CHAPTER XIV 



ROCKS AND SOIL 



284. The Earth's Crust. We. have already learned 

 (cf. 38, Fig. 30) about the four parts that appear in a 

 cross section of the earth. The crust is merely that por- 

 tion of the core that is known to us ; so far as we know, the 

 core contains the same materials as the crust, but in a 

 more compact form. 



If we examine the crust, as it is exposed in a railroad 

 cut, a cellar excavation, or, best, in a quarry or gorge, we 

 find that it consists of two parts : 



(1) The surface materials, or mantle rock; 



(2) Bedrock. 



Mantle rock is made up of such materials as clay, sand, 

 gravel, and pebbles, all of which are usually loose enough 

 to be worked by the spade and the pick. That upper 

 portion of the mantle rock which we can use for growing 

 crops is called soil. 



Ordinary excavations may not be deep enough to go 

 through the mantle rock; but everywhere, and at only a 

 moderate depth, the mantle stops, and we have bed rock. 

 When bed rock is exposed at the surface, we have an 

 outcrop. 



285. Some Common Rocks. Bed rock usually con- 

 sists of fragments of some mineral or minerals (cf. 102) 

 cemented together. The cementing material hardens, or 



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