A TREATISE ON METAMORPHISM. 



By Charles Richard Van Hise. 



CHAPTER I. 



INTRODUCTION. 



Following Powell, I shall regard the earth as composed of four 

 spheres — the atmosphere, the hydrosphere, the lithosphere, and the centro- 

 sphere." The terms atmosphere and hydrosphere need no definition. The 

 term lithosphere, as here used, will be confined to that portion of the outer 

 part of the earth which is within the limits of observation. How far below 

 the surface observation extends is somewhat uncertain; but it is certain that, 

 in consequence of deformation and denudation, we may observe rocks 

 which have been several thousands of meters below the surface. Clarke 

 suggests that the zone of observation be defined as extending to a depth of 

 10 miles (16 kilometers) below the level of the sea,. 1 Whatever distance 

 be taken as the limit of the zone of observation, it is certain that such 

 distance is but a very small fraction of the radius of the earth. All the 

 earth below this fraction will be considered as the centrosphere ; but no 

 hypotheses are advanced in respect to any essential difference in character 

 between the material of the lower part of the lithosphere and that of the 

 upper part of the centrosphere. 



GENBEAL NATURE OF ALTERATIONS. 



The data of geology have become so numerous as to be almost unman- 

 ageable. Not many decades ago it was possible for a geologist to have a 



"Powell, J. W., Physiographic processes: Nat. Geog. Mon., vol. 1, No. 1, 1895, p. 1. 

 6 Clarke, F. W., The relative abundance of the chemical elements: Bull. U. S. Geol. Survey No. 

 78, 1891, p. 34. 



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