PART II 



STRUCTURAL GEOLOGY 



CHAPTER X 

 THE ROCKS OF THE EARTH'S CRUST — IGNEOUS ROCKS 



In the first section of this book we made a study of the processes 

 and agencies which are still at work upon and within the earth, 

 tending to modify it in one or other particular. We there found 

 that slow but ceaseless cycles of change take place on the earth's 

 surface and that a continual circulation of material is going on. 



We have now to take up the second branch of our subject, that 

 of structural geology, which deals with the materials of the earth's 

 crust, their mode of occurrence, and their arrangement into great 

 masses. Structural geology is, however, not merely a descriptive 

 study ; hand in hand with the examination of the rock-masses 

 must go the attempt to explain their structure, and to show how 

 they have come to be as we find them. Dynamical principles 

 must be continually called in to interpret the facts of structure, and 

 many of the principles of the construction, destruction, and recon- 

 struction of rocks find their application in the study of structure. 



This application cannot, in all cases, be made with confidence 

 and certainty, both because a given structure may often be re- 

 ferred, with equal probability, to different processes, and because 

 certain of the great dynamical agencies are so slow and gradual in 

 their mode of operation, that no one has ever been able to observe 

 them at work. In this latter class of cases the agency must be 



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