GEOLOGY (Gr. ge and logos), the science of the earth, should include all the sciences that treat of the constitution and distribution of the inorganic matter of the earth, as well as those which describe the living beings that inhabit it; just as astronomy includes the whole science of the heavenly bodies. In this wide sense, as comprising all the physical sciences, it has sometimes been used. As usually employed, however, it has a much more limited meaning, being confined to that section of the sciences which takes cognizance of the hard crust of the earth—of the materials of which it is composed, and of the manner in which these materials are arranged.
The structure of the earth received little attention from the ancients: the extent of its surface known was limited, and the changes upon it were neither so speedy nor violent as to excite special attention. The only opinions deserving to be noticed, that have come clown to us, are those of Pythagoras and Strabo. They both observed the phenomena which were then altering the surface of the earth, and proposed theories for explaining the changes that had taken place in geological time. The first held that, in addition to volcanic action, the change in the level of sea and land was owing to the retiring of the sea; while the other maintained that the land changed its level, and not the sea, and that such changes happened more easily to the land below the sea because of its humidity.
From the fall of the Roman empire, during the dark ages. the cultivation of the physical sciences was neglected. In the 10th c., Avicenna, Omar, and other Arabian writers. commented on the works of the Romans, but added little of their own.
Geological phenomena attracted attention in Italy iu the 16th c., the absorbing ques tion then being as to the nature of fossils. On the one side, it was held that they were the results of the fermentation of fatty matter, or of terrestrial exhalations, or of the influence of the heavenly bodies, or that they were mere earthy concretions or sports of nature; while only a few maintained that they were the remains of animals. Two centuries elapsed before this opinion was generally adopted. At the outset, it was un fortunately linked to the belief that the fossils were relics of the Noachian deluge.
Steno (1669). observed a succession in the strata, and asserted that there were rocks elder than the fossiliferous strata hi which no organic', remains occur; he also distill guished between marine and fluviatile formations. He was not able, however, to free himself from the absurd hypotheses of his day.
In England, the diluvialists were busy framing idle theories, to give a plausibility to their creed that the Noachian deluge was the cause of all the past-changes on the earth's surface. Differing somewhat in detail, they all agreed in the notion of an interior
abyss, whence the waters rushed, breaking up and bursting through the crust of the earth, to cover its surface, and whither, after the deluge, they returned again. Such absurd dreams, obviously opposed to the observed order of nature, greatly hindered the progress of true science.
Leibnitz (1680) proposed the bold theory that the earth was originally in a molten. state from heat, and that the primary rocks were formed by the cooling of the surface, which also produced the primeval ocean, by condensing the surrounding vapors. The sedimentary strata resulted from the subsiding of the waters that had been put in motion. from the collapse of the crust on the contracting nucleus. This process was several times repeated, until at last an equilibrium was established.
Hooke (1688) and Ray (1690,) differing as much from Burnet as from Leilsnitz, advo cated views similar to those of Pythagoras. They considered the essential condition of the globe to be one of change, and that the forces now in action would, if allowed sufficient time, produce changes as great as those of geological date. They were followed in the same direction by Vallisneri (1720), Moro (1740). Buffon (1749), Lehman (1756), and Fuchsel (1773), each contributing something additional. Werner (1780) greatly advanced the science by establishing the superposition of certain groups, by giving a system and names, and by showing the practical applications of geology to mining, agriculture, and medicine. He had very crude notions regarding the origin of the strata, supposing that the various formations were precipitated over the earth in succession from a chaotic fluid; even the igneous rocks he held to be chemical precipi tates from the waters. Hutton (1788), rejecting all theories as to the beginning of the world, returned to the opinions of Pythagoras and Ray. He held that the strata which now compose the continents were once beneath the sea, and were formed out of the waste of pre-existing continents by the action of the same forces which are now destroy ing even the hardest rocks. He introduced the notion of a periodical elevation of the sedimentary deposits from the internal heat raising the bed of the sea. Lyell, in our own day, adopted and improved these views, eliminating the baseless theories which were mixed up with them, and demonstrating that existing forces might produce all the phenomena of geology.