Geology (Gr. the earth, anddiscourse), the science which treats of the structure of the earth, and of the methods by which its materials have been arranged. Under this term are confounded two distinct branches of study, the one being that of the chemical, physical, and biological laws which have presided over the development of the globe, and the other the natural history of the earth as displayed in its physical structure, its stratigraphy, mineralogy, and palaeontology. The name of geognosy, employed by some authors, may be very appropriately retained for the latter, while that of geogeny may be restricted to the first or theoretical division of geology. A knowledge of physical geography, of the distribution of land and water in past and present times, and of the laws of winds, currents, and climates, is one of the first requisites in the study of geology. Then comes the investigation of the various kinds of rocks, their arrangement and structure, their succession and relative antiquity, their chemical and mineralo-gical history.
The investigation of the chemical agencies which have presided over the formation of the various kinds of rocks and minerals belongs to chemical geology, while the laws which have regulated their deposition, structure, and arrangement constitute dynamical geology. The student finds that organic life in past time played a part in the earth not less important than it does to-day, and the study of the organic remains found in the various rocky strata, and known as fossil plants and animals, gives rise to departments of botany and zoology which are sometimes called paleobotany and palaeozoology, but are more generally included under the common term of paleontology. The changes that have taken place in the inorganic and organic world introduce in their study considerations of time and progress, and the science is found to be largely of a historical character; the geologist, as Cuvier remarked, being an antiquary of a new order. Its historical element is regarded by Lyell as so prominent that he de-tines geology simply as the science which investigates the successive changes that have taken place in the organic and inorganic kingdoms of nature." In the present article little more will be attempted than to present a general sketch of the history and progress of geological science, a reference to some principal objects of its pursuit, and the system of classifying the groups of rocks generally adopted.
The history of the science as developed in Europe is minutely traced in the familiar work of Lyell, "Principles of Geology," in which the whole subject may also be most advantageously studied.-From the earliest times the structure of the earth has been an object of interest to man, not merely on account of the useful materials he obtained from its rocky formations, but also for the curiosity awakened by the strange objects it presented to his notice. The south and west of Asia and much of the country bordering the Mediterranean were particularly favorable for directing attention to geological phenomena. Earthquakes were frequent, changing the relative positions of sea and land; volcanoes were seen in operation, adding layers of molten rock to those of sand and mud filled with the shells of the Mediterranean; the strata in the hills abounded in evidences of similar collections of vestiges of marine life far removed from access of the sea, and yet unchanged during the period of human observation and tradition; the Ganges and the Nile, pouring forth their vast sedimentary accumulations, were plainly building up the deltas at their mouths, and the broad valleys reaching far up their course were unmistakable productions of the same series of operations in remote periods.
These phenomena could not escape the attention of the philosophers among the ancient Egyptians and Indian races; and their influence is perceived in the strange mixtures of correct observation and extravagant conceit which make up their cosmogonies or universal theories of the creation. In the first chapter of the ordinances of Manu alternating periods of destruction and of renovation are distinctly recognized, extending in eternal succession throughout the whole assemblage of locomotive and immovable creatures, each period comprehending a duration of many thousand ages. The Greek schools of philosophy recognized these phenomena, which were clearly enunciated by Ovid in presenting the doctrines of Pythagoras. Remarkably free from extravagant statements, they were applied to prove a system of perpetual change slowly modifying the surface of the earth. Aristotle recognized the interchanges constantly taking place between land and sea by the action of running water and of earthquakes, and remarked how little man, in the short span of his life, can perceive of operations extending through the eternity of time.
Strabo distinctly applied the raising up of land, not merely of small tracts, but of continents also, by earthquake convulsions, to account for the perplexing phenomenon of beds of marine shells contained in the interior of hills far distant from the sea. Arabian philosophers of the 10th century are also cited who entertained similar views of the changes going on and their causes.-The Italian philosophers in the early part of the 10th century were the first to engage in systematic investigations concerning the true nature of fossil shells. Their abundance in the strata of the sub-Apennine range could not fail to arrest attention and excite inquiries, which were the more perplexing from the limited time allowed in popular belief to the past duration of the earth, and from the general persuasion that no great catastrophe except the Noachian deluge could have occurred to modify its surface. Various fanciful explanations were therefore adopted in the spirit of the scholastic disputations, and for three centuries argumentations were sustained with much spirit on the questions: first, whether fossil remains had ever belonged to living creatures; and secondly, admitting this, whether all the phenomena could not be explained by the deluge of Noah. Among those distinguished for the soundness of their views in the commencement of this controversy are Leonardo da Vinci, the celebrated painter, who died in 1519, and Fracastoro, whose attention was engaged by the multitude of curious petrifactions which were brought to light in 1517 in the mountains of Verona, in quarrying materials for repairing the city, He exposed the absurdities of the theories which referred the petrifactions to a certain plastic force in nature that could fashion stones into organic forms, and showed the inadequacy of the traditional deluge to bring together the marine fossils that form solid strata of the earth.