« PreviousContinue »
THE CEUST WE DWELL UPON.
NATURE OF THE EARTH'S CRUST OR SOLID EXTERIOR—DIFFERS FROM THE INTERIOR—COMPOSED OF ROCKS AND ROCK-FORMATIONS— THESE THE THEMES OF GEOLOGICAL INVESTIGATION—TECHNICAL MEANING OF THE TERM "ROCK"—STRATIFIED AND UNSTRATIFIED
, ROCKS—HOW AND BY WHAT PROCESSES FORMED—OLDER AND YOUNGER ROCKS—EXAMPLES OF—HOW DISTINGUISHED—CHRONOLOGICAL ARRANGEMENT OF ROCK-FORMATIONS—EACH A CHAPTER OF WORLD-HISTORY—ATTRACTIVE NATURE OF THIS HISTORY— FACTS ARRIVED AT BY A STUDY OF THE EARTH'S CRUST—THEIR THEORETICAL AND PRACTICAL IMPORTANCE.
Wherever we travel we find the land made up of rocks and rocky substances. If we go to the sea-shores, we find similar substances stretching away beneath the waters, or rising up in mid-ocean as reefs and islands. The fair inference therefore is, that all the exterior of our planet is composed of rocks and rock-formations, and that the ocean merely occupies the great hollows or depressions in the same way as the lesser lakes and tarns occupy the rockbasins of the continents and islands. To this rocky exterior geologists apply the term "crust," much in the same way as the housewife speaks of the crust of her loaf, or the
schoolboy of the crust of ice that forms on the stagnant pool during the frosts of winter. The crust is something hard and consistent, and may differ both in nature and consistency from the interior on which it rests, or over which it may be formed; and this is precisely the idea entertained by geologists when they speak of the outer shell or " crust of the globe."
The rocky exterior over which we travel, and into which we dig and mine and tunnel, is a thing we can see and investigate to a limited depth; but the interior, sinking away four thousand miles to the centre, is placed altogether beyond our reach and observation. It may consist of rocky substances, but if so, they must be in a condition as to density altogether different from those we find at the surface; for as a planet the earth has a certain astronomicallyascertained weight, and were the force of gravitation to exert itself to the centre on such rocks as we know, their compression would give to the earth's mass a weight far exceeding that which its astronomical relations will allow.* Again, as we descend into the earth by mines, shafts, and Artesian wells, the temperature seems to increase at a given ratio (about one degree Fahr. for every 60 feet of descent); and at this rate a depth would soon be reached at which every known substance would be held in a state of incandescent fusion, or even vaporiform dispersion. It is convenient, therefore, to draw a distinction between the " crust" we can examine, and the "interior," respecting which we can only form hypotheses. And it is this crust which constitutes the great theme of geological investigation. What is the nature of the rocks of which it is composed? how are
* The reader must guard against the idea that at extreme depths all substances suffer alike from mutual mechanical pressure. Their different compositions forbid this supposition; and their densities must continue to depend (no matter what the depth) more on their chemical nature than on the amount of compression to which they are subjected.
they arranged 1 by what agencies have they been formed 1 what changes are they now undergoing? and, reasoning from the known to the unknown, what changes do they seem to have undergone in former periods 1 If we can answer all these questions, or approach to anything like a reasonable answer, we then present something like a history of our planet; and such a history is the aim and object of all sound geology.
We have said that this earth-crust consists of rocks and rock-formations; and here we must explain that the term "rock " is applied by geologists to all the solid substances that enter into its composition. And there is good reason for this usage. The sand and gravel of the sea-shore are but comminuted rock-matters derived from the cliffs above; the sands and loams and clays of the valleys are merely rock-debris, worn and washed in course of ages from the hills and uplands. Be it gravel or sand, clay or mud, all are alike known to the geologist as "rocks ;" and there can be no doubt that, were these loose and soft matters consolidated by pressiire or other agency, they would become again compact and hard, like the rock-masses from which they were originally derived. It is necessary, then, to bear in mind this technical use of the term "rock;" and the least reflection upon the changes (mechanical and chemical) which all rock-matter is incessantly undergoing, will show the appropriateness of the application.
Understanding, then, what geologists mean by the term "rock," and bearing in mind that their labours are restricted to the accessible crust, let us inquire a little more narrowly into the nature of the rocks of which this crust is composed, and the modes of their arrangement. Wherever the structure of the crust is revealed—whether along the cliffs of the sea-shore, in ravines worn out by rivers, in shafts sunk for mining, or in railway-cuttings and tunnels —we see the rock-masses arranged in two great ways. A large and extensive class—like the sandstones and shales and limestones—lie in layers or beds one above another; and a second class—like the granites, greenstones, and basalts— exhibit no lines of bedding or layers, but occur in vast and indeterminate masses. These two modes of arrangement may be seen in almost every railway-cutting and sea-cliff, and must obviously have arisen from different causes. Now the great maxim in geology is to reason from the known to the unknown, and to appeal from the existing operations of nature to the operations of the past. For, as was long ago well remarked by Hutton, "when from a thing which is well known we explain another which is less so, we then investigate nature; but when we imagine things without a pattern or example in nature, instead of Natural History, we write merely fable." Abiding by this method, we find nature at the present day laying down, in every lake and estuary and sea, layers of mud and sand and gravel, varying in thickness and continuity, according to the extent of the areas and the magnitude of the in-flowing rivers; and were these layers consolidated, sand would form sandstone, gravel conglomerate, and mud shale. Here, then, as we cannot regard nature acting in time past otherwise than at present, we are entitled to infer that all rocks in the earth's crust occurring in layers have been formed through and by the agency of water—that is, that they are the sediments of former lakes and estuaries and seas, the particles of which they are composed having been worn down by water, transported by water, and deposited in water. Hence all such rocks are regarded as aquecms, sedimentary, or stratified, and indicate that the areas they now occupy were at some former period the sites of lakes, estuaries, and seas. In a similar manner we seek to explain the origin and nature of the basalts and greenstones that rise up in homo