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diff --git a/old/3772-h/files/ch29.html b/old/3772-h/files/ch29.html new file mode 100644 index 0000000..5b74aea --- /dev/null +++ b/old/3772-h/files/ch29.html @@ -0,0 +1,833 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Lyell/The Student’s Elements of Geology --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Student’s Elements of Geology: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv= +"Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel="stylesheet" href="geology.css" type="text/css"> +</head> +<body> +<p><b>The Student’s Elements of Geology</b></p> + +<hr> +<p class="page"><a name="page 520">[ 520 ]</a></p> + +<p> </p> + +<center><b>Chapter XXIX</b><br> +<br> +ON THE AGES OF VOLCANIC ROCKS.</center> + +<p class="intro">Tests of relative Age of Volcanic Rocks. — +Why ancient and modern Rocks can not be identical. — Tests by +Superposition and intrusion. — Test by Alteration of Rocks in +Contact. — Test by Organic Remains. — Test of Age by +Mineral Character. — Test by Included Fragments. — +Recent and Post-pliocene volcanic Rocks. — Vesuvius, +Auvergne, Puy de Côme, and Puy de Pariou. — Newer +Pliocene volcanic Rocks. — Cyclopean Isles, Etna, Dikes of +Palagonia, Madeira. — Older Pliocene volcanic Rocks. — +Italy. — Pliocene Volcanoes of the Eifel. — Trass.</p> + +<p>Having in the former part of this work referred the sedimentary +strata to a long succession of geological periods, we have now to +consider how far the volcanic formations can be classed in a +similar chronological order. The tests of relative age in this +class of rocks are four: first, superposition and intrusion, with +or without alteration of the rocks in contact; second, organic +remains; third, mineral characters; fourth, included fragments of +older rocks.</p> + +<p>Besides these four tests it may be said, in a general way, that +volcanic rocks of Primary or Palæozoic antiquity differ from +those of the Secondary or Mesozoic age, and these again from the +Tertiary and Recent. Not, perhaps, that they differed originally in +a greater degree than the modern volcanic rocks of one region, such +as that of the Andes, differ from those of another, such as +Iceland, but because all rocks permeated by water, especially if +its temperature be high, are liable to undergo a slow +transmutation, even when they do not assume a new crystalline form +like that of the hypogene rocks.</p> + +<p>Although subaërial and submarine denudation, as before +stated, remove, in the course of ages, large portions of the upper +or more superficial products of volcanoes, yet these are sometimes +preserved by subsidence, becoming covered by the sea or by +superimposed marine deposits. In this way they may be protected for +ages from the waves of the sea, or the destroying action of rivers, +while, at the same time, they may not sink so deep as to be exposed +to that Plutonic action (to be spoken of in Chapter XXXI) which +would convert them into crystalline rocks. But even in this case +they will not remain unaltered, because they will be percolated by +water often of high temperature, and charged with</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 521">[ 521 ]</a></p> + +<p>carbonate of lime, silex, iron, and other mineral ingredients, +whereby gradual changes in the constitution of the rocks may be +superinduced. Every geologist is aware how often silicified trees +occur in volcanic tuffs, the perfect preservation of their internal +structure showing that they have not decayed before the petrifying +material was supplied.</p> + +<p>The porous and vesicular nature of a large part, both of the +basaltic and trachytic lavas, affords cavities in which silex and +carbonate of lime are readily deposited. Minerals of the zeolite +family, the composition of which has already been alluded to, <a +href="ch28.html#page 500">p. 500</a>, occur in amygdaloids and +other trap-rocks in great abundance, and Daubrée’s +observations have proved that they are not always simple deposits +of substances held in solution by the percolating waters, being +occasionally products of the chemical action of that water on the +rock through which they are filtered, and portions of which are +decomposed. From these considerations it follows that the perfect +identity of very ancient and very modern volcanic formations is +scarcely possible.</p> + +<center><img src="../images5/fig597.jpg" width="337" height="83" alt= +"Fig. 597: Showing melted matter forced between two strata."> +</center> + +<p><b>Tests by Superposition.</b>—If a volcanic rock rest +upon an aqueous deposit, the volcanic must be the newest of the +two; but the like rule does not hold good where the aqueous +formation rests upon the volcanic, for melted matter, rising from +below, may penetrate a sedimentary mass without reaching the +surface, or may be forced in conformably between two strata, as <i> +b</i> below D in Fig. 597, after which it may cool down and +consolidate. Superposition, therefore, is not of the same value as +a test of age in the unstratified volcanic rocks as in +fossiliferous formations. We can only rely implicitly on this test +where the volcanic rocks are contemporaneous, not where they are +intrusive. Now, they are said to be contemporaneous if produced by +volcanic action which was going on simultaneously with the +deposition of the strata with which they are associated. Thus in +the section at D (Fig. 597), we may perhaps ascertain that the trap +<i>b</i> flowed over the fossiliferous bed <i>c,</i> and that, +after its consolidation, <i>a</i> was deposited upon it, <i>a</i> +and <i>c</i> both belonging to the same geological period. But, on +the other hand, we must conclude the trap to be intrusive, if the +stratum <i>a</i> be altered by <i>b</i> at the point of +contact,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 522">[ 522 ]</a></p> + +<p>or if, in pursuing <i>b</i> for some distance, we find at length +that it cuts through the stratum <i>a,</i> and then overlies it as +at E.</p> + +<img src="../images5/fig598.jpg" width="168" height="106" alt= +"Fig. 598: Section through sedimentary mass with melted matter." +align="left"> + +<p>We may, however, be easily deceived in supposing the volcanic +rock to be intrusive, when in reality it is contemporaneous; for a +sheet of lava, as it spreads over the bottom of the sea, can not +rest everywhere upon the same stratum, either because these have +been denuded, or because, if newly thrown down, they thin out in +certain places, thus allowing the lava to cross their edges. +Besides, the heavy igneous fluid will often, as it moves along, cut +a channel into beds of soft mud and sand. Suppose the submarine +lava F (Fig. 598) to have come in contact in this manner with the +strata <i>a, b, c,</i> and that after its consolidation the strata +<i>d, e</i> are thrown down in a nearly horizontal position, yet so +as to lie unconformably to F, the appearance of subsequent +intrusion will here be complete, although the trap is in fact +contemporaneous. We must not, therefore, hastily infer that the +rock F is intrusive, unless we find the overlying strata, <i>d, +e,</i> to have been altered at their junction, as if by heat.</p> + +<p>The test of age by superposition is strictly applicable to all +stratified volcanic tuffs, according to the rules already explained +in the case of sedimentary deposits (see <a href= +"ch8.html#page 124">p. 124</a>).</p> + +<p><b>Test of Age by Organic Remains.</b>—We have seen how, +in the vicinity of active volcanoes, scoriæ, pumice, fine +sand, and fragments of rock are thrown up into the air, and then +showered down upon the land, or into neighbouring lakes or seas. In +the tuffs so formed shells, corals, or any other durable organic +bodies which may happen to be strewed over the bottom of a lake or +sea will be imbedded, and thus continue as permanent memorials of +the geological period when the volcanic eruption occurred. +Tufaceous strata thus formed in the neighbourhood of Vesuvius, +Etna, Stromboli, and other volcanoes now in islands or near the +sea, may give information of the relative age of these tuffs at +some remote future period when the fires of these mountains are +extinguished. By evidence of this kind we can establish a +coincidence in age between volcanic rocks and the different +primary, secondary, and tertiary fossiliferous strata.</p> + +<p>The tuffs alluded to may not always be marine, but may include, +in some places, fresh-water shells; in others, the bones of +terrestrial quadrupeds. The diversity of organic remains in +formations of this nature is perfectly intelligible,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 523">[ 523 ]</a></p> + +<p>if we reflect on the wide dispersion of ejected matter during +late eruptions, such as that of the volcano of Coseguina, in the +province of Nicaragua, January 19, 1835. Hot cinders and fine +scoriæ were then cast up to a vast height, and covered the +ground as they fell to the depth of more than ten feet, for a +distance of eight leagues from the crater, in a southerly +direction. Birds, cattle, and wild animals were scorched to death +in great numbers, and buried in ashes. Some volcanic dust fell at +Chiapa, upward of 1200 miles, not to leeward of the volcano, as +might have been anticipated, but to windward, a striking proof of a +counter-current in the upper region of the atmosphere; and some on +Jamaica, about 700 miles distant to the north-east. In the sea, +also, at the distance of 1100 miles from the point of eruption, +Captain Eden of the “Conway” sailed 40 miles through +floating pumice, among which were some pieces of considerable +size.*</p> + +<p><b>Test of Age by Mineral Composition.</b>—As sediment of +homogeneous composition, when discharged from the mouth of a large +river, is often deposited simultaneously over a wide space, so a +particular kind of lava flowing from a crater during one eruption +may spread over an extensive area; thus in Iceland, in 1783, the +melted matter, pouring from Skaptar Jokul, flowed in streams in +opposite directions, and caused a continuous mass the extreme +points of which were 90 miles distant from each other. This +enormous current of lava varied in thickness from 100 feet to 600 +feet, and in breadth from that of a narrow river gorge to 15 +miles.† Now, if such a mass should afterwards be divided +into separate fragments by denudation, we might still, perhaps, +identify the detached portions by their similarity in mineral +composition. Nevertheless, this test will not always avail the +geologist; for, although there is usually a prevailing character in +lava emitted during the same eruption, and even in the successive +currents flowing from the same volcano, still, in many cases, the +different parts even of one lava-stream, or, as before stated, of +one continuous mass of trap, vary much in mineral composition and +texture.</p> + +<p>In Auvergne, the Eifel, and other countries where trachyte and +basalt are both present, the trachytic rocks are for the most part +older than the basaltic. These rocks do, indeed, sometimes +alternate partially, as in the volcano of Mont Dor, in Auvergne; +and in Madeira trachytic rocks overlie an older basaltic series; +but the trachyte occupies more generally an inferior position, and +is cut through and overflowed by</p> + +<p class="fnote">* Caldcleugh, Phil. Trans., 1836, p. 27.<br> +† See Principles, <i>Index,</i> “Skaptar +Jokul.”</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 524">[ 524 ]</a></p> + +<p>basalt. It can by no means be inferred that trachyte +predominated at one period of the earth’s history and basalt +at another, for we know that trachytic lavas have been formed at +many successive periods, and are still emitted from many active +craters; but it seems that in each region, where a long series of +eruptions have occurred, the lavas containing feldspar more rich in +silica have been first emitted, and the escape of the more augitic +kinds has followed. The hypothesis suggested by Mr. Scrope may, +perhaps, afford a solution of this problem. The minerals, he +observes, which abound in basalt are of greater specific gravity +than those composing the feldspathic lavas; thus, for example, +hornblende, augite, and olivine are each more than three times the +weight of water; whereas common feldspar and albite have each +scarcely more than 2½ times the specific gravity of water; +and the difference is increased in consequence of there being much +more iron in a metallic state in basalt and greenstone than in +trachyte and other allied feldspathic lavas. If, therefore, a large +quantity of rock be melted up in the bowels of the earth by +volcanic heat, the denser ingredients of the boiling fluid may sink +to the bottom, and the lighter remaining above would in that case +be first propelled upward to the surface by the expansive power of +gases. Those materials, therefore, which occupy the lowest place in +the subterranean reservoir will always be emitted last, and take +the uppermost place on the exterior of the earth’s crust.</p> + +<p><b>Test by Included Fragments.</b>—We may sometimes +discover the relative age of two trap-rocks, or of an aqueous +deposit and the trap on which it rests, by finding fragments of one +included in the other in cases such as those before alluded to, +where the evidence of superposition alone would be insufficient. It +is also not uncommon to find a conglomerate almost exclusively +composed of rolled pebbles of trap, associated with some +fossiliferous stratified formation in the neighbourhood of massive +trap. If the pebbles agree generally in mineral character with the +latter, we are then enabled to determine its relative age by +knowing that of the fossiliferous strata associated with the +conglomerate. The origin of such conglomerates is explained by +observing the shingle beaches composed of trap-pebbles in modern +volcanoes, as at the base of Etna.</p> + +<p><b>Recent and Post-pliocene Volcanic Rocks.</b>—I shall +now select examples of contemporaneous volcanic rocks of successive +geological periods, to show that igneous causes have been in +activity in all past ages of the world. They have been perpetually +shifting the places where they have broken out</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 525">[ 525 ]</a></p> + +<p>at the earth’s surface, and we can sometimes prove that +those areas which are now the great theatres of volcanic action +were in a state of perfect tranquillity at remote geological +epochs, and that, on the other hand, in places where at former +periods the most violent eruptions took place at the surface and +continued for a great length of time, there has been an entire +suspension of igneous action in historical times, and even, as in +the British Isles, throughout a large part of the antecedent +Tertiary Period.</p> + +<p>In the absence of British examples of volcanic rocks newer than +the Upper Miocene, I may state that in other parts of the world, +especially in those where volcanic eruptions are now taking place +from time to time, there are tuffs and lavas belonging to that part +of the Tertiary era the antiquity of which is proved by the +presence of the bones of extinct quadrupeds which co-existed with +terrestrial, fresh-water, and marine mollusca of species still +living. One portion of the lavas, tuffs, and trap-dikes of Etna, +Vesuvius, and the island of Ischia has been produced within the +historical era; another and a far more considerable part originated +at times immediately antecedent, when the waters of the +Mediterranean were already inhabited by the existing testacea, but +when certain species of elephant, rhinoceros, and other quadrupeds +now extinct, inhabited Europe.</p> + +<p><i>Vesuvius.</i>—I have traced in the “Principles of +Geology” the history of the changes which the volcanic region +of Campania is known to have undergone during the last 2000 years. +The aggregate effect of igneous operations during that period is +far from insignificant, comprising as it does the formation of the +modern cone of Vesuvius since the year 79, and the production of +several minor cones in Ischia, together with that of Monte Nuovo in +the year 1538. Lava-currents have also flowed upon the land and +along the bottom of the sea—volcanic sand, pumice, and +scoriæ have been showered down so abundantly that whole +cities were buried—tracts of the sea have been filled up or +converted into shoals—and tufaceous sediment has been +transported by rivers and land-floods to the sea. There are also +proofs, during the same recent period, of a permanent alteration of +the relative levels of the land and sea in several places, and of +the same tract having, near Puzzuoli, been alternately upheaved and +depressed to the amount of more than twenty feet. In connection +with these convulsions, there are found, on the shores of the Bay +of Baiæ, recent tufaceous strata, filled with articles +fabricated by the hands of man, and mingled with marine shells.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 526">[ 526 ]</a></p> + +<p>It has also been stated (<a href="ch13.html#page 206">p. +206</a>), that when we examine this same region, it is found to +consist largely of tufaceous strata, of a date anterior to human +history or tradition, which are of such thickness as to constitute +hills from 500 to more than 2000 feet in height. Some of these +strata contain marine shells which are exclusively of living +species, others contain a slight mixture, one or two per cent of +species not known as living.</p> + +<p>The ancient part of Vesuvius is called Somma, and consists of +the remains of an older cone which appears to have been partly +destroyed by explosion. In the great escarpment which this remnant +of the ancient mountain presents towards the modern cone of +Vesuvius, there are many dikes which are for the most part +vertical, and traverse the inclined beds of lava and scoriæ +which were successively superimposed during those eruptions by +which the old cone was formed. They project in relief several +inches, or sometimes feet, from the face of the cliff, being +extremely compact, and less destructible than the intersected tuffs +and porous lavas. In vertical extent they vary from a few yards to +500 feet, and in breadth from one to twelve feet. Many of them cut +all the inclined beds in the escarpment of Somma from top to +bottom, others stop short before they ascend above halfway. In +mineral composition they scarcely differ from the lavas of Somma, +the rock consisting of a base of leucite and augite, through which +large crystals of augite and some of leucite are scattered.</p> + +<p>Nothing is more remarkable than the usual parallelism of the +opposite sides of the dikes, which correspond almost as regularly +as the two opposite faces of a wall of masonry. This character +appears at first the more inexplicable, when we consider how jagged +and uneven are the rents caused by earthquakes in masses of +heterogeneous composition, like those composing the cone of Somma. +In explanation of this phenomenon, M. Necker refers us to Sir W. +Hamilton’s account of an eruption of Vesuvius in the year +1779, who records the following fact: “The lavas, when they +either boiled over the crater, or broke out from the conical parts +of the volcano, constantly formed channels as regular as if they +had been cut by art down the steep part of the mountain; and whilst +in a state of perfect fusion, continued their course in those +channels, which were sometimes full to the brim, and at other times +more or less so, according to the quantity of matter in motion.</p> + +<p>”These channels (says the same observer), I have found, +upon examination after an eruption, to be in general from</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 527">[ 527 ]</a></p> + +<p>two to five or six feet wide, and seven or eight feet deep. They +were often hid from the sight by a quantity of scoriæ that +had formed a crust over them; and the lava, having been conveyed in +a covered way for some yards, came out fresh again into an open +channel. After an eruption, I have walked in some of those +subterraneous or covered galleries, which were exceedingly curious, +the sides, top, and bottom <i>being worn perfectly smooth and +even</i> in most parts by the violence of the currents of the +red-hot lavas which they had conveyed for many weeks +successively.” I was able to verify this phenomenon in 1858, +when a stream of lava issued from a lateral cone.* Now, the walls +of a vertical fissure, through which lava has ascended in its way +to a volcanic vent, must have been exposed to the same erosion as +the sides of the channels before adverted to. The prolonged and +uniform friction of the heavy fluid, as it is forced and made to +flow upward, can not fail to wear and smooth down the surfaces on +which it rubs, and the intense heat must melt all such masses as +project and obstruct the passage of the incandescent fluid.</p> + +<p>The rock composing the dikes both in the modern and ancient part +of Vesuvius is far more compact than that of ordinary lava, for the +pressure of a column of melted matter in a fissure greatly exceeds +that in an ordinary stream of lava; and pressure checks the +expansion of those gases which give rise to vesicles in lava. There +is a tendency in almost all the Vesuvian dikes to divide into +horizontal prisms, a phenomenon in accordance with the formation of +vertical columns in horizontal beds of lava; for in both cases the +divisions which give rise to the prismatic structure are at right +angles to the cooling surfaces. (See <a href="ch28.html#page 510"> +p. 510</a>.)</p> + +<p><i>Auvergne.</i>—Although the latest eruptions in central +France seem to have long preceded the historical era, they are so +modern as to have a very intimate connection with the present +superficial outline of the country and with the existing valleys +and river-courses. Among a great number of cones with perfect +craters, one called the Puy de Tartaret sent forth a lava-current +which can be traced up to its crater, and which flowed for a +distance of thirteen miles along the bottom of the present valley +to the village of Nechers, covering the alluvium of the old valley +in which were preserved the bones of an extinct species of horse, +and of a lagomys and other quadrupeds all closely allied to recent +animals, while the associated land-shells were of species now +living, such as <i>Cyclostoma elegans, Helix hortensis, H. +nemoralis,</i></p> + +<p class="fnote">* Principles of Geology, vol. i, p. 626.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 528">[ 528 ]</a></p> + +<p><i>H. lapicida,</i> and <i>Clausilia rugosa.</i> That the +current which has issued from the Puy de Tartaret may, +nevertheless, be very ancient in reference to the events of human +history, we may conclude, not only from the divergence of the +mammiferous fauna from that of our day, but from the fact that a +Roman bridge of such form and construction as continued in use only +down to the fifth century, but which may be older, is now seen at a +place about a mile and a half from St. Nectaire. This ancient +bridge spans the river Couze with two arches, each about fourteen +feet wide. These arches spring from the lava of Tartaret, on both +banks, showing that a ravine precisely like that now existing had +already been excavated by the river through that lava thirteen or +fourteen centuries ago.</p> + +<p>While the river Couze has in most cases, as at the site of this +ancient bridge, been simply able to cut a deep channel through the +lava, the lower portion of which is shown to be columnar, the same +torrent has in other places, where the valley was contracted to a +narrow gorge, had power to remove the entire mass of basaltic rock, +causing for a short space a complete breach of continuity in the +volcanic current. The work of erosion has been very slow, as the +basalt is tough and hard, and one column after another must have +been undermined and reduced to pebbles, and then to sand. During +the time required for this operation, the perishable cone of +Tartaret, occupying the lowest part of the great valley descending +from Mont Dor (see <a href="ch30.html#page 542">p. 542</a>), and +damming up the river so as to cause the Lake of Chambon, has stood +uninjured, proving that no great flood or deluge can have passed +over this region in the interval between the eruption of Tartaret +and our own times.</p> + +<p><i>Puy de Côme.</i>—The Puy de Côme and its +lava-current, near Clermont, may be mentioned as another minor +volcano of about the same age. This conical hill rises from the +granitic platform, at an angle of between 30° and 40°, to +the height of more than 900 feet. Its summit presents two distinct +craters, one of them with a vertical depth of 250 feet. A stream of +lava takes its rise at the western base of the hill instead of +issuing from either crater, and descends the granitic slope towards +the present site of the town of Pont Gibaud. Thence it pours in a +broad sheet down a steep declivity into the valley of the Sioule, +filling the ancient river-channel for the distance of more than a +mile. The Sioule, thus dispossessed of its bed, has worked out a +fresh one between the lava and the granite of its western bank; and +the excavation</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 529">[ 529 ]</a></p> + +<p>has disclosed, in one spot, a wall of columnar basalt about +fifty feet high.*</p> + +<p>The excavation of the ravine is still in progress, every winter +some columns of basalt being undermined and carried down the +channel of the river, and in the course of a few miles rolled to +sand and pebbles. Meanwhile the cone of Côme remains +unimpaired, its loose materials being protected by a dense +vegetation, and the hill standing on a ridge not commanded by any +higher ground, so that no floods of rain-water can descend upon it. +There is no end to the waste which the hard basalt may undergo in +future, if the physical geography of the country continue +unchanged—no limit to the number of years during which the +heap of incoherent and transportable materials called the Puy de +Côme may remain in an almost stationary condition.</p> + +<p><i>Puy de Pariou.</i>—The brim of the crater of the Puy de +Pariou, near Clermont, is so sharp, and has been so little blunted +by time, that it scarcely affords room to stand upon. This and +other cones in an equally remarkable state of integrity have stood, +I conceive, uninjured, not <i>in spite</i> of their loose porous +nature, as might at first be naturally supposed, but in consequence +of it. No rills can collect where all the rain is instantly +absorbed by the sand and scoriæ, as is remarkably the case on +Etna; and nothing but a water-spout breaking directly upon the Puy +de Pariou could carry away a portion of the hill, so long as it is +not rent or ingulfed by earthquakes.</p> + +<p><b>Newer Pliocene Volcanic Rocks.</b>—The more ancient +portion of Vesuvius and Etna originated at the close of the Newer +Pliocene period, when less than ten, sometimes only one, in a +hundred of the shells differed from those now living. In the case +of Etna, it was before stated (<a href="ch13.html#page 205">p. +205</a>) that Post-pliocene formations occur in the neighbourhood +of Catania, while the oldest lavas of the great volcano are +Pliocene. These last are seen associated with sedimentary deposits +at Trezza and other places on the southern and eastern flanks of +the great cone (see <a href="ch13.html#page 205">p. 205</a>).</p> + +<p><i>Cyclopean Islands.</i>—The Cyclopean Islands, called by +the Sicilians Dei Faraglioni, in the sea-cliffs of which these beds +of clay, tuff, and associated lava are laid open to view, are +situated in the Bay of Trezza, and may be regarded as the extremity +of a promontory severed from the main land. Here numerous proofs +are seen of submarine eruptions, by which the argillaceous and +sandy strata were invaded and cut through, and tufaceous breccias +formed. Inclosed in</p> + +<p class="fnote">* Scrope’s Central France, p. 60, and +plate.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 530">[ 530 ]</a></p> + +<p>these breccias are many angular and hardened fragments of +laminated clay in different states of alteration by heat, and +intermixed with volcanic sands.</p> + +<center><img src="../images5/fig599.jpg" width="350" height="255" alt= +"Fig. 599: View of the Isle of Cyclops, in the Bay of Trezza."> +</center> + +<p>The loftiest of the Cyclopean islets, or rather rocks, is about +200 feet in height, the summit being formed of a mass of stratified +clay, the laminæ of which are occasionally subdivided by thin +arenaceous layers. These strata dip to the N.W., and rest on a mass +of columnar lava (see Fig. 599) in which the tops of the pillars +are weathered, and so rounded as to be often hemispherical.</p> + +<img src="../images5/fig600.jpg" width="172" height="294" alt= +"Fig. 600: Contortions of strata in the largest of the Cyclopean Islands." + align="left"> + +<p>In some places in the adjoining and largest islet of the group, +which lies to the north-eastward of that represented in Figure +599), the overlying clay has been greatly altered and hardened by +the igneous rock, and occasionally contorted in the most +extraordinary manner; yet the lamination has not been obliterated, +but, on the contrary, rendered much more conspicuous, by the +indurating process.</p> + +<p>In Fig. 600 I have represented a portion of the altered rock, a +few feet square, where the alternating thin laminæ of sand +and clay are contorted in a manner often observed in ancient +metamorphic schists. A great fissure, running</p> + +<br> + + +<p> </p> + +<hr> +<p class="page"><a name="page 531">[ 531 ]</a></p> + +<img src="../images5/fig601.jpg" width="184" height="248" alt= +"Fig. 601: Newer pliocene strata invaded by lava. Isle of Cyclops (horizontal section)." + align="right"> + +<p>from east to west, nearly divides this larger island into two +parts, and lays open its internal structure. In the section thus +exhibited, a dike of lava is seen, first cutting through an older +mass of lava, and then penetrating the superincumbent tertiary +strata. In one place the lava ramifies and terminates in thin +veins, from a few feet to a few inches in thickness (see Fig. 601). +The arenaceous laminæ are much hardened at the point of +contact, and the clays are converted into siliceous schist. In this +island the altered rocks assume a honey-comb structure on their +weathered surface, singularly contrasted with the smooth and even +outline which the same beds present in their usual soft and +yielding state. The pores of the lava are sometimes coated, or +entirely filled with carbonate of lime, and with a zeolite +resembling analcime, which has been called cyclopite. The latter +mineral has also been found in small fissures traversing the +altered marl, showing that the same cause which introduced the +minerals into the cavities of the lava, whether we suppose +sublimation or aqueous infiltration, conveyed it also into the open +rents of the contiguous sedimentary strata.</p> + +<p><i>Dikes of Palagonia.</i>—Dikes of vesicular and +amygdaloidal lava are also seen traversing marine tuff or peperino, +west of Palagonia, some of the pores of the lava being empty, while +others are filled with carbonate of lime. In such cases we may +suppose the tuff to have resulted from showers of volcanic sand and +scoriæ, together with fragments of limestone, thrown out by a +submarine explosion, similar to that which gave rise to Graham +Island in 1831. When the mass was, to a certain degree, +consolidated, it may have been rent open, so that the lava ascended +through fissures, the walls of which were perfectly even and +parallel. In one case, after the melted matter that filled the rent +(Fig. 602) had cooled down, it must have been fractured and shifted +horizontally by a lateral movement.</p> + +<p>In Fig. 603, the lava has more the appearance of a vein, which +forced its way through the peperino. It is highly probable that +similar appearances would be seen, if we could examine the floor of +the sea in that part</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 532">[ 532 ]</a></p> + +<img src="../images5/fig602.jpg" width="237" height="201" alt= +"Figs. 602 and 603: Ground-plan of dikes near Palagonia." align= +"left"> + +<p>of the Mediterranean where the waves have recently washed away +the new volcanic island; for when a superincumbent mass of ejected +fragments has been removed by denudation, we may expect to see +sections of dikes traversing tuff, or, in other words, sections of +the channels of communication by which the subterranean lavas +reached the surface.</p> + +<p><i>Madeira.</i>—Although the more ancient portion of the +volcanic eruptions by which the island of Madeira and the +neighbouring one of Porto Santo were built up occurred, as we shall +presently see, in the Upper Miocene Period, a still larger part of +the island is of Pliocene date. That the latest outbreaks belonged +to the Newer Pliocene Period, I infer from the close affinity to +the present flora of Madeira of the fossil plants preserved in a +leaf-bed in the north-eastern part of the island. These fossils, +associated with some lignite in the ravine of the river San Jorge, +can none of them be proved to be of extinct species, but their +antiquity may be inferred from the following considerations: +Firstly—The leaf-bed, discovered by Mr. Hartung and myself in +1853, at the height of 1000 feet above the level of the sea, crops +out at the base of a cliff formed by the erosion of a gorge cut +through alternating layers of basalt and scoriæ, the product +of a vast succession of eruptions of unknown date, piled up to a +thickness of 1000 feet, and which were all poured out after the +plants, of which about twenty species have been recognised, +flourished in Madeira. These lavas are inclined at an angle of +about 15° to the north, and came down from the great central +region of eruption. Their accumulation implies a long period of +intermittent volcanic action, subsequently to which the ravine of +San Jorge was hollowed out. Secondly—Some few of the plants, +though perhaps all of living species, are supposed to be of genera +not now existing in the island. They have been described by Sir +Charles Bunbury and Professor Heer, and the former first pointed +out that many of the leaves are of the laurel type, and analogous +to those now flourishing in the modern forests of Madeira. He also +recognised among them the leaves of <i>Woodwardia radicans,</i></p> + +<p> </p> + +<hr> +<p class="page"><a name="page 532">[ 532 ]</a></p> + +<p>and <i>Davallia Canariensis,</i> ferns now abundant in Madeira. +Thirdly—the great age of this leaf-bed of San Jorge, which +was perhaps originally formed in the crater of some ancient +volcanic cone afterwards buried under lava, is proved by its +belonging to a part of the eastern extremity of Madeira, which, +after the close of the igneous eruptions, became covered in the +adjoining district of Caniçal with blown sand in which a +vast number of land-shells were buried. These fossil shells +belonged to no less than 36 species, among which are many now +extremely rare in the island, and others, about five per cent, +extinct or unknown in any part of the world. Several of these of +the genus <i>Helix</i> are conspicuous from the peculiarity of +their forms, others from their large dimensions. The geographical +configuration of the country shows that this shell-bed is +considerably more modern than the leaf-bed; it must therefore be +referred to the Newer Pliocene, according to the definition of this +period given in a former chapter (<a href="ch9.html#page 143">p. +143</a>).</p> + +<p><b>Older Pliocene Period.</b>—<i>Italy.</i>—In +Tuscany, as at Radicofani, Viterbo, and Aquapendente, and in the +Campagna di Roma, submarine volcanic tuffs are interstratified with +the Older Pliocene strata of the Sub-apennine hills in such a +manner as to leave no doubt that they were the products of +eruptions which occurred when the shelly marls and sands of the +Sub-appenine hills were in the course of deposition. This opinion I +expressed* after my visit to Italy in 1828 and it has recently +(1850) been confirmed by the argument adduced by Sir R. Murchison +in favour of the submarine origin of the tertiary volcanic rocks of +Italy.† These rocks are well-known to rest conformably on +the Sub-apennine marls, even as far south as Monte Mario, in the +suburbs of Rome. On the exact age of the deposits of Monte Mario +new light has recently been thrown by a careful study of their +marine fossil shells, undertaken by MM. Rayneval, Van den Hecke, +and Ponzi. They have compared no less than 160 species with the +shells of the Coralline Crag of Suffolk, so well described by Mr. +Searles Wood; and the specific agreement between the British and +Italian fossils is so great, if we make due allowance for +geographical distance and the difference of latitude, that we can +have little hesitation in referring both to the same period, or to +the Older Pliocene of this work. It is highly probable that, +between the oldest trachytes of Tuscany and the newest rocks in the +neighbourhood of Naples, a</p> + +<p class="fnote">* See 1st edit. of Principles of Geology, vol. +iii, chaps. xiii and xiv, 1833; and former editions of this work, +chap. xxxi.<br> +† Quart. Geol. Journ., vol. vi, p. 281.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 534">[ 534 ]</a></p> + +<p>series of volcanic products might be detected of every age from +the Older Pliocene to the historical epoch.</p> + +<p><i>Pliocene Volcanoes of the Eifel.</i>—Some of the most +perfect cones and craters in Europe, not even excepting those of +the district round Vesuvius, may be seen on the left or west bank +of the Rhine, near Bonn and Andernach. They exhibit characters +distinct from any which I have observed elsewhere, owing to the +large part which the escape of aqueous vapour has played in the +eruptions and the small quantities of lava emitted. The fundamental +rocks of the district are grey and red sandstones and shales, with +some associated limestones, replete with fossils of the Devonian or +Old Red Sandstone group. The volcanoes broke out in the midst of +these inclined strata, and when the present systems of hills and +valleys had already been formed. The eruptions occurred sometimes +at the bottom of deep valleys, sometimes on the summit of hills, +and frequently on intervening platforms. In travelling through this +district we often come upon them most unexpectedly, and may find +ourselves on the very edge of a crater before we had been led to +suspect that we were approaching the site of any igneous outburst. +Thus, for example, on arriving at the village of Gemund, +immediately south of Daun, we leave the stream, which flows at the +bottom of a deep valley in which strata of sandstone and shale crop +out. We then climb a steep hill, on the surface of which we see the +edges of the same strata dipping inward towards the mountain. When +we have ascended to a considerable height, we see fragments of +scoriæ sparingly scattered over the surface; until at length, +on reaching the summit, we find ourselves suddenly on the edge of a +<i>tarn,</i> or deep circular lake-basin called the Gemunder Maar. +In it we recognise the ordinary form of a crater, for which we have +been prepared by the occurrence of scoriæ scattered over the +surface of the soil. But on examining the walls of the crater we +find precipices of sandstone and shale which exhibit no signs of +the action of heat; and we look in vain for those beds of lava and +scoriæ, dipping outward on every side, which we have been +accustomed to consider as characteristic of volcanic vents. As we +proceed, however, to the opposite side of the lake, we find a +considerable quantity of scoriæ and some lava, and see the +whole surface of the soil sparkling with volcanic sand, and strewed +with ejected fragments of half-fused shale, which preserves its +laminated texture in the interior, while it has a vitrified or +scoriform coating.</p> + +<p>Other crater lakes of circular or oval form, and hollowed out of +similar ancient strata, occur in the Upper Eifel, where</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 535">[ 535 ]</a></p> + +<p>copious aëriform discharges have taken place, throwing out +vast heaps of pulverized shale into the air. I know of no other +extinct volcanoes where gaseous explosions of such magnitude have +been attended by the emission of so small a quantity of lava. Yet I +looked in vain in the Eifel for any appearances which could lend +support to the hypothesis that the sudden rushing out of such +enormous volumes of gas had ever lifted up the stratified rocks +immediately around the vent so as to form conical masses, having +their strata dipping outward on all sides from a central axis, as +is assumed in the theory of elevation craters, alluded to in the +last chapter.</p> + +<p>I have already given (<a href="../images4/fig590.jpg">Fig. 590</a>) +an example in the Eifel of a small stream of lava which issued from +one of the craters of that district at Bertrich-Baden. It shows +that when some of these volcanoes were in action the valleys had +already been eroded to their present depth.</p> + +<p><i>Trass.</i>—The tufaceous alluvium called <i>trass,</i> +which has covered large areas in the Eifel, and choked up some +valleys now partially re-excavated, is unstratified. Its base +consists almost entirely of pumice, in which are included fragments +of basalt and other lavas, pieces of burnt shale, slate, and +sandstone, and numerous trunks and branches of trees. If, as is +probable, this trass was formed during the period of volcanic +eruptions, it may have originated in the manner of the moya of the +Andes.</p> + +<p>We may easily conceive that a similar mass might now be +produced, if a copious evolution of gases should occur in one of +the lake-basins. If a breach should be made in the side of the +cone, the flood would sweep away great heaps of ejected fragments +of shale and sandstone, which would be borne down into the +adjoining valleys. Forests might be torn up by such a flood, and +thus the occurrence of the numerous trunks of trees dispersed +irregularly through the trass can be explained. The manner in which +this trass conforms to the shape of the present valleys implies its +comparatively modern origin, probably not dating farther back than +the Pliocene Period.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch28.html"> +Chapter XXVIII</a> / <a href="ch30.html">Chapter XXX</a></small> +</body> +</html> + |