188 INTRODUCTION TO GENERAL SCIENCE 



b. 1202 : 1-3. Definition of Rocks. 



c. 1202 : 9-12. The Minerals Constituting Rocks. 



d. 1206 : 33-34. Minerals Have a Definite Chemical Com- 



position. 



e. 1208 : 36-37. Rock, a Mixture of Minerals. 

 /. 1209 : 192-193. Properties of Minerals. 



g. 1210:233-234. Mineral Veins. 



h. 1301 : 221-225. Minerals of the Crust. 



i. 1305 : 180-181. Minerals and Rocks. 



j. 1312 : 239-246. Minerals. 



135. ROCKS OF THE EARTH'S CRUST 



Sedimentary Rocks. The rain washes pebbles, sand, and 

 pieces of rock down from the hills; these are carried by rivers 

 into lakes and the ocean, where they settle, forming layers 

 along the bottom. The next time there is a heavy rush of 

 water there will be another layer formed, and thus the ma- 

 terial is built up, layer upon layer, until great depths have 

 been filled. These layers are called strata, and the rock is said 

 to be stratified. The thickness of the strata depends upon the 

 amount of material which was brought down at one time. 

 These strata are naturally horizontal where they have not 

 been disturbed. We will study the result of their disturbance 

 under the formation of mountains. As time goes on, the 

 great pressure and the action of cementing material contained 

 in some of the rocks cause these rock strata to become solidi- 

 fied into one huge rock mass, maintaining, however, the defi- 

 nite strata which went to make it. 



Igneous Rocks. Melted rock which has come up from 

 within the earth may appear either on the surface, or may fill 

 crevices and caverns below the surface. When cool, it is called 

 igneous rock, receiving its name from the fact that it was once 



