Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/146



No study illustrates better than Geology not only the advantage but the absolute necessity of general knowledge for the thorough understanding of any particular subject. Geology means literally a discourse on the earth. The student of so vast a theme ought naturally therefore to be familiar with at least the general conclusions offered by Astronomers, Physicists, Chemists, Mineralogists, Botanists, and Zoologists, so far as they relate to the history of this planet, since by astronomical data we picture our earth as a once gaseous, chaotic mass. The study of the cooling of heated bodies underpressure, implying a knowledge of the laws of Heat and Chemistry, furnishes the clue to the explanation of the origin of many formations. Mineralogy distinguishes the different rocks of which the crust of the earth is composed, while Botany and Zoology supply the means by which the life of bygone days is revivified, enabling us to interpret the structure and relations of plants and animals long since extinct. Geology, therefore, is not a separate science, since it consists only of the conclusions of many sciences applied to the investigation of the past and present history of the earth. As three-fourths of the earth are covered with water, with our present resources only a very small portion is susceptible of geological examination. And notwithstanding the great number of surveys and scientific expeditions which have been made during the present century, with the exception of Great (106)