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| author | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:22:20 -0700 |
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| committer | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:22:20 -0700 |
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diff --git a/old/3772-h/files/ch35.html b/old/3772-h/files/ch35.html new file mode 100644 index 0000000..8c84941 --- /dev/null +++ b/old/3772-h/files/ch35.html @@ -0,0 +1,431 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<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 597">[ 597 ]</a></p> + +<p> </p> + +<center><b>Chapter XXXV</b><br> +<br> +ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS.</center> + +<p class="intro">Difficulty of ascertaining the Age of metamorphic +Strata. — Metamorphic Strata of Eocene date in the Alps of +Switzerland and Savoy. — Limestone and Shale of Carrara. +— Metamorphic Strata of older date than the Silurian and +Cambrian Rocks. — Order of Succession in metamorphic Rocks. +— Uniformity of mineral Character. — Supposed Azoic +Period. — Connection between the Absence of Organic Remains +and the Scarcity of calcareous Matter in metamorphic Rocks.</p> + +<p>According to the theory adopted in the last chapter, the +metamorphic strata have been deposited at one period, and have +become crystalline at another. We can rarely hope to define with +exactness the date of both these periods, the fossils having been +destroyed by Plutonic action, and the mineral characters being the +same, whatever the age. Superposition itself is an ambiguous test, +especially when we desire to determine the period of +crystallisation. Suppose, for example, we are convinced that +certain metamorphic strata in the Alps, which are covered by +cretaceous beds, are altered lias; this lias may have assumed its +crystalline texture in the cretaceous or in some tertiary period, +the Eocene for example.</p> + +<p>When discussing the ages of the Plutonic rocks, we have seen +that examples occur of various primary, secondary, and tertiary +deposits converted into metamorphic strata near their contact with +granite. There can be no doubt in these cases that strata once +composed of mud, sand, and gravel, or of clay, marl, and shelly +limestone, have for the distance of several yards, and in some +instances several hundred feet, been turned into gneiss, +mica-schist, hornblende-schist, chlorite-schist, quartz rock, +statuary marble, and the rest. (See the two preceding chapters.) It +may be easy to prove the identity of two different parts of the +same stratum; one, where the rock has been in contact with a +volcanic or Plutonic mass, and has been changed into marble or +hornblende-schist, and another not far distant, where the same bed +remains unaltered and fossiliferous; but when hydrothermal action, +as described in Chapter XXXIII, has operated gradually on a more +extensive scale, it may have finally destroyed</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 598">[ 598 ]</a></p> + +<p>all monuments of the date of its development throughout a whole +mountain chain, and all the labour and skill of the most practised +observers are required, and may sometimes be at fault. I shall +mention one or two examples of alteration on a grand scale, in +order to explain to the student the kind of reasoning by which we +are led to infer that dense masses of fossiliferous strata have +been converted into crystalline rocks.</p> + +<p><b>Eocene Strata rendered metamorphic in the Alps.</b>—In +the eastern part of the Alps, some of the Palæozoic strata, +as well as the older Mesozoic formations, including the oolitic and +cretaceous rocks, are distinctly recognisable. Tertiary deposits +also appear in a less elevated position on the flanks of the +Eastern Alps; but in the Central or Swiss Alps, the Palæozoic +and older Mesozoic formations disappear, and the Cretaceous, +Oolitic, Liassic, and at some points even the Eocene strata, +graduate insensibly into metamorphic rocks, consisting of granular +limestone, talc-schist, talcose-gneiss, micaceous schist, and other +varieties.</p> + +<p>As an illustration of the partial conversion into gneiss of +portions of a highly inclined set of beds, I may cite Sir R. +Murchison’s memoir on the structure of the Alps. Slates +provincially termed “flysch” (see <a href= +"ch16.html#page 278">p. 278</a>), overlying the nummulite limestone +of Eocene date, and comprising some arenaceous and some calcareous +layers, are seen to alternate several times with bands of granitoid +rock, answering in character to gneiss. In this case heat, vapour, +or water at a high temperature may have traversed the more +permeable beds, and altered them so far as to admit of an internal +movement and re-arrangement of the molecules, while the adjoining +strata did not give passage to the same heated gases or water, or, +if so, remained unchanged because they were composed of less +fusible or decomposable materials. Whatever hypothesis we adopt, +the phenomena establish beyond a doubt the possibility of the +development of the metamorphic structure in a tertiary deposit in +planes parallel to those of stratification. The strata appear +clearly to have been affected, though in a less intense degree, by +that same Plutonic action which has entirely altered and rendered +metamorphic so many of the subjacent formations; for in the Alps +this action has by no means been confined to the immediate vicinity +of granite. Granite, indeed, and other Plutonic rocks, rarely make +their appearance at the surface, notwithstanding the deep ravines +which lay open to view the internal structure of these mountains. +That they exist below at no great depth we can not doubt, for at +some points, as in</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 599">[ 599 ]</a></p> + +<p>the Valorsine, near Mont Blanc, granite and granitic veins are +observable, piercing through talcose gneiss, which passes +insensibly upward into secondary strata.</p> + +<p>It is certainly in the Alps of Switzerland and Savoy, more than +in any other district in Europe, that the geologist is prepared to +meet with the signs of an intense development of Plutonic action; +for here strata thousands of feet thick have been bent, folded, and +overturned, and marine secondary formations of a comparatively +modern date, such as the Oolitic and Cretaceous, have been upheaved +to the height of 12,000, and some Eocene strata to elevations of +10,000 feet above the level of the sea; and even deposits of the +Miocene era have been raised 4000 or 5000 feet, so as to rival in +height the loftiest mountains in Great Britain. In one of the +sections described by M. Studer in the highest of the Bernese Alps, +namely in the Roththal, a valley bordering the line of perpetual +snow on the northern side of the Jungfrau, there occurs a mass of +gneiss 1000 feet thick, and 15,000 feet long, which I examined, not +only resting upon, but also again covered by strata containing +oolitic fossils. These anomalous appearances may partly be +explained by supposing great solid wedges of intrusive gneiss to +have been forced in laterally between strata to which I found them +to be in many sections unconformable. The superposition, also, of +the gneiss to the oolite may, in some cases, be due to a reversal +of the original position of the beds in a region where the +convulsions have been on so stupendous a scale.</p> + +<p><b>Northern Apennines.—Carrara.</b>—The celebrated +marble of Carrara, used in sculpture, was once regarded as a type +of primitive limestone. It abounds in the mountains of Massa +Carrara, or the “Apuan Alps,” as they have been called, +the highest peaks of which are nearly 6000 feet high. Its great +antiquity was inferred from its mineral texture, from the absence +of fossils, and its passage downward into talc-schist and +garnetiferous mica-schist; these rocks again graduating downward +into gneiss, which is penetrated, at Forno, by granite veins. But +the researches of MM. Savi, Boué, Pareto, Guidoni, De la +Beche, Hoffman, and Pilla demonstrated that this marble, once +supposed to be formed before the existence of organic beings, is, +in fact, an altered limestone of the Oolitic period, and the +underlying crystalline schists are secondary sandstones and shales, +modified by Plutonic action. In order to establish these +conclusions it was first pointed out that the calcareous rocks +bordering the Gulf of Spezia, and abounding in Oolitic fossils, +assume a texture like that of Carrara marble, in proportion as they +are more and more</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 600">[ 600 ]</a></p> + +<p>invaded by certain trappean and Plutonic rocks, such as diorite, +serpentine, and granite, occurring in the same country.</p> + +<p>It was then observed that, in places where the secondary +formations are unaltered, the uppermost consist of common Apennine +limestone with nodules of flint, below which are shales, and at the +base of all, argillaceous and siliceous sandstones. In the +limestone fossils are frequent, but very rare in the underlying +shale and sandstone. Then a gradation was traced laterally from +these rocks into another and corresponding series, which is +completely metamorphic; for at the top of this we find a white +granular marble, wholly devoid of fossils, and almost without +stratification, in which there are no nodules of flint, but in its +place siliceous matter disseminated through the mass in the form of +prisms of quartz. Below this, and in place of the shales, are +talc-schists, jasper, and hornstone; and at the bottom, instead of +the siliceous and argillaceous sandstones, are quartzite and +gneiss.* Had these secondary strata of the Apennines undergone +universally as great an amount of transmutation, it would have been +impossible to form a conjecture respecting their true age; and +then, according to the method of classification adopted by the +earlier geologists, they would have ranked as primary rocks. In +that case the date of their origin would have been thrown back to +an era antecedent to the deposition of the Lower Silurian or +Cambrian strata, although in reality they were formed in the +Oolitic period, and altered at some subsequent and perhaps much +later epoch.</p> + +<p><b>Metamorphic Strata of older date than the Silurian and +Cambrian Rocks.</b>—It was remarked (<a href= +"../images5/fig617.jpg">Fig. 617</a>) that as the hypogene rocks, both +stratified and unstratified, crystallise originally at a certain +depth beneath the surface, they must always, before they are +upraised and exposed at the surface, be of considerable antiquity, +relatively to a large portion of the fossiliferous and volcanic +rocks. They may be forming at all periods; but before any of them +can become visible, they must be raised above the level of the sea, +and some of the rocks which previously concealed them must have +been removed by denudation.</p> + +<p>In Canada, as we have seen (<a href="ch27.html#page 491">p. +491</a>), the Lower Laurentian gneiss, quartzite, and limestone may +be regarded as metamorphic, because, among other reasons, organic +remains (<i>Eozoon Canadense</i>) have been detected in a part of +one of the calcareous masses. The Upper Laurentian or Labrador</p> + +<p class="fnote">* See notices of Savi, Hoffman, and others, +referred to by Boué, Bull. de la Soc. Géol. de +France, tome v, p. 317 and tome iii, p. 44; also Pilla, cited by +Murchison, Quart. Geol. Journ., vol. v, p. 266.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 601">[ 601 ]</a></p> + +<p>series lies unconformably upon the Lower, and differs from it +chiefly in having as yet yielded no fossils. It consists of gneiss +with Labrador-feldspar and feldstones, in all 10,000 feet thick, +and both its composition and structure lead us to suppose that, +like the Lower Laurentian, it was originally of sedimentary origin +and owes its crystalline condition to metamorphic action. The +remote date of the period when some of these old Laurentian strata +of Canada were converted into gneiss may be inferred from the fact +that pebbles of that rock are found in the overlying Huronian +formation, which is probably of Cambrian age (<a href= +"ch27.html#page 490">p. 490</a>).</p> + +<p>The oldest stratified rock of Scotland is the hornblendic gneiss +of Lewis, in the Hebrides, and that of the north-west coast of +Ross-shire, represented at the base of the section given at <a +href="../images/fig82.jpg">Fig. 82</a>. It is the same as that +intersected by numerous granite veins which forms the cliffs of +Cape Wrath, in Sutherlandshire (see <a href="../images5/fig613.jpg"> +Fig. 613</a>), and is conjectured to be of Laurentian age. Above +it, as shown in the section (<a href="../images/fig82.jpg">Fig. +82</a>), lie unconformable beds of a reddish or purple sandstone +and conglomerate, nearly horizontal, and between 3000 and 4000 feet +thick. In these ancient grits no fossils have been found, but they +are supposed to be of Cambrian date, for Sir R. Murchison found +Lower Silurian strata resting unconformably upon them. These strata +consist of quartzite with annelid burrows already alluded to (<a +href="ch7.html#page 112">p. 112</a>), and limestone in which Mr. +Charles Peach was the first to find, in 1854, three or four species +of <i>Orthoceras,</i> also the genera <i>Cyrtoceras</i> and <i> +Lituites,</i> two species of <i>Murchisonia,</i> a <i> +Pleurotomaria,</i> a species of <i>Maclurea,</i> one of <i> +Euomphalus,</i> and an <i>Orthis.</i> Several of the species are +believed by Mr. Salter to be identical with Lower Silurian fossils +of Canada and the United States.</p> + +<p>The discovery of the true age of these fossiliferous rocks was +one of the most important steps made of late years in the progress +of British Geology, for it led to the unexpected conclusion that +all the Scotch crystalline strata to the eastward, once called +primitive, which overlie the limestone and quartzite in question, +are referable to some part of the Silurian series.</p> + +<p>These Scotch metamorphic strata are of gneiss, mica-schist, and +clay-slate of vast thickness, and having a strike from north-east +to south-west almost at right angles to that of the older +Laurentian gneiss before mentioned. The newer crystalline series, +comprising the crystalline rocks of Aberdeenshire, Perthshire, and +Forfarshire, were inferred by Sir R. Murchison to be altered +Silurian strata; and his opinion</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 602">[ 602 ]</a></p> + +<p>has been since confirmed by the observations of three able +geologists, Messrs. Ramsay, Harkness, and Geikie. The newest of the +series is a clay-slate, on which, along the southern borders of the +Grampians, the Lower Old Red, containing <i>Cephalaspis Lyelli, +Pterygotus Anglicus,</i> and <i>Parka decipiens,</i> rests +unconformably.</p> + +<p><b>Order of Succession in Metamorphic Rocks.</b>—There is +no universal and invariable order of superposition in metamorphic +rocks, although a particular arrangement may prevail throughout +countries of great extent, for the same reason that it is traceable +in those sedimentary formations from which crystalline strata are +derived. Thus, for example, we have seen that in the Apennines, +near Carrara, the descending series, where it is metamorphic, +consists of, first, saccharine marble; second, talcose-schist; and +third, of quartz-rock and gneiss: where unaltered, of, first, +fossiliferous limestone; second, shale; and third, sandstone.</p> + +<p>But if we investigate different mountain chains, we find gneiss, +mica-schist, hornblende-schist, chlorite-schist, hypogene +limestone, and other rocks, succeeding each other, and alternating +with each other in every possible order. It is, indeed, more common +to meet with some variety of clay-slate forming the uppermost +member of a metamorphic series than any other rock; but this fact +by no means implies, as some have imagined, that all clay-slates +were formed at the close of an imaginary period when the deposition +of the crystalline strata gave way to that of ordinary sedimentary +deposits. Such clay-slates, in fact, are variable in composition, +and sometimes alternate with fossiliferous strata, so that they may +be said to belong almost equally to the sedimentary and metamorphic +order of rocks. It is probable that, had they been subjected to +more intense Plutonic action, they would have been transformed into +hornblende-schist, foliated chlorite-schist, scaly talcose-schist, +mica-schist, or other more perfectly crystalline rocks, such as are +usually associated with gneiss.</p> + +<p><i>Uniformity of Mineral Character in Hypogene +Rocks.</i>—It is true, as Humboldt has happily remarked, that +when we pass to another hemisphere, we see new forms of animals and +plants, and even new constellations in the heavens; but in the +rocks we still recognise our old acquaintances—the same granite, +the same gneiss, the same micaceous schist, quartz-rock, and the +rest. There is certainly a great and striking general resemblance +in the principal kinds of hypogene rocks in all countries, however +different their ages; but each of them, as we have seen, must be +regarded as geological</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 603">[ 603 ]</a></p> + +<p>families of rocks, and not as definite mineral compounds. They +are more uniform in aspect than sedimentary strata, because these +last are often composed of fragments varying greatly in form, size, +and colour, and contain fossils of different shapes and mineral +composition, and acquire a variety of tints from the mixture of +various kinds of sediment. The materials of such strata, if they +underwent metamorphism, would be subject to chemical laws, simple +and uniform in their action, the same in every climate, and wholly +undisturbed by mechanical and organic causes. It would, however, be +a great error to assume, as some have done, that the hypogene +rocks, considered as aggregates of simple minerals, are really more +homogeneous in their composition than the several members of the +sedimentary series. Not only do the proportional quantities of +feldspar, quartz, mica, hornblende, and other minerals, vary in +hypogene rocks bearing the same name; but what is still more +important, the ingredients, as we have seen, of the same simple +mineral are not always constant (see <a href="ch28.html#page 503"> +p. 503</a> and table, <a href="ch28.html#page 499">p. 499</a>).</p> + +<p><b>Supposed Azoic Period.</b>—The total absence of any +trace of fossils has inclined many geologists to attribute the +origin of the most ancient strata to an azoic period, or one +antecedent to the existence of organic beings. Admitting, they say, +the obliteration, in some cases, of fossils by Plutonic action, we +might still expect that traces of them would oftener be found in +certain ancient systems of slate which can scarcely be said to have +assumed a crystalline structure. But in urging this argument it +seems to have been forgotten that there are stratified formations +of enormous thickness, and of various ages, some of them even of +Tertiary date, and which we know were formed after the earth had +become the abode of living creatures, which are, nevertheless, in +some districts, entirely destitute of all vestiges of organic +bodies. In some, the traces of fossils may have been effaced by +water and acids, at many successive periods; indeed the removal of +the calcareous matter of fossil shells is proved by the fact of +such organic remains being often replaced by silex or other +minerals, and sometimes by the space once occupied by the fossil +being left empty, or only marked by a faint impression.</p> + +<p>Those who believed the hypogene rocks to have originated +antecedently to the creation of organic beings, imputed the absence +of lime, so remarkable in metamorphic strata, to the non-existence +of those mollusca and zoophytes by which shells and corals are +secreted; but when we ascribe the crystalline formations to +Plutonic action, it is natural to inquire whether this action +itself may not tend to expel carbonic</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 604">[ 604 ]</a></p> + +<p>acid and lime from the materials which it reduces to fusion or +semi-fusion. Not only carbonate of lime, but also free carbonic +acid gas, is given off plentifully from the soil and crevices of +rocks in regions of active and spent volcanoes, as near Naples and +in Auvergne. By this process, fossil shells or corals may often +lose their carbonic acid, and the residual lime may enter into the +composition of augite, hornblende, garnet, and other hypogene +minerals. Although we can not descend into the subterranean regions +where volcanic heat is developed, we can observe in regions of +extinct volcanoes, such as Auvergne and Tuscany, hundreds of +springs, both cold and thermal, flowing out from granite and other +rocks, and having their waters plentifully charged with carbonate +of lime.</p> + +<p>If all the calcareous matter transferred in the course of ages +by these and thousands of other springs from the lower part of the +earth’s crust to the atmosphere could be presented to us in a +solid form, we should find that its volume was comparable to that +of many a chain of hills. Calcareous matter is poured into lakes +and the ocean by a thousand springs and rivers; so that part of +almost every new calcareous rock chemically precipitated, and of +many reefs of shelly and coralline stone, must be derived from +mineral matter subtracted by Plutonic agency, and driven up by gas +and steam from fused and heated rocks in the bowels of the +earth.</p> + +<p>The scarcity of limestone in many extensive regions of +metamorphic rocks, as in the Eastern and Southern Grampians of +Scotland, may have been the result of some action of this kind; and +if the limestones of the Lower Laurentian in Canada afford a +remarkable exception to the general rule, we must not forget that +it is precisely in this most ancient formation that the <i>Eozoon +Canadense</i> has been found. The fact that some distinct bands of +limestone from 700 to 1500 feet thick occur here, may be connected +with the escape from destruction of some few traces of organic +life, even in a rock in which metamorphic action has gone so far as +to produce serpentine, augite, and other minerals found largely +intermixed with the carbonate of lime.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch34.html"> +Chapter XXXIV</a> / <a href="ch36.html">Chapter XXXVI</a></small> +</body> +</html> + |