C. THE LITHOSPHERE 



CHAPTER VI. 



CLASSIFICATION OF THE ROCKS OF THE EARTH'S CRUST. 



As has been outlined in the introductory chapter, the stratig- 

 rapher deals with the genesis of the rocks of the earth's crust and 

 considers their structure and composition, or the petrographical 

 characters of rocks, chiefly from the point of view of their bearing 

 on lithogenesis. The dynamic forces active in the production of 

 rocks are likewise considered by him in their bearing upon this 

 subject. Naturally, those rocks which bear the most satisfactory 

 evidence of chronological succession, i. e., the stratified rocks, re- 

 ceive most attention, for it is the business of the stratigrapher to 

 establish the events in the history of the earth in the order of their 

 occurrence. 



SUBDIVISION OF ROCKS. 



Numerous subdivisions of rocks have been proposed, most of 

 them being largely or entirely artificial in character. The most 

 familiar divisions are : igneous, aqueous, ceolian, and organic. The 

 last three are commonly classed together as stratified or sedimentary, 

 as opposed to the unstratified or igneous rocks. Metamorphic rocks 

 are generally grouped by themselves, though in the beginning they 

 were grouped with the stratified (6). The chief objection to this 

 classification lies in the fact that, under the aqueous as well as 

 under the organic groups, rocks of chemical and of mechanical 

 origin are included. Nor are all of the rocks so classed truly sedi- 

 mentary or stratified. One of the earliest attempts at a rational 

 classification based on origin was that of Carl Friedrich Naumann, 

 who. in the second edition of his Lehrhuch der Gcognosie, 1858 

 (21), recognized two classes in which all rocks are included. These 

 are : I, The Protogenous, or original ; and IT, the Deuterogenous, 

 or derived, terms corresponding essentially to the endogenetic and 



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