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diff --git a/old/3772-h/files/ch32.html b/old/3772-h/files/ch32.html new file mode 100644 index 0000000..450b2b1 --- /dev/null +++ b/old/3772-h/files/ch32.html @@ -0,0 +1,578 @@ +<!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 564">[ 564 ]</a></p> + +<p> </p> + +<center><b>Chapter XXXII</b><br> +<br> +ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS.</center> + +<p class="intro">Difficulty in ascertaining the precise Age of a +Plutonic Rock. — Test of Age by Relative Position. — +Test by Intrusion and Alteration. — Test by Mineral +Composition. — Test by included Fragments. — Recent and +Pliocene Plutonic Rocks, why invisible. — Miocene Syenite of +the Isle of Skye. — Eocene Plutonic Rocks in the Andes. +— Granite altering Cretaceous Rocks. — Granite altering +Lias in the Alps and in Skye. — Granite of Dartmoor altering +Carboniferous Strata. — Granite of the Old Red Sandstone +Period. — Syenite altering Silurian Strata in Norway. — +Blending of the same with Gneiss. — Most ancient Plutonic +Rocks. — Granite protruded in a solid Form.</p> + +<p>When we adopt the igneous theory of granite, as explained in the +last chapter, and believe that different Plutonic rocks have +originated at successive periods beneath the surface of the planet, +we must be prepared to encounter greater difficulty in ascertaining +the precise age of such rocks than in the case of volcanic and +fossiliferous formations. We must bear in mind that the evidence of +the age of each contemporaneous volcanic rock was derived either +from lavas poured out upon the ancient surface, whether in the sea +or in the atmosphere, or from tuffs and conglomerates, also +deposited at the surface, and either containing organic remains +themselves or intercalated between strata containing fossils. But +the same tests entirely fail, or are only applicable in a modified +degree, when we endeavour to fix the chronology of a rock which has +crystallised from a state of fusion in the bowels of the earth. In +that case we are reduced to the tests of relative position, +intrusion, alteration of the rocks in contact, included fragments, +and mineral character; but all these may yield at best a somewhat +ambiguous result.</p> + +<p><b>Test of Age by Relative Position.</b>—Unaltered +fossiliferous strata of every age are met with reposing immediately +on Plutonic rocks; as at Christiania, in Norway, where the +Post-pliocene deposits rest on granite; in Auvergne, where the +fresh-water Miocene strata, and at Heidelberg, on the Rhine, where +the New Red sandstone occupy a similar place. In all these, and +similar instances, inferiority in position is connected with the +superior antiquity of granite. The crystalline rock was solid +before the sedimentary beds were superimposed, and the latter +usually contain in them rounded pebbles of the subjacent +granite.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 565">[ 565 ]</a></p> + +<p><b>Test by Intrusion and Alteration.</b>—But when Plutonic +rocks send veins into strata, and alter them near the point of +contact, in the manner before described (<a href= +"ch31.html#page 559">p. 559</a>), it is clear that, like intrusive +traps, they are newer than the strata which they invade and alter. +Examples of the application of this test will be given in the +sequel.</p> + +<p><b>Test by Mineral Composition.</b>—Notwithstanding a +general uniformity in the aspect of Plutonic rocks, we have seen in +the last chapter that there are many varieties, such as syenite, +talcose granite, and others. One of these varieties is sometimes +found exclusively prevailing throughout an extensive region, where +it preserves a homogeneous character; so that, having ascertained +its relative age in one place, we can recognise its identity in +others, and thus determine from a single section the chronological +relations of large mountain masses. Having observed, for example, +that the syenitic granite of Norway, in which the mineral called +zircon abounds, has altered the Silurian strata wherever it is in +contact, we do not hesitate to refer other masses of the same +zircon-syenite in the south of Norway to a post-Silurian date. Some +have imagined that the age of different granites might, to a great +extent, be determined by their mineral characters alone; syenite, +for instance, or granite with hornblende, being more modern than +common or micaceous granite. But modern investigations have proved +these generalisations to have been premature.</p> + +<p><b>Test by Included Fragments.</b>—This criterion can +rarely be of much importance, because the fragments involved in +granite are usually so much altered that they can not be referred +with certainty to the rocks whence they were derived. In the White +Mountains, in North America, according to Professor Hubbard, a +granite vein, traversing granite, contains fragments of slate and +trap which must have fallen into the fissure when the fused +materials of the vein were injected from below,* and thus the +granite is shown to be newer than those slaty and trappean +formations from which the fragments were derived.</p> + +<p><b>Recent and Pliocene Plutonic Rocks, why +invisible.</b>—The explanations already given in the 28th and +in the last chapter of the probable relation of the Plutonic to the +volcanic formations, will naturally lead the reader to infer that +rocks of the one class can never be produced at or near the surface +without some members of the other being formed below. It is not +uncommon for lava-streams to require more than ten years to cool in +the open air; and where they are of great</p> + +<p class="fnote">* Silliman’s Journ., No. 69, p. 123.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 566">[ 566 ]</a></p> + +<p>depth, a much longer period. The melted matter poured from +Jorullo, in Mexico, in the year 1759, which accumulated in some +places to the height of 550 feet, was found to retain a high +temperature half a century after the eruption.* We may conceive, +therefore, that great masses of subterranean lava may remain in a +red-hot or incandescent state in the volcanic foci for immense +periods, and the process of refrigeration may be extremely gradual. +Sometimes, indeed, this process may be retarded for an indefinite +period by the accession of fresh supplies of heat; for we find that +the lava in the crater of Stromboli, one of the Lipari Islands, has +been in a state of constant ebullition for the last two thousand +years; and we may suppose this fluid mass to communicate with some +caldron or reservoir of fused matter below. In the Isle of Bourbon, +also, where there has been an emission of lava once in every two +years for a long period, the lava below can scarcely fail to have +been permanently in a state of liquefaction. If then it be a +reasonable conjecture, that about 2000 volcanic eruptions occur in +the course of every century, either above the waters of the sea or +beneath them,† it will follow that the quantity of Plutonic +rock generated or in progress during the Recent epoch must already +have been considerable.</p> + +<p>But as the Plutonic rocks originate at some depth in the +earth’s crust, they can only be rendered accessible to human +observation by subsequent upheaval and denudation. Between the +period when a Plutonic rock crystallises in the subterranean +regions and the era of its protrusion at any single point of the +surface, one or two geological periods must usually intervene. +Hence, we must not expect to find the Recent or even the Pliocene +granites laid open to view, unless we are prepared to assume that +sufficient time has elapsed since the commencement of the Pliocene +period for great upheaval and denudation. A Plutonic rock, +therefore, must, in general, be of considerable antiquity +relatively to the fossiliferous and volcanic formations, before it +becomes extensively visible. As we know that the upheaval of land +has been sometimes accompanied in South America by volcanic +eruptions and the emission of lava, we may conceive the more +ancient Plutonic rocks to be forced upward to the surface by the +newer rocks of the same class formed successively +below—subterposition in the Plutonic, like superposition in the +sedimentary rocks, being usually characteristic of a newer +origin.</p> + +<p class="fnote">* See “Principles,” <i>Index,</i> +“Jorullo.”<br> +† Ibid., “Volcanic Eruptions.”</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 567">[ 567 ]</a></p> + +<center><img src="../images5/fig617.jpg" width="611" height="317" alt= +"Fig. 617: Diagram showing the relative position which the Plutonic and sedimentary formations of different ages may occupy."> +</center> + +<p>In Fig. 617 an attempt is made to show the inverted order in +which sedimentary and Plutonic formations may occur in the +earth’s crust. The oldest Plutonic rock, No. I, has been +upheaved at successive periods until it has become exposed to view +in a mountain-</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 568">[ 568 ]</a></p> + +<p>chain. This protrusion of No. I has been caused by the igneous +agency which produced the newer Plutonic rocks Nos. II, III and IV. +Part of the primary fossiliferous strata, No. I, have also been +raised to the surface by the same gradual process. It will be +observed that the Recent <i>strata</i> No. 4 and the Recent <i> +granite</i> or Plutonic rock No. IV are the most remote from each +other in position, although of contemporaneous date. According to +this hypothesis, the convulsions of many periods will be required +before Recent or Post-tertiary granite will be upraised so as to +form the highest ridges and central axes of mountain-chains. During +that time the <i>recent</i> strata No. 4 might be covered by a +great many newer sedimentary formations.</p> + +<p><b>Miocene Plutonic Rocks.</b>—A considerable mass of +syenite, in the Isle of Skye, is described by Dr. MacCulloch as +intersecting limestone and shale, which are of the age of the lias. +The limestone, which at a greater distance from the granite +contains shells, exhibits no traces of them near its junction, +where it has been converted into a pure crystalline marble.* +MacCulloch pointed out that the syenite here, as in Raasay, was +newer than the secondary rocks, and Mr. Geikie has since shown that +there is a strong probability that this Plutonic rock may be of +Miocene age, because a similar Syenite having a true granitic +character in its crystallisation has modified the Tertiary volcanic +rocks of Ben More, in Mull, some of which have undergone +considerable metamorphism.</p> + +<p><b>Eocene Plutonic Rocks.</b>—In a former part of this +volume (Chapter 16), the great nummulitic formation of the Alps and +Pyrenees was referred to the Eocene period, and it follows that +vast movements which have raised those fossiliferous rocks from the +level of the sea to the height of more than 10,000 feet above its +level have taken place since the commencement of the Tertiary +epoch. Here, therefore, if anywhere, we might expect to find +hypogene formations of Eocene date breaking out in the central axis +or most disturbed region of the loftiest chain in Europe. +Accordingly, in the Swiss Alps, even the <i>flysch,</i> or upper +portion of the nummulitic series, has been occasionally invaded by +Plutonic rocks, and converted into crystalline schists of the +hypogene class. There can be little doubt that even the talcose +granite or gneiss of Mont Blanc itself has been in a fused or pasty +state since the <i>flysch</i> was deposited at the bottom of the +sea; and the question as to its age is not so much whether it be a +secondary or tertiary granite or gneiss, as whether it should be +assigned to the Eocene or Miocene epoch.</p> + +<p class="fnote">* “Western Islands,” vol. i, p. +330.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 569">[ 569 ]</a></p> + +<p>Great upheaving movements have been experienced in the region of +the Andes, during the Post-tertiary period. In some part, +therefore, of this chain, we may expect to discover tertiary +Plutonic rocks laid open to view; and Mr. Darwin’s account of +the Chilian Andes, to which the reader may refer, fully realises +this expectation: for he shows that we have strong ground to +presume that Plutonic rocks there exposed on a large scale are of +later date than certain Secondary and Tertiary formations.</p> + +<p>But the theory adopted in this work of the subterranean origin +of the hypogene formations would be untenable, if the supposed fact +here alluded to, of the appearance of tertiary granite at the +surface, was not a rare exception to the general rule. A +considerable lapse of time must intervene between the formation of +Plutonic and metamorphic rocks in the nether regions and their +emergence at the surface. For a long series of subterranean +movements must occur before such rocks can be uplifted into the +atmosphere or the ocean; and, before they can be rendered visible +to man, some strata which previously covered them must have been +stripped off by denudation.</p> + +<p>We know that in the Bay of Baiæ in 1538, in Cutch in 1819, +and on several occasions in Peru and Chili, since the commencement +of the present century, the permanent upheaval or subsidence of +land has been accompanied by the simultaneous emission of lava at +one or more points in the same volcanic region. From these and +other examples it may be inferred that the rising or sinking of the +earth’s crust, operations by which sea is converted into +land, and land into sea, are a part only of the consequences of +subterranean igneous action. It can scarcely be doubted that this +action consists, in a great degree, of the baking, and occasionally +the liquefaction, of rocks, causing them to assume, in some cases a +larger, in others a smaller volume than before the application of +heat. It consists also in the generation of gases, and their +expansion by heat, and the injection of liquid matter into rents +formed in superincumbent rocks. The prodigious scale on which these +subterranean causes have operated in Sicily since the deposition of +the Newer Pliocene strata will be appreciated when we remember that +throughout half the surface of that island such strata are met +with, raised to the height of from 50 to that of 2000 and even 3000 +feet above the level of the sea. In the same island also the older +rocks which are contiguous to these marine tertiary strata must +have undergone, within the same period, a similar amount of +upheaval.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 570">[ 570 ]</a></p> + +<p>The like observations may be extended to nearly the whole of +Europe, for, since the commencement of the Eocene Period, the +entire European area, including some of the central and very lofty +portions of the Alps themselves, as I have elsewhere shown,* has, +with the exception of a few districts, emerged from the deep to its +present altitude. There must, therefore, have been at great depths +in the earth’s crust, within the same period, an amount of +subterranean change corresponding to this vast alteration of level +affecting a whole continent.</p> + +<p>The principal effect of subterranean movements during the +Tertiary Period seems to have consisted in the upheaval of hypogene +formations of an age anterior to the Carboniferous. The repetition +of another series of movements, of equal violence, might upraise +the Plutonic and metamorphic rocks of many secondary periods; and, +if the same force should still continue to act, the next +convulsions might bring up to the day the <i>tertiary</i> and <i> +recent</i> hypogene rocks. In the course of such changes many of +the existing sedimentary strata would suffer greatly by denudation, +others might assume a metamorphic structure, or become melted down +into Plutonic and volcanic rocks. Meanwhile the deposition of a +great thickness of new strata would not fail to take place during +the upheaval and partial destruction of the older rocks. But I must +refer the reader to the last chapter but one of this volume for a +fuller explanation of these views.</p> + +<img src="../images5/fig618.jpg" width="175" height="107" alt= +"Fig. 618: Section through three layers (b, c, d) of the Cretaceous series over granite (A)." + align="left"> + +<p><b>Plutonic Rocks of Cretaceous Period.</b>—It will be +shown in the next chapter that chalk, as well as lias, has been +altered by granite in the eastern Pyrenees. Whether such granite be +cretaceous or tertiary, can not easily be decided. Suppose <i>b, c, +d,</i> Fig. 618, to be three members of the Cretaceous series, the +lowest of which, <i>b,</i> has been altered by the granite A, the +modifying influence not having extended so far as <i>c,</i> or +having but slightly affected its lowest beds. Now it can rarely be +possible for the geologist to decide whether the beds <i>d</i> +existed at the time of the intrusion of A, and alteration of <i> +b</i> and <i>c,</i> or whether they were subsequently thrown down +upon <i>c.</i> But as some Cretaceous and even Tertiary rocks have +been raised to the height of more than 9000 feet in the Pyrenees, +we must not assume that plutonic formations of the same periods may +not have been brought up and</p> + +<p class="fnote">* See map of Europe, and explanation, in +Principles, book i.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 571">[ 571 ]</a></p> + +<p>exposed by denudation, at the height of 2000 or 3000 feet on the +flanks of that chain.</p> + +<img src="../images5/fig619.jpg" width="237" height="281" alt= +"Fig. 619: Junction of granite with Jurassic or Oolite strata in the Alps, near Champoleon." + align="right"> + +<p><b>Plutonic Rocks of the Oolite and Lias.</b>—In the +Department of the Hautes Alpes, in France, M. Élie de +Beaumont traced a black argillaceous limestone, charged with +belemnites, to within a few yards of a mass of granite. Here the +limestone begins to put on a granular texture, but is extremely +fine-grained. When nearer the junction it becomes grey, and has a +saccharoid structure. In another locality, near Champoleon, a +granite composed of quartz, black mica, and rose-coloured feldspar +is observed partly to overlie the secondary rocks, producing an +alteration which extends for about 30 feet downward, diminishing in +the beds which lie farthest from the granite. (See Fig. 619.) In +the altered mass the argillaceous beds are hardened, the limestone +is saccharoid, the grits quartzose, and in the midst of them is a +thin layer of an imperfect granite. It is also an important +circumstance that near the point of contact, both the granite and +the secondary rocks become metalliferous, and contain nests and +small veins of blende, galena, iron, and copper pyrites. The +stratified rocks become harder and more crystalline, but the +granite, on the contrary, softer and less perfectly crystallised +near the junction.* Although the granite is incumbent in the +section (Fig. 619), we can not assume that it overflowed the +strata, for the disturbances of the rocks are so great in this part +of the Alps that their original position is often inverted.</p> + +<p>At Predazzo, in the Tyrol, secondary strata, some of which are +limestones of the Oolitic period, have been traversed and altered +by Plutonic rocks, one portion of which is an augitic porphyry, +which passes insensibly into granite. The limestone</p> + +<p class="fnote">* Élie de Beaumont sur les Montagnes de +l’Oisans, etc. Mém. de la Soc. d’Hist. Nat. de +Paris, tome v.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 572">[ 572 ]</a></p> + +<p>is changed into granular marble, with a band of serpentine at +the junction.*</p> + +<p><b>Plutonic Rocks of Carboniferous Period.</b>—The granite +of Dartmoor, in Devonshire, was formerly supposed to be one of the +most ancient of the Plutonic rocks, but is now ascertained to be +posterior in date to the culm-measures of that county, which from +their position, and, as containing true coal-plants, are now known +to be members of the true Carboniferous series. This granite, like +the syenitic granite of Christiania, has broken through the +stratified formations, on the north-west side of Dartmoor, the +successive members of the culm-measures abutting against the +granite, and becoming metamorphic as they approach. These strata +are also penetrated by granite veins, and Plutonic dikes, called +“elvans.”† The granite of Cornwall is probably +of the same date, and, therefore, as modern as the Carboniferous +strata, if not newer.</p> + +<center><img src="../images5/fig620.jpg" width="338" height="94" alt= +"Fig. 620: Section through Silurian strata and Granite."></center> + +<p><b>Plutonic Rocks of Silurian Period.</b>—It has long been +known that a very ancient granite near Christiania, in Norway, is +posterior in date to the Lower Silurian strata of that region, +although its exact position in the Palæozoic series can not +be defined. Von Buch first announced, in 1813, that it was of newer +origin than certain limestones containing orthocerata and +trilobites. The proofs consist in the penetration of granite veins +into the shale and limestone, and the alteration of the strata, for +a considerable distance from the point of contact, both of these +veins and the central mass from which they emanate. (See <a href= +"ch31.html#page 562">p. 562</a>) Von Buch supposed that the +Plutonic rock alternated with the fossiliferous strata, and that +large masses of granite were sometimes incumbent upon the strata; +but this idea was erroneous, and arose from the fact that the beds +of shale and limestone often dip towards the granite up to the +point of contact, appearing as if they would pass under it in mass, +as at <i>a,</i> Fig. 620, and then again on the opposite side of +the same mountain, as at <i>b,</i> dip away from the same granite. +When the junctions, however, are carefully examined, it is found +that the Plutonic rock</p> + +<p class="fnote">* Von Buch, Annales de Chimie, etc.<br> +† Proceed. Geol. Soc., vol. ii, p. 562; and Trans., 2nd +series, vol. v, p. 686.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 573">[ 573 ]</a></p> + +<p>intrudes itself in veins, and nowhere covers the fossiliferous +strata in large overlying masses, as is so commonly the case with +trappean formations.*</p> + +<p>Now this granite, which is more modern than the Silurian strata +of Norway, also sends veins in the same country into an ancient +formation of gneiss; and the relations of the Plutonic rock and the +gneiss, at their junction, are full of interest when we duly +consider the wide difference of epoch which must have separated +their origin.</p> + +<center><img src="../images5/fig621.jpg" width="334" height="146" alt= +"Fig. 621: Granite sending veins into Silurian strata and gneiss. Christiania, Norway."> +</center> + +<p>The length of this interval of time is attested by the following +facts: The fossiliferous, or Silurian, beds rest unconformably upon +the truncated edges of the gneiss, the inclined strata of which had +been denuded before the sedimentary beds were superimposed (see +Figure 621). The signs of denudation are twofold; first, the +surface of the gneiss is seen occasionally, on the removal of the +newer beds containing organic remains, to be worn and smoothed; +secondly, pebbles of gneiss have been found in some of these +Silurian strata. Between the origin, therefore, of the gneiss and +the granite there intervened, first, the period when the strata of +gneiss were denuded; secondly, the period of the deposition of the +Silurian deposits upon the denuded and inclined gneiss, a. Yet the +granite produced after this long interval is often so intimately +blended with the ancient gneiss, at the point of junction, that it +is impossible to draw any other than an arbitrary line of +separation between them; and where this is not the case, tortuous +veins of granite pass freely through gneiss, ending sometimes in +threads, as if the older rock had offered no resistance to their +passage. These appearances may probably be due to hydrothermal +action (see <a href="ch33.html#page 584">p. 584</a>). I shall +merely observe in this place that had such junctions alone been +visible, and had we not learnt, from other sections, how long a +period elapsed between the consolidation of the gneiss and the +injection of this granite, we might have suspected that the gneiss +was scarcely solidified,</p> + +<p class="fnote">* See the Gæa Norvegica and other works of +Keilhau, with whom I examined this country.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 574">[ 574 ]</a></p> + +<p>or had not yet assumed its complete metamorphic character when +invaded by the Plutonic rock. From this example we may learn how +impossible it is to conjecture whether certain granites in +Scotland, and other countries, which send veins into gneiss and +other metamorphic rocks, are primary, or whether they may not +belong to some secondary or tertiary period.</p> + +<p><b>Oldest Granites.</b>—It is not half a century since the +doctrine was very general that all granitic rocks were <i> +primitive,</i> that is to say, that they originated before the +deposition of the first sedimentary strata, and before the creation +of organic beings (see <a href="ch1.html#page 34">p. 34</a>). But +so greatly are our views now changed, that we find it no easy task +to point out a single mass of granite demonstrably more ancient +than known fossiliferous deposits. Could we discover some +Laurentian strata resting immediately on granite, there being no +alterations at the point of contact, nor any intersecting granitic +veins, we might then affirm the Plutonic rock to have originated +before the oldest known fossiliferous strata. Still it would be +presumptuous, as we have already pointed out (<a href= +"ch26.html#page 464">p. 464</a>), to suppose that when a small part +only of the globe has been investigated, we are acquainted with the +oldest fossiliferous strata in the crust of our planet. Even when +these are found, we can not assume that there never were any +antecedent strata containing organic remains, which may have become +metamorphic. If we find pebbles of granite in a conglomerate of the +Lower Laurentian system, we may then feel assured that the parent +granite was formed before the Laurentian formation. But if the +incumbent strata be merely Cambrian or Silurian, the fundamental +granite, although of high antiquity, may be posterior in date to +<i>known</i> fossiliferous formations.</p> + +<p><b>Protrusion of Solid Granite.</b>—In part of +Sutherlandshire, near Brora, common granite, composed of feldspar, +quartz, and mica is in immediate contact with Oolitic strata, and +has clearly been elevated to the surface at a period subsequent to +the deposition of those strata.* Professor Sedgwick and Sir R. +Murchison conceive that this granite has been upheaved in a solid +form; and that in breaking through the submarine deposits, with +which it was not perhaps originally in contact, it has fractured +them so as to form a breccia along the line of junction. This +breccia consists of fragments of shale, sandstone, and limestone, +with fossils of the oolite, all united together by a calcareous +cement. The secondary strata at some distance from the granite are +but slightly disturbed, but in proportion to their proximity the +amount of dislocation becomes greater.</p> + +<p class="fnote">* Murchison, Geol. Trans., 2nd series, vol. ii, p. +307.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 575">[ 575 ]</a></p> + +<p>Mr. T. McKenney Hughes has suggested to me in explanation of +these phenomena that they may be the effect of the association of +more pliant strata with hard unyielding rocks, the whole of which +were subjected simultaneously to great movements, whether of +elevation or subsidence, and of lateral pressure, during which the +more solid granite, being incapable of compression, was forced +through the softer beds of shale, sandstone, and limestone. He +remarks that similar breccias with slickensides are observed on a +minor scale where rocks of different composition and rigidity are +contorted together. Such protrusion may have been brought about by +degrees by innumerable shocks of earthquakes repeated after long +intervals of time along the same tract of country. The opening of +new fissures in the hardest rocks is a frequent accompaniment of +such convulsions, and during the consequent vibrations, breccias +must often be caused. But these catastrophes, as we well know, do +not imply that the land or sea of the disturbed region are rendered +uninhabitable by living beings, and by no means indicate a state of +things different from that witnessed in the ordinary course of +nature.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch31.html"> +Chapter XXXI</a> / <a href="ch33.html">Chapter XXXIII</a></small> +</body> +</html> + |