=Varieties of rock in crust.=—If the mantle of soil, subsoil, and glacial rubbish were stripped from the land, the surface beneath would be found to be made up of a great variety of rocks, all of which may be grouped into two great classes. About four-fifths of the land surface would be of rock arranged in layers, and the other fifth would be of crystalline rock, generally without distinct stratification, and often bearing evidence of the effects of high temperature. =Stratified rocks.=—The composition of most stratified rocks corresponds somewhat closely with the composition of sediments now being carried from the land and being deposited in the sea. Their arrangement in layers is the same, and the markings on the surfaces of the layers, such as ripple-marks, rill-marks, wave-marks, etc., are identical. Furthermore, the stratified rocks of the land, like the recent sediments of the sea, frequently contain the shells and skeletons of animals, and sometimes the impressions of plants. Most of the relics of life found in the stratified rocks belonged to animals or plants which lived in salt water. Because of their structure, their composition, their distinctive markings, and the remains of life which they contain, it is confidently inferred that most of the stratified rocks which lie beneath the mantle rock of the land were originally laid down in beds beneath the sea, and that the familiar processes of the present time furnish the key to their history. [Illustration: +Fig.+ 1.—Beds of (Cambrian) sandstone, _a_, are conformable with one another, but unconformable on beds of (Huronian) quartzite, _b_, Near Ableman, Wis.] =Conformability.=—When the stratified rocks exposed by the removal of the mantle rock are examined, the successive beds are sometimes found to lie on one another in regular succession, showing that they were laid down one after another, without change in the attitude of the surface on which they were deposited. Such rocks are _conformable_ (the beds of series _a_, Fig. 1). In other cases it would be seen that certain beds overlie the worn surfaces of lower beds, the layers of which may have a different angle of inclination (series _a_, Fig. 1, is unconformable on series _b_). Such relations show that the lower series of beds was disturbed and eroded before the overlying beds were deposited on them. Such series of rocks are _unconformable_.