GEOLOGY has for its object the struc ture and formation of this globe : it, of course, embraces the consideration of the materials of which it is composed, and the circumstances peculiar to its original for mation, as well as the different states tin der which it has existed, and the various changes which it has undergone.
It necessarily follows, from the very li mited depth within which our actual exa minations have been made, that our facts and real observations are confined to what may be considered, comparatively, as merely the crust of the globe With re spect to its more internal part, we have hitherto only been aided by conjecture, which, it must be admitted, has too fre quently led to theories the most extrava gant and absurd. From the experiments of several learned men, it, however, ap pears, that the density of the globe is greatest towards its centre. Boscovich is of opinion, from his very ingenious calcu lations, that the centre is a spherical nu cleus, possessing an equal degree of den sity to within some leagues of the earth's surface ; but although it is thus conclud ed, that the interior of the earth is solid, contrary to the conjectures of several an cient philosophers, yet it is by no means pretended, that even in this, its more solid parts, there may not exist cavities of a greater or less size, connected, perhaps, with each other, and extending consider ably, in all probability, towards the sur face.
The solid masses of the globe, which have come within our examination, have been distinguished into primitive and se condary ; among the former were placed the rocks of granite, gneiss, porphyry, serpentine, and limestone, of a peculiar character ; and, among the latter, were considered the rocks of secondary lime stone, of phosphate of lime, of gypsum, and of some of the sandstones; of chalk, and of silex. This division is not, how ever, at present universally adopted ; other divisions having been assumed, which have appeared to agree better with the different systems which have been proposed: these divisions we shall there fore more fully notice, after pointing out the peculiarities of these several systems. The water is supposed, at present, to co ver about three-fifths of the whole earth; but undoubted evidence exists, of its having extended over a much wider sur face ; and it is the opinion of many of the most eminent geologists, of its having covered the whole of the earth. As the necessity of ascertaining this latter cir cumstance is much urged, by those who have endeavoured to form correct opi nions respecting the mode in which this globe was originally formed, it will be proper here to notice some of the evi dence which has been adduced respect ing this circumstance.
Herodotus relates, that, according to the priests of Vulcan, the whole of Egypt, except in the neighbourhood of Thebes, had been covered with water. Hero dotus himself also noticed the exist ence, even in his time, of lakes of salt water in different parts of Egypt, as well as of the saline matter, mingled with the vast tracts of sand with which the coun try is covered; which observations are confirmed by the accounts that have been given, by those who have examined these parts in modern times. The diminution of the ocean is also rendered in the high est degree probable, from various facts related also by Strabo, Pliny, Diodorus the Sicilian, and several other early wri ters; and in the present day, the obser vations of Pallas, Celsius, Linnaeus, and others; seem to establish the fact, of the diminution and sinking both of the Baltic and of the Caspian Seas.
On the other hand, innumerable facts may be adduced, which seem to prove that the water has actually increased, in its proportion, over the dry land. From the relations of Plaucus, Bryden, Bar ral, Fortis, and others, there can no doubt exist of the Mediterranean Sea having very much encroached on its shores ; temples, and other edifices of different descriptions, which are known to have been erected at considerable distances from the sea, being now buried beneath its waves. In explanation of this vary ing evidence, it is necessary to state, al though it may not affect the general question, that it cannot be doubted, that whilst the land is gaining on the sea, in some parts, similar encroachments are observable in others, of the sea on the dry land. Instances of this, on the small scale, may be observed on almost all flat, and on many precipitous shores : on the former, large embankments of sand are sometimes suddenly thrown up by derable and violent inundations, and which, in consequence of alteration in the shape of the coasts, and of the direc tion of currents, may still remain, and ap pear to 'manifest an increase of the dry land ; on precipitous shores, the reverse is observable ; undermined by the conti nual and powerful action of the waves, large masses are perpetually falling, and, broken by their fall and by the action of the water, are so reduced as to easily al low of their removal by the waves : thus is occasioned a considerable reduction of the level of the .shore ; and thus an oppor tunity is given for the extension of the waters of the ocean on such particular spots. The balance, however, of this seem ingly contradictory evidence is undoubt edly in favour of the opinion, that the wa ter has considerably diminished, and is, perhaps, lessening at the present period.