186 INTRODUCTION TO GENERAL SCIENCE 



turn these rocks have been raised above the surface of the 

 ocean to form new land. Plants have lived, borne fruit, died, 

 decayed, become soil again, as food for other plants; animals 

 have eaten of plant food and died and fertilized the ground 

 for coming generations of plants. It is this continual change, 

 which, acting through a length of time that is almost im- 

 possible for the human brain to conceive, has produced our 

 present earth. 



References : 



1. 1205 : 5. Slowness of Change. 



2. 1304 : 42-44. Results of Weathering. 



3. 1304 : 45-46. Age of the Earth. 



4. 1601 : 21-26. Over and Over Again. 



a. 1207 : 343-348. Results of Slowness of Change as Applied 



to Agriculture. 

 6. 1301 : 255. Slowness of Change. 



c. 1312 : 270-273. The Geographic Cycle. 



d. 1612 : 7-8. Slowness of Change. 



133. ROCKS DEFINED AND CLASSIFIED 



We call the material of which the earth is made, rock, and 

 we may consider that the whole earth is made of rock. This 

 rock has been changed through several kinds of processes into 

 a large variety of sands, clays, and soils. The kinds of rocks 

 which compose the surface of the earth are divided into sev- 

 eral classes, according to the way in which they were formed. 

 They are: sedimentary, chemically formed, organic, igneous, 

 metamorphic, and seolian. In addition, there are various 

 combinations of these rocks which cannot be classified. 



On the surface of the earth rocks occur which are called 

 mantle rock on account of being loose, and forming a 

 mantle over the surface of the earth, while underneath is the 



