[Gk.] The science which treats of the history and structure of the earth. The rocks which compose the crust of the earth have not all been formed in the same way. Some have cooled from a state of fusion, and are known as igneous rocks. This class is represented by such rocks as granite, syenite, traps, and lavas. Metamorphic rocks are those which have either originally been deposited in water and have become changed into crystalline rock, or those which,originally crystalline, have become foliated In structure under great pressure. Sedimentary rocks comprise the various deposits which are laid down on the bed of the sea or on the land.
They have all been formed from sediment washed by rain and streams from the land,and, be-ing thus derivative, imply the existence of older rock. This division forms the larger part of the earth's crust, and is the most important for the geologist, since it contains most of the materials from which the geological history of the earth is worked out. These rocks are arranged in strata or layers, which have been much lifted, contorted and broken. When they are bent with curve upwards, they are called Anticlinal. They include sandstones, limestones, slates or claystones, etc., their ages depending on their relative position. When the age of a rock strata is in doubt it is frequently learned from the animal or plant fossils it may contain.