GEOLOGY (from Gk. 7i), ge, earth + -Xcryta, -logic, account, from Xe-yap, legein, to say). Geology is the science which investigates the history of the earth. The rocks of the earth's crust contain the records of this history. Many of the pages of the rock book are lost, others are obscured through partial destruction, and many, like the hieroglyphics on ancient monuments, re quire great care, patience, and intelligence to decipher; yet, in spite of these difficulties, we are already in possession of a vast fund of infor mation concerning the history of the earth.
Geological study shows that forces similar to those of the present have been operating in the past. Therefore to interpret properly the past history presupposes a knowledge of the forces working at the present time to modify the earth. One class of forces, depending upon energy from within the earth, causes the earth's surface to rise and fall, volcanoes to erupt, and the rocks to be disturbed; another, deriving its energy from without the earth, and mainly from the sun, sets in operation winds, waves, rain, rivers, glaciers, and tides, which wear away the surface of the land and distribute the waste in the oceans. The
effect of life on the globe is another geological factor. Many geological changes are influenced by animals and plants. It is of great impor tance, therefore, that in many instances the layers of rock forming the crust contain remains of animals and plants of past ages. The study of these original remains has given much informa tion concerning past life and thd conditions amid which the life existed. Moreover, since life has developed in ordinary succession, the study of the fossils of animals and plants has given a basis for the division of the earth's history into periods, or ages.
Although geology stands as a distinct science, with numerous subdivisions, to master it thor oughly requires a broad knowledge of several allied sciences. Botany and zoology are indispen sable to the student of fossils; physics and chem istry, to the student of rocks; and astronomy and geography, to all who would broadly grasp the subject of geological history. Each of these sciences furnishes tools with which the geologist works out the varied and complex earth history.