CHAPTER XII 

 THE STRUCTURE OF ROCK MASSES— STRATIFIED ROCKS 



In the preceding chapter we have studied the rocks which make 

 up the crust of the earth, so far as that is accessible to observation. 

 It remains for us to inquire how these rocks are arranged on a 

 large scale, and to what displacements and dislocations they have 

 been subjected since the time of their formation. Examined with 

 reference to the simplest and broadest facts of structure, we find 

 that rock masses fall into two categories : (i) Stratified Rocks, and 

 (2) Unstratified or Massive Rocks. A very brief examination will 

 show us that these two categories correspond respectively to the 

 sedimentary and igneous divisions of the classification according to 

 mode of origin, neglecting, for the present, the metamorphic class. 



We shall begin our study of rock masses with the stratified 

 series, because their structure and mode of occurrence are, on the 

 whole, the simplest and most intelligible, and tell their own story. 

 The unstratified series, on the other hand, can be understood only 

 by determining their relation to the former. 



The stratified rocks form more than nine-tenths of the earth's 

 surface, and if the entire series of them were present at any one 

 place, they would have a maximum thickness of about thirty miles, 

 but no such place is known. The regions of greatest sedimentary 

 accumulation are the shallower parts of the oceans, while those 

 regions which have remained as dry land, through long ages, have 

 not only had no important additions to their surfaces, but have 

 lost immense thicknesses of rock through denudation. The great 

 oceanic abysses are also areas of excessively slow sedimentation, 

 and thus the thickness of the stratified rocks varies much from 

 point to point, a variation which has been increased by the irregu- 

 larities of upheaval and depression and of different rates of denu- 



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