Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/148

108 are composed being easily distinguishable, the convulsions to which others have been subjected have been so great, and the effects of heat so intense, that no sign of such stratification is visible, if it ever existed. The old Geologists resembled the knights who fought about the color of the shield. The early German school, influenced by the character of the rocks in that part of the world, attributed a great deal to the action of water; while the Scotch school, equally impressed by the features of the formations in their country, attached great importance to the effects of heat. Hence arose the sects of the Neptunists and Plutonists. Both were right in attributing the formation of the rocks in their respective countries to the action of water and heat. Both were wrong in applying to the whole world conclusions drawn from such local data. Modern Geologists steer a middle course,—avoiding these extremes,—considering the effects of the combined action of water and heat, as well as admitting the influence exerted by these agents separately. Rocks the origin of which is supposed to be due to the gradual deposition under water, in layers or strata, of the materials composing such formations, are called Aqueous Rocks; while those of which the structure clearly testifies to the action of heat in producing them are known as Plutonic Rocks. Finally, the Metamorphic Rocks illustrate the alternate action of water and heat. Geologists classify rocks according to their mineral composition, their organic remains, and the order in which they follow or overlie one another. The Geologist, starting in Canada, and traveling through New York and Pennsylvania, notices continually as he advances southward the change in the minerals composing the rocks, and the different aspects of their organic remains. Thus, in Canada and the east of New York, granite, gneiss, and syenite are common minerals. These rocks were originally called Azoic, or without life; improperly, however, as within a few years the Eozoon, or