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diff --git a/old/3772-h/files/ch1.html b/old/3772-h/files/ch1.html new file mode 100644 index 0000000..5ba7b92 --- /dev/null +++ b/old/3772-h/files/ch1.html @@ -0,0 +1,575 @@ +<!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> +<hr> +<p class="page"><a name="page 25">[ 25 ]</a></p> + +<p> </p> + +<center> +<h3>STUDENT’S</h3> + +<br> +<h2>ELEMENTS OF GEOLOGY</h2> +</center> + +<hr width="20%"> +<br> +<br> + + +<center><b>Chapter I</b><br> +<br> + ON THE DIFFERENT CLASSES OF ROCKS.</center> + +<p class="intro">Geology defined. — Successive Formation of +the Earth’s Crust. — Classification of Rocks according to +their Origin and Age. — Aqueous Rocks. — Their +Stratification and imbedded Fossils. — Volcanic Rocks, with +and without Cones and Craters. — Plutonic Rocks, and their +Relation to the Volcanic. — Metamorphic Rocks, and their +probable Origin. — The term Primitive, why erroneously +applied to the Crystalline Formations. — Leading Division of +the Work.</p> + +<p>Of what materials is the earth composed, and in what manner are +these materials arranged? These are the first inquiries with which +Geology is occupied, a science which derives its name from the +Greek <i>ge</i>, the earth, and <i>logos</i>, a discourse. +Previously to experience we might have imagined that investigations +of this kind would relate exclusively to the mineral kingdom, and +to the various rocks, soils, and metals, which occur upon the +surface of the earth, or at various depths beneath it. But, in +pursuing such researches, we soon find ourselves led on to consider +the successive changes which have taken place in the former state +of the earth’s surface and interior, and the causes which have +given rise to these changes; and, what is still more singular and +unexpected, we soon become engaged in researches into the history +of the animate creation, or of the various tribes of animals and +plants which have, at different periods of the past, inhabited the +globe.</p> + +<p>All are aware that the solid parts of the earth consist of +distinct substances, such as clay, chalk, sand, limestone, coal, +slate, granite, and the like; but previously to observation it is +commonly imagined that all these had remained from the first in the +state in which we now see them—that they were created in +their present form, and in their present position. The geologist +soon comes to a different conclusion, discovering</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 26">[ 26 ]</a></p> + +<br> + + +<p>proofs that the external parts of the earth were not all +produced in the beginning of things in the state in which we now +behold them, nor in an instant of time. On the contrary, he can +show that they have acquired their actual configuration and +condition gradually, under a great variety of circumstances, and at +successive periods, during each of which distinct races of living +beings have flourished on the land and in the waters, the remains +of these creatures still lying buried in the crust of the +earth.</p> + +<p>By the “earth’s crust,” is meant that small portion of the +exterior of our planet which is accessible to human observation. It +comprises not merely all of which the structure is laid open in +mountain precipices, or in cliffs overhanging a river or the sea, +or whatever the miner may reveal in artificial excavations; but the +whole of that outer covering of the planet on which we are enabled +to reason by observations made at or near the surface. These +reasonings may extend to a depth of several miles, perhaps ten +miles; and even then it may be said, that such a thickness is no +more than 1/400 part of the distance from the surface to the +centre. The remark is just: but although the dimensions of such a +crust are, in truth, insignificant when compared to the entire +globe, yet they are vast, and of magnificent extent in relation to +man, and to the organic beings which people our globe. Referring to +this standard of magnitude, the geologist may admire the ample +limits of his domain, and admit, at the same time, that not only +the exterior of the planet, but the entire earth, is but an atom in +the midst of the countless worlds surveyed by the astronomer.</p> + +<p>The materials of this crust are not thrown together confusedly; +but distinct mineral masses, called rocks, are found to occupy +definite spaces, and to exhibit a certain order of arrangement. The +term <i>rock</i> is applied indifferently by geologists to all +these substances, whether they be soft or stony, for clay and sand +are included in the term, and some have even brought peat under +this denomination. Our old writers endeavoured to avoid offering +such violence to our language, by speaking of the component +materials of the earth as consisting of rocks and <i>soils.</i> But +there is often so insensible a passage from a soft and incoherent +state to that of stone, that geologists of all countries have found +it indispensable to have one technical term to include both, and in +this sense we find <i>roche</i> applied in French, <i>rocca</i> in +Italian, and <i>felsart</i> in German. The beginner, however, must +constantly bear in mind that the term rock by no means implies that +a mineral mass is in an indurated or stony condition.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 27">[ 27 ]</a></p> + +<br> + + +<p>The most natural and convenient mode of classifying the various +rocks which compose the earth’s crust, is to refer, in the first +place, to their origin, and in the second to their relative age. I +shall therefore begin by endeavouring briefly to explain to the +student how all rocks may be divided into four great classes by +reference to their different origin, or, in other words, by +reference to the different circumstances and causes by which they +have been produced.</p> + +<p>The first two divisions, which will at once be understood as +natural, are the aqueous and volcanic, or the products of watery +and those of igneous action at or near the surface.</p> + +<p><b>Aqueous Rocks.</b>—The aqueous rocks, sometimes called +the sedimentary, or fossiliferous, cover a larger part of the +earth’s surface than any others. They consist chiefly of mechanical +deposits (pebbles, sand, and mud), but are partly of chemical and +some of them of organic origin, especially the limestones. These +rocks are <i>stratified,</i> or divided into distinct layers, or +strata. The term <i>stratum</i> means simply a bed, or any thing +spread out or <i>strewed</i> over a given surface; and we infer +that these strata have been generally spread out by the action of +water, from what we daily see taking place near the mouths of +rivers, or on the land during temporary inundations. For, whenever +a running stream charged with mud or sand, has its velocity +checked, as when it enters a lake or sea, or overflows a plain, the +sediment, previously held in suspension by the motion of the water, +sinks, by its own gravity to the bottom. In this manner layers of +mud and sand are thrown down one upon another.</p> + +<p>If we drain a lake which has been fed by a small stream, we +frequently find at the bottom a series of deposits, disposed with +considerable regularity, one above the other; the uppermost, +perhaps, may be a stratum of peat, next below a more dense and +solid variety of the same material; still lower a bed of +shell-marl, alternating with peat or sand, and then other beds of +marl, divided by layers of clay. Now, if a second pit be sunk +through the same continuous lacustrine <i>formation</i> at some +distance from the first, nearly the same series of beds is commonly +met with, yet with slight variations; some, for example, of the +layers of sand, clay, or marl, may be wanting, one or more of them +having thinned out and given place to others, or sometimes one of +the masses first examined is observed to increase in thickness to +the exclusion of other beds.</p> + +<p>The term <i>formation,</i> which I have used in the above +explanation, expresses in geology any assemblage of rocks which +have some character in common, whether of origin,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 28">[ 28 ]</a></p> + +<br> + + +<p>age, or composition. Thus we speak of stratified and +unstratified, fresh-water and marine, aqueous and volcanic, ancient +and modern, metalliferous and non-metalliferous formations.</p> + +<p>In the estuaries of large rivers, such as the Ganges and the +Mississippi, we may observe, at low water, phenomena analogous to +those of the drained lakes above mentioned, but on a grander scale, +and extending over areas several hundred miles in length and +breadth. When the periodical inundations subside, the river hollows +out a channel to the depth of many yards through horizontal beds of +clay and sand, the ends of which are seen exposed in perpendicular +cliffs. These beds vary in their mineral composition, or colour, or +in the fineness or coarseness of their particles, and some of them +are occasionally characterised by containing drift-wood. At the +junction of the river and the sea, especially in lagoons nearly +separated by sand-bars from the ocean, deposits are often formed in +which brackish and salt-water shells are included.</p> + +<p>In Egypt, where the Nile is always adding to its delta by +filling up part of the Mediterranean with mud, the newly deposited +sediment is <i>stratified,</i> the thin layer thrown down in one +season differing slightly in colour from that of a previous year, +and being separable from it, as has been observed in excavations at +Cairo and other places.*</p> + +<p>When beds of sand, clay, and marl, containing shells and +vegetable matter, are found arranged in a similar manner in the +interior of the earth, we ascribe to them a similar origin; and the +more we examine their characters in minute detail, the more exact +do we find the resemblance. Thus, for example, at various heights +and depths in the earth, and often far from seas, lakes, and +rivers, we meet with layers of rounded pebbles composed of flint, +limestone, granite, or other rocks, resembling the shingles of a +sea-beach or the gravel in a torrent’s bed. Such layers of pebbles +frequently alternate with others formed of sand or fine sediment, +just as we may see in the channel of a river descending from hills +bordering a coast, where the current sweeps down at one season +coarse sand and gravel, while at another, when the waters are low +and less rapid, fine mud and sand alone are carried +seaward.†</p> + +<p>If a stratified arrangement, and the rounded form of pebbles, +are alone sufficient to lead us to the conclusion that certain +rocks originated under water, this opinion is farther confirmed by +the distinct and independent evidence of</p> + +<p class="fnote">* See Principles of Geology, by the Author, Index, +“Nile,” “Rivers,” etc.<br> +† See p. 44, Fig. 7.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 29">[ 29 ]</a></p> + +<br> + + +<p><i>fossils,</i> so abundantly included in the earth’s crust. By +a <i>fossil</i> is meant any body, or the traces of the existence +of any body, whether animal or vegetable, which has been buried in +the earth by natural causes. Now the remains of animals, especially +of aquatic species, are found almost everywhere imbedded in +stratified rocks, and sometimes, in the case of limestone, they are +in such abundance as to constitute the entire mass of the rock +itself. Shells and corals are the most frequent, and with them are +often associated the bones and teeth of fishes, fragments of wood, +impressions of leaves, and other organic substances. Fossil shells, +of forms such as now abound in the sea, are met with far inland, +both near the surface, and at great depths below it. They occur at +all heights above the level of the ocean, having been observed at +elevations of more than 8000 feet in the Pyrenees, 10,000 in the +Alps, 13,000 in the Andes, and above 18,000 feet in the +Himalaya.*</p> + +<p>These shells belong mostly to marine testacea, but in some +places exclusively to forms characteristic of lakes and rivers. +Hence it is concluded that some ancient strata were deposited at +the bottom of the sea, and others in lakes and estuaries.</p> + +<p>We have now pointed out one great class of rocks, which, however +they may vary in mineral composition, colour, grain, or other +characters, external and internal, may nevertheless be grouped +together as having a common origin. They have all been formed under +water, in the same manner as modern accumulations of sand, mud, +shingle, banks of shells, reefs of coral, and the like, and are all +characterised by stratification or fossils, or by both.</p> + +<p><b>Volcanic Rocks.</b>—The division of rocks which we may +next consider are the volcanic, or those which have been produced +at or near the surface whether in ancient or modern times, not by +water, but by the action of fire or subterranean heat. These rocks +are for the most part unstratified, and are devoid of fossils. They +are more partially distributed than aqueous formations, at least in +respect to horizontal extension. Among those parts of Europe where +they exhibit characters not to be mistaken, I may mention not only +Sicily and the country round Naples, but Auvergne, Velay, and +Vivarais, now the departments of Puy de Dome, Haute Loire, and +Ardêche, towards the centre and south of France, in which are +several hundred conical hills having the forms of modern volcanoes, +with craters more or less perfect on many of their summits. These +cones are composed moreover</p> + +<p class="fnote">* Col. R. J. Strachey found oolitic fossils 18,400 +feet high in the Himalaya.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 30">[ 30 ]</a></p> + +<br> + + +<p>of lava, sand, and ashes, similar to those of active volcanoes. +Streams of lava may sometimes be traced from the cones into the +adjoining valleys, where they have choked up the ancient channels +of rivers with solid rock, in the same manner as some modern flows +of lava in Iceland have been known to do, the rivers either flowing +beneath or cutting out a narrow passage on one side of the lava. +Although none of these French volcanoes have been in activity +within the period of history or tradition, their forms are often +very perfect. Some, however, have been compared to the mere +skeletons of volcanoes, the rains and torrents having washed their +sides, and removed all the loose sand and scoriæ, leaving +only the harder and more solid materials. By this erosion, and by +earthquakes, their internal structure has occasionally been laid +open to view, in fissures and ravines; and we then behold not only +many successive beds and masses of porous lava, sand, and +scoriæ, but also perpendicular walls, or <i>dikes,</i> as +they are called, of volcanic rock, which have burst through the +other materials. Such dikes are also observed in the structure of +Vesuvius, Etna, and other active volcanoes. They have been formed +by the pouring of melted matter, whether from above or below, into +open fissures, and they commonly traverse deposits of <i>volcanic +tuff,</i> a substance produced by the showering down from the air, +or incumbent waters, of sand and cinders, first shot up from the +interior of the earth by the explosions of volcanic gases.</p> + +<p>Besides the parts of France above alluded to, there are other +countries, as the north of Spain, the south of Sicily, the Tuscan +territory of Italy, the lower Rhenish provinces, and Hungary, where +spent volcanoes may be seen, still preserving in many cases a +conical form, and having craters and often lava-streams connected +with them.</p> + +<p>There are also other rocks in England, Scotland, Ireland, and +almost every country in Europe, which we infer to be of igneous +origin, although they do not form hills with cones and craters. +Thus, for example, we feel assured that the rock of Staffa, and +that of the Giant’s Causeway, called basalt, is volcanic, because +it agrees in its columnar structure and mineral composition with +streams of lava which we know to have flowed from the craters of +volcanoes. We find also similar basaltic and other igneous rocks +associated with beds of <i>tuff</i> in various parts of the British +Isles, and forming <i>dikes,</i> such as have been spoken of; and +some of the strata through which these dikes cut are occasionally +altered at the point of contact, as if they had been exposed to the +intense heat of melted matter.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 31">[ 31 ]</a></p> + +<br> + + +<p>The absence of cones and craters, and long narrow streams of +superficial lava, in England and many other countries, is +principally to be attributed to the eruptions having been +submarine, just as a considerable proportion of volcanoes in our +own times burst out beneath the sea. But this question must be +enlarged upon more fully in the chapters on Igneous Rocks, in which +it will also be shown, that as different sedimentary formations, +containing each their characteristic fossils, have been deposited +at successive periods, so also volcanic sand and scoriæ have +been thrown out, and lavas have flowed over the land or bed of the +sea, at many different epochs, or have been injected into fissures; +so that the igneous as well as the aqueous rocks may be classed as +a chronological series of monuments, throwing light on a succession +of events in the history of the earth.</p> + +<p><b>Plutonic Rocks</b> (<i>Granite,</i> etc).—We have now +pointed out the existence of two distinct orders of mineral masses, +the aqueous and the volcanic: but if we examine a large portion of +a continent, especially if it contain within it a lofty mountain +range, we rarely fail to discover two other classes of rocks, very +distinct from either of those above alluded to, and which we can +neither assimilate to deposits such as are now accumulated in lakes +or seas, nor to those generated by ordinary volcanic action. The +members of both these divisions of rocks agree in being highly +crystalline and destitute of organic remains. The rocks of one +division have been called Plutonic, comprehending all the granites +and certain porphyries, which are nearly allied in some of their +characters to volcanic formations. The members of the other class +are stratified and often slaty, and have been called by some the +<i>crystalline schists,</i> in which group are included gneiss, +micaceous-schist (or mica-slate), hornblende-schist, statuary +marble, the finer kinds of roofing slate, and other rocks +afterwards to be described.</p> + +<p>As it is admitted that nothing strictly analogous to these +crystalline productions can now be seen in the progress of +formation on the earth’s surface, it will naturally be asked, on +what data we can find a place for them in a system of +classification founded on the origin of rocks. I can not, in reply +to this question, pretend to give the student, in a few words, an +intelligible account of the long chain of facts and reasonings from +which geologists have been led to infer the nature of the rocks in +question. The result, however, may be briefly stated. All the +various kinds of granites which constitute the Plutonic family are +supposed to be of igneous or aqueo-igneous origin, and to have been +formed</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 32">[ 32 ]</a></p> + +<br> + + +<p>under great pressure, at a considerable depth in the earth, or +sometimes, perhaps, under a certain weight of incumbent ocean. Like +the lava of volcanoes, they have been melted, and afterwards cooled +and crystallised, but with extreme slowness, and under conditions +very different from those of bodies cooling in the open air. Hence +they differ from the volcanic rocks, not only by their more +crystalline texture, but also by the absence of tuffs and breccias, +which are the products of eruptions at the earth’s surface, or +beneath seas of inconsiderable depth. They differ also by the +absence of pores or cellular cavities, to which the expansion of +the entangled gases gives rise in ordinary lava.</p> + +<p><b>Metamorphic, or Stratified Crystalline Rocks.</b>—The +fourth and last great division of rocks are the crystalline strata +and slates, or schists, called gneiss, mica-schist, clay-slate, +chlorite-schist, marble, and the like, the origin of which is more +doubtful than that of the other three classes. They contain no +pebbles, or sand, or scoriæ, or angular pieces of imbedded +stone, and no traces of organic bodies, and they are often as +crystalline as granite, yet are divided into beds, corresponding in +form and arrangement to those of sedimentary formations, and are +therefore said to be stratified. The beds sometimes consist of an +alternation of substances varying in colour, composition, and +thickness, precisely as we see in stratified fossiliferous +deposits. According to the Huttonian theory, which I adopt as the +most probable, and which will be afterwards more fully explained, +the materials of these strata were originally deposited from water +in the usual form of sediment, but they were subsequently so +altered by subterranean heat, as to assume a new texture. It is +demonstrable, in some cases at least, that such a complete +conversion has actually taken place, fossiliferous strata having +exchanged an earthy for a highly crystalline texture for a distance +of a quarter of a mile from their contact with granite. In some +cases, dark limestones, replete with shells and corals, have been +turned into white statuary marble; and hard clays, containing +vegetable or other remains, into slates called mica-schist or +hornblende-schist, every vestige of the organic bodies having been +obliterated.</p> + +<p>Although we are in a great degree ignorant of the precise nature +of the influence exerted in these cases, yet it evidently bears +some analogy to that which volcanic heat and gases are known to +produce; and the action may be conveniently called Plutonic, +because it appears to have been developed in those regions where +Plutonic rocks are generated, and under similar circumstances of +pressure and depth in the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 33">[ 33 ]</a></p> + +<br> + + +<p>earth. Intensely heated water or steam permeating stratified +masses under great pressure have no doubt played their part in +producing the crystalline texture and other changes, and it is +clear that the transforming influence has often pervaded entire +mountain masses of strata.</p> + +<p>In accordance with the hypothesis above alluded to, I proposed +in the first edition of the Principles of Geology (1833) the term +“Metamorphic” for the altered strata, a term derived from meta, <i> +trans,</i> and morphe, <i>forma.</i></p> + +<p>Hence there are four great classes of rocks considered in +reference to their origin—the aqueous, the volcanic, the +Plutonic, and the metamorphic. In the course of this work it will +be shown that portions of each of these four distinct classes have +originated at many successive periods. They have all been produced +contemporaneously, and may even now be in the progress of formation +on a large scale. It is not true, as was formerly supposed, that +all granites, together with the crystalline or metamorphic strata, +were first formed, and therefore entitled to be called “primitive,” +and that the aqueous and volcanic rocks were afterwards +superimposed, and should, therefore, rank as secondary in the order +of time. This idea was adopted in the infancy of the science, when +all formations, whether stratified or unstratified, earthy or +crystalline, with or without fossils, were alike regarded as of +aqueous origin. At that period it was naturally argued that the +foundation must be older than the superstructure; but it was +afterwards discovered that this opinion was by no means in every +instance a legitimate deduction from facts; for the inferior parts +of the earth’s crust have often been modified, and even entirely +changed, by the influence of volcanic and other subterranean +causes, while superimposed formations have not been in the +slightest degree altered. In other words, the destroying and +renovating processes have given birth to new rocks below, while +those above, whether crystalline or fossiliferous, have remained in +their ancient condition. Even in cities, such as Venice and +Amsterdam, it cannot be laid down as universally true that the +upper parts of each edifice, whether of brick or marble, are more +modern than the foundations on which they rest, for these often +consist of wooden piles, which may have rotted and been replaced +one after the other, without the least injury to the buildings +above; meanwhile, these may have required scarcely any repair, and +may have been constantly inhabited. So it is with the habitable +surface of our globe, in its relation to large masses of rock +immediately below; it may continue the same for ages, while +subjacent materials, at</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 34">[ 34 ]</a></p> + +<br> + + +<p>a great depth, are passing from a solid to a fluid state, and +then reconsolidating, so as to acquire a new texture.</p> + +<p>As all the crystalline rocks may, in some respects, be viewed as +belonging to one great family, whether they be stratified or +unstratified, metamorphic or Plutonic, it will often be convenient +to speak of them by one common name. It being now ascertained, as +above stated, that they are of very different ages, sometimes newer +than the strata called secondary, the terms primitive and primary +which were formerly used for the whole must be abandoned, as they +would imply a manifest contradiction. It is indispensable, +therefore, to find a new name, one which must not be of +chronological import, and must express, on the one hand, some +peculiarity equally attributable to granite and gneiss (to the +Plutonic as well as the <i>altered</i> rocks), and, on the other, +must have reference to characters in which those rocks differ, both +from the volcanic and from the <i>unaltered</i> sedimentary strata. +I proposed in the Principles of Geology (first edition, vol. iii) +the term “hypogene” for this purpose, derived from upo, <i> +under,</i> and ginomai, <i>to be,</i> or <i>to be born</i>; a word +implying the theory that granite, gneiss, and the other crystalline +formations are alike <i>netherformed</i> rocks, or rocks which have +not assumed their present form and structure at the surface. They +occupy the lowest place in the order of superposition. Even in +regions such as the Alps, where some masses of granite and gneiss +can be shown to be of comparatively modern date, belonging, for +example, to the period hereafter to be described as tertiary, they +are still <i>underlying</i> rocks. They never repose on the +volcanic or trappean formations, nor on strata containing organic +remains. They are <i>hypogene,</i> as “being under” all the +rest.</p> + +<p>From what has now been said, the reader will understand that +each of the four great classes of rocks may be studied under two +distinct points of view; first, they may be studied simply as +mineral masses deriving their origin from particular causes, and +having a certain composition, form, and position in the earth’s +crust, or other characters both positive and negative, such as the +presence or absence of organic remains. In the second place, the +rocks of each class may be viewed as a grand chronological series +of monuments, attesting a succession of events in the former +history of the globe and its living inhabitants.</p> + +<p>I shall accordingly proceed to treat of each family of rocks; +first, in reference to those characters which are not +chronological, and then in particular relation to the several +periods when they were formed.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch2.html"> +Chapter II</a></small> +</body> +</html> + |