The Lithosphere 



In nature we find that when molten rock, of composi- 

 tion similar to that of granite, is poured out from a 

 volcano in the form of a lava-stream, it cools quickly and 

 forms a natural glass called obsidian ; but when the same 

 material has filled cracks in the Earth's crust, and the 

 escape of heat has been retarded, it has formed crystalline 

 rocks similar in composition to granite, but much finer in 

 the grain. Granite itself occurs in large masses, and 

 there is often evidence to show that at the time of its 

 consolidation it was covered by thick masses of rock, 

 which prevented the escape of the heat, and so prolonged 

 the period of cooling, and gave time for the formation of 

 large crystals. 



The question of the origin of the great masses of 

 igneous rock found amongst the other rocks of the 

 Earth's crust will be further considered in a subsequent 

 chapter. 



The sedimentary rocks differ so widely in composition, 

 and may contain such a great variety of minerals, that 

 they have to be classified on other lines than those 

 employed in the case of the igneous rocks. 



Both structural and chemical features are taken into 

 consideration in this classification and, to some extent 

 also, mode of origin. 



Commencing with the finest-grained sediments, we 

 will now enumerate and describe some of the commoner 

 types of sedimentary rock. 



Clay is the name given to extremely fine-grained 

 sediments which contain sufficient water to render them 

 plastic. By the drying and hardening of this material 

 such rocks as shale and slate are produced. Shale can 

 be readily divided into thin flakes along the original 



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