252 THE EARTH'S CRUST 



Characteristics of Land Surfaces. The surface of the 

 land differs from that of the sea in being at least com- 

 paratively immovable. It is rough and irregular, and is 

 composed of many different kinds of rocks and soils. For 

 the larger part of its area it rises above the level of the sea, 

 but in a few places it sinks below, as in the Salton Sea, 

 a part of Imperial Valley, California, and near the Dead 

 Sea in Palestine. Its surface is eroded by wind and water 

 and is thus constantly but slowly changing its features. 



Materials Composing the Land. Experiment 81. Obtain 

 specimens of the igneous rocks, lava, obsidian, basalt, granite ; of the 

 sedimentary rocks, sandstone, fossiliferous limestone, conglom- 

 erate, peat; of the metamorphic rocks, gneiss, schist, marble, 

 anthracite coal. Examine these carefully with the eye and with 

 a lens, noting whether they have a uniform composition or are 

 made up of different particles. Are the particles composing the 

 rocks crystalline? Are they scattered irregularly or arranged in 

 layers? Test with a file or knife-blade the hardness of the rock as 

 a whole and of its different constituents. Try a drop of hydro- 

 chloric acid on the different rocks to see whether they are 

 affected by it. Describe in a general way the characteristics of 

 each specimen. 



The composition of different land areas varies greatly. 

 Many different kinds of rocks are often found crowded to- 

 gether, or it may happen that the same kind of rock covers 

 a large area. There is no uniformity. The soil on top of 

 the rock is also variable. In some places it contains the 

 minerals which are in the rock below and in other places 

 its composition is not at all dependent upon the bed rock. 



The great variety of rocks of which the crust of the earth 

 is composed has been divided into three great groups in 

 accordance with the manner in which they were formed. 

 These groups are igneous, sedimentary, and metamorphic. 



