GEOLOGY (from Greek ge, earth, and 'ogle, account). Geology is that science which treats of the history of the earth. It begins with the remotest periods and traces in orderly manner all those changes in structure, material and external form which our planet has under gone. The record is read from the rocks themselves and interpreted in the light of proc esses now known to be taking place. Among those processes earthquakes and volcanoes were early recognized as important by reason of their cataclysmic nature; but later and more detailed studies show that quite as important, perhaps vastly more so, are the unobtrusive activities of rain, wind, running water, glaciers, waves, tides, and even organic agencies.
In following out these processes it must be always borne in mind that they operate with exceeding slowness. Indeed it is only through the conception of a history enormously long, expressed in millions of years, that we can un derstand the truly stupendous character of the work accomplished. (See section on Age of the Earth). With this vast length of time emphasized in the thought it must be further borne in mind that the great changes of the geologic past have been produced for the most part by the operation of those forces now going on before our very eyes. For that reason, we
study closely the changes now taking place in order that in their light we may properly inter pret the records read in the rock pages of the geologic volume.
In its broadest sense geology is one of the most inclusive sciences. The life records of the past can be read only in terms of modern biology; the remoter history of the earth's be• ginnings are inextricably interwoven with as tronomy; physics and mechanics must be invoked to explain tides, interior rigidity, earth heat, and many other problems; the ultimate analysis of the materials of which our globe is composed must be referred to the chemist, as must also many of the changes involved in weathering and metamorphism to be detailed later; meteorology and climatology furnish the only rational background for the adequate study of those external forces now modifying the earth's surface. And so in nearly every respect it is seen that geology overlaps other funda mental sciences.