical position of the columns of the Giant's Causeway is
+rather enigmatical. The Causeway cannot be a dyke, as has often been
+supposed, otherwise the columns would have been horizontal, _i.e._, at
+right angles to the sides of the dyke. Mr. R. G. Symes, of the
+Geological Survey, has suggested that the Causeway columns have been
+vertically lowered between two lines of fault, and that originally they
+formed a portion of the tier of beautiful columns seen in the cliff
+above, and known as "The Organ."
+
+[6] Sleamish and several other of the Antrim vents are described by Sir
+A. Geikie in the monograph already referred to, _loc. cit._, p. 101, _et
+seq._ Also in the _Expl. Memoirs of the Geological Survey of Ireland_.
+
+[7] A diagrammatised section of the Carrick-a-raide volcanic neck is
+given by Sir A. Geikie, _loc. cit._, p. 105.
+
+[8] Geikie, _loc. cit._, p. 29, _et seq._
+
+[9] P. 32. The view that the crust of the earth has been horizontally
+extended by the intrusion of dykes is noticed by McCulloch in reference
+to the dykes of Skye.
+
+[10] Hopkins, _Cambridge Phil. Trans._, vol. vi. p. 1 (1836).
+
+[11] As suggested in my Presidential Address to Section C. of the
+British Association at Belfast, 1874.
+
+
+
+
+CHAPTER III.
+
+ISLAND OF MULL AND ADJOINING COAST.
+
+
+The Island of Mull, with the adjoining districts of Morvern and
+Ardnamurchan, forms the more southern of the two chief centres of
+Tertiary volcanic eruptions in the West of Scotland, that of Skye being
+the more northern. These districts have been the subject of critical and
+detailed study by several geologists, from McCulloch down to the present
+day; and amongst the more recent, Sir Archibald Geikie and Professor
+Judd hold the chief place. Unfortunately, the interpretation of the
+volcanic phenomena by these two accomplished observers has led them to
+very different conclusions as regards several important points in the
+volcanic history of these groups of islands; as, for example, regarding
+the relative ages of the plateau-basalts and the acid rocks, such as the
+trachytes and granophyres; again as regards the presence of distinct
+centres of eruption; and also as regards the relations of the gabbros of
+Skye to the basaltic sheets. Such being the case, it would appear the
+height of rashness on the part of the writer, especially in the absence
+of a detailed examination of the sections over the whole region, to
+venture on a statement of opinion regarding the points at issue; and he
+must, therefore, content himself with a brief account of the phenomena
+as gathered from a perusal of the writings of these and other
+observers,[1] guided also to some extent by the analogous phenomena
+presented by the volcanic region of the North-east of Ireland.
+
+(_a._) _General Features._--As in the case of the Antrim district, the
+Island of Mull and adjoining tracts present us with the spectacle of a
+vast accumulation of basaltic lava-flows, piled layer upon layer, with
+intervening beds of bole and tuff, up to a thickness, according to
+Geikie, of about 3,500 feet. At the grand headland of Gribon, on the
+west coast, the basaltic sheets are seen to rise in one sheer sweep to a
+height of 1,600 feet, and then to stretch away with a slight easterly
+dip under Ben More at a distance of some eight miles. This mountain, the
+upper part of which is formed of beds of ashes, reaches an elevation of
+3,169 feet, so that the accumulated thickness of the beds of basalt
+under the higher part of the mountain must be at least equal to the
+amount stated above--that is, twice as great as the representative
+masses of Antrim. The base of the volcanic series is seen at Carsaig and
+Gribon to rest on Cretaceous and Jurassic rocks, like those of Antrim;
+hence the Tertiary age is fully established by the evidence of
+superposition. This was further confirmed by the discovery by the Duke
+of Argyll,[2] some years ago (1850), of bands of flint-gravel and tuff,
+with dicotyledonous leaves amongst the basalts of Ardtun Head. The
+basement beds of tuff and gravel contain, besides pebbles of flint and
+chalk, others of sanidine trachyte, showing that highly acid lavas had
+been extruded and consolidated before the first eruption of the
+plateau-basalts; another point of analogy between the volcanic
+phenomenon of Antrim and the Inner Hebrides. These great sheets of
+augitic lava extend over the whole of the northern tract of Mull, the
+Isles of Ulva and Staffa, and for a distance of several miles inwards
+from the northern shore of the Sound of Mull, covering the wild
+moorlands of Morvern and Ardnamurchan, where they terminate in
+escarpments and outlying masses, indicating an originally much more
+extended range than at the present day. The summits of Ben More and its
+neighbouring height, Ben Buy, are formed of beds of ash and tuff. The
+volcanic plateau is, according to Judd, abruptly terminated along the
+southern side by a large vault, bringing the basalt in contact with
+Palaeozoic rocks.[3]
+
+(_b._) _Granophyres._--The greater part of the tract lying to the south
+of Loch na Keal, which almost divides Mull into two islands, and
+extending southwards and eastwards to the shores of the Firth of Lorn
+and the Sound of Mull, is formed of a peculiar group of acid (or highly
+silicated) rocks, classed under the general term of "Granophyres." These
+rocks approach towards true granites in one direction, and through
+quartz-porphyry and felsite to rhyolite in another--probably depending
+upon the conditions of cooling and consolidation. In their mode of
+weathering and general appearance on a large scale, they present a
+marked contrast to the basic lavas with which they are in contact from
+the coast of L. na Keal to that of L. Buy. The nature of this contact,
+whether indicating the priority of the granophyres to the
+plateau-basalts or otherwise, is a matter of dispute between the two
+observers above named; but the circumstantial account given by Sir A.
+Geikie,[4] accompanied by drawings of special sections showing this
+contact, appears to prove that the granophyre is the newer of the two
+masses of volcanic rock, and that it has been intruded amongst the
+basaltic-lavas at a late period in the volcanic history of these
+islands. A copy of one of these sketches is here given (Fig. 33),
+according to which the felsite is shown to penetrate the basaltic sheets
+at Alt na Searmoin in Mull; other sections seen at Cruach Torr an
+Lochain, and on the south side of Beinn Fada, appear to lead to similar
+conclusions. These rocks are penetrated by numerous basaltic dykes.
+
+[Illustration: Fig. 33.--Section at Alt na Searmoin, Mull, to show the
+intrusion of felsite (or granophyre) (_b_) into basalt and dolerite
+(_a_) of the plateau-basalt series.--(Geikie.)]
+
+(_c._) _Representative Rocks of Mourne and Carlingford,
+Ireland._--Assuming Sir A. Geikie's view to be correct, it is possible
+that we may have in the granite and quartz-porphyries of Mourne and
+Carlingford representatives of the granites, granophyres, and other acid
+rocks of the later period of Mull. The granite of Mourne is peculiar in
+structure, and differs from the ordinary type of that rock in which the
+silica forms the ground mass. In the case of the granite of the Mourne
+Mountains, the rock consists of a crystalline granular aggregate of
+orthoclase, albite, smoke-quartz, and mica; it is also full of drusy
+cavities, in which the various minerals crystallise out in very perfect
+form. As far as regards direct evidence, the age of this rock can only
+be stated to be post-Carboniferous, and earlier than certain Tertiary
+basaltic dykes by which it is traversed. The granophyres of Mull are
+traversed by similar dykes, which are representatives of the very latest
+stage of volcanic action in the British Islands. The author is therefore
+inclined to concur with Sir A. Geikie in assigning to the granite of the
+Mourne Mountains, and the representative felsitic rocks of the
+Carlingford Mountains, a Tertiary age--in which case the analogy between
+the volcanic phenomena of the Inner Hebrides and of the North-east of
+Ireland would seem to be complete.[5]
+
+[1] Geikie, _Proc. Roy. Soc. Edinburgh_ (1867); _Brit. Assoc. Rep._
+(Dundee, 1867); "Tertiary Volcanic Rocks of the British Isles," _Quart.
+Journ. Geol. Soc._, vol. xxvii. p. 279; also, "History of Volcanic
+Action in British Isles," _Trans. Roy. Soc. Edin._ (1888); Judd, "On the
+Ancient Volcanoes of the Highlands," etc., _Quart. Journ. Geol. Soc._,
+vol. xxx. p. 233; and _Volcanoes_, p. 139.
+
+[2] _Brit. Assoc. Rep._ for 1850, p. 70.
+
+[3] Judd, _Quart. Jour. Geol. Soc._, vol. xxx. p. 242.
+
+[4] _History of Volcanic Action, etc._, _loc. cit._ p. 153, _et seq._
+The "Granophyres" of Geikie come under the head of "Felsites," passing
+into "granite" in one direction and quartz-trachyte in another,
+according to Judd; the proportion of silica from 69 to 75 per
+cent.--_Quart. Jour. Geol. Soc._, vol. xxx. p. 235.
+
+[5] This view the author has expressed in a recent edition of _The
+Physical Geology of Ireland_, p. 177 (1891).
+
+
+
+
+CHAPTER IV.
+
+ISLE OF SKYE.
+
+
+This is the largest and most important of all the Tertiary volcanic
+districts, but owing to the extensive denudation to which, in common
+with other Tertiary volcanic regions of the British Isles, it has been
+subjected, its present limits are very restricted comparatively to its
+original extent. Not only is this evident from the manner in which the
+basaltic sheets terminate along the sea-coast in grand mural cliffs, as
+opposite "Macleod's Maidens," and at the entrance to Lough Bracadale on
+the western coast, but the evidence is, according to Sir A. Geikie,
+still more striking along the eastern coast; showing that the Jurassic,
+and other older rocks there visible, were originally buried deep under
+the basaltic sheets which have been stripped from off that part of the
+country. These great plateau-basalts occupy about three-fourths of the
+entire island along the western and northern areas, rising into terraced
+mountains over 2,000 feet in height, and are deeply furrowed by glens
+and arms of the sea, along which the general structure of the tableland
+is laid open, sometimes for leagues at a time.
+
+It is towards the south-eastern part of the island that the most
+interesting and important phenomena are centred; for here we meet with
+representatives of the acid (or highly silicated) group of rocks, and
+of remarkable beds of gabbro, which have long attracted the attention of
+petrologists. These latter beds, throughout a considerable distance
+round the flanks of the Cuillin Hills, are interposed between the acid
+rocks and the plateau-basalts; but towards the north, on approaching
+Lough Sligahan, the acid rocks, consisting of granophyres,
+quartz-porphyries, and hornblendic-granitites, are in direct contact
+with the plateau-basalts; and, according to the very circumstantial
+account of Sir A. Geikie, are intrusive into them; not only sending
+veins into the basaltic sheets, but also producing a marked alteration
+in their structure where they approach the newer intrusive mass. Equally
+circumstantial is the same author's account of the relations of the
+granophyres to the gabbros,[1] as seen at Meall Dearg and the western
+border of the Cuillin Hills--where the former rock may be seen to send
+numerous veins into the latter. Not only is this so, but the granophyre
+is frequently seen to truncate, and abruptly terminate some of the
+basaltic dykes by which the basic sheets are traversed--as in the
+neighbourhood of Beinn na Dubhaic. All these phenomena strongly remind
+us of the conditions of similar rocks amongst the mountains of Mourne
+and Carlingford in Ireland; where, at Barnaveve, the syenite (or
+hornblendic quartz-felsite) is seen to break through the masses of
+olivine gabbro, and send numerous veins into this latter rock.[2]
+
+The interpretation here briefly sketched differs widely from that
+arrived at by Professor Judd. The granitoid masses of the Red Mountains
+(Beinn Dearg) and the neighbouring heights are, in his view, the roots
+of the great volcano from which were erupted the various lavas; the
+earlier eruptions producing the acid lavas, to be followed by the
+gabbros, and these by the plateau-basaltic sheets, which stretch away
+towards the north and west into several peninsulas. Thus he holds that
+"the rocks of basic composition were ejected subsequently to those of
+the acid variety," and appeals to various sections in confirmation of
+this view.[3] To reconcile these views is at present impossible; but as
+the controversy between these two observers is probably not yet closed,
+there is room for hope that the true interpretation of the relations of
+these rocks to each other will ere long be fully established.
+
+[1] Geikie, _loc. cit._, p. 161, etc.
+
+[2] _Physical Geology of Ireland_, 2nd edition, p. 174 (Fig. 21).
+Professor Judd has also come to the conclusion that the granite of
+Mourne is of Tertiary age, _Quart. Jour. Geol. Soc._, vol. xxx. p. 275.
+
+[3] Judd, _loc. cit._, p. 254.
+
+
+
+
+CHAPTER V.
+
+THE SCUIR OF EIGG.
+
+
+Amongst the more remarkable of the smaller islets are those of Eigg,
+Rum, Canna, and Muck, lying between Mull on the south and Skye on the
+north, and undoubtedly at one time physically connected together. The
+Island of Eigg is especially remarkable for the fact, as stated by
+Geikie, that here we have the one solitary case of "a true superficial
+stream of acid lava--that of the Scuir of Eigg."[1] (Fig. 34.) This
+forms a sinuous ridge, composed of pitchstone of several kinds, of over
+two miles in length, rising from the midst of a tableland of bedded
+basalt and tuff to a height of 1,289 feet above the ocean; the
+plateau-basalt is traversed by basaltic dykes, ranging in a N.W.-S.E.
+direction. But what is specially remarkable is the evidence afforded by
+an examination of the course of the Scuir, that it follows the channel
+of an ancient river-valley, which has been hollowed out in the surface
+of the plateau. The course of this channel is indicated by the presence
+of a deposit of river-gravel, which in some places forms a sort of
+cushion between the base of the Scuir and the side of the channel. Over
+this gravel-bed the viscous pitchstone-lava appears to have flowed,
+taking possession of the river-channel, and also of the beds of several
+small tributary streams which flowed into the channel of the Scuir. The
+recent date of the pitchstone forming this remarkable mural ridge, once
+occupying the bed of a river-channel, is shown by the fact that the
+basaltic dykes which traverse the plateau-basalts are truncated by the
+river-gravel, which is, therefore, more recent; and, as we have seen,
+the pitchstone stream is more recent than the river-gravel. But at the
+time when this last volcanic eruption took place, the physical geography
+of the whole region must have been very different from that of the
+present time. From the character and composition of the pebbles in the
+old river-bed, amongst which are Cambrian sandstone, quartzite,
+clay-slate, and white Jurassic limestone, Sir A. Geikie concludes that
+when the river was flowing, the island must have been connected with the
+mainland to the east where the parent masses of these pebbles are found.
+
+[Illustration: Fig. 34.--View of the Scuir of Eigg from the east. The
+lower portion of the mountain is formed of bedded basalt, or dolerite
+with numerous dykes and veins of basalt, felstone, and pitchstone; the
+upper cliff, or Scuir, is composed of pitchstone of newer age, the
+remnant of a lava flow which once filled a river channel in the basaltic
+sheets. A dyke, or sheet, of porphyry is seen to be interposed between
+the Scuir and the basaltic sheets.--(After Geikie.)]
+
+_Effects of Denudation._--The position of the Scuir of Eigg and its
+relations to the basaltic sheets show the enormous amount of denudation
+which these latter have undergone since the stream of pitchstone-lava
+filled the old river channel. The walls, or banks, of the channel have
+been denuded away, thus converting the pitchstone casting into a
+projecting wall of rock. That it originally extended outwards into the
+ocean to a far greater distance than at present is evident from the
+abruptly truncated face of the cliff; and yet this remarkable volcanic
+mass seems to have been, perhaps, the most recent exhibition of volcanic
+action to be found in the British Isles. It is perhaps, on this account,
+the most striking of the numerous examples exhibited throughout the West
+of Scotland and the North-east of Ireland of the enormous amount of
+denudation to which these districts have been subjected since the
+extinction of the volcanic fires; and this at a period to which we
+cannot assign a date more ancient than that of the Pliocene. Yet, let us
+consider for a moment to what physical vicissitudes these districts have
+been subjected since that epoch. Assuming, as we may with confidence,
+that the volcanic eruptions were subaerial, and that the tracts covered
+by the plateau-basalts were in the condition of dry land when the
+eruptions commenced, in this condition they continued in the main
+throughout the period of volcanic activity. But the eruptions had
+scarcely ceased, and the lava floods and dykes become consolidated,
+before the succeeding glacial epoch set in; when the snows and glaciers
+of the Scottish Highlands gradually descending from their original
+mountain heights, and spreading outwards in all directions, ultimately
+enveloped the whole of the region we are now considering until it was
+entirely concealed beneath a mantle of ice moving slowly, but
+irresistibly, outwards towards the Atlantic, crossing the deep channels,
+such as the Sound of Mull and the Minch, climbing up the sides of
+opposing rocks and islands until even the Outer Hebrides and the
+North-east of Ireland were covered by one vast mantle of ice and snow.
+The movement of such a body of ice over the land must have been attended
+with a large amount of abrasion of the rocky floor; nor have the
+evidences of that abrasion entirely disappeared even at the present day.
+We still detect the grooves and scorings on the rock-surfaces where they
+have been protected by a coating of boulder clay; and we still find the
+surface strewn with the blocks and _debris_ of that mighty ice-flood.
+
+But whatever may have been the amount of erosion caused by the great
+ice-sheet, it was chiefly confined to the more or less horizontal
+surface-planes. Erosion of another kind was to succeed, and to produce
+more lasting effects on the configuration of the surface. On the
+disappearance of the ice-sheet, an epoch characterised by milder
+conditions of climate set in. This was accompanied by subsidence and
+submersion of large tracts of the land during the Interglacial stage; so
+that the sea rose to heights of several hundred feet above the present
+level, and has left behind stratified gravels with shells at these
+elevations in protected places. During this period of depression and of
+subsequent re-emergence the wave-action of the Atlantic waters must have
+told severely on the coast and islands, wearing them into cliffs and
+escarpments, furrowing out channels and levelling obstructions. Such
+action has gone on down to the present day. The North-west of Scotland
+and of Ireland has been subjected throughout a very lengthened period to
+the wear and tear of the Atlantic billows. In the case of the former,
+the remarkable breakwater which nature has thrown athwart the North-west
+Highlands in the direction of the waves, forming the chain of islands
+constituting the Outer Hebrides, and composed of very tough Archaean
+gneiss and schist, has done much to retard the inroads which the waves
+might otherwise have made on the Isle of Skye; while Coll and Tiree,
+composed of similar materials, have acted with similar beneficent effect
+for Mull and the adjoining coasts. But such is the tremendous power of
+the Atlantic billows when impelled by westerly winds, that to their
+agency must be mainly attributed the small size of the volcanic
+land-surfaces as compared with their original extent, and the formation
+of those grand headlands which are presented by the igneous masses of
+Skye, Ardnamurchan, and Mull towards the west. Rain and river action,
+supplemented by that of glaciers, have also had a share in eroding
+channels and wearing down the upper surface of the ground, with the
+result we at present behold in the wild and broken scenery of the Inner
+Hebrides and adjoining coast.
+
+[1] Geikie, _loc. cit._, p. 178; also _Quart. Jour. Geol. Soc._, vol.
+xxvii. p. 303.
+
+
+
+
+CHAPTER VI.
+
+ISLE OF STAFFA.
+
+
+Reference has been made to this remarkable island in a former page, but
+some more extended notice is desirable before leaving the region of the
+Inner Hebrides. Along with the islands of Pladda, Treshnish, and
+Blackmore, Staffa is one of the outlying volcanic islands of the group,
+being distant about six miles from the coast of Mull, and indicates the
+minimum distance to which the plateau-basaltic sheets originally
+extended in the direction of the old marginal lands of Tiree and Coll.
+The island consists of successive sheets of bedded basaltic lava, with
+partings of tuff, one of which of considerable thickness is shown to lie
+at the base of the cliff on the south-west side of the island.[1] The
+successive lava-sheets present great varieties of structure, like those
+on the north coast of Antrim; some being amorphous, others columnar,
+with either straight or bent columns. The lava-sheet out of which
+Fingal's Cave is excavated consists of vertical prisms, beautifully
+formed, and surmounted by an amorphous mass of the same material. At the
+entrance of the Boat Cave we have a somewhat similar arrangement of the
+columns;[2] but at the Clam-shell Cave the prisms are curved, indicating
+some movement in the viscous mass before they had been fully
+consolidated.
+
+Fingal's Cave is called after the celebrated prince of Morvern (or
+Morven), a province of ancient Caledonia. He is supposed to have been
+the father of Ossian, the Celtic bard rendered famous by Macpherson. The
+cave, one of many which pierce the coast-cliffs of Western Scotland, is
+227 feet in length, 166 feet in height, and 40 feet in width. On all
+sides regular columns of basalt, some entire, others broken, rise out of
+the water and support the roof. The cave is only accessible in calm
+weather.
+
+[1] A drawing of this cliff is given by Geikie in the _Manual of
+Geology_ (Jukes and Geikie), 3rd edition, p. 277.
+
+[2] Prestwich, _Geology_, vol. i. p. 281, where a view of this cave is
+given.
+
+
+
+
+PART V.
+
+PRE-TERTIARY VOLCANIC ROCKS.
+
+
+
+
+CHAPTER I.
+
+THE DECCAN TRAP-SERIES OF INDIA.
+
+
+The great outpourings of augitic lava of Tertiary and recent times which
+we have been considering appear to have been anticipated in several
+parts of the world, more especially in Peninsular India and in Africa,
+and it is desirable that we should devote a few pages to the description
+of these remarkable volcanic formations, as they resemble, both in their
+mode of occurrence and general structure, some of the great lava-floods
+of a more recent period we have been considering. Of the districts to be
+described, the first which claims our notice is the Deccan.
+
+(_a._) _Extent of the Volcanic Plateau._--The volcanic plateau of the
+Deccan stretches from the borders of the Western Ghats and the sea-coast
+near Bombay inland to Amarantak, at the head of the Narbudda River
+(long. 82 deg. E.), and from Belgaum (lat. 15 deg. 31' N.) to near Goona (lat.
+24 deg. 30'). The vast area thus circumscribed is far from representing the
+original extent of the tract overspread by the lava-floods, as outlying
+fragments of these lavas are found as far east as long. 84 deg. E. in one
+direction, and at Kattiwar and Cutch in another. The present area,
+however, is estimated to be not less than 200,000 square miles.[1]
+
+(_b._) _Nature and Thickness of the Lava-flows._--This tract is
+overspread almost continuously by sheets of basaltic lava, with
+occasional bands of fresh-water strata containing numerous shells,
+figured and described by Hislop, and believed by him to be of Lower
+Eocene age. The lava-sheets vary considerably in character, ranging from
+finest compact basalt to coarsely crystalline dolerite, in which olivine
+is abundant. The columnar structure is not prevalent, the rock being
+either amorphous, or weathering into concentric shells. Volcanic ash, or
+bole, is frequently found separating the different lava-flows; and in
+the upper amygdaloidal sheets numerous secondary minerals are found,
+such as quartz, agate and jasper, stilbite and chlorite. The total
+thickness of the whole series, where complete, is about 6,000 feet,
+divided as follows:
+
+1. Upper trap; with ash and inter-trappean beds 1,500 feet
+2. Middle trap; sheets of basalt and ash 4,000 "
+3. Lower trap; basalt with inter-trappean beds 500 "
+ --------
+ 6,000 "
+ ========
+
+Throughout the region here described these great sheets of volcanic rock
+are everywhere approximately horizontal, and constitute a table-land of
+3,000 to 4,000 feet in elevation, breaking off in terraced escarpments,
+and penetrated by deep river-valleys, of which the Narbudda is the most
+important. The foundation rock is sometimes metamorphic schist, or
+gneiss, at other times sandstone referred by Hislop to Jurassic age; and
+in no single instance has a volcanic crater or focus of eruption been
+observed. But outside the central trappean area volcanic foci are
+numerous, as in Cutch, the Rajhipla Hills and the Lower Narbudda valley.
+The original excessive fluidity of the Deccan trap is proved by the
+remarkable horizontality of the beds over large areas, and the extensive
+regions covered by very thin sheets of basalt or dolerite.
+
+(_c._) _Geological Age._--As regards the geological age of this great
+volcanic series much uncertainty exists, owing to the absence of marine
+forms in the inter-trappean beds. One single species, _Cardita
+variabilis_, has been observed as occurring in these beds, and in the
+limestone below the base of the trap at Dudukur. The _facies_ of the
+forms in this limestone is Tertiary; but there is a remarkable absence
+of characteristic genera. On the other hand, Mr. Blanford states that
+the bedded traps are seen to underlie the Eocene Tertiary strata with
+_Nummulites_ in Guzerat and Cutch,[2] which would appear to determine
+the limit of their age in one direction. On balancing the evidence,
+however, it is tolerably clear that the volcanic eruptions commenced
+towards the close of the Cretaceous period, and continued into the
+commencement of the Tertiary, thus bridging over the interval between
+the two epochs; and since the greater sheets have been exposed
+throughout the whole of the Tertiary and Quarternary periods, it is not
+surprising if they have suffered enormously from denuding agencies, and
+that any craters or cones of eruption that may once have existed have
+disappeared.
+
+[1] The Deccan Traps have been described by Sykes, _Geol. Trans._, 2nd
+Series, vol. iv.; also Rev. S. Hislop, "On the Geology of the
+Neighbourhood of Nagpur, Central India," _Quart. Journ. Geol. Soc._,
+vol. x. p. 274; and _Ibid._, vol. xvi. p. 154. Also, H. B. Medlicott and
+W. T. Blanford, _Manual of the Geology of India_, vol. i. (1879).
+
+[2] Blanford, _Geology of Abyssinia_, p. 185.
+
+
+
+
+CHAPTER II.
+
+ABYSSINIAN TABLE-LANDS.
+
+
+Another region in which the volcanic phenomena bear a remarkable analogy
+to those of Central India, just described, is that of Abyssinia. Nor are
+these tracts so widely separated that they may not be considered as
+portions of one great volcanic area extending from Abyssinia, through
+Southern Arabia, into Cutch and the Deccan, in the one direction, while
+the great volcanic cones of Kenia and Kilimanjaro, with their
+surrounding tracts of volcanic matter, may be the extreme prolongations
+in the other. Along this tract volcanic operations are still active in
+the Gulf of Aden; and cones quite unchanged in form, and evidently of
+very recent date, abound in many places along the coast both of Arabia
+and Africa. The volcanic formations of this tract are, however, much
+more recent than those which occupy the high plateaux of Central and
+Southern Abyssinia of which we are about to speak.
+
+(_a._) _Physical Features._--Abyssinia forms a compact region of lofty
+plateaux intersected by deep valleys, interposed between the basin of
+the Nile on the west, and the low-lying tract bordering the Red Sea and
+the Indian Ocean on the east. The plateaux are deeply intersected by
+valleys and ravines, giving birth to streams which feed the head waters
+of the Blue Nile (Bahr el Arak) and the Atbara. Several fine lakes lie
+in the lap of the mountains, of which the Zana, or Dembia, is the
+largest, and next Ashangi, visited by the British army on its march to
+Magdala in 1868, and which, from its form and the volcanic nature of the
+surrounding hills, appears to occupy the hollow of an extinct crater.
+The table-land of Abyssinia reaches its highest elevation along the
+eastern and southern margin, where its average height may be 8,000 to
+10,000 feet; but some peaks rise to a height of 12,000 to 15,000 feet in
+Shoa and Ankobar.[1]
+
+(_b._) _Basaltic Lava Sheets._--An enormous area of this country seems
+to be composed of volcanic rocks chiefly in the form of sheets of
+basaltic lava, which rise into high plateaux, and break off in
+steep--sometimes precipitous--mural escarpments along the sides of the
+valleys. These are divisible into the following series:--
+
+(1) _The Ashangi Volcanic Series._--The earliest forerunners of the more
+recent lavas seem to have been erupted in Jurassic times, in the form of
+sheets of contemporaneous basalt or dolerite amongst the Antola
+limestones which are of this period. But the great mass of the volcanic
+rocks are much more recent, and may be confidently referred to the late
+Cretaceous or early Tertiary epochs. Their resemblance to the great
+trappean series of Western India, even in minute particulars, is
+referred to by Mr. Blanford, who suggests the view that they belong to
+one and the same great series of lava-flows extruded over the surface of
+this part of the globe. This view is inherently probable. They consist
+of basalts and dolerites, generally amygdaloidal, with nodules of agate
+and zeolite, and are frequently coated with green-earth (chlorite). Beds
+of volcanic ash or breccia also frequently occur, and often contain
+augite crystals. At Senafe, hills of trachyte passing into claystone and
+basalt were observed by Mr. Blanford, but it is not clear what are their
+relations to the plateau-basaltic sheets.[2]
+
+(2) _Magdala Volcanic Series._--This is a more recent group of volcanic
+lavas, chiefly distinguished from the lower, or Ashangi, group, by the
+occurrence of thick beds of trachyte, usually more or less crystalline,
+and containing beautiful crystals of sanidine. The beds of trachyte
+break off in precipitous scarps, and being of great thickness and
+perfectly horizontal, are unusually conspicuous. Mr. Blanford says, with
+regard to this group, that there is a remarkable resemblance in its
+physical aspect to the scenery of the Deccan and the higher valleys of
+the Western Ghats of India, but the peculiarities of the landscape are
+exaggerated in Abyssinia. Many of the trachytic beds are brecciated and
+highly columnar; sedimentary beds are also interstratified with those of
+volcanic origin. The Magdala group is unconformable to that of Ashangi
+in some places. A still more recent group of volcanic rocks appears to
+occur in the neighbourhood of Senafe, consisting of amorphous masses of
+trachyte, often so fine-grained and compact as to pass into claystone
+and to resemble sandstone. At Akub Teriki the rocks appear to be in the
+immediate vicinity of an ancient vent of eruption.
+
+From what has been said, it will be apparent that Abyssinia offers
+volcanic phenomena of great interest for the observer. There is
+considerable variety in the rock masses, in their mode of distribution,
+and in the scenery which they produce. The extensive horizontal sheets
+of lava are suggestive of fissure-eruption rather than of eruption
+through volcanic craters; and although these may have once been in
+existence, denudation has left no vestiges of them at the present day.
+In all these respects the resemblance of the volcanic phenomena to those
+of Peninsular India is remarkably striking; it suggests the view that
+they are contemporaneous as regards the time of their eruption, and
+similar as regards their mode of formation.
+
+[1] W. T. Blanford, _Geology of Abyssinia_, pp. 151-2.
+
+[2] Blanford, _loc. cit._, p. 182.
+
+
+
+
+CHAPTER III.
+
+CAPE COLONY.
+
+
+_Basalt of the Plateau._--The extensive sheets of plateau-basalt forming
+portions of the Neuweld range and the elevated table-land of Cape
+Colony, may be regarded as forerunners of those just described, and
+possibly contemporaneous with the Ashangi volcanic series of Abyssinia.
+The great basaltic sheets of the Cape Colony are found capping the
+highest elevations of the Camderboo and Stormberg ranges, as well as
+overspreading immense areas of less elevated land, to an extent,
+according to Professor A. H. Green, of at least 120,000 square miles.[1]
+Amongst these sheets, innumerable dykes, and masses of solid lava which
+filled the old vents of eruption, are to be observed. The floor upon
+which the lava-floods have been poured out generally consists of the
+"Cave Sandstone," the uppermost of a series of deposits which had
+previously been laid down over the bed of an extensive lake which
+occupied this part of Africa during the Mesozoic period. After the
+deposition of this sandstone, the volcanic forces appear to have burst
+through the crust, and from vents and fissures great floods of augitic
+lava, with beds of tuff, invaded the region occupied by the waters of
+the lake. The lava-sheets have since undergone extensive denudation, and
+are intersected by valleys and depressions eroded down through them into
+the sandstone floor beneath; and though the precise geological period at
+which they were extruded must remain in doubt, it appears probable that
+they may be referred to that of the Trias.[2]
+
+[1] Green. "On the Geology of the Cape Colony," _Quart. Jour. Geol.
+Soc._, vol. xliv. (1888).
+
+[2] The district lying along the south coast of Africa is described by
+Andrew G. Bain, in the _Trans. Geol. Soc._, vol. vii. (1845); but there
+is little information regarding the volcanic region here referred to.
+
+
+
+
+CHAPTER IV.
+
+VOLCANIC ROCKS OF PAST GEOLOGICAL PERIODS OF THE BRITISH ISLES.
+
+
+It is beyond the scope of this work to describe the volcanic rocks of
+pre-Tertiary times over various parts of the globe. The subject is far
+too large to be treated otherwise than in a distinct and separate essay.
+I will therefore content myself with a brief enumeration of the
+formations of the British Isles in which contemporaneous volcanic action
+has been recognised.[1]
+
+There is little evidence of volcanic action throughout the long lapse of
+time extending backwards from the Cretaceous to the Triassic epochs,
+that is to say, throughout the Mesozoic or Secondary period, and it is
+not till we reach the Palaeozoic strata that evidence of volcanic action
+unmistakably presents itself.
+
+_Permian Period._--In Ayrshire, and in the western parts of Devonshire,
+beds of felspathic porphyry, felstone and ash are interstratified with
+strata believed to be of Permian age. In Devonshire these have only
+recently been recognised by Dr. Irving and the author as of Permian age,
+the strata consisting of beds of breccia, lying at the base of the New
+Red Sandstone. Those of Ayrshire have long been recognised as of the
+same period; as they rest unconformably on the coal measures, and
+consist of porphyrites, melaphyres, and tuffs of volcanic origin.
+
+_Carboniferous Period._--Volcanic rocks occur amongst the coal-measures
+of England and Scotland, while they are also found interbedded with the
+Carboniferous Limestone series in Derbyshire, Scotland, and Co. Limerick
+in Ireland. The rocks consist chiefly of basalt, dolerite, melaphyre and
+felstone.
+
+_Devonian Period._--Volcanic rocks of Devonian age occur in the South of
+Scotland, consisting of felstone-porphyries and melaphyres; also at
+Boyle, in Roscommon, and amongst the Glengariff beds near Killarney in
+Ireland.
+
+_Upper Silurian Period._--Volcanic rocks of this stage are only known in
+Ireland, on the borders of Cos. Mayo and Galway, west of Lough Mask, and
+at the extreme headland of the Dingle Promontory in Co. Kerry. They
+consist of porphyrites, felstones and tuffs, or breccias,
+contemporaneously erupted during the Wenlock and Ludlow stages. Around
+the flanks of Muilrea, beds of purple quartz-felstone with tuff are
+interstratified with the Upper Silurian grits and slates.
+
+_Lower Silurian Period._--Volcanic action was developed on a grand scale
+during the Arenig and Caradoc-Bala stages, both in Wales and the Lake
+district, and in the Llandeilo stage in the South of Scotland. The
+felspathic lavas, with their associated beds of tuff and breccia, rise
+into some of the grandest mountain crests of North Wales, such as those
+of Cader Idris, Aran Mowddwy, Arenig and Moel Wyn. A similar series is
+also represented in Ireland, ranging from Wicklow to Waterford, forming
+a double group of felstones, porphyries, breccias, and ash-beds, with
+dykes of basalt and dolerite. The same series again appears amidst the
+Lower Silurian beds of Co. Louth, near Drogheda.
+
+_Metamorphic Series presumably of Lower Silurian Age._--If, as seems
+highly probable, the great metamorphic series of Donegal and Derry are
+the representatives in time of the Lower Silurian series, some of the
+great sheets of felspathic and hornblendic trap which they contain are
+referable to this epoch. These rocks have undergone a change in
+structure along with the sedimentary strata of which they were
+originally formed, so that the sheets of (presumably) augitic lava have
+been converted into hornblende-rock and schist. Similar masses occur in
+North Mayo, south of Belderg Harbour.
+
+_Cambrian Period._--In the Pass of Llanberis, along the banks of Llyn
+Padarn, masses of quartz-porphyry, felsite and agglomerate, or breccia,
+indicate volcanic action during this stage. These rocks underlie beds of
+conglomerate, slate and grit of the Lower Cambrian epoch, and, as Mr.
+Blake has shown, are clearly of volcanic origin, and pass upwards into
+the sedimentary strata of the period. A similar group, first recognised
+by Professor Sedgwick, stretches southwards from Bangor along the
+southern shore of the Menai Straits. Again, we find the volcanic
+eruptions of this epoch at St. David's, consisting of diabasic and
+felsitic lava, with beds of ash; and in the centre of England, amongst
+the grits and slates of Charnwood Forest presumably of Cambrian age,
+various felstones, porphyries, and volcanic breccias are found.
+
+Thus it will be seen that every epoch, from the earliest stage of the
+Cambrian to the Permian, in the British Isles, gives evidence of the
+existence of volcanic action; from which we may infer that the
+originating cause, whatever it may be, has been in operation throughout
+all past geological time represented by living forms. The question of
+the condition of our globe in Archaean times, and earlier, is one which
+only can be discussed on theoretic ground, and is beyond the scope of
+this work.
+
+[1] The reader is referred to Sir A. Geikie's Presidential Address to
+the Geological Society (1891) for the latest view of this subject.
+
+[Illustration: VOLCANIC BAND OF THE MOLUCCAS.
+Map showing the volcanic belt to which Krakatoa belongs. The shaded
+portion is volcanic.]
+
+
+
+
+PART VI.
+
+SPECIAL VOLCANIC AND SEISMIC PHENOMENA.
+
+
+
+
+CHAPTER I.
+
+THE ERUPTION OF KRAKATOA IN 1883.
+
+
+I propose to introduce here some account of one of the most terrible
+outbursts of volcanic action that have taken place in modern times;
+namely, the eruption of the volcano of Krakatoa (a corruption of Rakata)
+in the strait of Sunda, between the islands of Sumatra and Java, in the
+year 1883. The Malay Archipelago, of which this island once formed a
+member, is a region where volcanic action is constant, and where the
+outbursts are exceptionally violent. With the great island of Borneo as
+a solid, non-volcanic central core, a line of volcanic islands extends
+from Chedooba off the coast of Pegu through Sumatra, Java, Sumbawa,
+Flores, and, reaching the Moluccas, stretches northwards through the
+Philippines into Japan and Kamtschatka. This is probably the most active
+volcanic belt in the world, and the recent terrible earthquake and
+eruption in Japan (November, 1891) gives proof that the volcanic forces
+are as powerful and destructive as ever.[1]
+
+(_a._) _Dormant Condition down to 1680._--Down to the year 1680, this
+island, although from its form and structure evidently volcanic, appears
+to have been in a dormant state; its sides were covered with luxuriant
+forests, and numerous habitations dotted its shore. But in May of that
+year an eruption occurred, owing to which the aspect of Krakatoa as
+described by Vogel was entirely changed; the surface of the island when
+this writer passed on his voyage to Sumatra appeared burnt up and arid,
+while blocks of incandescent rock were being hurled into the air from
+four distinct points. After this first recorded eruption the island
+relapsed into a state of repose, and except for a stream of molten lava
+which issued from the northern extremity, there was no evidence of its
+dangerous condition. The luxuriant vegetation of the tropics speedily
+re-established itself, and the volcano was generally regarded as
+"extinct."[2] History repeats itself; and the history of Vesuvius was
+repeated in the case of Krakatoa.
+
+[Illustration: Fig. 35.--Map Of The Krakatoa Group Of Islands Before
+The Eruption Of August 1883 (From Admiralty Chart)]
+
+(_b._) _Eruption of May, 1883._[3]--On the morning of May 20, 1883, the
+inhabitants of Batavia, of Buitenzorg, and neighbouring localities, were
+surprised by a confused noise, mingled with detonations resembling the
+firing of artillery. The phenomena commenced between ten and eleven
+o'clock in the morning, and soon acquired such intensity as to cause
+general alarm. The detonations were accompanied by tremblings of the
+ground, of buildings and various objects contained in dwellings; but it
+was generally admitted that these did not proceed from earthquake
+shocks, but from atmospheric vibrations. No deviation of the magnetic
+needle was observed at the Meteorological Institute of Batavia; but a
+vertical oscillation was apparent, and persons who listened with the ear
+placed on the ground, even during the most violent detonations, could
+hear no subterranean noise whatever. It became clear that the sounds
+came from some volcano burst into activity; but it is strange that for
+two whole days it remained uncertain what was the particular volcano to
+which the phenomena were to be referred. The detonations appeared,
+indeed, to come from the direction of Krakatoa; but from Serang, Anjer,
+and Merak, localities situated much nearer Krakatoa than Batavia, the
+telegraph announced that neither detonations nor atmospheric vibrations
+had been perceived. The distance between Batavia and Krakatoa is
+ninety-three English miles. The doubts thus experienced were, however,
+soon put to rest by the arrival of an American vessel under the command
+of A. R. Thomas, and of other ships which hailed from the straits of
+Sunda. From their accounts it was ascertained that in the direction of
+Krakatoa the heavens were clouded with ashes, and that a grand column of
+smoke, illumined from time to time by flashes of flame, arose from above
+the island. Thus after a repose of more than two hundred years, "the
+peaceable isle of Krakatoa, inhabited, and covered by thick forests, was
+suddenly awakened from its condition of fancied security."
+
+[Illustration: Fig. 36.--Section from Verlaten Island through Krakatoa,
+to show the outline before and after the eruption of August, 1888. The
+continuous line shows the former; the dotted line and shading, the
+latter; from which it will be observed that the original island has to a
+large extent disappeared. The line of section is shown in Fig. 35.]
+
+(_c._) _Form and Appearance of the Island before the Eruption of
+1883._--From surveys made in 1849 and 1881, it would appear that the
+island of Krakatoa consisted of three mountains or groups of mountains
+(Figs. 35, 36); the southern formed by the cone of Rakata (properly so
+called), rising with a scarped face above the sea to a height of over
+800 metres (2,622 feet). Adjoining this cone, and rising from the centre
+of the island, came the group of Danan, composed of many summits,
+probably forming part of the _enceinte annulaire_ of a crater. And near
+the northern extremity of the isle, a third group of mammelated heights
+could be recognised under the general name of Perboewatan, from which
+issued several obsidian lava-flows, with a steep slope; these dated back
+perhaps to the period of the first known eruption of 1680. This large
+and mountainous island as it existed at the beginning of May, 1883, has
+been entirely destroyed by the terrible eruptions of that year, with the
+exception of the peripheric rim (composed of the most ancient of the
+volcanic rocks, andesite), of which Verlaten Island and Rakata formed a
+part, and one very small islet, which is noted on the maps as "rots"
+(rock), and on the new map of the Straits of Sunda of the Dutch Navy as
+that of "Bootsmansrots."[4]
+
+As shown by the map in the Report of the Royal Society, the group of
+islands which existed previous to 1883 were but the unsubmerged portions
+of one vast volcanic crater, built up of a remarkable variety of lava
+allied to the andesite of the Java volcanoes, but having a larger
+percentage of silica, and hence falling under the head of
+"enstatite-dacite."[5] That these volcanic rocks are of very recent
+origin is shown by the fact, ascertained by Verbeek, that beneath them
+occur deposits of Post-Tertiary age, and that these in turn rest on the
+Tertiary strata which are widely distributed through Sumatra, Java, and
+the adjoining islands. According to the reasoning of Professor Judd, the
+Krakatoa group at an early period of its history presented the form of a
+magnificent crater-cone, several miles in circumference at the base,
+which subsequent eruptions shattered into fragments or blew into the
+air in the form of dust, ashes, and blocks of lava, while the central
+part collapsed and fell in, leaving a vast circular ring like the
+ancient crater of Somma (see Fig. 6, p. 43), and he supposes the former
+eruptions to have been on a scale exceeding in magnificence those which
+have caused such world-wide interest within the last few years.
+
+(_d._) _Eruption of 26th to 28th of August._--It was, as we have seen,
+in the month of May that, in the language of Chev. Verbeek, "the volcano
+of Krakatoa chose to announce in a high voice to the inhabitants of the
+Archipelago that, although almost nothing amongst the many colossal
+volcanic mountains of the Indies, it yielded to none of them in regard
+to its power." These eruptions were, however, only premonitory of the
+tremendous and terrible explosion which was to commence on Sunday, the
+26th of August, and which continued for several days subsequently. A
+little after noon of that day, a rumbling noise accompanied by short and
+feeble explosions was heard at Buitenzorg, coming from the direction of
+Krakatoa; and similar sounds were heard at Anjer and Batavia a little
+later. Soon these detonations augmented in intensity, especially about
+five o'clock in the evening; and news was afterwards received that the
+sounds had been heard in the isle of Java. These sounds increased still
+more during the night, so that few persons living on the west side of
+the isle of Java were able to sleep. At seven in the morning there came
+a crash so formidable, that those who had hoped for a little sleep at
+Buitenzorg leaped from their beds. Meanwhile the sky, which had up to
+this time been clear, became overcast, so that by ten o'clock it became
+necessary to have recourse to lamps, and the air became charged with
+vapour. Occasional shocks of earthquake were now felt. Darkness became
+general all over the straits and the bordering coasts. Showers of ashes
+began to fall. The repeated shocks of earthquake, and the rapid
+discharges of subterranean artillery, all combined to show that an
+eruption of even greater violence than that of May was in progress at
+the isle of Krakatoa.
+
+But the most interested witnesses to this terrible outburst were those
+on board the ships plying through the straits. Amongst these was the
+_Charles Bal_, a British vessel under the command of Captain Watson.
+This ship was ten miles south of the volcano on Sunday afternoon, and
+therefore well in sight of the island at the time when the volcano had
+entered upon its paroxysmal state of action. Captain Watson describes
+the island as being covered by a dense black cloud, while sounds like
+the discharges of artillery occurred at intervals of a second of time;
+and a crackling noise (probably arising from the impact of fragments of
+rock ascending and descending in the atmosphere) was heard by those on
+board. These appearances became so threatening towards five o'clock in
+the evening, that the commander feared to continue his voyage and began
+to shorten sail. From five to six o'clock a rain of pumice in large
+pieces, quite warm, fell upon the ship, which was one of those that
+escaped destruction during this terrible night.[6]
+
+(_e._) _Electrical Phenomena._--During this eruption, electrical
+phenomena of great splendour were observed. Captain Wooldbridge,
+viewing the eruption in the afternoon of the 26th from a distance of
+forty miles, speaks of a great vapour-cloud looking like an immense wall
+being momentarily lighted up "by bursts of forked lightning like large
+serpents rushing through the air. After sunset this dark wall resembled
+a blood-red curtain, with edges of all shades of yellow, the whole of a
+murky tinge, through which gleamed fierce flashes of lightning." As
+Professor Judd observes, the abundant generation of atmospheric
+electricity is a familiar phenomenon in all volcanic eruptions on a
+grand scale. The steam-jets rushing through the orifices of the earth's
+crust constitute an enormous hydro-electrical engine, and the friction
+of the ejected materials striking against one another in their ascent
+and descent also does much in the way of generating electricity.[7] It
+has been estimated by several observers that the column of watery vapour
+ascended to a height of from twelve to seventeen and even twenty-three
+miles; and on reaching the upper strata of the atmosphere, it spread
+itself out in a vast canopy resembling "the pine-tree" form of Vesuvian
+eruptions; and throughout the long night of the 27th this canopy
+continued to extend laterally, and the particles of dust which it
+enclosed began to descend slowly through the air.
+
+(_f._) _Formation of Waves._--This tremendous outburst of volcanic
+forces, which to a greater or less extent influenced the entire surface
+of the globe, gave rise to waves which traversed both air and ocean; and
+in consequence of the large number of observatories scattered all over
+the globe, and the excellence and delicacy of the instruments of
+observation, we are put in possession of the remarkable results which
+have been obtained from the collection of the observations in the hands
+of competent specialists. The results are related _in extenso_ in the
+Report of the Royal Society, illustrated by maps and diagrams, and are
+worthy of careful study by those interested in terrestrial phenomena. A
+brief summary is all that can be given here, but it will probably
+suffice to bring home to the reader the magnitude and grandeur of the
+eruption.
+
+The vibrations or waves generated in August, 1883, at Krakatoa may be
+arranged under three heads: (1) Atmospheric Waves; (2) Sound Waves; and
+(3) Oceanic Waves; which I will touch upon in the order here stated.
+
+(1) _Atmospheric Waves._--These phenomena have been ably handled by
+General Strachey,[8] from a large number of observations extending all
+over the globe. From these it has been clearly established that an
+atmospheric wave, originating at Krakatoa as a centre, expanded outwards
+in a circular form and travelled onwards till it became a great circle
+at a distance of 180 degrees from its point of origin, after which it
+still advanced, but now gradually contracting to a node at the antipodes
+of Krakatoa; that is to say, at a point over the surface of North
+America, situated in lat. 6 deg. N. and long. 72 deg. W. (or thereabout). Having
+attained this position, the wave was reflected or reproduced, expanding
+outwards for 180 degrees and travelling backwards again to Krakatoa,
+from which it again started, and returning to its original form again
+overspread the globe. This wonderful repetition, due to the spherical
+form of the earth, was observed no fewer than seven times, though with
+such diminished force as ultimately to be outside the range of
+observation by the most sensitive instruments. It is one of the triumphs
+of modern scientific appliances that the course of such a wave,
+generated in a fluid surrounding a globe, which might be demonstrated on
+mathematical principles, has been actually determined by experiments
+carried on over so great an area.
+
+(2) _Sound Waves._--If the sound-waves produced at the time of maximum
+eruption were not quite as far-reaching as those of the air, they were
+certainly sufficiently surprising to be almost incredible, were it not
+that they rest, both as regards time and character, upon incontestible
+authority. The sound of the eruption, resembling that of the discharge
+of artillery, was heard not only over nearly all parts of Sumatra, Java,
+and the coast of Borneo opposite the Straits of Sunda, but at places
+over two thousand miles distant from the scene of the explosions.
+Detailed accounts, collected with great care, are given in the Report of
+the Royal Society, from which the following are selected as examples:--
+
+ 1. At the port of Acheen, at the northern extremity of Sumatra,
+ distant 1,073 miles, it was supposed that the port was being
+ attacked, and the troops were put under arms.
+
+ 2. At Singapore, distant 522 miles, two steamers were dispatched to
+ look out for the vessel which was supposed to be firing guns as
+ signals of distress.
+
+ 3. At Bankok, in Siam, distant 1,413 miles, the report was heard on
+ the 27th; as also at Labuan, in Borneo, distant 1,037 miles.
+
+ 4. At places in the Philippine Islands, distant about 1,450 miles,
+ detonations were heard on the 27th, at the time of the eruption.
+
+The above places lie northwards of Krakatoa. In the opposite direction,
+we have the following examples:--
+
+ 5. At Perth, in Western Australia, distant 1,092 miles, sounds as
+ of guns firing at sea were heard; and at the Victorian Plains,
+ distant about 1,700 miles, similar sounds were heard.
+
+ 6. In South Australia, at Alice's Springs, Undoolga, and other
+ places at distances of over 2,000 miles, the sounds of the eruption
+ were also heard.
+
+ 7. In a westerly direction at Dutch Bay, Ceylon, distant 2,058
+ miles, the sounds were heard between 7 a.m. and 10 a.m. on the
+ morning of the 27th of August.
+
+ 8. Lastly, at the Chagos Islands, distant 2,267 miles, the
+ detonations were audible between 10 and 11 a.m. of the same day.
+
+Some of the above distances are so great that we may fail to realise
+them; but they will be more easily appreciated, perhaps, if we change
+the localities to our own side of the globe, and take two or three cases
+with similar distances. Then, if the eruption had taken place amongst
+the volcanoes of the Canaries, the detonations would have been heard at
+Gibraltar, at Lisbon, at Portsmouth, Southampton, Cork, and probably at
+Dublin and Liverpool; or, again, supposing the eruption had taken place
+on the coast of Iceland, the report would have been heard all over the
+western and northern coasts of the British Isles, as well as at
+Amsterdam and the Hague. The enormous distance to which the sound
+travelled in the case of Krakatoa was greatly due to the fact that the
+explosions took place at the surface of the sea, and the sound was
+carried along that surface uninterruptedly to the localities recorded; a
+range of mountains intervening would have cut off the sound-wave at a
+comparatively short distance from its source.
+
+(3) _Oceanic Waves._--As may be supposed, the eruption gave rise to
+great agitation of the ocean waters with various degrees of vertical
+oscillation; but according to the conclusions of Captain Wharton,
+founded on numerous data, the greatest wave seems to have originated at
+Krakatoa about 10 a.m. on the 27th of August, rising on the coasts of
+the Straits of Sunda to a height of fifty feet above the ordinary
+sea-level. This wave appears to have been observed over at least half
+the globe. It travelled westwards to the coast of Hindostan and Southern
+Arabia, ultimately reaching the coasts of France and England. Eastwards
+it struck the coast of Australia, New Zealand, the Sandwich Islands,
+Alaska, and the western coast of North America; so that it was only the
+continent of North and South America which formed a barrier (and that
+not absolute) to the circulation of this oceanic wave all over the
+globe. The destruction to life and property caused by this wave along
+the coasts of Sunda was very great. Combined with the earthquake shocks
+(which, however, were not very severe), the tremendous storm of wind,
+the fall of ashes and cinders, and the changes in the sea-bed, it
+produced in the Straits of Sunda for some time after the eruption a
+disastrous transformation. Lighthouses had been swept away; all the old
+familiar landmarks on the shore were obscured by a vast deposit of
+volcanic dust; the sea itself was encumbered with enormous quantities of
+floating pumice, in many places of such thickness that no vessel could
+force its way through them; and for months after the eruption one of the
+principal channels was greatly obstructed by two islands which had
+arisen in its midst. The Sebesi channel was completely blocked by banks
+composed of volcanic materials, and two portions of these banks rose
+above the sea as islands, which received the name of "Steers Island" and
+"Calmeyer Island"; but these, by the action of the waves, have since
+been completely swept away, and the materials strewn over the bed of the
+sea.[9]
+
+(_g._) _Atmospheric Effects._--But the face of nature, even in her most
+terrific and repulsive aspect, is seldom altogether unrelieved by some
+traces of beauty. In contrast to the fearful and disastrous phenomena
+just described, is to be placed the splendour of the heavens, witnessed
+all over the central regions of the globe throughout a period of several
+months after the eruption of 1883, which has been ably treated by the
+Hon. Rollo Russell and Mr. C. D. Archibald, in the Royal Society's
+Report.
+
+When the particles of lava and ashes mingled with vapour were projected
+into the air with a velocity greater than that of a ball discharged from
+the largest Armstrong gun, these materials were carried by the prevalent
+trade-winds in a westerly direction, and some of them fell on the deck
+of ships sailing in the Indian Ocean as far as long. 80 deg. E., as in the
+case of the _British Empire_--on which the particles fell on the 29th of
+August, at a distance of 1,600 miles from Krakatoa. But far beyond this
+limit, the finer particles of dust (or rather minute crystals of felspar
+and other minerals), mingled with vapour of water, were carried by the
+higher currents of the air as far as the Seychelles and Africa,--not
+only the East coast, but also the West, as Cape Coast Castle on the Gold
+Coast; to Paramaribo, Trinidad, Panama, the Sandwich Isles, Ceylon and
+British India, at all of which places during the month of September the
+sun assumed tints of blue or green, as did also the moon just before and
+after the appearance of the stars;[10] and from the latter end of
+September and for several months, the sky was remarkable for its
+magnificent coloration; passing from crimson through purple to yellow,
+and melting away in azure tints which were visible in Europe and the
+British Isles; while a large corona was observed round both the sun and
+moon. These beautiful sky effects were objects of general observation
+throughout the latter part of the year 1883 and commencement of the
+following year.
+
+The explanation of these phenomena may be briefly stated. The fine
+particles, consisting for the most part of translucent crystals, or
+fragments of crystals, formed a canopy high up in the atmosphere, being
+gradually spread over both sides of the equator till it formed a broad
+belt, through which the rays of the sun and moon were refracted. Towards
+dawn and sunset they were refracted and reflected from the facets of the
+crystal, and thus underwent decomposition into the prismatic colours; as
+do the rays of the sun when refracted and reflected from the particles
+of moisture in a rain-cloud. The subject is one which cannot be fully
+dealt with here, and is rather outside the scope of this work.
+
+(_h._) _Origin of the Eruption._--The ultimate cause of volcanic
+eruptions is treated in a subsequent chapter, nor is that of Krakatoa
+essentially different from others. It was remarkable, however, both for
+the magnitude of the forces evoked and the stupendous scale of the
+resulting phenomena. It is evident that water played an important part
+in these phenomena, though not as the prime mover;--any more than water
+in the boiler of a locomotive is the prime mover in the generation of
+the steam. Without the fuel in the furnace the steam would not be
+produced; and the amount of steam generated will be proportional to the
+quantity and heat of the fuel in the furnace and the quantity of water
+in the boiler. In the case of Krakatoa, both these elements were
+enormous and inexhaustible. The volcanic chimney (or system of
+chimneys), being situated on an island, was readily accessible to the
+waters of the ocean when fissures gave them access to the interior
+molten matter. That such fissures were opened we may well believe. The
+earthquakes which occurred at the beginning of May, and later on, on the
+27th of that month, may indicate movements of the crust by which water
+gained access. It appears that in May the only crater in a state of
+activity was that of Perboewatan; in June another crater came into
+action, connected with Danan in the centre of the island, and in August
+a third burst forth. Thus there was progressive activity up to the
+commencement of the grand eruption of the 26th of that month.[11] During
+this last paroxysmal stage, the centre of the island gave way and sunk
+down, when the waters of the ocean gained free access, and meeting with
+the columns of molten matter rising from below originated the prodigious
+masses of steam which rose into the air.
+
+(_i._) _Cause of the Detonations._--The detonations which accompanied
+the last great eruption are repeatedly referred to in all the accounts.
+These may have been due, not only to the sudden explosions of steam
+directly produced by the ocean water coming in contact with the molten
+lava, but by dissociation of the vapour of water at the critical point
+of temperature into its elements of oxygen and hydrogen; the reunion of
+these elements at the required temperature would also result in
+explosions.
+
+The phenomena attending this great volcanic eruption, so carefully
+tabulated and critically examined, will henceforth be referred to as
+constituting an epoch in the history of volcanic action over the globe,
+and be of immense value for reference and comparison.
+
+[1] The eruption of Krakatoa has been the subject of an elaborate Report
+published by the Royal Society, and is also described in a work by
+Chevalier R. D. M. Verbeek, Ingenieur en Chef des Mines, and published
+by order of the Governor-General of the Netherland Indies (1886). See
+also an Article by Sir R. S. Ball in the _Contemporary Review_ for
+November, 1888.
+
+[2] Verbeek, _loc. cit._, p. 4.
+
+[3] The account of this eruption is a free translation from Verbeek.
+
+[4] Verbeek, _loc. cit._, p. 160.
+
+[5] Judd, _Rep. R. S._
+
+[6] A fuller account by Prof. Judd will be found in the _Report of the
+Royal Society_, p. 14. Several vessels at anchor were driven ashore on
+the straits owing to the strong wind which arose.
+
+[7] Judd, _Report_, p. 21.
+
+[8] _Report_, Part ii.
+
+[9] In this eruption, 36,380 human beings perished, of whom 37 were
+Europeans; 163 villages (_kampoengs_) were entirely, and 132 partially,
+destroyed.--Verbeek, _loc. cit._, p. 79.
+
+[10] Verbeek, _loc. cit._, p. 144-5. The dust put a girdle round the
+earth in thirteen days.
+
+[11] Verbeek, _loc. cit._, p. 30.
+
+
+
+
+CHAPTER II.
+
+EARTHQUAKES.
+
+
+_Connection of Earthquakes with Volcanic Action._--The connection
+between earthquake shocks and volcanic eruptions is now so generally
+recognised that it is unnecessary to insist upon it here. All volcanic
+districts over the globe are specially liable to vibrations of the
+crust; but at the same time it is to be recollected that these movements
+visit countries occasionally from which volcanoes, either recent or
+extinct, are absent; in which cases we may consider earthquake shocks to
+be abortive attempts to originate volcanic action.
+
+(_a._) _Origin._--From the numerous observations which have been made
+regarding the nature of these phenomena by Hopkins, Lyell, and others,
+it seems clearly established that earthquakes have their origin in some
+sudden impact of gas, steam, or molten matter impelled by gas or steam
+under high pressure, beneath the solid crust.[1] How such impact
+originates we need not stop to inquire, as the cause is closely
+connected with that which produces volcanic eruptions. The effect,
+however, of such impact is to originate a wave of translation through
+the crust, travelling outwards from the point, or focus, on the surface
+immediately over the point of impact.[2] These waves of translation can
+in some cases be laid down on a map, and are called "isoseismal curves,"
+each curve representing approximately an equal degree of seismal
+intensity; as shown on the chart of a part of North America affected by
+the great Charleston earthquake. (Fig. 37.) Mr. Hopkins has shown that
+the earthquake-wave, when it passes through rocks differing in density
+and elasticity, changes in some degree not only its velocity, but its
+direction; being both refracted and reflected in a manner analogous to
+that of light when it passes from one medium to another of different
+density.[3] When a shock traverses the crust through a thickness of
+several miles it will meet with various kinds of rock as well as with
+fissures and plications of the strata, owing to which its course will be
+more or less modified.
+
+(_b._) _Formation of Fissures._--During earthquake movements, fissures
+may be formed in the crust, and filled with gaseous or melted matter
+which may not in all cases reach the surface; and, on the principle that
+volcanoes are safety-valves for regions beyond their immediate
+influence, we may infer that the earthquake shock, which generally
+precedes the outburst of a volcano long dormant, finds relief by the
+eruption which follows; so that whatever may be the extent of the
+disastrous results of such an eruption, they would be still more
+disastrous if there had been no such safety-valve as that afforded by a
+volcanic vent. Thus, probably, owing to the extinction of volcanic
+activity in Syria, the earthquakes in that region have been peculiarly
+destructive. For example, on January 1, 1837, the town of Safed west of
+the Jordan valley was completely destroyed by an earthquake in which
+most of the inhabitants perished. The great earthquakes of Aleppo in the
+present century, and of Syria in the middle of the eighteenth, were of
+exceptional severity. In that of Syria, which took place in 1759, and
+which was protracted during a period of three months, an area of 10,000
+square leagues was affected. Accon, Saphat, Baalbeck, Damascus, Sidon,
+Tripoli, and other places were almost entirely levelled to the ground;
+many thousands of human beings lost their lives.[4] Other examples might
+be cited.
+
+(_c._) _Earthquake Waves._--We have now to return to the phenomena
+connected with the transmission of earthquake-waves. The velocity of
+transmission through the earth is very great, and several attempts have
+been made to measure this velocity with accuracy. The most valuable of
+such attempts are those connected with the Charleston and Riviera
+shocks. Fortunately, owing to the perfection of modern appliances, and
+the number of observers all over the globe, these results are entitled
+to great confidence. The phenomena connected with the Charleston
+earthquake, which took place on the 31st of August, 1886, are described
+in great detail by Captain Clarence E. Dutton, of the U.S. Ordnance
+Corps.[5] The conclusions arrived at are;--that as regards the depth of
+the focal point, this is estimated at twelve miles, with a probable
+error of less than two miles; while, as regards the rate of travel of
+the earthquake-wave, the estimate is (in one case) about 3.236 miles per
+second; and in another about 3.226 miles per second.
+
+On the other hand, in the case of the earthquake of the Riviera, which
+took place on the 23rd of February, 1887, at 5.30 a.m. (local time), the
+vibrations of which appear to have extended across the Atlantic, and to
+have sensibly affected the seismograph in the Government Signal Office
+at Washington, the rate of travel was calculated at about 500 miles per
+hour, less than one-half that determined in the case of Charleston; but
+Captain Dutton claims, and probably with justice, that the results
+obtained in the latter case are far more reliable than any hitherto
+arrived at.
+
+(_d._) _Oceanic Waves._--When the originating impact takes place under
+the bed of the ocean--either by a sudden up-thrust of the crust to the
+extent, let us suppose, of two or three feet, or by an explosion from a
+submarine volcano--a double wave is formed, one travelling through the
+crust, the other through the ocean; and as the rate of velocity of the
+former is greatly in excess of that of the latter, the results on their
+reaching the land are often disastrous in the extreme. It is the
+ocean-wave, however, which is the more important, and calls for special
+consideration. If the impact takes place in very deep water, the whole
+mass of the water is raised in the form of a low dome, sloping equally
+away in all directions; and it commences to travel outwards as a wave
+with an advancing crest until it approaches the coast and enters shallow
+water. The wave then increases in height, and the water in front is
+drawn in and relatively lowered; so that on reaching a coast with a
+shelving shore the form of the surface consists of a trough in front
+followed by an advancing crest. These effects may be observed on a small
+scale in the case of a steamship advancing up a river, or into a harbour
+with a narrow channel, but are inappreciable in deep water, or along a
+precipitous open coast.
+
+(_e._) _The Earthquake of Lisbon, 1755._--The disastrous results of a
+submarine earthquake upon the coast have never been more terribly
+illustrated than in the case of the earthquake of Lisbon which took
+place on November 1, 1755. The inhabitants had no warning of the coming
+danger, when a sound like that of thunder was heard underground, and
+immediately afterwards a violent shock threw down the greater part of
+their city; this was the land-wave. In the course of about six minutes,
+sixty thousand persons perished. The sea first retired and left the
+harbour dry, so forming the trough in front of the crest; immediately
+after the water rolled in with a lofty crest, some 50 feet above the
+ordinary level, flooding the harbour and portions of the city bordering
+the shore. The mountains of Arrabida, Estrella, Julio, Marvan, and
+Cintra, were impetuously shaken, as it were, from their very
+foundations; and according to the computation of Humboldt, a portion of
+the earth's surface four times the extent of Europe felt the effects of
+this great seismic shock, which extended to the Alps, the shores of the
+Baltic, the lakes of Scotland, the great lakes of North America, and
+the West Indian Islands. The velocity of the sea-wave was estimated at
+about 20 miles per minute.
+
+(_f._) _Earthquake of Lima and Callao, 28th October, 1746._--Of somewhat
+similar character was the terrible catastrophe with which the cities of
+Lima and Callao were visited in the middle of the last century,[6] in
+which the former city, then one of great magnificence, was overthrown;
+and Callao was inundated by a sea-wave, in which out of 23 ships of all
+sizes in the harbour the greater number foundered; several, including a
+man-of-war, were lifted bodily and stranded, and all the inhabitants
+with the exception of about two hundred were drowned. A volcano in
+Lucanas burst forth the same night, and such quantities of water
+descended from the cone that the whole country was overflowed; and in
+the mountain near Pataz, called Conversiones de Caxamarquilla, three
+other volcanoes burst forth, and torrents of water swept down their
+sides. In the case of these cities, the land-wave, or shock, preceded
+the sea-wave, which of course only reached the port of Callao.
+
+[Illustration: Fig. 37.--The lines represent isoseismal curves, or
+curves of equal intensity, the force decreasing outwards from the focus
+at Charleston, No. 10.]
+
+(_g._) _Earthquake of Charleston, 31st August, 1886._--I shall close
+this account of some remarkable earthquakes with a few facts regarding
+that of Charleston, on the Atlantic seaboard of Carolina.[7] At 9.51
+a.m. of this day, the inhabitants engaged in their ordinary occupations
+were startled by the sound of a distant roar, which speedily deepened in
+volume so as to resemble the noise of cannon rattling along the road,
+"spreading into an awful noise, that seemed to pervade at once the
+troubled earth below and the still air above." At the same time the
+floors began to heave underfoot, the walls visibly swayed to and fro,
+and the crash of falling masonry was heard on all sides, while
+universal terror took possession of the populace, who rushed into the
+streets, the black portion of the community being the most demonstrative
+of their terror. Such was the commencement of the earthquake, by which
+nearly all the houses of Charleston were damaged or destroyed, many of
+the public buildings seriously injured or partially demolished. The
+effects were felt all over the States as far as the great lakes of
+Canada and the borders of the Rocky Mountains. Two epicentral _foci_
+appear to have been established; one lying about 15 miles to the N.W. of
+Charleston, called the _Woodstock focus_; the other about 14 miles due
+west of Charleston, called the _Rantowles focus_; around each of these
+_foci_ the isoseismic curves concentrated,[8] but in the map (Fig. 37)
+are combined into the area of one curve. The position of these _foci_
+clearly shows that the origin of the Charleston earthquake was not
+submarine, though occurring within a short distance of the Atlantic
+border; the curves of equal intensity (isoseismals) are drawn all over
+the area influenced by the shock.
+
+As a general result of these detailed observations, Captain Dutton
+states that there is a remarkable coincidence in the phenomena with
+those indicated by the theory of wave-motion as the proper one for an
+elastic, nearly homogeneous, solid medium, composed of such materials as
+we know to constitute the rocks of the outer portions of the earth; but
+on the other hand he states that nothing has been disclosed which seems
+to bring us any nearer to the precise nature of the forces which
+generated the disturbance.[9]
+
+[1] The views of Mr. R. Mallet, briefly stated, are somewhat as
+follows:--Owing to the secular cooling of the earth, and the consequent
+lateral crushing of the surface, this crushing from time to time
+overcomes the resistance; in which case shocks are experienced along the
+lines of fracture and faulting by which the crust is intersected. These
+shocks give rise to earthquake waves, and as the crushing of the walls
+of the fissure developes heat, we have here the _vera causa_ both of
+volcanic eruptions and earthquake shocks--the former intensified into
+explosions by access of water through the fissures.--"On the Dynamics of
+Earthquakes," _Trans. Roy. Irish Acad._, vol. xxi.
+
+[2] Illustration of the mode of propagation of earthquake shocks will be
+found in Lyell's _Principles of Geology_, vol. ii. p. 136, or in the
+author's _Physiography_, p. 76, after Hopkins.
+
+[3] "Rep. on Theories of Elevation and Earthquakes," _Brit. Ass. Rep._
+1847, p. 33. In the map prepared by Prof. J. Milne and Mr. W. K. Burton
+to show the range of the great earthquake of Japan (1891), similar
+isoseismal lines are laid down.
+
+[4] Lyell, _loc. cit._, p. 163. Two Catalogues of Earthquakes have been
+drawn up by Prof. O'Reilly, and are published in the _Trans. Roy. Irish
+Academy_, vol. xxviii. (1884 and 1886).
+
+[5] _Ninth Annual Report, U.S. Geological Survey_ (1888).
+
+[6] _A True and Particular Account of the Dreadful Earthquake_, 2nd
+edit. The original published at Lima by command of the Viceroy. London,
+1748. Translated from the Spanish.
+
+[7] I take the account from that of Capt. Dutton above cited, p. 220.
+
+[8] Dutton, _Report_, Plate xxvi., p. 308.
+
+[9] _Ibid._, p. 211. On the connection between the moon's position and
+earthquake shocks, see Mr. Richardson's paper on Scottish earthquakes,
+_Trans. Edin. Geol. Soc._, vol. vi. p. 194 (1892).
+
+
+
+
+PART VII.
+
+VOLCANIC AND SEISMIC PROBLEMS.
+
+
+
+
+CHAPTER I.
+
+THE ULTIMATE CAUSE OF VOLCANIC ACTION.
+
+
+Volcanic phenomena are the outward manifestations of forces deep-seated
+beneath the crust of the globe; and in seeking for the causes of such
+phenomena we must be guided by observation of their nature and mode of
+action. The universality of these phenomena all over the surface of our
+globe, in past or present times, indicates the existence of a general
+cause beneath the crust. It is true that there are to be found large
+tracts from which volcanic rocks (except those of great geological
+antiquity) are absent, such as Central Russia, the Nubian Desert, and
+the Central States of North America; but such absence by no means
+implies the non-existence of the forces which give rise to volcanic
+action beneath those regions, but only that the forces have not been
+sufficiently powerful to overcome the resistance offered by the crust
+over those particular tracts. On the other hand, the similarity of
+volcanic lavas over wide regions is strong evidence that they are drawn
+from one continuous magma, consisting of molten matter beneath the
+solid exterior crust.
+
+(_a._) _Lines of Volcanic Action._--It has been shown in a previous page
+that volcanic action of recent or Tertiary times has taken place mainly
+along certain lines which may be traced on the surface of a map or
+globe. One of these lines girdles the whole globe, while others lie in
+certain directions more or less coincident with lines of flexure,
+plication or faulting. The Isle of Sumatra offers a remarkable example
+of the coincidence of such lines with those of volcanic vents. Not only
+the great volcanic cones, but also the smaller ones, are disposed in
+chains which run parallel to the longitudinal axis of the island
+(N.W.-S.E.). The sedimentary rocks are bent and faulted in lines
+parallel to the main axis, and also to the chains of volcanic mountains,
+and the observation holds good with regard to different geological
+periods.[1] Another remarkable case is that of the Jordan Valley.
+Nowhere can the existence of a great fracture and vertical displacement
+of the strata be more clearly determined than along this remarkable line
+of depression; and it is one which is also coincident with a zone of
+earthquake and volcanic disturbances.
+
+(_b._) _Such Lines generally lie along the Borders of the Ocean._--But
+even where, from some special cause, actual observation on the relations
+of the strata are precluded, the general configuration of the ground and
+the relations of the boundaries between land and sea to those of
+volcanic chains, evidently point in many cases to their mutual
+interdependence. The remarkable straightness of the coast of Western
+America, and of the parallel chain of the Andes, affords presumptive
+evidence that this line is coincident with a fracture or system of
+faults, along which the continent has been bodily raised out of the
+waters of the ocean. Of this elevation within very recent times we have
+abundant evidence in the existence of raised coral-reefs and oceanic
+shell-beds at intervals all along the coast; rising in Peru to a level
+of no less than 3,000 feet above the ocean, as shown by Alexander
+Agassiz.[2] Such elevations probably occurred at a time when the
+volcanoes of the Andes were much more active than at present. Considered
+as a whole, these great volcanic mountains may be regarded as in a
+dormant, or partially moribund, condition; and if the volcanic forces
+have to some extent lost their strength, so it would appear have those
+of elevation.
+
+(_c._) _Areas of Volcanic Action in the British Isles._--In the case of
+the British Islands it may be observed that the later Tertiary volcanic
+districts lie along very ancient depressions, which may indicate zones
+of weakness in the crust. Thus the Antrim plateau, as originally
+constituted, lay in the lap of a range of hills formed of crystalline,
+or Lower Silurian, rocks; while the volcanic isles of the Inner Hebrides
+were enclosed between the solid range of the Archaean rocks of the Outer
+Hebrides on the one side, and the Silurian and Archaean ranges of the
+mainland on the other. And if we go back to the Carboniferous period, we
+find that the volcanic district of the centre of Scotland was bounded by
+ranges of solid strata both to the north and south, where the resistance
+to interior pressure from molten matter would have been greater than in
+the Carboniferous hollow-ground, where such molten matter has been
+abundantly extruded. In all these cases, the outflow of molten matter
+was in a direction somewhat parallel to the plications of the strata.
+
+(_d._) _Special Conditions under which the Volcanic Action
+operates._--Assuming, then, that the molten matter, forming an interior
+magma or shell, is constantly exerting pressure against the inner
+surface of the solid crust, and can only escape where the crust is too
+weak (owing to faults, plications, or fissures) to resist the pressure,
+we have to inquire what are the special conditions under which outbursts
+of volcanic matter take place, and what are the general results as
+regards the nature of the _ejecta_ dependent on those conditions.
+
+(_e._) _Effect of the Presence or Absence of Water._--The two chief
+conditions determining the nature of volcanic products, considered in
+the mass, are the presence or absence of water. Such presence or absence
+does not of course affect the essential chemical composition of the
+_ejecta_, but it materially influences the form in which the matter is
+erupted. The agency of water in volcanic eruptions is a very interesting
+and important subject in connection with the history of volcanic action,
+and has been ably treated by Professor Prestwich.[3] At one time it was
+considered that water was essential to volcanic activity; and the fact
+that the great majority of volcanic cones are situated in the vicinity
+of the oceanic waters, or of inland seas, was pointed to in confirmation
+of this theory. But the existence in Western America and other volcanic
+countries of fissures of eruption along which molten lava has been
+extruded without explosions of steam, shows that water is not an
+essential factor in the production of volcanic phenomena; and, as
+Professor Prestwich has clearly demonstrated, it is to be regarded as an
+element in volcanic explosions, rather than as a prime cause of volcanic
+action. The main difficulty he shows to be thermo-dynamical; and
+calculating the rate of increase in the elastic force of steam on
+descending to greater and greater depths and reaching strata of higher
+and higher temperatures, as compared with the force of capillarity, he
+comes to the conclusion that water cannot penetrate to depths of more
+than seven or eight miles, and therefore cannot reach the seat of the
+eruptive forces. Professor Prestwich also points out that if the
+extrusion of lava were due to the elastic force of vapour of water there
+should be a distinct relation between the discharge of the lava and of
+the vapour; whereas the result of an examination of a number of
+well-recorded eruptions shows that the two operations are not related,
+and are, in fact, perfectly independent. Sometimes there has been a
+large discharge of lava, and little or no escape of steam; at other
+times there have been paroxysmal explosive eruptions with little
+discharge of lava. Even in the case of Vesuvius, which is close to the
+sea, there have been instances when the lava has welled out almost with
+the tranquillity of a water-spring.
+
+(_f._) _Access of Surface Water to Molten Lava during Eruptions._--The
+existence of water during certain stages in eruptions is too frequent a
+phenomena to be lost sight of; but its presence may be accounted for in
+other ways, besides proximity to the sea or ocean. Certain volcanic
+mountains, such as Etna and Vesuvius, are built upon water-bearing
+strata, receiving their supplies from the rainfall of the surrounding
+country, or perhaps partly from the sea. In addition to this the ashes
+and scoriae of the mountain sides are highly porous, and rain or snow can
+penetrate and settle downwards around the pipe or throat through which
+molten lava wells up from beneath. In such cases it is easy to
+understand how, at the commencement of a period of activity, molten lava
+ascending through one or more fissures, and meeting with water-charged
+strata or scoriae, will convert the water into steam at high pressure,
+resulting in explosions more or less violent and prolonged, in
+proportion to the quantity of water and the depth to which it has
+penetrated. In this manner we may suppose that ashes, scoriae, and blocks
+of rock torn from the sides of the crater-throat, and hurled into the
+air, are piled around the vent, and accumulate into hills or mountains
+of conical form. After the explosion has exhausted itself, the molten
+lava quietly wells up and fills the crater, as in the cases of those of
+Auvergne and Syria, and other places. We may, therefore, adopt the
+general principle that in volcanic eruptions _where water in large
+quantities is present, we shall have crater-cones built up of ashes,
+scoriae, and pumice; but where absent, the lava will be extravasated in
+sheets to greater or less distances without the formation of such cones;
+or if cones are fanned, they will be composed of solidified lava only,
+easily distinguishable from crater-cones of the first class_.
+
+(_g._) _Nature of the Interior Reservoir from which Lavas are
+derived._--We have now to consider the nature of the interior reservoir
+from which lavas are derived, and the physical conditions necessary for
+their eruption at the surface.
+
+Without going back to the question of the original condition of our
+globe, we may safely hold the view that at a very early period of
+geological history it consisted of a solidified crust at a high
+temperature, enfolding a globe of molten matter at a still higher
+temperature. As time went on, and the heat radiated into space from the
+surface of the globe, while at the same time slowly ascending from the
+interior by conduction, the crust necessarily contracted, and pressing
+more and more on the interior molten magma, this latter was forced from
+time to time to break through the contracting crust along zones of
+weakness or fissures.
+
+(_h._) _The Earth's Crust in a State of both Exterior Thrust and of
+Interior Tension._--As has been shown by Hopkins,[4] and more recently
+by Mr. Davison,[5] an exterior crust in such a condition must eventually
+result in being under a state of horizontal thrust towards the exterior
+and of tension towards the interior surface. For the exterior portion,
+having cooled down, and consequently contracted to its normal state,
+will remain rigid up to a certain point of resistance; but the interior
+portion still continuing to contract, owing to the conduction of the
+heat towards the exterior, would tend to enter upon a condition of
+tension, as becoming too small for the interior molten magma; and such a
+state of tension would tend to produce rupture of the interior part. In
+this manner fissures would be formed into which the molten matter would
+enter; and if the fissures happened to extend to the surface, owing to
+weakness of the crust or flexuring of the strata, or other cause, the
+molten matter would be extruded either in the form of dykes or volcanic
+vents. In this way we may account for the numerous dykes of trap by
+which some volcanic districts are intersected, such as those of the
+north of Ireland and centre of Scotland.
+
+From the above considerations, it follows that the earth's crust must be
+in a condition both of pressure (or lateral thrust) towards the exterior
+portion, and of tension towards the interior, the former condition
+resulting in faulting and flexuring of the rocks, the latter in the
+formation of open fissures, through which lava can ascend under high
+pressure. These operations are of course the attempt of the natural
+forces to arrive at a condition of equilibrium, which is never attained
+because the processes are never completed; in other words, radiation and
+convection of heat are constantly proceeding, giving rise to new forces
+of thrust and tension.
+
+It now remains for us to consider what may be the condition of the
+interior molten magma; and in doing so we must be guided to a large
+extent by considerations regarding the nature of the extruded matter at
+the surface.
+
+(_i._) _Relative Densities of Lavas._--Now, observation shows that, as
+bearing on the subject under consideration, lavas occur mainly under two
+classes as regards their density. The most dense (or basic) are those in
+which silica is deficient, but iron is abundant; the least dense (or
+acid) are those which are rich in silica, but in which iron occurs in
+small quantity. This division corresponds with that proposed by Bunsen
+and Durocher[6] for volcanic rocks, upon the results of analyses of a
+large number of specimens from various districts. Rocks may be thus
+arranged in groups:
+
+ (1) _The Basic_ (Heavier)--poor in silica, rich in iron; containing
+ silica 45-58 per cent. Examples: Basalt, Dolerite,
+ Hornblende rock, Diorite, Diabase, Gabbro,
+ Melaphyre, and Leucite lava.
+
+ (2) _The Acid_ (Lighter)--rich in silica, poor in iron; containing
+ silica 62-78 per cent. Examples: Trachyte, Rhyolite,
+ Obsidian, Domite, Felsite, Quartz-porphyry, Granite.
+
+The Andesite group forms a connecting link between the highly acid and
+the basic groups, and there are many varieties of the above which it is
+not necessary to enumerate. Durocher supposes that the molten magmas of
+these various rocks are arranged in concentric shells within the solid
+crust in order of their respective densities, those of the lighter
+density, namely the acid magmas, being outside those of greater density,
+namely the basic; and this is a view which seems not improbable from a
+consideration not only of the principle itself, but of the succession of
+the varieties of lava in many districts. Thus we find that acid lavas
+have been generally extruded first, and basic afterwards--as in the
+cases of Western America, of Antrim, the Rhine and Central France. And
+if the interior of our globe had been in a condition of equilibrium from
+the time of the consolidation of the crust to the present, reason would
+induce us to conclude that the lavas would ultimately have arranged
+themselves in accordance with the conditions of density beneath that
+crust. But the state of equilibrium has been constantly disturbed. Every
+fresh outburst of volcanic force, and every fresh extrusion of lava,
+tends to disturb it, and to alter the relations of the interior viscous
+or molten magmas. Owing to this it happens, as we may suppose, that the
+order of eruption according to density is sometimes broken, and we find
+such rocks as granophyre (a variety of andesite) breaking through the
+plateau-basalts of Mull and Skye, as explained in a former chapter.
+Notwithstanding such variations, however, the view of Durocher may be
+considered as the most reasonable we can arrive at on a subject which is
+confessedly highly conjectural.
+
+(_j._) _Conclusion as regards the Ultimate Cause of Volcanic
+Action_.--Notwithstanding, however, the complexity of the subject, and
+the uncertainties which must attend an inquiry where some of the data
+are outside the range of our observation, sufficient evidence can be
+adduced to enable us to arrive at a tolerably clear view of the ultimate
+cause of volcanic action. So tempting a subject was sure to evoke
+numerous essays, some of great ingenuity, such as that of Mr. Mallet;
+others of great complexity, such as that of Dr. Daubeny. But more recent
+consideration and wider observation have tended to lead us to the
+conclusion that the ultimate cause is the most simple, the most
+powerful, and the most general which can be suggested; namely, _the
+contraction of the crust due to secular cooling of the more deeply
+seated parts by conduction and radiation of heat into space_. Owing to
+this cause, the enclosed molten matter is more or less abundantly
+extruded from time to time along the lines and vents of eruption, so as
+to accommodate itself to the ever-contracting crust. Nor can we doubt
+that this process has been going on from the very earliest period of the
+earth's history, and formerly at a greater rate than at present. When
+the crust was more highly heated, the radiation and conduction must have
+been proportionately more rapid. Owing to this cause also the
+contraction of the crust was accelerated. To such irresistible force we
+owe the wonderful flexuring, folding, and horizontal overthrusting which
+the rocks have undergone in some portions of the globe--such as in the
+Alps, the Highlands of Scotland and of Ireland, and the Alleghannies of
+America. It is easy to show that the acceleration of the earth's
+rotation must be a consequence of such contraction; but, after all, this
+is but one of those compensatory forces of which we see several examples
+in the world around us. It can also be confidently inferred that at an
+early period of the earth's history, when the moon was nearer to our
+planet than at present, the tides were far more powerful, and their
+effect in retarding the earth's rotation was consequently greater.
+During this period the acceleration due to contraction was also greater;
+and the two forces probably very nearly balanced each other. Both these
+forces (those of acceleration and retardation) have been growing weaker
+down to the present day, though there appears to have been a slight
+advantage on the side of the retarding force.[7]
+
+[1] R. D. M. Verbeek, _Krakatau_, p. 105 (1886); also, J. Milne, _The
+Great Earthquake of Japan_, 1891.
+
+[2] _Bull. Mus. Comp. Zool._, vol. iii.
+
+[3] _Proc. Roy. Soc._, No. 237 (1885); also, _Rep. Brit. Assoc._ (1881).
+
+[4] Hopkins, _supra cit._, p. 218.
+
+[5] C. Davison and G. H. Darwin, _Phil. Trans._, vol. 178, p; 241.
+
+[6] Durocher, _Ann. des Mines_, vol. ii. (1857).
+
+[7] See on this subject the author's _Textbook of Physiography_ (Deacon
+and Co., 1888), pp. 56 and 122.
+
+
+
+
+CHAPTER II.
+
+LUNAR VOLCANOES.
+
+
+The surface of the moon presented to our view affords such remarkable
+indications of volcanic phenomena of a special kind, that we are
+justified in devoting a chapter to their consideration. It is very
+tantalising that our beautiful satellite only permits us to look at and
+admire one half of her sphere; but it is not a very far-fetched
+inference if we feel satisfied that the other half bears a general
+resemblance to that which is presented to the earth. It is scarcely
+necessary to inform the reader why it is that we never see but one face;
+still, for the sake of those who have not thought out the subject I may
+state that it is because the moon rotates on her axis exactly in the
+time that she performs a revolution round the earth. If this should not
+be sufficiently clear, let the reader perform a very simple experiment
+for himself, which will probably bring conviction to his mind that the
+explanation here given is correct. Let him place an orange in the centre
+of a round table, and then let him move round the table from a
+starting-point sideways, ever keeping his face directed towards the
+orange; and when he has reached his starting-point, he will find that he
+has rotated once round while he has performed one revolution round the
+table. In this case the performer represents the moon and the orange
+the earth.
+
+Now this connection between the earth and her satellite is sufficiently
+close to be used as an argument (if not as actual demonstration) that
+the earth and the moon were originally portions of the same mass, and
+that during some very early stage in the development of the solar system
+these bodies parted company, to assume for ever after the relations of
+planet and satellite. At the epoch referred to, we may also suppose that
+these two masses of matter were in a highly incandescent, if not even
+gaseous, state; and we conclude, therefore, that having been once
+portions of the same mass, they are composed of similar materials. This
+conclusion is of great importance in enabling us to reason from analogy
+regarding the origin of the physical features on the moon's surface, and
+for the purpose of comparison with those which we find on the surface of
+our globe; because it is evident that, if the composition of the moon
+were essentially different from that of our earth, we should have no
+basis whatever for a comparison of their physical features.
+
+When the moon started on her career of revolution round the earth, we
+may well suppose that her orbit was much smaller than at present. She
+was influenced by counteracting forces, those of gravitation drawing her
+towards the centre of gravity of the earth,[1] and the centrifugal
+force, which in the first instance was the stronger, so that her orbit
+for a lengthened period gradually increased until the two forces, those
+of attraction and repulsion, came into a condition of equilibrium, and
+she now performs her revolution round the earth at a mean distance of
+240,000 miles, in an orbit which is only very slightly elliptical.[2]
+How the period of the moon's rotation is regulated by the earth's
+attraction on her molten lava-sheets, first at the surface, and now
+probably below the outer crust, has been graphically shown by Sir Robert
+Ball,[3] but it cannot be doubted that once the moon was appreciably
+nearer to our globe than at present. The attraction of her mass produced
+tides in the ocean of correspondingly greater magnitude, and capable of
+effecting results, both in eroding the surface and in transporting
+masses of rock, far beyond the bounds of our every-day experience.
+
+Of all the heavenly bodies, the sun excepted, the moon is the most
+impressive and beautiful. As we catch her form, rising as a fair
+crescent in the western sky after sunset, gradually increasing in size
+and brilliancy night after night till from her circular disk she throws
+a full flood of light on our world and then passes through her
+decreasing phases, we recognise her as "the Governor of the night," or
+in the words of our own poet, when in her crescent phase, "the Diadem of
+night." Seen through a good binocular glass, her form gains in
+rotundity; but under an ordinary telescope with a four-inch objective,
+she appears like a globe of molten gold. Yet all this light is
+derivative, and is only a small portion of that she receives from the
+sun. That her surface is a mass of rigid matter destitute of any
+inherent brilliancy, appears plain enough when we view a portion of her
+disk through a very large telescope. It was the good fortune of the
+author to have an opportunity for such a view through one of the largest
+telescopes in the world. The 27-inch refractor manufactured by Sir
+Howard Grubb of Dublin, for the Vienna observatory, a few years ago, was
+turned on a portion of the moon's disk before being finally sent off to
+its destination; and seen by the aid of such enormous magnifying power,
+nothing could be more disappointing as regards the appearance of our
+satellite. The sheen and lustre of the surface was now observed no
+longer; the mountains and valleys, the circular ridges and hollows were,
+indeed, wonderfully defined and magnified, but the matter of which they
+seemed to be constituted resembled nothing so much as the pale plaster
+of a model. One could thus fully realise the fact that the moon's light
+is only derivative. Still we must recollect that the most powerful
+telescope can only bring the surface of the moon to a distance from us
+of about 250 miles; and it need not be said that objects seen at such a
+distance on our earth present very deceptive appearances; so that we
+gain little information regarding the composition of the moon's crust,
+or exterior surface, simply from observation by the aid of large
+telescopes.
+
+Reasoning from analogy with our globe, we may infer that the exterior
+shell of the moon consists of crystalline volcanic matter of the highly
+silicated, or acid, varieties resting upon another of a denser
+description, rich in iron, and resembling basalt. This hypothesis is
+hazarded on the supposition that the composition of the matter of the
+moon's mass resembles in the main that of our globe. During the process
+of cooling from a molten condition, the heavier lavas would tend to fall
+inwards, and allow the lighter to come to the surface, and form the
+outer shell in both cases. Thus, the outer crust would resemble the
+trachytic lavas of our globe, and their pale colour would enable the
+sun's rays to be reflected to a greater extent than if the material were
+of the blackness of basalt.[4] So much for the material. We have now to
+consider the structure of the moon's surface, and here we find ourselves
+treading on less speculative and safer ground. All astronomers since the
+time of Schroter seem to be of accord in the opinion that the remarkable
+features of the moon's surface are in some measure of volcanic origin,
+and we shall presently proceed to consider the character of these forms
+more in detail.
+
+But first, and as leading up to the discussion of these physical
+features, we must notice one essential difference between the
+constitution of the moon and of the earth; namely, the absence of water
+and of an atmosphere in the case of the moon. The sudden and complete
+occultation of the stars when the moon's disk passes between them and
+the place of the observer on the earth's surface, is sufficient evidence
+of the absence of air; and, as no cloud has ever been noticed to veil
+even for a moment any part of our satellite's face, we are pretty safe
+in concluding that there is no water; or at least, if there be any, that
+it is inappreciable in quantity.[5] Hence we infer that there is no
+animal or vegetable life on the moon's surface; neither are there
+oceans, lakes or rivers, snowfields or glaciers, river-valleys or
+canyons, islands, stratified rocks, nor volcanoes of the kind most
+prevalent on our own globe.
+
+[Illustration: Fig. 38.--Photograph of the moon's surface (in part)
+showing the illuminated "spots," and ridges, and the deep hollows. The
+position of "Tycho" is shown near the upper edge, and some of the
+volcanic craters are very clearly seen near the margin.]
+
+Now on looking at a photographic picture of the moon's surface (Fig.
+38), we observe that there are enormous dark spaces, irregular in
+outline, but more or less approaching the circular form, surrounded by
+steep and precipitous declivities, but with sides sloping outwards.
+These were supposed at one time to be seas; and they retain the name,
+though it is universally admitted that they contain no water. Some of
+these hollows are four English miles in depth. The largest of these,
+situated near the north pole of the moon, is called _Mare Imbrium_; next
+to it is _Mare Serenitatis_; next, _Mare Tranquilitatis_, with several
+others.[6] Mare Imbrium is of great depth, and from its floor rise
+several conical mountains with circular craters, the largest of which,
+_Archimedes_, is fifty miles in diameter; its vast smooth interior being
+divided into seven distinct zones running east and west. There is no
+central mountain or other obvious internal sign of former volcanic
+activity, but its irregular wall rises into abrupt towers, and is marked
+outside by decided terraces.[7]
+
+The Mare Imbrium is bounded along the east by a range of mountains
+called the _Apennines_, and towards the north by another range called
+the _Alps_; while a third range, that of the _Caucasus_, strikes
+northward from the junction of the two former ranges. Several circular
+or oval craters are situated on, and near to, the crest of these ridges.
+
+[Illustration: Fig. 39.--A magnified portion of the moon's surface,
+showing the forms of the great craters with their outer ramparts. The
+white spot with shadow is a cone rising from the centre of one of the
+larger craters to a great height and thus becoming illuminated by the
+sun's light.]
+
+But the greater part of the moon's hemisphere is dotted over by almost
+innumerable circular crater-like hollows; sometimes conspicuously
+surmounting lofty conical mountains, at other times only sinking below
+the general outer surface of the lunar sphere. On approaching the
+margin, these circular hollows appear oval in shape owing to their
+position on the sphere; and the general aspect of those that are visible
+leads to the conclusion that there are large numbers of smaller craters
+too small to be seen by the most powerful telescopes. These cones and
+craters are the most characteristic objects on the whole of the visible
+surface, and when highly magnified present very rugged outlines,
+suggestive of slag, or lava, which has consolidated on cooling, as in
+the case of most solidified lava-streams on our earth.[8] One of the
+most remarkable of these crateriform mountains is that named
+_Copernicus_, situated in a line with the southern prolongation of the
+Apennines. Of this mountain Sir R. Ball says: "It is particularly well
+known through Sir John Herschel's drawing, so beautifully reproduced in
+the many editions of the _Outlines of Astronomy_. The region to the west
+is dotted over with innumerable minute craterlets. It has a central,
+many-peaked mountain about 2,400 feet in height. There is good reason to
+believe that the terracing shown in its interior is mainly due to the
+repeated alternate rise, partial congealation and retreat of a vast sea
+of lava. At full moon it is surrounded by radiating streaks."[9] The
+view regarding the structure of Copernicus here expressed is of
+importance, as it is probably applicable to all the craters of our
+satellite.
+
+"When the moon is five or six days old," says Sir Robert Ball, "a
+beautiful group of three craters will be readily found on the boundary
+line between night and day. These are _Catharina_, _Cyrillus_, and
+_Theophilus_. Catharina is the most southerly of the group, and is more
+than 16,000 feet deep and connected to Cyrillus by a wide valley; but
+between Cyrillus and Theophilus there is no such connection. Indeed
+Cyrillus looks as if its huge surrounding ramparts, as high as Mont
+Blanc, had been completely finished when the volcanic forces commenced
+the formation of Theophilus, the rampart of which encroaches
+considerably on its older neighbour. Theophilus stands as a well-defined
+round crater, about 64 miles in diameter, with an internal depth of
+14,000 to 18,000 feet, and a beautiful central group of mountains,
+one-third of that height, on its floor. This proves that the last
+eruptive efforts in this part of the moon fully equalled in intensity
+those that had preceded them. Although Theophilus is on the whole the
+deepest crater we can see in the moon, it has received little or no
+deformation by secondary eruptions."
+
+But perhaps the most remarkable object on the whole hemisphere of the
+moon is "the majestic Tycho," which rises from the surface near the
+south pole, and at a distance of about 1/6th of the diameter of the
+sphere from its margin. Its depth is stated by Ball to be 17,000 feet,
+and its diameter 50 miles. But its special distinction amongst the other
+volcanic craters lies in the streaks of light which radiate from it in
+all directions for hundreds and even thousands of miles, stretching with
+superb indifference across vast plains, into the deepest craters, and
+over the highest opposing ridges. When the sun rises on Tycho these
+streaks are invisible, but as soon as it has reached a height of 25 deg. to
+30 deg. above the horizon, the rays emerge from their obscurity, and
+gradually increase in brightness until full moon, when they become the
+most conspicuous objects on her surface. As yet no satisfactory
+explanation has been given of the origin of these illuminated rays,[10]
+but I may be permitted to add that their form and mode of occurrence are
+eminently suggestive of gaseous exhalations from the volcano illumined
+by the sun's rays; and owing to the absence of an atmosphere, spreading
+themselves out in all directions and becoming more and more attenuated
+until they cease to be visible.
+
+The above account will probably suffice to give the reader a general
+idea of the features and inferential structure of the moon's surface.
+That she was once a molten mass is inferred from her globular form; but,
+according to G. F. Chambers, the most delicate measurements indicate no
+compression at the poles.[11] That her surface has cooled and become
+rigid is also a necessary inference; though Sir J. Herschel considered
+that the surface still retains a temperature _possibly_ exceeding that
+of boiling water.[12] However this may be, it is pretty certain that
+whatever changes may occur upon her surface are not due to present
+volcanic action, all evidence of such action being admittedly absent.
+If, when the earth and moon parted company, their respective
+temperatures were equal, the moon being so much the smaller of the two
+would have cooled more rapidly, and the surface may have been covered by
+a rigid crust when as yet that of the earth may have been molten from
+heat. Hence the rigidity of the moon's surface may date back to an
+immensely distant period, but she may still retain a high temperature
+within this crust. Having arrived at this stage of our narrative, we are
+in a position to consider by what means, and under what conditions, the
+cones and craters which diversify the lunar surface have been developed.
+
+In doing so it may be desirable, in the first place, to determine what
+form of crater on our earth's surface those of the moon do not
+represent; and we are guided in our inquiry by the consideration of the
+absence of water on the lunar surface. Now there are large numbers of
+crateriform mountains on our globe in the formation of which water has
+played an important, indeed essential, part. As we have already seen,
+water, though not the ultimate cause of volcanic eruptions, has been the
+chief agent, when in the form of steam at high pressure, in producing
+the explosions which accompany these eruptions, and in tearing up and
+hurling into the air the masses of rock, scoriae, and ashes, which are
+piled around the vents of eruption in the form of craters during periods
+of activity. To this class of craters those of Etna, Vesuvius, and
+Auvergne belong. These mountains and conical hills (the domes excepted)
+are all built up of accumulations of fragmental material, with
+occasional sheets and dykes of lava intervening; and where eruptions
+have taken place in recent times, observation has shown that they are
+accompanied by outbursts of vast quantities of aqueous vapour, which has
+been the chief agent (along with various gases) in piling up the
+circular walls of the crater.
+
+It has also been shown that in many instances these crater-walls have
+been breached on one side, and that streams of molten lava which once
+occupied the cup to a greater or less height, have poured down the
+mountain side. Hence the form or outline of many of these fragmental
+craters is crescent-shaped. Such breached craters are to be found in all
+parts of the world, and are not confined to any one district, or even
+continent, so that they may be considered as characteristic of the class
+of volcanic crater-cones to which I am now referring. In the case of the
+moon, however, we fail to observe any decided instances of breached
+craters, with lava-streams, such as those I have described.[13] In
+nearly all cases the ramparts appear to extend continuously round the
+enclosed depression, solid and unbroken; or at least with no large gap
+occupying a very considerable section of the circumference. (See Fig.
+38.) Hence we are led to suspect that there is some essential
+distinction between the craters on the surface of the moon and the
+greater number of those on the surface of our earth.
+
+It is scarcely necessary to add that the volcanic mountains of the moon
+offer no resemblance whatever to the dome-shaped volcanic mountains of
+our globe. If it were otherwise, the lunar mountains would appear as
+simple luminous points rising from a dark floor, over which they would
+cast a conical shadow. But the form of the lunar volcanic mountains is
+essentially different; as already observed, they consist in general of a
+circular rampart enclosing a depressed floor, sometimes terraced as in
+the case of Copernicus, from which rise one or more conical mountains,
+which are in effect the later vents of eruption.
+
+In our search, therefore, for analogous forms on our own earth, we must
+leave out the craters and domes of the type furnished by the European
+volcanoes and their representatives abroad, and have recourse to others
+of a different type. Is there then, we may ask, any type of volcanic
+mountain on our globe comparable with those on the moon? In all
+probability there is.
+
+If the reader will turn to the description of the volcanoes of the
+Hawaiian group in the Pacific, especially that of Mauna Loa, as given by
+Professor Dana and others, and compare it with that of Copernicus, he
+will find that in both cases we have a circular rampart of solid lava
+enclosing a vast plain of the same material from which rise one or more
+lava-cones. The interiors in both cases are terraced. So that, allowing
+for differences in magnitude, it would seem that there is no essential
+distinction between lunar craters and terrestrial craters of the type of
+Mauna Loa. Dana calls these Hawaiian volcanoes "basaltic," basalt being
+the prevalent material of which they are formed. Those of the moon may
+be composed of similar material, or otherwise; but in either case we may
+suppose they are built up of lava, erupted from vents connected with the
+molten reservoirs of the interior. Thus we conclude that they belong to
+an entirely different type, and have been built up in a different
+manner, from those represented by Etna, Vesuvius, and most of the
+extinct volcanoes of Auvergne, the Eifel, and of other districts
+considered in these pages.
+
+Let us now endeavour to picture to ourselves the stages through which
+the moon may be supposed to have passed from the time her surface began
+to consolidate owing to the radiation of her heat into space; for there
+is every probability that some of the craters now visible on her disk
+were formed at a very early period of her physical history.
+
+When the surface began to consolidate, it must also have contracted; and
+the interior molten matter, pressed out by the contracting crust, must
+have been over and over again extruded through fissures produced over
+the solidified surface, until the solid crust extended over the whole
+lunar surface, and became of considerable thickness.
+
+It is from this epoch that, in all probability, we should date the
+commencement of what may be termed "the volcanic history" of the moon.
+We must bear in mind that although the moon's surface had become solid,
+its temperature may have remained high for a very long period. But the
+continuous radiation of the surface-heat into space would produce
+continuous contraction, while the convection of the interior heat would
+tend to increase the thickness of the outer solid shell; and this, ever
+pressing with increasing force on the interior molten mass, would result
+in frequent ruptures of the shell, and the extrusion of molten lava
+rising from below. Hence we may suppose the fissure-eruptions of lava
+were of frequent occurrence for a lengthened period during the early
+stage of consolidation of the lunar crust; but afterwards these may be
+supposed to have given place to eruptions through pipes or vents,
+resulting in the formation of the circular craters which form such
+striking and characteristic objects in the physical aspect of our
+satellite.[14]
+
+It is not to be supposed that the various physical features on the lunar
+surface have all originated in the same way. The great ranges of
+mountains previously described may have originated by a process of
+piling up of immense masses of molten lava extruded from the interior
+through vents or fissures; while the great hollows (or "seas") are
+probably due to the falling inwards of large spaces owing to the escape
+of the interior lava.
+
+But it is with the circular craters that we are most concerned. Judging
+from analogy with the lava-craters present on our globe, we must suppose
+them to be due to the extrusion, and piling up, of lava through central
+pipes, followed in some cases by the subsidence of the floor of the
+crater. It seems not improbable that it was in this way the greater
+number of the circular craters lying around Tycho, and dotting so large
+a space round the margin of the moon, were constructed. (See Fig. 38.)
+In general they appear to consist of an elevated rim, enclosing a
+depressed plain, out of which a central cone arises. The rim may be
+supposed to have been piled up by successive discharges of lava from a
+central orifice; and after the subsidence of the paroxysm the lava still
+in a molten condition may have sunk down, forming a seething lake within
+the vast circular rampart, as in the case of the Hawaiian volcanoes. The
+terraces observable within the craters in some instances have probably
+been left by subsequent eruptions which have not attained to the level
+of preceding ones; and where a central crater-cone is seen to rise
+within the caldron, we may suppose this to have been built up by a later
+series of eruptions of lava through the original pipe after the
+consolidation of the interior sea of lava. The mamelons of the Isle of
+Bourbon,[15] and some of the lava-cones of Hawaii, appear to offer
+examples on our earth's surface of these peculiar forms.
+
+Such are the views of the origin of the physical features of our
+satellite which their form and inferred constitution appear to suggest.
+They are not offered with any intention of dogmatising on a subject
+which is admittedly obscure, and regarding which we have by no means all
+the necessary data for coming to a clear conclusion. All that can be
+affirmed is, that there is a great deal to be said in support of them,
+and that they are to some extent in harmony with phenomena within range
+of observation on the surface of our earth.
+
+The far greater effects of lunar vulcanicity, as compared with those of
+our globe, may be accounted for to some extent by the consideration that
+the force of gravity on the surface of the moon is only one-sixth of
+that on the surface of the earth. Hence the eruptive forces of the
+interior of our satellite have had less resistance to overcome than in
+the case of our planet; and the erupted materials have been shot forth
+to greater distances, and piled up in greater magnitude, than with us.
+We have also to recollect that the abrading action of water has been
+absent from the moon; so that, while accumulations of matter had been
+proceeding throughout a prolonged period over its surface, there was no
+counteracting agency of denudation at work to modify or lessen the
+effects of the ruptive forces.
+
+[1] Correctly speaking, each attracts the other towards its centre of
+gravity with a force proportionate to its mass, and inversely as the
+square of the distance; but the earth being by much the larger body, its
+attraction is far greater than that of the moon.
+
+[2] The variation in the distance is only under rare circumstances
+40,000 miles, but ordinarily about 13,000 miles.
+
+[3] _Story of the Heavens_, 2nd edition, p. 525, _et seq._
+
+[4] A series of researches made by Zoellner, of Leipzig, led him to
+assign to the light-reflecting capacity of the full-moon a result
+intermediate between that obtained by Bouguer, which gave a brightness
+equal to 1/300000 part of that of the sun, and of Wollaston, which gave
+1/801070 part. We may accept 1/618000 of Zoellner as sufficiently close;
+so that it would require 600,000 full moons to give the same amount of
+light as that of the sun.
+
+[5] Schroter, however, came to the conclusion that the moon has an
+atmosphere.
+
+[6] A chart of the moon's surface, with the names of the principal
+physical features, will be found in Ball's _Story of the Heavens_, 2nd
+edit., p. 60. It must be remembered that the moon as seen through a
+telescope appears in reversed position.
+
+[7] _Ibid._, p. 66.
+
+[8] As represented by Nasmyth's models in plaster.
+
+[9] Ball, _loc. cit._, p. 67.
+
+[10] Ball, _loc. cit._, p. 69.
+
+[11] _Astronomy_, p. 78.
+
+[12] _Outlines of Astronomy_, p. 285.
+
+[13] At rare intervals a few crescent-shaped ridges are discernible on
+the lunar sphere, but it is very doubtful if they are to be regarded as
+breached craters.
+
+[14] The number of "spots" on the moon was considered to be 244 until
+Schroter increased it to 6,000, and accurately described many of them.
+Schroter seems to have been the earliest observer who identified the
+circular hollows on the moon's surface as volcanic craters.
+
+[15] Drawings of these very curious forms are given by Judd,
+_Volcanoes_, p. 127.
+
+
+
+
+CHAPTER III.
+
+ARE WE LIVING IN AN EPOCH OF SPECIAL VOLCANIC ACTIVITY?
+
+
+The question which we are about to discuss in the concluding chapter of
+this volume is one to which we ought to be able to offer a definite
+answer. This can only be arrived at by a comparison of the violence and
+extent of volcanic and seismic phenomena within the period of history
+with those of pre-historic periods.
+
+At first sight we might be disposed to give to the question an
+affirmative reply when we remember the eruptions of the last few years,
+and add to these the volcanic outbursts and earthquake shocks which
+history records. The cases of the earthquake and eruption in Japan of
+November, 1891, where in one province alone two thousand people lost
+their lives and many thousand houses were levelled[1]; that of Krakatoa,
+in 1883; of Vesuvius, in 1872; and many others of recent date which
+might be named, added to those which history records;--the recollection
+of such cases might lead us to conclude that our epoch is one in which
+the subterranean volcanic forces had broken out with extraordinary
+energy over the earth's surface. Still, when we come to examine into
+the cases of recorded eruptions--especially those of great violence--we
+find that they are limited to very special districts; and even if we
+extend our retrospect into the later centuries of our era, we shall find
+that the exceptionally great eruptions have been confined to certain
+permanently volcanic regions, such as the chain of the Andes, that of
+the Aleutian, Kurile, Japanese, and Philippine and Sunda Islands, lying
+for the most part along the remarkable volcanic girdle of the world to
+which I have referred in a previous page. Add to these the cases of
+Iceland and the volcanic islands of the Pacific, and we have almost the
+whole of the very active volcanoes of the world.
+
+Then for the purposes of our inquiry we have to ascertain how these
+active vents of eruption compare, as regards the magnitude of their
+operations, with those of the pre-historic and later Tertiary times. But
+before entering into this question it maybe observed, in the first
+place, that a large number of the vents of eruption, even along the
+chain of the earth's volcanic girdle, are dormant or extinct. This
+observation applies to many of the great cones and domes of the Andes,
+including Chimborazo and other colossal mountains in Ecuador, Columbia,
+Chili, Peru, and Mexico. The region between the eastern Rocky Mountains
+and the western coast of North America was, as we have seen, one over
+which volcanic eruptions took place on a vast scale in later Tertiary
+times; but one in which only the after-effects of volcanic action are at
+present in operation. We have also seen that the chain of volcanoes of
+Japan and of the Kurile Islands are only active to a slight extent as
+compared with former times, and the same observation applies to those
+of New Zealand. Out of 130 volcanoes in the Japanese islands, only 48
+are now believed to be active.
+
+Again, if we turn to other districts we have been considering, we find
+that in the Indian Peninsula, in Arabia, in Syria and the Holy Land, in
+Persia, in Abyssinia and Asia Minor--regions where volcanic operations
+were exhibited on a grand scale throughout the Tertiary period, and in
+some cases almost down into recent times--we are met by similar
+evidences either of decaying volcanic energy, or of an energy which, as
+far as surface phenomena are concerned, is a thing of the past. Lastly,
+turning our attention to the European area, notwithstanding the still
+active condition of Etna, Vesuvius, and a few adjoining islands, we see
+in all directions throughout Southern Italy evidences of volcanic
+operations of a past time,--such as extinct crater-cones, lakes
+occupying the craters of former volcanoes, and extensive deposits of
+tuff or streams of lava--all concurring in giving evidence of a period
+now past, when vulcanicity was widespread over regions where its
+presence is now never felt except when some earthquake shock, like that
+of the Riviera, brings home to our minds the fact that the motive force
+is still beneath our feet, though under restrained conditions as
+compared with a former period.
+
+Similar conclusions are applicable with even greater force to other
+parts of the European area. The region of the Lower Rhine and Moselle,
+of Hungary and the Carpathians, of Central France, of the North of
+Ireland and the Inner Hebrides, all afford evidence of volcanic
+operations at a former period on an extensive scale; and the contrast
+between the present physically silent and peaceful condition of these
+regions, as regards any outward manifestations of sub-terrestrial
+forces, compared with those which were formerly prevalent, cannot fail
+to impress our minds irresistibly with the idea that volcanic energy has
+well-nigh exhausted itself over these tracts of the earth's surface.
+
+From this general survey of the present condition of the earth's
+surface, as regards the volcanic operations going on over it, and a
+comparison with those of a preceding period, we are driven to the
+conclusion that, however violent and often disastrous are the volcanic
+and seismic phenomena of the present day, they are restricted to
+comparatively narrow limits; and that even within these limits the
+volcanic forces are less powerful than they were in pre-historic times.
+
+The middle part of the Tertiary period appears, in fact, to have been
+one of extraordinary volcanic activity, whether we regard the wide area
+over which this activity manifested itself, or the results as shown by
+the great amount of the erupted materials. Many of the still active
+volcanic chains, or groups, probably had their first beginnings at the
+period referred to; but in the majority of cases the eruptive forces
+have become dormant or extinct. With the exception of the lavas of the
+Indian-Peninsular area, which appear, at least partially, to belong to
+the close of the Cretaceous epoch, the specially volcanic period may be
+considered to extend from the beginning of the Miocene down to the close
+of the Pliocene stage. During the Eocene stage, volcanic energy appears
+to have been to a great degree dormant; but plutonic energy was
+gathering strength for the great effort of the Miocene epoch, when the
+volcanic forces broke out with extraordinary violence over Europe, the
+British Isles, and other regions, and continued to develop throughout
+the succeeding Pliocene epoch, until the whole globe was surrounded by a
+girdle of fire.
+
+ * * * * *
+
+The reply, therefore, to the question with which we set out is very
+plain; and is to the effect that the present epoch is one of
+comparatively low volcanic activity. The further question suggests
+itself, whether the volcanic phenomena of the middle Tertiary period
+bear any comparison with those of past geological times. This, though a
+question of great interest, is one which is far too large to be
+discussed here; and it is doubtful if we have materials available upon
+which to base a conclusion. But it may be stated with some confidence,
+in general terms, that the history of the earth appears to show that,
+throughout all geological time, our world has been the theatre of
+intermittent geological activity, periods of rest succeeding those of
+action; and if we are to draw a conclusion regarding the present and
+future, it would be that, owing to the lower rate of secular cooling of
+the crust, volcanic action ought to become less powerful as the world
+grows older.
+
+[1] Admirably illustrated in Prof. J. Milne's recently published work,
+_The Great Earthquake of Japan, 1891_.
+
+
+
+
+APPENDIX.
+
+A BRIEF ACCOUNT OF THE PRINCIPAL VARIETIES OF VOLCANIC ROCKS.
+
+
+The text-books on this subject are so numerous and accessible, that a
+very brief account of the volcanic rocks is all that need be given here
+for the purposes of reference by readers not familiar with petrological
+details.
+
+Let it be observed, in the first place, that there is no hard and fast
+line between the varieties of igneous and volcanic rocks. In this as in
+other parts of creation--_natura nil facit per saltum_; there are
+gradations from one variety to the other. At the same time a systematic
+arrangement is not only desirable, but necessary; and the most important
+basis of arrangement is that founded on the proportion of _silica_ (or
+quartz) in the various rocks, as first demonstrated by Durocher and
+Bunsen, who showed that silica plays the same part in the inorganic
+kingdom that carbon does in the organic. Upon this hypothesis, which is
+a very useful one to work with, these authors separated all igneous and
+volcanic rocks into two classes, viz., the Basic and the Acid; the
+former containing from 45-58 per cent., the latter 62-78 per cent. of
+that mineral. But there are a few intermediate varieties which serve to
+bridge over the space between the Basic and Acid Groups. The following
+is a generalised arrangement of the most important rocks under the above
+heads:--
+
+_Tabular View of Chief Igneous and Volcanic Rocks._
+
+BASIC GROUP.
+
+1. Basalt and Dolerite.
+2. Gabbro.
+3. Diorite.
+4. Diabase and Melaphyre.
+5. Porphyrite.
+
+INTERMEDIATE GROUP.
+
+6. Syenite.
+7. Mica-trap, or Lampophyre.
+8. Andesite.
+
+ACID GROUP.
+
+9. Trachyte, Domite, and Phonolite.
+10. Rhyolite and Obsidian.
+11. Granophyre.
+12. Granite.
+
+In the above grouping, and in the following definitions, I have not been
+able to follow any special authority. But the most serviceable
+text-books are those of Mr. Frank Rutley, _Study of Rocks_, and Dr.
+Hatch, _Petrology_; also H. Rosenbusch, _Mikroskopische Physiographie
+der Mineralien_, and F. Zirkel's _Untersuchungen ueber mikroskopische
+Structur der Basaltgesteine_. We shall consider these in the order above
+indicated:--
+
+1. BASALT.--The most extensively distributed of all volcanic rocks. It
+is a dense, dark rock of high specific gravity (2.4-2.8), consisting of
+plagioclase felspar (Labradorite or anorthite), augite, and
+titano-ferrite (titaniferous magnetite). Olivine is often present; and
+when abundant the rock is called "olivine-basalt." In the older rocks,
+basalt has often undergone decomposition into melaphyre; and amongst the
+metamorphic rocks it has been changed into diorite or hornblende rock;
+the augite having been converted into hornblende.
+
+When leucite or nepheline replaces plagioclase, the rock becomes a
+leucite-basalt,[1] or nepheline-basalt. Some basalts have a glass paste,
+or "ground-mass," in which the minerals are enclosed.
+
+The lava of Vesuvius may be regarded as a variety of basalt in which
+leucite replaces plagioclase, although this latter mineral is also
+present. Zirkel calls it "Sanidin-leucitgestein," as both the
+macroscopic and microscopic structure reveal the presence of leucite,
+sanidine, plagioclase, nephiline, augite, mica, olivine, apatite, and
+magnetite.[2]
+
+_Dolerite_ does not differ essentially from basalt in composition or
+structure, but is a largely crystalline-granular variety, occurring more
+abundantly than basalt amongst the more ancient rocks, and the different
+minerals are distinctly visible to the naked eye.
+
+A remarkable variety of this rock occurs at Slieve Gullion in Ireland,
+in which mica is so abundant as to constitute the rock a "micaceous
+dolerite."
+
+2. GABBRO.--A rather wide group of volcanic rocks with variable
+composition. Essentially it is a crystalline-granular compound of
+plagioclase, generally Labradorite and diallage. Sometimes the pyroxenic
+mineral becomes hypersthene, giving rise to _hypersthene-gabbro_; or
+when hornblende is present, to _hornblende-gabbro_; when olivine, to
+_olivine-gabbro_. Magnetite is always present.
+
+These rocks occur in the Carlingford district in Ireland, in the Lizard
+district of Cornwall, the Inner Hebrides (Mull, Skye, etc.) of Scotland,
+and in Saxony.
+
+3. DIORITE.--A crystalline-granular compound of plagioclase and
+hornblende with magnetite. When quartz is present it becomes (according
+to the usual British acceptation) a _syenite_; but this view is
+gradually giving place to the German definition of syenite, which is a
+compound of orthoclase and hornblende; and it may be better to
+denominate the variety as _quartz-diorite_. The diorites are abundant as
+sheets and dykes amongst the older palaeozoic and metamorphic rocks, and
+are sometimes exceedingly rich in magnetite. Mica, epidote, and chlorite
+are also present as accessories.
+
+The rock occurs in North Wales, Charnwood Forest, Wicklow, Galway, and
+Donegal, and the Highlands of Scotland. There can be little doubt that
+amongst the metamorphic rocks of Galway, Mayo, and Donegal the great
+beds of (often columnar) diorite were originally augitic lavas, which
+have since undergone transformation.
+
+4. DIABASE.--It is very doubtful if "Diabase" ought to be regarded as a
+distinct species of igneous rock, as it seems to be simply an altered
+variety of basalt or dolerite, in which chlorite, a secondary
+alteration-product, has been developed by the decomposition of the
+pyroxene or olivine of the original rock. It is a convenient name for
+use in the field when doubt occurs as to the real nature of an igneous
+rock. Melaphyre is a name given to the very dark varieties of altered
+augitic lavas, rich in magnetite and chlorite.
+
+5. PORPHYRITE (or quartzless porphyry).--A basic variety of
+felstone-porphyry, consisting of a felspathic base with distinct
+crystals of felspar, with which there may be others of hornblende, mica,
+or augite. The colour is generally red or purple, and it weathers into
+red clay, in contrast to the highly acid or silicated felsites which
+weather into whitish sand.
+
+6. SYENITE.--As stated above, this name has been variously applied. Its
+derivation is from Syene (Assouan) in Egypt, and the granitic rocks of
+that district were called "syenites," under the supposition (now known
+to be erroneous) that they differ from ordinary granites in that they
+were supposed to be composed of quartz, felspar, and hornblende, instead
+of quartz, felspar, and mica. From this it arose that syenite was
+regarded as a variety of granite in which the mica is replaced by
+hornblende, and this has generally been the British view of the
+question. But the German definition is applied to an entirely different
+rock, belonging to the felstone family; and according to this
+classification syenite consists of a crystalline-granular compound of
+orthoclase and hornblende, in which quartz may or may not be present.
+From this it will be seen that, according to Zirkel, syenite is
+essentially distinct from diorite in the species of its felspar.[3] It
+seems desirable to adopt the German view; and as regards diorites
+containing quartz as an accessory, to apply to them the name of
+_quartz-diorite_, as stated above, the name syenite as used by British
+geologists having arisen from a misconception.
+
+7. MICA-TRAP (LAMPOPHYRE).--A rock, allied to the felstone family, in
+which mica is an abundant and essential constituent, thus consisting of
+plagioclase and mica, with a little magnetite. Quartz may be an
+accessory. This rock occurs amongst the Lower Silurian strata of
+Ireland, Cumberland, and the South of Scotland; it is not volcanic in
+the ordinary acceptation of that term. The term _lampophyre_ was
+introduced by Guembel in describing the mica-traps of Fichtelgebirge.
+
+8. ANDESITE.--This is a dark-coloured, compact or vesicular,
+semi-vitreous group of volcanic rocks, composed essentially of a glassy
+plagioclase felspar, and a ferro-magnesian constituent enclosed in a
+glassy base. According to the nature of the ferro-magnesian constituent,
+the group may be divided into _hornblende-andesite_, _biotite-andesite_,
+and _augite-andesite_. Quartz is sometimes present, and when this
+mineral becomes an essential it gives rise to a variety called
+_quartz-andesite_ or _dacite_.
+
+These rocks are the principal constituents of the lavas of the Andes,
+and the name was first applied to them by Leopold von Buch; but their
+representatives also occur in the British Isles, Germany, and elsewhere.
+Dacite is the lava of Krakatoa and some of the volcanoes of Japan.
+
+9, 10. TRACHYTE and DOMITE, etc.--These names include very numerous
+varieties of highly silicated volcanic rock, and in their general form
+consist of a white felsitic paste with distinct crystals of sanidine,
+together with plagioclase, augite, biotite, hornblende, and accessories.
+When crystalline grains or blebs of quartz occur, we have a
+quartz-trachyte; when tridymite is abundant, as in the trachyte of Co.
+Antrim, we have "tridymite-trachyte."
+
+The trachytes occupy a position between the pitchstone lavas on the one
+hand, and the andesites and granophyres on the other.
+
+(_b._) _Domite_ is the name applied to the trachytic rocks of the
+Auvergne district and the Puy de Dome particularly. They do not contain
+free quartz, though they are highly acid rocks, containing sometimes as
+much as 68 per cent. of silica.
+
+(_c._) _Phonolite (Clinkstone)_ is a trachytic rock, composed
+essentially of sanidine, nepheline, and augite or hornblende. It is
+usually of a greenish colour, hard and compact, so as to ring under the
+hammer; hence the name. The Wolf Rock is composed of phonolite, and it
+occurs largely in Auvergne.
+
+(_d._) _Rhyolites_ are closely connected with the _quartz-trachytes_,
+but present a marked fluidal, spherulitic, or perlitic structure. They
+consist of a trachytic ground-mass in which grains or crystals of
+quartz and sanidine, with other accessory minerals, are imbedded. They
+occur amongst the volcanic rocks of the British Isles, Hungary, and the
+Lipari Islands, from which the name _Liparite_ has been derived.
+
+(_e._) _Obsidian (Pitchstone)._--This is a vitreous, highly acid rock,
+which has become a volcanic glass in consequence of rapid cooling,
+distinct minerals not having had time to form. It has a conchoidal
+fracture, various shades of colour from grey to black; and under the
+microscope is seen to contain crystallites or microliths, often
+beautifully arranged in stellate or feathery groups. Spherulitic
+structure is not infrequent; and occasionally a few crystals of
+sanidine, augite, or hornblende are to be seen imbedded in the glassy
+ground-mass. The rock occurs in dykes and veins in the Western Isles of
+Scotland, in Antrim, and on the borders of the Mourne Mountains, near
+Newry, in Ireland.
+
+11. GRANOPHYRE.--This term, according to Geikie, embraces the greater
+portion of the acid volcanic rocks of the Inner Hebrides. They are
+closely allied to the quartz-porphyries, and vary in texture from a fine
+felsitic or crystalline-granular quartz-porphyry, in the ground-mass of
+which porphyritic turbid felspar and quartz may generally be detected,
+to a granitoid rock of medium grain, in which the component dull felspar
+and clear quartz can be readily distinguished by the naked eye.
+Throughout all the varieties of texture there is a strong tendency to
+the development of minute irregularly-shaped cavities, inside of which
+quartz or felspar has crystallised out--a feature characteristic of the
+granites of Arran and of the Mourne Mountains.
+
+12. GRANITE.--A true granite consists of a crystalline-granular rock
+consisting of quartz, felspar (orthoclase), and mica; the quartz is the
+paste or ground-mass in which the felspar and mica crystals are
+enclosed. This is the essential distinction between a granite and a
+quartz-porphyry or a granophyre. Owing to the presence of highly-heated
+steam under pressure in the body of the mass when in a molten condition,
+the quartz has been the last of the minerals to crystallise out, and
+hence does not itself occur with the crystalline form.
+
+True granite is not a volcanic rock, and its representatives amongst
+volcanic ejecta are to be found in the granophyres, quartz-porphyries,
+felsites, trachytes, and rhyolites so abundant in most volcanic
+countries, and to one or other of these the so-called granites of the
+Mourne Mountains, of Arran Island, and of Skye are to be referred.
+Granite is a rock which has been intruded in a molten condition
+amongst the deep-seated parts of the crust, and has consolidated
+under great pressure in presence of aqueous vapour and with extreme
+slowness, resulting in the formation of a rock which is largely
+crystalline-granular. Its presence at the surface is due to denudation
+of the masses by which it was originally overspread.
+
+[Illustration: Plate I.]
+
+EXPLANATION OF PLATE I.
+
+MAGNIFIED SECTIONS OF VESUVIAN MINERALS.
+
+ Fig. 1. Section of leucite crystal from the lava of 1868, with
+ fluid cavities. Mag., 350 diams.
+
+ " 2, 3, 4, and 5. Sections of nepheline crystals from the lava of
+ 1767, 1834, and 1854.
+
+ " 6. Section of sodalite crystal from the lava of 1794, with
+ belonites and crystals of magnetite enclosed.
+
+ " 7, 8, 9. Crystals of leucite with microliths and cavities
+ darkened by magnetite dust; also, containing crystals of
+ magnetite.
+
+ " 10. Group of leucite crystals of irregular form from the lava of
+ 1855, congregated around a nucleus of crystals of plagioclase
+ and magnetite.
+
+[Illustration: Plate II.]
+
+EXPLANATION OF PLATE II.
+
+MAGNIFIED SECTIONS OF VESUVIAN MINERALS.
+
+ Fig. 1. Section of augite crystal from the lava of 1794, with
+ numerous gas cells and delicately banded walls. The interior
+ contains two long prisms, probably of apatite.
+
+ " 2. Crystal of augite with banded walls, and indented by leucite
+ crystals, from the lava of 1794. Mag., 40 diams.
+
+ " 3, 4, 5. Sections of augite crystals from the lavas of 1794 and
+ 1820.
+
+ " 6. Group of augite crystals from the lava of 1835.
+
+ " 7. Ditto from the lava of 1822, with encluded mica-flake (_a_)
+ and portion of the glass paste, or ground-mass, of the rock
+ (_b_), containing microliths and grains of magnetite.
+
+ Fig. 8. Two crystals of olivine from the lava of 1855; they are
+ intersected on one side by the plane of the thin section, and
+ are remarkable for showing lines of gas cells, and bands of
+ growth sometimes cellular. Mag., 40 diams.
+
+ " 9. Section of rock-crystal (quartz), with double terminal
+ pyramids, from the lava of 1850.
+
+ " 10. Twin crystal of sanidine from the lava of 1858. Mag., 40
+ diams.
+
+ " 11, 12, 13. Sections of plagioclase crystals (probably
+ labradorite) from the lava of 1855. Mag., 100 diams.
+
+ " 14. Section of olivine crystal from the lava of 1631--imperfectly
+ formed. Mag., 30 diams.
+
+ " 15. Section of mica-flake from the lava of 1822. Mag., 30 diams.
+
+[Illustration: Plate III.]
+
+EXPLANATION OF PLATE III.
+
+MAGNIFIED SECTIONS OF VOLCANIC ROCKS.
+
+ 1. Diorite dyke, traversing Assynt limestone, North Highlands.
+
+ 2. Basalt from upper beds, near Giant's Causeway, County Antrim.
+
+ 3. Hornblende-hypersthene-augite Andesite, from Pichupichu, Andes.
+
+ 4. Augite-Andesite from Pichupichu, Andes.
+
+ 5. Olivine dolerite, with hornblende and biotite, Madagascar.
+
+ 6. Leucite basalt, with mellilite, Capo di Bove, Italy.
+
+[Illustration: Plate IV.]
+
+EXPLANATION OF PLATE IV.
+
+MAGNIFIED SECTIONS OF VOLCANIC ROCKS.
+
+ 1. Vesuvian lava, glass paste with numerous crystals of leucite;
+ others of augite and nepheline porphyritically developed; also
+ small grains of magnetite.
+
+ 2. Vesuvian lava, glass paste with numerous crystals of leucite;
+ others of olivine, hornblende, and sanidine, porphyritically
+ developed; small grains of magnetite.
+
+ 3. Trachyte from Hungary; felsitic paste with crystals of
+ hornblende and sanidine, and a little magnetite.
+
+ 4. Gabbro, from Carlingford Hill, Ireland, consisting of anorthite,
+ augite, a little olivine, and magnetite.
+
+ 5. Dolerite, from old volcanic neck, Scalot Hill, near Lame,
+ consisting of labradorite, augite, olivine, and magnetite.
+
+ 6. Dolerite, Ballintoy, County Antrim, showing ophetic structure,
+ consisting of augite, labradorite, and magnetite.
+
+[1] Mr. S. Allport has discovered this in the rock called the "Wolf
+Rock" off the coast of Cornwall. The most important work on basalt is
+that by F. Zirkel, _Unters. ueber mikros. Zusammensetzung und Structur
+der Basaltgesteine_. Bonn (1870).
+
+[2] Zirkel, _Die mikroskopische Beschaffenheit der Mineralien und
+Gesteine_, p. 153. Leipsig (1873).
+
+[3] Zirkel, _Petrog._, i. 578; B. von Cotta, p. 178 (Eng. Trans.).
+
+
+
+
+INDEX.
+
+
+
+
+INDEX.
+
+
+Abyssinian table-lands, 190 _et seq._
+
+Albano, Lake, 89
+
+America, volcanic regions of North, 136 _et seq._;
+ of Western, 144
+
+Andes, 18, 27, 227, 254
+
+Andesite, 263
+
+Antrim, 154 _et seq._
+
+Arabia, dormant volcanoes of, 126-135
+
+Arabian desert, 134
+
+Archibald, C. D., 213
+
+Arizona, volcanoes of, 137
+
+Argyll, Duke of, 173
+
+Ascension, 36
+
+Ashangi, volcanic series of, 192
+
+Atmospheric effects of Krakatoa eruption, 213-214
+
+Auckland district, volcanoes of, 147
+
+Auvergne, volcanic regions of, 14, 16, 92 _et seq._
+
+Azores, 32
+
+
+Ball, Sir R. S., 242, 244
+
+Basalt, 260
+
+Blanford, W. T., 188, 189
+
+Bonneville, Lake, 141-142
+
+British Isles,
+ Tertiary volcanic districts of, 154 _et seq._, 227;
+ pre-Tertiary volcanic districts of, 196 _et seq._
+
+Buch, L. von, 6, 11, 24
+
+
+California, volcanoes of, 140
+
+Callirrhoe, springs of, 133
+
+Canyon, the Grand, 138
+
+Cantal, volcanoes of the, 99-101
+
+Cape Colony, Basalts of, 194
+
+Charleston earthquake, 218, 222, 224
+
+Chambers, G. F., 246
+
+Charnwood Forest, 198
+
+Chimborazo, 18
+
+Clermont, vale of, 96-97
+
+Clinkstone, 263
+
+Cordilleras of Quito, 25
+
+Cotopaxi, 16-18, 24, 26
+
+Crater-cones, Lava, 19
+
+Crateriform cones, 13
+
+Craterless domes, 15
+
+
+Dana, Prof. J. D., 19, 39, 249
+
+Darwin, 28, 30
+
+Darwin, Prof. G. H., 9, 231
+
+Daubeny, 7, 61, 69
+
+Davison, C., 9, 231
+
+Davy, Sir H., 11
+
+Deccan trap-series, 187 _et seq._
+
+Demavend, Mount, 24
+
+Diabase, 262
+
+Diorite, 261
+
+Dolerite, 261
+
+Domite, 263
+
+Dore, volcanoes of Mont, 100-101
+
+Doughty, C. M., 127
+
+Durocher, 232
+
+Dutton, Capt. C. E., 9, 220, 222
+
+Dykes in Ireland, 169-170
+
+
+Earthquakes, 217 _et seq._
+
+Errigal, 10
+
+Etna, 14, 61 _et seq._, 229
+
+
+Fingal's Cave, 185
+
+Forbes, D., 27
+
+France, extinct volcanoes of, 92 _et seq._
+
+
+Gabbro, 261
+
+Gardner, J. S., 156
+
+Geikie, Sir A., 8, 29, 143, 156, 160, 169, 172, 176, 177, 196
+
+Giant's Causeway, 165-166
+
+Granite, 264
+
+Granophyre, 264; of Mull, 174
+
+Green, Prof. A. H., 194
+
+
+Hatch, Dr., 260
+
+Haughton, Prof., 68
+
+Hauran, volcanoes of the, 22, 129
+
+Haute Loire, volcanic districts of, 101-105
+
+Hawaii, volcanoes of, 39, 249, 251
+
+Hecla, 32
+
+Herschel, Sir J., 244
+
+Hibbert, Dr. S., 6, 114, 124
+
+Hochstetter, F. von, 147
+
+Hopkins, 171, 217
+
+Hull, Dr. E. G., 110
+
+Humboldt, A. von, 20, 25
+
+Hutton, James, 5
+
+
+Iceland, volcanoes of, 30-32
+
+Ireland, volcanic Tertiary rocks of, 154 _et seq._
+
+
+Jaulan, 129
+
+Johnston-Lavis, 52
+
+Jordan valley, 126 _et seq._, 226
+
+Jorullo, 24
+
+Judd, Prof., 8, 68, 69, 71, 172, 178, 208
+
+
+Krakatoa, eruption of, 206 _et seq._
+
+Kurile Islands, volcanoes of, 28
+
+
+Laacher See, 121-123
+
+Lampophyre, 262
+
+Lancerote, 34
+
+Lasaulx, Prof. von, 68
+
+Lavas, relative density of, 232-234
+
+Lima in 1746, earthquake of, 222
+
+Lipari Islands, volcanoes of, 69 _et seq._
+
+Lisbon, earthquake of, 221
+
+Lister, J. J., 38
+
+Lunar volcanoes, 236 _et seq._
+
+Lyell, Sir C., 30, 62, 78, 217
+
+
+Mackowen, Col., 74
+
+Magdala, volcanic series of, 192-193
+
+Mallet, R., 9, 217
+
+Mauna Loa, 19, 39, 249
+
+Mica-trap, 262
+
+Milne, Prof., 28, 218, 253
+
+Moab, volcanic regions of, 132
+
+Moon, volcanoes of, 236 _et seq._
+
+Monte Nuovo, 85
+
+Mull, 172 _et seq._
+
+
+Neapolitan group of volcanoes, 28
+
+New Zealand, volcanoes of, 146
+
+
+Obsidian, 264
+
+Ocean waves of seismic origin, 208, 220
+
+O'Reilly, Prof., 9, 219
+
+Orizaba, 21
+
+Ovid, 3
+
+
+Pacific, volcanic islands of, 37
+
+Palestine, dormant volcanoes of, 126-135
+
+Palmieri, Prof., 55
+
+Pantelleria, 74
+
+Phlegraean fields, 85
+
+Phonolite, 263
+
+Pitchstone, 264
+
+Pliny, 2, 4
+
+Porphyrite, 262
+
+Powell, Major, 138
+
+Pre-Tertiary volcanic rocks, 187 _et seq._;
+ of British Isles, 196 _et seq._
+
+Puy de Dome, 105-110
+
+Pythagoreans on volcanoes, 2-3
+
+
+Quito, Cordilleras of, 25
+
+
+Rangitoto, 19, 149
+
+Reyer, Dr. E., 17
+
+Rhine valley, volcanoes of, 113 _et seq._
+
+Rhyolite, 263
+
+Riviera in 1887, earthquake of, 219
+
+Rocca Monfina, 80
+
+Roderberg, 119, 120
+
+Rome, 88-89
+
+Rosenbusch, H., 260
+
+Roto Mahana, 151
+
+Ruapahu, 151
+
+Russell, Hon. Rollo, 213
+
+Rutley, F., 260
+
+
+St. Helena, 37
+
+San Francisco, Mount, 138
+
+Santorin, 76-83
+
+Schehallion, 10
+
+Schumacher, 127
+
+Scotland, volcanic districts of, 172 _et seq._
+
+Scrope, Poulett, 5, 73, 93, 98
+
+Scuir of Eigg, 180-184
+
+Seismic phenomena, special, 201 _et seq._, 217 _et seq._
+
+Shasta, Mount, 140
+
+Siebengebirge, 116-120
+
+Skye, 177-179
+
+Sleamish, 168
+
+Smyth, Piazzi, 33
+
+Snake River, volcanoes of, 142
+
+Staffa, 185-186
+
+Strabo on volcanoes, 3
+
+Stromboli, 71-73
+
+Sumatra, volcanic action in, 226
+
+Syenite, 262
+
+Symes, R. G., 167
+
+Syria, earthquakes in, 219
+
+
+Taupo Lake, 150
+
+Taylor, Mount, 138
+
+Tell el Ahmar, 131
+
+Tell el Akkasheh, 131
+
+Tell el Farras, 131
+
+Tell Abu en Neda, 130
+
+Tell Abu Nedir, 129
+
+Templepatrick, quarry at, 160
+
+Teneriffe, 33
+
+Tertiary period, volcanic activity of, 255
+
+Thucydides, 2
+
+Tonga Islands, volcanoes of, 38
+
+Tongariro, 151
+
+Trachyte, 263
+
+Trass of Bruehl Valley, 123-125
+
+Tristan da Cunha, 37
+
+Tristram, Canon, 127, 131
+
+
+Utah, volcanoes of, 137
+
+
+Verbeek, R. D. M., 202
+
+Vesuvius, 4, 14, 41-60, 67, 229
+
+Volcanoes,
+ historic notices of, 1-5;
+ form, structure, and composition of, 10-19;
+ lines and groups of active, 20-29;
+ of mid-ocean, 30-40;
+ extinct or dormant, 84 _et seq._;
+ special volcanic and seismic phenomena, 201 _et seq._;
+ the ultimate cause of volcanic action, 225 _et seq._;
+ whether we are living in an epoch of special volcanic activity, 253-256;
+ brief account of volcanic rocks, 259-265
+
+Vulcanists, 5
+
+Vulcano, 69, 71
+
+
+Wallace, A. R., 81
+
+Waltershausen, W. S. von, 7, 61
+
+Wellington, Mount, 149
+
+Wharton, Capt., 212
+
+Whymper, E., 18
+
+
+Yarmuk, valley of the, 129, 131
+
+Yellowstone Park, 145
+
+
+Zirkel, F., 260
+
+Zoellner, 240
+
+
+THE WALTER SCOTT PRESS, NEWCASTLE-ON-TYNE.
+
+
+
+
+The Contemporary Science Series.
+
+EDITED BY HAVELOCK ELLIS.
+
+ * * * * *
+
+Crown 8vo, Cloth, 3s. 6d. per vol.; Half Morocco, 6s. 6d.
+
+I. THE EVOLUTION OF SEX. By Professor PATRICK GEDDES and J. ARTHUR
+ THOMSON. With 90 Illustrations. Second Edition.
+
+"The authors have brought to the task--as indeed their names
+guarantee--a wealth of knowledge, a lucid and attractive method of
+treatment, and a rich vein of picturesque language."--_Nature._
+
+II. ELECTRICITY IN MODERN LIFE. By G. W. DE TUNZELMANN. With 88
+ Illustrations.
+
+"A clearly-written and connected sketch of what is known about
+electricity and magnetism, the more prominent modern applications, and
+the principles on which they are based."--_Saturday Review._
+
+III. THE ORIGIN OF THE ARYANS. By Dr. ISAAC TAYLOR. Illustrated. Second
+ Edition.
+
+"Canon Taylor is probably the most encyclopaedic all-round scholar now
+living. His new volume on the Origin of the Aryans is a first-rate
+example of the excellent account to which he can turn his exceptionally
+wide and varied information.... Masterly and exhaustive."--_Pall Mall
+Gazette._
+
+IV. PHYSIOGNOMY AND EXPRESSION. By P. MANTEGAZZA. Illustrated.
+
+"Professor Mantegazza is a writer full of life and spirit, and the
+natural attractiveness of his subject is not destroyed by his scientific
+handling of it."--_Literary World_ (Boston).
+
+V. EVOLUTION AND DISEASE. By J. B. SUTTON, F.R.C.S. With 135
+ Illustrations.
+
+"The work is of special value to professional men, yet educated persons
+generally will find much in it which it is both interesting and
+important to know."--_The Scottish Weekly._
+
+VI. THE VILLAGE COMMUNITY. By G. L. GOMME. Illustrated.
+
+"His book will probably remain for some time the best work of reference
+for facts bearing on those traces of the village community which have
+not been effaced by conquest, encroachment, and the heavy hand of Roman
+law."--_Scottish Leader._
+
+VII. THE CRIMINAL. By HAVELOCK ELLIS. Illustrated.
+
+"An ably written, an instructive, and a most entertaining book."--_Law
+Quarterly Review._
+
+ * * * * *
+
+London: WALTER SCOTT, LIMITED, 24 Warwick Lane.
+
+The Contemporary Science Series--continued.
+
+VIII. SANITY AND INSANITY. By Dr. CHARLES MERCIER. Illustrated.
+
+"Taken as a whole, it is the brightest book on the physical side of
+mental science published in our time."--_Pall Mall Gazette._
+
+IX. HYPNOTISM. By Dr. ALBERT MOLL. Second Edition.
+
+"Marks a step of some importance in the study of some difficult
+physiological and psychological problems which have not yet received
+much attention in the scientific world of England."--_Nature._
+
+X. MANUAL TRAINING. By Dr. C. M. WOODWARD, Director of the Manual
+ Training School, St. Louis. Illustrated.
+
+"There is no greater authority on the subject than Professor
+Woodward."--_Manchester Guardian._
+
+XI. THE SCIENCE OF FAIRY TALES. By E. SIDNEY HARTLAND.
+
+"Mr. Hartland's book will win the sympathy of all earnest students, both
+by the knowledge it displays, and by a thorough love and appreciation of
+his subject, which is evident throughout."--_Spectator._
+
+XII. PRIMITIVE FOLK. By ELIE RECLUS.
+
+"For an introduction to the study of the questions of property,
+marriage, government, religion,--in a word, to the evolution of
+society,--this little volume will be found most convenient."--_Scottish
+Leader._
+
+XIII. THE EVOLUTION OF MARRIAGE. By Professor LETOURNEAU.
+
+"Among the distinguished French students of sociology, Professor
+Letourneau has long stood in the first rank. He approaches the great
+study of man free from bias and shy of generalisations. To collect,
+scrutinise, and appraise facts is his chief business."--_Science._
+
+XIV. BACTERIA AND THEIR PRODUCTS. By Dr. G. SIMS WOODHEAD. Illustrated.
+
+"An excellent summary of the present state of knowledge of the
+subject."--_Lancet._
+
+XV. EDUCATION AND HEREDITY. By J. M. GUYAU.
+
+"It is a sign of the value of this book that the natural impulse on
+arriving at its last page is to turn again to the first, and try to
+gather up and coordinate some of the many admirable truths it
+presents."--_Anti-Jacobin._
+
+XVI. THE MAN OF GENIUS. By Professor LOMBROSO. Illustrated.
+
+"By far the most comprehensive and fascinating collection of facts and
+generalisations concerning genius which has yet been brought
+together."--_Journal of Mental Science._
+
+XVII. THE GRAMMAR OF SCIENCE. By Professor KARL PEARSON. Illustrated.
+
+XVIII. PROPERTY: ITS ORIGIN AND DEVELOPMENT. By CH. LETOURNEAU, General
+ Secretary to the Anthropological Society, Paris, and Professor in
+ the School of Anthropology, Paris.
+
+An ethnological account of the beginnings of property among animals, of
+its communistic stages among primitive races, and of its later
+individualistic developments, together with a brief sketch of its
+probable evolution in the future.
+
+ * * * * *
+
+London: WALTER SCOTT, LIMITED, 24 Warwick Lane.
+
+
+
+
+
+ * * * * *
+
+
+
+
+Transcriber's note:
+
+
+Changed 'Kilarrea' to 'Kilauea' on page 19: Mauna Loa and Kilarrea.
+
+Changed 'Kilanea' to 'Kilauea' on page 39: Kilanea, 4158 feet.
+
+Made punctuation (semi-colons) consistent in caption to figure 16.
+
+Changed 'Brionde' to 'Brioude' on page 94: till at Brionde it becomes.
+
+Changed 'occuping' to 'occupying' on page 96: occuping a hollow.
+
+Changed 'Rodesberg' to 'Roderberg' on page 118: old extinct volcano of
+Rodesberg.
+
+Changed 'Wolkenberg' to 'Wolkenburg' on page 118: and that of the
+Wolkenberg.
+
+Left the reference to Jeremiah, l. 25. in footnote to Part III Chapter
+I, although Jeremiah, li. 25. seems more appropriate.
+
+Changed 'fumarols' to 'fumaroles' on page 137: fumarols give evidence.
+
+Removed extra comma on page 153: of the present, epoch.
+
+Changed 'columnal' to 'columnar' on page 176: the columnal structure.
+
+Changed 'groves' to 'grooves' on page 183: the groves and scorings.
+
+Changed 'Angust' to 'August' on page 212: the 27th of Angust.
+
+Changed 'mikroskopischen' to 'mikroskopische' on page 260: ueber
+mikroskopischen Structur.
+
+Changed 'become' to 'becomes' on page 260: the rock become a
+leucite-basalt.
+
+Left inconsistent spellings of 'Baalbec' and 'Baalbeck'; 'Harrat' and
+'Harrat'; 'metres' and 'metres'; 'pitchstone' and 'pitch-stone';
+'prehistoric' and 'pre-historic'; 'Rhone' and 'Rhone'; 'sub-aerial',
+'subaerial' and 'subaerial'; 'tableland' and 'table-land'.
+
+Greek words were replaced with their transliterations: 'meson pyr' and
+'Peri kosmou'.
+
+The oe-ligature was expanded to the two separate characters: 'Euboea'
+and 'Boeotia'.
+
+Left the list numbering as is at the beginning of Chapter II of Part IV,
+even though the list begins at item c, as if it continues the list which
+began in the previous chapter.
+
+
+
+***END OF THE PROJECT GUTENBERG EBOOK VOLCANOES: PAST AND PRESENT***
+
+
+******* This file should be named 31627.txt or 31627.zip *******
+
+
+This and all associated files of various formats will be found in:
+http://www.gutenberg.org/dirs/3/1/6/2/31627
+
+
+
+Updated editions will replace the previous one--the old editions
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