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diff --git a/old/3772-h/files/ch33.html b/old/3772-h/files/ch33.html new file mode 100644 index 0000000..b8cd236 --- /dev/null +++ b/old/3772-h/files/ch33.html @@ -0,0 +1,637 @@ +<!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 576">[ 576 ]</a></p> + +<p> </p> + +<center><b>Chapter XXXIII</b><br> +<br> +METAMORPHIC ROCKS.</center> + +<p class="intro">General Character of Metamorphic Rocks. — +Gneiss. — Hornblende-schist. — Serpentine. — +Mica-schist. — Clay-slate. — Quartzite. — +Chlorite-schist. — Metamorphic Limestone. — Origin of +the metamorphic Strata. — Their Stratification. — +Fossiliferous Strata near intrusive Masses of Granite converted +into Rocks identical with different Members of the metamorphic +Series. — Arguments hence derived as to the Nature of +Plutonic Action. — Hydrothermal Action, or the Influence of +Steam and Gases in producing Metamorphism. — Objections to +the metamorphic Theory considered.</p> + +<p>We have now considered three distinct classes of rocks: first, +the aqueous, or fossiliferous; secondly, the volcanic; and, +thirdly, the Plutonic; and it remains for us to examine those +crystalline (or hypogene) strata to which the name of <i> +metamorphic</i> has been assigned. The last-mentioned term +expresses, as before explained, a theoretical opinion that such +strata, after having been deposited from water, acquired, by the +influence of heat and other causes, a highly crystalline texture. +They who still question this opinion may call the rocks under +consideration the stratified hypogene formations or crystalline +schists.</p> + +<p>These rocks, when in their characteristic or normal state, are +wholly devoid of organic remains, and contain no distinct fragments +of other rocks, whether rounded or angular. They sometimes break +out in the central parts of mountain chains, but in other cases +extend over areas of vast dimensions, occupying, for example, +nearly the whole of Norway and Sweden, where, as in Brazil, they +appear alike in the lower and higher grounds. However crystalline +these rocks may become in certain regions, they never, like granite +or trap, send veins into contiguous formations. In Great Britain, +those members of the series which approach most nearly to granite +in their composition, as gneiss, mica-schist, and +hornblende-schist, are confined to the country north of the rivers +Forth and Clyde.</p> + +<p>Many attempts have been made to trace a general order of +succession or superposition in the members of this family; +clay-slate, for example, having been often supposed to hold +invariably a higher geological position than mica-schist, and</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 577">[ 577 ]</a></p> + +<p>mica-schist to overlie gneiss. But although such an order may +prevail throughout limited districts, it is by no means universal. +To this subject, however, I shall again revert, in Chapter XXXV, +where the chronological relations of the metamorphic rocks are +pointed out.</p> + +<p><b>Principal Metamorphic Rocks.</b>—The following may be +enumerated as the principal members of the metamorphic +class:—gneiss, mica-schist, hornblende-schist, clay-slate, +chlorite-schist, hypogene or metamorphic limestone, and certain +kinds of quartz-rock or quartzite.</p> + +<center><img src="../images5/fig622.jpg" width="302" height="176" alt= +"Fig. 622: Fragment of gneiss; section made at right angles to the planes of foliation."> +</center> + +<p><i>Gneiss.</i>—The first of these, gneiss, may be called +stratified—or by those who object to that term, foliated—granite, +being formed of the same materials as granite, namely, feldspar, +quartz, and mica. In the specimen in Fig. 622, the white layers +consist almost exclusively of granular feldspar, with here and +there a speck of mica and grain of quartz. The dark layers are +composed of grey quartz and black mica, with occasionally a grain +of feldspar intermixed. The rock splits most easily in the plane of +these darker layers, and the surface thus exposed is almost +entirely covered with shining spangles of mica. The accompanying +quartz, however, greatly predominates in quantity, but the most +ready cleavage is determined by the abundance of mica in certain +parts of the dark layer. Instead of consisting of these thin +laminæ, gneiss is sometimes simply divided into thick beds, +in which the mica has only a slight degree of parallelism to the +planes of stratification.</p> + +<p>Hand specimens may often be obtained from such gneiss which are +undistinguishable from granite, affording an argument to which we +shall allude in the concluding part of this chapter, in favour of +those who regard all granite and syenite not as igneous rocks, but +as aqueous formations so altered as to have lost all signs of their +original stratified arrangement. Gneiss in geology is commonly used +to designate not merely</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 578">[ 578 ]</a></p> + +<p>stratified and foliated rocks having the same component +materials as granite or syenite, but also in a wider sense to +embrace the formation with which other members of the metamorphic +series, such as hornblende-schist, may alternate, and which are +then considered subordinate to the true gneiss.</p> + +<p>The different varieties of rock allied to gneiss, into which +feldspar enters as an essential ingredient, will be understood by +referring to what was said of granite. Thus, for example, +hornblende may be superadded to mica, quartz, and feldspar, forming +a hornblendic or syenitic gneiss; or talc may be substituted for +mica, constituting talcose gneiss (called stratified protogine by +the French), a rock composed of feldspar, quartz, and talc, in +distinct crystals or grains.</p> + +<p><i>Eurite,</i> which has already been mentioned as a Plutonic +rock, occurs also with precisely the same composition in beds +subordinate to gneiss or mica-slate.</p> + +<p><i>Hornblende-schist</i> is usually black, and composed +principally of hornblende, with a variable quantity of feldspar, +and sometimes grains of quartz. When the hornblende and feldspar +are in nearly equal quantities, and the rock is not slaty, it +corresponds in character with the greenstones of the trap family, +and has been called “primitive greenstone.” It may be +termed hornblende rock, or amphibolite. Some of these hornblendic +masses may really have been volcanic rocks, which have since +assumed a more crystalline or metamorphic texture.</p> + +<p><i>Serpentine</i> is a greenish rock, a silicate of magnesia, in +which there is sometimes from 30 to 40 per cent of magnesia. It +enters largely into the composition of a trap dike cutting through +Old Red Sandstone in Forfarshire, and in that case is probably an +altered basaltic dike which had contained much olivine. The theory +of its having been originally a volcanic product subsequently +altered by metamorphism may at first sight seem inconsistent with +its occurrence in large and regularly stratified masses in the +metamorphic series in Scotland, as in Aberdeenshire. But it has +been suggested in explanation that such serpentine may have been +originally regularly-bedded trap tuff, and volcanic breccia, with +much olivine, which would still retain a stratified appearance +after their conversion into a metamorphic rock.</p> + +<p><i>Actinolite Schist</i> is a slaty foliated rock, composed +chiefly of actinolite, an emerald-green mineral, allied to +hornblende, with some admixture of garnet, mica, and quartz.</p> + +<p><i>Mica-schist</i> or <i>Micaceous Schist</i> is, next to +gneiss, one of the most abundant rocks of the metamorphic series. +It is slaty, essentially composed of mica and quartz, the mica</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 579">[ 579 ]</a></p> + +<p>sometimes appearing to constitute the whole mass. Beds of pure +quartz also occur in this formation. In some districts, garnets in +regular twelve-sided crystals form an integrant part of +mica-schist. This rock passes by insensible gradations into +clay-slate.</p> + +<p><i>Clay-slate—Argillaceous +Schist—Argillite.</i>—This rock sometimes resembles an +indurated clay or shale. It is for the most part extremely fissile, +often affording good roofing-slate. Occasionally it derives a +shining and silky lustre from the minute particles of mica or talc +which it contains. It varies from greenish or bluish-grey to a lead +colour; and it may be said of this, more than of any other schist, +that it is common to the metamorphic and fossiliferous series, for +some clay-slates taken from each division would not be +distinguishable by mineral characters alone. It is not uncommon to +meet with an argillaceous rock having the same composition, without +the slaty cleavage, which may be called argillite.</p> + +<p><i>Chlorite Schist</i> is a green slaty rock, in which chlorite +is abundant in foliated plates, usually blended with minute grains +of quartz, or sometimes with feldspar or mica; often associated +with, and graduating into, gneiss and clay-slate.</p> + +<p><i>Quartzite,</i> or <i>Quartz Rock,</i> is an aggregate of +grains of quartz which are either in minute crystals, or in many +cases slightly rounded, occurring in regular strata, associated +with gneiss or other metamorphic rocks. Compact quartz, like that +so frequently found in veins, is also found together with granular +quartzite. Both of these alternate with gneiss or mica-schist, or +pass into those rocks by the addition of mica, or of feldspar and +mica.</p> + +<p><i>Crystalline,</i> or <i>Metamorphic Limestone.</i>—This +hypogene rock, called by the earlier geologists <i>primary +limestone,</i> is sometimes a white crystalline granular marble, +which when in thick beds can be used in sculpture; but more +frequently it occurs in thin beds, forming a foliated schist much +resembling in colour and arrangement certain varieties of gneiss +and mica-schist. When it alternates with these rocks, it often +contains some crystals of mica, and occasionally quartz, feldspar, +hornblende, talc, chlorite, garnet, and other minerals. It enters +sparingly into the structure of the hypogene districts of Norway, +Sweden, and Scotland, but is largely developed in the Alps.</p> + +<p><b>Origin of the Metamorphic Strata.</b>—Having said thus +much of the mineral composition of the metamorphic rocks, I may +combine what remains to be said of their structure and history with +an account of the opinions entertained of their probable origin. At +the same time, it may be well to</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 580">[ 580 ]</a></p> + +<p>forewarn the reader that we are here entering upon ground of +controversy, and soon reach the limits where positive induction +ends, and beyond which we can only indulge in speculations. It was +once a favourite doctrine, and is still maintained by many, that +these rocks owe their crystalline texture, their want of all signs +of a mechanical origin, or of fossil contents, to a peculiar and +nascent condition of the planet at the period of their formation. +The arguments in refutation of this hypothesis will be more fully +considered when I show, in Chapter XXXV, to how many different ages +the metamorphic formations are referable, and how gneiss, +mica-schist, clay-slate, and hypogene limestone (that of Carrara, +for example) have been formed, not only since the first +introduction of organic beings into this planet, but even long +after many distinct races of plants and animals had flourished and +passed away in succession.</p> + +<p>The doctrine respecting the crystalline strata implied in the +name metamorphic may properly be treated of in this place; and we +must first inquire whether these rocks are really entitled to be +called stratified in the strict sense of having been originally +deposited as sediment from water. The general adoption by +geologists of the term stratified, as applied to these rocks, +sufficiently attests their division into beds very analogous, at +least in form, to ordinary fossiliferous strata. This resemblance +is by no means confined to the existence in both occasionally of a +laminated structure, but extends to every kind of arrangement which +is compatible with the absence of fossils, and of sand, pebbles, +ripple-mark, and other characters which the metamorphic theory +supposes to have been obliterated by Plutonic action. Thus, for +example, we behold alike in the crystalline and fossiliferous +formations an alternation of beds varying greatly in composition, +colour, and thickness. We observe, for instance, gneiss alternating +with layers of black hornblende-schist or of green chlorite-schist, +or with granular quartz or limestone; and the interchange of these +different strata may be repeated for an indefinite number of times. +In the like manner, mica-schist alternates with chlorite-schist, +and with beds of pure quartz or of granular limestone. We have +already seen that, near the immediate contact of granitic veins and +volcanic dikes, very extraordinary alterations in rocks have taken +place, more especially in the neighbourhood of granite. It will be +useful here to add other illustrations, showing that a texture +undistinguishable from that which characterises the more +crystalline metamorphic formations has actually been superinduced +in strata once fossiliferous.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 581">[ 581 ]</a></p> + +<p><b>Fossiliferous Strata rendered metamorphic by intrusive Masses +of Granite.</b>—In the southern extremity of Norway there is +a large district, on the west side of the fiord of Christiania, +which I visited in 1837 with the late Professor Keilhau, in which +syenitic granite protrudes in mountain masses through fossiliferous +strata, and usually sends veins into them at the point of contact. +The stratified rocks, replete with shells and zoophytes, consist +chiefly of shale, limestone, and some sandstone, and all these are +invariably altered near the granite for a distance of from 50 to +400 yards. The aluminous shales are hardened, and have become +flinty. Sometimes they resemble jasper. Ribboned jasper is produced +by the hardening of alternate layers of green and +chocolate-coloured schist, each stripe faithfully representing the +original lines of stratification. Nearer the granite the schist +often contains crystals of hornblende, which are even met with in +some places for a distance of several hundred yards from the +junction; and this black hornblende is so abundant that eminent +geologists, when passing through the country, have confounded it +with the ancient hornblende-schist, subordinate to the great gneiss +formation of Norway. Frequently, between the granite and the +hornblende-slate above-mentioned, grains of mica and crystalline +feldspar appear in the schist, so that rocks resembling gneiss and +mica-schist are produced. Fossils can rarely be detected in these +schists, and they are more completely effaced in proportion to the +more crystalline texture of the beds, and their vicinity to the +granite.</p> + +<center><img src="../images5/fig623.jpg" width="341" height="219" alt= +"Fig. 623: Ground-plan of altered slate and limestone near granite. Christiania. The arrows indicate the dip, and the oblique lines the strike of the beds."> +</center> + +<p>In some places the siliceous matter of the schist becomes a +granular quartz; and when hornblende and mica are added, the +altered rock loses its stratification, and passes into a kind of +granite. The limestone, which at points remote</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 582">[ 582 ]</a></p> + +<p>from the granite is of an earthy texture and blue colour, and +often abounds in corals, becomes a white granular marble near the +granite, sometimes siliceous, the granular structure extending +occasionally upward of 400 yards from the junction; the corals +being for the most part obliterated, though sometimes preserved, +even in the white marble. Both the altered limestone and hardened +slate contain garnets in many places, also ores of iron, lead, and +copper, with some silver. These alterations occur equally whether +the granite invades the strata in a line parallel to the general +strike of the fossiliferous beds, or in a line at right angles to +their strike, both of which modes of junction will be seen by the +ground-plan in Fig. 623.*</p> + +<p>The granite of Cornwall sends forth veins into a coarse +argillaceous-schist, provincially termed killas. This killas is +converted into hornblende-schist near the contact with the veins. +These appearances are well seen at the junction of the granite and +killas, in St. Michael’s Mount, a small island nearly 300 +feet high, situated in the bay, at a distance of about three miles +from Penzance. The granite of Dartmoor, in Devonshire, says Sir H. +De la Beche, has intruded itself into the Carboniferous slate and +slaty sandstone, twisting and contorting the strata, and sending +veins into them. Hence some of the slate rocks have become +“micaceous; others more indurated, and with the characters of +mica-slate and gneiss; while others again appear converted into a +hard zoned rock strongly impregnated with +feldspar.”†</p> + +<p>We learn from the investigation of M. Dufrenoy that in the +eastern Pyrenees there are mountain masses of granite posterior in +date to the formations called lias and chalk of that district, and +that these fossiliferous rocks are greatly altered in texture, and +often charged with iron-ore, in the neighbourhood of the granite. +Thus in the environs of St. Martin, near St. Paul de Fenouillet, +the chalky limestone becomes more crystalline and saccharoid as it +approaches the granite, and loses all trace of the fossils which it +previously contained in abundance. At some points, also, it becomes +dolomitic, and filled with small veins of carbonate of iron, and +spots of red iron-ore. At Rancie the lias nearest the granite is +not only filled with iron-ore, but charged with pyrites, tremolite, +garnet, and a new mineral somewhat allied to feldspar, called, from +the place in the Pyrenees where it occurs, +“couzeranite.”</p> + +<p>“Hornblende-schist,” says Dr. MacCulloch, “may +at first have been mere clay; for clay or shale is found altered +by</p> + +<p class="fnote">* Keilhau, Gæa Norvegica, pp. 61-63.<br> +† Geol. Manual, p. 479.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 583">[ 583 ]</a></p> + +<p>trap into Lydian stone, a substance differing from +hornblende-schist almost solely in compactness and uniformity of +texture.”* “In Shetland,” remarks the same +author, “argillaceous-schist (or clay-slate), when in contact +with granite, is sometimes converted into hornblende-schist, the +schist becoming first siliceous, and ultimately, at the contact, +hornblende-schist.” In like manner gneiss and mica-schist may +be nothing more than altered micaceous and argillaceous sandstones, +granular quartz may have been derived from siliceous sandstone, and +compact quartz from the same materials. Clay-slate may be altered +shale, and granular marble may have originated in the form of +ordinary limestone, replete with shells and corals, which have +since been obliterated; and, lastly, calcareous sands and marls may +have been changed into impure crystalline limestones.</p> + +<p>The anthracite and plumbago associated with hypogene rocks may +have been coal; for not only is coal converted into anthracite in +the vicinity of some trap dikes, but we have seen that a like +change has taken place generally even far from the contact of +igneous rocks, in the disturbed region of the Appalachians. At +Worcester, in the State of Massachusetts, 45 miles due west of +Boston, a bed of plumbago and impure anthracite occurs, +interstratified with mica-schist. It is about two feet in +thickness, and has been made use of both as fuel, and in the +manufacture of lead pencils. At the distance of 30 miles from the +plumbago, there occurs, on the borders of Rhode Island, an impure +anthracite in slates containing impressions of coal-plants of the +genera <i>Pecopteris, Neuropteris, Calamites,</i> etc. This +anthracite is intermediate in character between that of +Pennsylvania and the plumbago of Worcester, in which last the +gaseous or volatile matter (hydrogen, oxygen, and nitrogen) is to +the carbon only in the proportion of three per cent. After +traversing the country in various directions, I came to the +conclusion that the carboniferous shales or slates with anthracite +and plants, which in Rhode Island often pass into mica-schists, +have at Worcester assumed a perfectly crystalline and metamorphic +texture; the anthracite having been nearly transmuted into that +state of pure carbon which is called plumbago or +graphite.†</p> + +<p>Now the alterations above described as superinduced in rocks by +volcanic dikes and granite veins prove incontestably that powers +exist in nature capable of transforming fossiliferous into +crystalline strata, a very few simple elements</p> + +<p class="fnote">* Syst. of Geol., vol. i, pp. 210, 211.<br> + † See Lyell, Quart. Geol. Journ., vol. i, p. 199.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 584">[ 584 ]</a></p> + +<p>constituting the component materials common to both classes of +rocks. These elements, which are enumerated in the table at <a +href="ch28.html#page 499">p. 499</a>, may be made to form new +combinations by what has been termed Plutonic action, or those +chemical changes which are no doubt connected with the passage of +heat, unusually heated steam and waters, through the strata.</p> + +<p><b>Hydrothermal Action, or the Influence of Steam and Gases in +producing Metamorphism.</b>—The experiments of Gregory Watt, +in fusing rocks in the laboratory, and allowing them to consolidate +by slow cooling, prove distinctly that a rock need not be perfectly +melted in order that a re-arrangement of its component particles +should take place, and a partial crystallisation ensue.* We may +easily suppose, therefore, that all traces of shells and other +organic remains may be destroyed, and that new chemical +combinations may arise, without the mass being so fused as that the +lines of stratification should be wholly obliterated. We must not, +however, imagine that heat alone, such as may be applied to a stone +in the open air, can constitute all that is comprised in Plutonic +action. We know that volcanoes in eruption not only emit fluid +lava, but give off steam and other heated gases, which rush out in +enormous volume, for days, weeks, or years continuously, and are +even disengaged from lava during its consolidation.</p> + +<p>We also know that long after volcanoes have spent their force, +hot springs continue for ages to flow out at various points in the +same area. In regions, also, subject to violent earthquakes such +springs are frequently observed issuing from rents, usually along +lines of fault or displacement of the rocks. These thermal waters +are most commonly charged with a variety of mineral ingredients, +and they retain a remarkable uniformity of temperature from century +to century. A like uniformity is also persistent in the nature of +the earthy, metallic, and gaseous substances with which they are +impregnated. It is well ascertained that springs, whether hot or +cold, charged with carbonic acid, especially with hydrofluoric +acid, which is often present in small quantities, are powerful +causes of decomposition and chemical reaction in rocks through +which they percolate.</p> + +<p>The changes which Daubrée has shown to have been produced +by the alkaline waters of Plombières in the Vosges, are more +especially instructive.† These waters have a heat of +160° F., or an excess of 109° above the average temperature +of ordinary springs in that district. They were</p> + +<p class="fnote">* Phil. Trans., 1804.<br> + † Daubrée, Sur le Métamorphisme. Paris, +1860.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 585">[ 585 ]</a></p> + +<p>conveyed by the Romans to baths through long conduits or +aqueducts. The foundations of some of their works consisted of a +bed of concrete made of lime, fragments of brick, and sandstone. +Through this and other masonry the hot waters have been percolating +for centuries, and have given rise to various zeolites—apophyllite +and chabazite among others; also to calcareous spar, arragonite, +and fluor spar, together with siliceous minerals, such as opal—all +found in the inter-spaces of the bricks and mortar, or constituting +part of their re-arranged materials. The quantity of heat brought +into action in this instance in the course of 2000 years has, no +doubt, been enormous, but the intensity of it developed at any one +moment has been always inconsiderable.</p> + +<p>From these facts and from the experiments and observations of +Sénarmont, Daubrée, Delesse, Scheerer, Sorby, Sterry +Hunt, and others, we are led to infer that when in the bowels of +the earth there are large volumes of matter containing water and +various acids intensely heated under enormous pressure, these +subterranean fluid masses will gradually part with their heat by +the escape of steam and various gases through fissures, producing +hot springs; or by the passage of the same through the pores of the +overlying and injected rocks. Even the most compact rocks may be +regarded, before they have been exposed to the air and dried, in +the light of sponges filled with water. According to the +experiments of Henry, water, under a hydrostatic pressure of 96 +feet, will absorb three times as much carbonic acid gas as it can +under the ordinary pressure of the atmosphere. There are other +gases, as well as the carbonic acid, which water absorbs, and more +rapidly in proportion to the amount of pressure. Although the +gaseous matter first absorbed would soon be condensed, and part +with its heat, yet the continual arrival of fresh supplies from +below might, in the course of ages, cause the temperature of the +water, and with it that of the containing rock, to be materially +raised; the water acts not only as a vehicle of heat, but also by +its affinity for various silicates, which, when some of the +materials of the invaded rocks are decomposed, form quartz, +feldspar, mica, and other minerals. As for quartz, it can be +produced under the influence of heat by water holding alkaline +silicates in solution, as in the case of the Plombières +springs. The quantity of water required, according to +Daubrée, to produce great transformations in the mineral +structure of rocks, is very small. As to the heat required, +silicates may be produced in the moist way at about incipient red +heat, whereas to form the same in the dry way would require a much +higher temperature.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 586">[ 586 ]</a></p> + +<p>M. Fournet, in his description of the metalliferous gneiss near +Clermont, in Auvergne, states that all the minute fissures of the +rock are quite saturated with free carbonic acid gas; which gas +rises plentifully from the soil there and in many parts of the +surrounding country. The various elements of the gneiss, with the +exception of the quartz, are all softened; and new combinations of +the acid with lime, iron, and manganese are continually in +progress.*</p> + +<p>The power of subterranean gases is well illustrated by the +stufas of St. Calogero in the Lipari Islands, where the horizontal +strata of tuffs, forming cliffs 200 feet high, have been +discoloured in places by the jets of steam often above the boiling +point, called “stufas,” issuing from the fissures; and +similar instances are recorded by M. Virlet of corrosion of rocks +near Corinth, and by Dr. Daubeny of decomposition of trachytic +rocks by sulphureted hydrogen and muriatic acid gases in the +Solfatara, near Naples. In all these instances it is clear that the +gaseous fluids must have made their way through vast thicknesses of +porous or fissured rocks, and their modifying influence may spread +through the crust for thousands of yards in thickness.</p> + +<p>It has been urged as an argument against the metamorphic theory, +that rocks have a small power of conducting heat, and it is true +that when dry, and in the air, they differ remarkably from metals +in this respect. The syenite of Norway, as we have seen (<a href= +"ch31.html#page 558">p. 558</a>), has sometimes altered +fossiliferous strata both in the direction of their dip and strike +for a distance of a quarter of a mile, but the theory of gneiss and +mica-schist above proposed requires us to imagine that the same +influence has extended through strata miles in thickness. Professor +Bischof has shown what changes may be superinduced, on black marble +and other rocks, by the steam of a hot spring having a temperature +of no more than 133° to 167° Fahrenheit, and we are +becoming more and more acquainted with the prominent part which +water is playing in distributing the heat of the interior through +mountain masses of incumbent strata, and of introducing into them +various mineral elements in a fluid or gaseous state. Such facts +may induce us to consider whether many granites and other rocks of +that class may not sometimes represent merely the extreme of a +similar slow metamorphism. But, on the other hand, the heat of lava +in a volcanic crater when it is white and glowing like the sun must +convince us that the temperature of a column of such a fluid at the +depth of many miles exceeds any heat which can ever be witnessed at +the surface.</p> + +<p class="fnote">* See Principles, <i>Index,</i> “Carbonated +Springs,” etc.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 587">[ 587 ]</a></p> + +<p>That large portions of the Plutonic rocks had been formed under +the influence of such intense heat is in perfect accordance with +their great volume, uniform composition, and absence of +stratification. The forcing also of veins into contiguous +stratified or schistose rocks is a natural consequence of the +hydrostatic pressure to which columns of molten matter many miles +in height must give rise.</p> + +<p><b>Objections to the Metamorphic Theory considered.</b>—It +has been objected to the metamorphic theory that the crystalline +schists contain a considerable proportion of potash and soda, +whilst the sedimentary strata out of which they are supposed to +have been formed are usually wanting in alkaline matter. But this +reasoning proceeds on mistaken data, for clay, marl, shale, and +slate often contain a considerable proportion of alkali, so much so +as to make them frequently unfit to be burnt into bricks or +pottery, and the Old Red Sandstone in Forfarshire and other parts +of Scotland, derived from disintegration of granite, contains much +triturated feldspar rich in potash. In the common salt by which +strata are often largely impregnated, as in Patagonia, much soda is +present, and potash enters largely into the composition of fossil +sea-weeds, and recent analysis has also shown that the +carboniferous strata in England, the Upper and Lower Silurian in +East Canada, and the oldest clay-slates in Norway, all contain as +much alkali as is generally present in metamorphic rocks.</p> + +<p>Another objection has been derived from the alternation of +highly crystalline strata with others less crystalline. The heat, +it is said, in its ascent from below, must have traversed the less +altered schists before it reached a higher and more crystalline +bed. In answer to this, it may be observed, that if a number of +strata differing greatly in composition from each other be +subjected to equal quantities of heat, or hydrothermal action, +there is every probability that some will be much more fusible or +soluble than others. Some, for example, will contain soda, potash, +lime, or some other ingredient capable of acting as a flux or +solvent; while others may be destitute of the same elements, and so +refractory as to be very slightly affected by the same causes. Nor +should it be forgotten that, as a general rule, the less +crystalline rocks do really occur in the upper, and the more +crystalline in the lower part of each metamorphic series.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch32.html"> +Chapter XXXII</a> / <a href="ch34.html">Chapter XXXIV</a></small> +</body> +</html> + |