CLASSES OF ROCKS 9 



By far the larger part of the land surface would be of stratified rock, 

 and the remainder of rocks without distinct stratification. The 

 latter are divided into two great groups, igneous rocks, and meta- 

 morphic rocks. 



The essential feature of stratified rock (Fig. 2) is its arrangement 

 in layers. The layers may be distinct or indistinct, and thick or 

 thin. In many cases thick layers are made up of many thinner 

 ones. In composition, most stratified rock corresponds somewhat 

 closely with sediments now being carried from land and deposited 

 in the sea; that is, these rocks are made up of gravel, sand, or mud, 

 the particles of which are cemented together. The bedded arrange- 

 ment of stratified rocks and of recent sediments is the same, and the 



Fig. 3. Diagrammatic representation of the relations of igneous rock to 

 stratified rock. The igneous rocks, represented in black, have been forced up from 

 beneath. 



markings on the surfaces of the layers, such as ripple-marks, rill- 

 marks, wave-marks, etc., are identical. Furthermore, many of the 

 stratified rocks of the land, like the recent sediments of the sea, 

 contain the shells and skeletons of animals, and some of them the 

 impressions of plants. Many of the relics of life found in the strat- 

 ified 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 confi- 

 dently inferred that many, if not most, of the stratified rocks which 

 fie beneath the mantle rock of the land originally were laid down 

 in beds beneath the sea, and that the familiar processes of the pres- 

 ent time furnish the key to their origin. 



Igneous rocks may be defined as hardened lavas. They sustain 

 various relations to stratified rocks, as illustrated by Fig. 3, in which 

 some of the igneous rock is represented as lying beneath the stratified 

 rock, some above it, and some interbedded with it, while some cuts 

 across its layers. From these relations it is possible to tell some- 



