CHAPTER V. 



MINERALS. 



In the previous chapters I have discussed the forces and agents of 

 alteration, and the general nature of the alterations in the zones of 

 anamorphism and katamorphism, including the two helts of the latter zone. 

 We are now prepared to consider the particular alterations which affect the 

 individual minerals in reference to these forces, agents, zones, and belts. 



SECTION 1.— CHEMICAL AND MINERAL COMPOSITION OF THE KNOWN 

 CRUST OF THE EARTH. 



For convenience the outer part of the crust of the earth of which we 

 have positive knowledge will be called the crust. Clarke," for the purpose of 

 considering the chemical composition of the outer part of the earth, confines 

 this term "crust" to the part of the earth which extends from the tops 

 of the mountains to 10 miles below sea level. He thinks it fair to assume 

 that we may infer the approximate composition of this small part of the 

 earth by the parts of it which may be observed at or near the surface. 1 The 

 term "crust" in this treatise will be used in the restricted sense of Clarke- 

 But in so using- the term there is no intention to imply that there is any 

 sharp division between the crust and the deeper part of the earth, to which 

 the term "centrosphere" is applied. 



Below is a table which gives the relative proportions of the twenty-one 

 elements composing as much as 0.01 per cent of the crust of the earth as 

 above defined, including the lithosphere, hydrosphere, and atmosphere, and 

 also their atomic weights/ 



"Clarke, F. W., Relative abundance of the chemical elements: Bull. U. S. Geol. Survey No. 78, 

 1891, p. 34. 



i-Clarke, cit, pp. 34-37. 



« Clarke, F. W., Analyses of rocks, laboratory of the United States Geological Survey, 1880-1899: 

 Bull. U. S. Geol. Survey No. 168, 1900, p. 15. 

 192 



