INTERNAL STRUCTURE. OF THE EARTH.
Having in the preceding Chapters desct ibed the va rious inequalities observable on the earth's surface, and stated the means which nature appears to have employ ed in forming them, we come now to the consideration of the second branch of Geognosy, which makes us ac quainted with the internal structure of the earth.
At first sight the solid mass of the earth appears to be a confused assemblage of rocky masses piled on each other without order or regularity : to the superficial ob server, nature appears, in the apparently rude matter of the inorganic kingdom, to present us as only with a pic ture of chaos, where none of those admirable displays of skill and contrivance, which, in the structure of ani mals and vegetables, so powerfully excite our attention, and claim our admiration, are to be observed. It is not surprising that this unfavourable opinion should have long continued to be prevalent, when we consider the experience, skill and judgment, which are necessary for enabling us to unravel all the variety of apparently un connected relations, which are observable in the internal structure of the earth. In ancient writers we find no thing on this important subject ; and it is only by the light of modern discoveries, that we have been enabled to trace out those beautiful arrangements that prevail in the mineral kingdom. Of these, we shall now give a concise and accurate description, and at the same time state some of those inferences that appear to be deduci ble from these various relations and appearances. But these descriptions and inferences can only be fully un derstood, and the gratification derived from them com pletely enjoyed, by an intimate acquaintance with nature het self, not in cabinets alone, but in mines and among mountains. On descending into mines, we are not only gratified by displays of human ingenuity and skill, but we also receive much information respecting the stt uc lure of the earth, and the changes it: has experienced during the different perio,ls of its fOrmation. Our re searches on the surface earth, on the other hand, often leads us among the grandest and most sublime works of nature ; and amid alpine groups, the geologist is, as it were, conducted nearer to the scene of those great operations, which it is his business to explore. In
the midst of such scenes, he. feels his mind invigorated; the magnitude of the appearances before him extin guishes all the little and contracted notions he may have formed in the closet ; and he learns, that it is only by visiting and studying these stupendous works, that he can form an adequate conception of the great relations of the crust of the globe, and of its mode of formation.
Nature appears, in her formations, frequently to pro ceed from the most special to the most general ; or, we observe the most general, including a series of gradually diminishing subordinate differences. In simple mine rals, many instances of this occur, as in the distinct con cretions, fracture, and external surface. But it is more strikingly the case in the structure of the crust of the earth, for here there is a series from that of the simple mountain rock to those grand and extensive arrange ments in the formations that form whole mountains and chains of mountains.
Four different kinds of structure are observable in the crust of the earth.—The first is, that which is to be observed in hand specimens ; it is the smallest kind of structure, and occurs in what are termed Mountain Rocks or Stones.—The second kind of structure, or that of Mountain iWasses, is more on the great scale, and is not to be observed in hand specimens, but only in sin gle masses of rock. To this structure belongs stratifi cation, and the seams of distinct concretions.—The third kind of structure is that of Rock Formations, or those great masses of which the crust of the earth is composed. To examine this kind of structure, we must traverse considerable tracts of country.—The fourth kind of structure is that of the Earth itself, which is formed by the junction of various formations. To examine this structure, we must travel through many countries.