CHAPTER XVI 



GEOGNOSY AND FORMATION OF THE 



CRUST RESUME OF THE PAST PAGES. 



THE Science of Mineralogy teaches us to read in 

 the minerals that the crust of the Earth exposes to 

 us the records of past chemical actions that now rest 

 quiet in their affinities; satisfied and permanent under 

 the present conditions of temperature and atmospheric 

 pressure. How many of these minerals were formed 

 it is often impossible to conceive. In what way the 

 Carbon of the Diamond, for instance, was enabled to 

 crystallize into the relatively large masses in which it 

 is found is a problem hard to solve. The crust of the 

 earth, so far as accessible to us, is composed almost en- 

 tirely of oxidized bodies ; the compounds of Chlorine 

 with the alkaline metals, with Calcium and Mag- 

 nesium more rarely native Copper and the Metallic 

 Sulphides being nearly the only exceptions. The 

 great density of the earth, as a whole, compared to 

 that of the materials forming the crust which alone 

 is accessible to us, is as 5.5 is to 2.7, while water, 



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