CHAPTER VII. 



THE ORIGIN AND DESCENT OF ROCKS. 



It has been the current opinion that the earth was once in a molten 

 state, and thence cooled to a sohd condition, and hence that all the 

 primitive rocks were igneous. Even those who think that the earth 

 may never have passed through a molten state agree that the oldest 

 known rocks are either true igneous rocks, or rocks of very similar nature. 

 A molten magma may, therefore, be taken as the mother state of the 

 rocks. Starting with this conception, the natural order of events sug- 

 gests the inquiries (1) how rocks are formed from molten magmas, 

 (2) what natures they assume, (3) how other rocks are derived from 

 them, (4) how still other rocks are derived from these derivatives, 

 and so on. To answer these inquiries is to trace out the generations 

 of rocks and learn the general history of rock-formation. 



(1) The process by which igneous rocks are formed from lavas is 

 actually taking place in existing volcanoes. As these are widely scat- 

 tered over the face of the earth, the material poured out by them repre- 

 sents different parts of the interior and varies in nature accordingly. 

 This affords the means of studying the differences that arise from differ- 

 ences of material. This is a radical consideration, for variations in 

 composition give rise to the most fundamental distinctions between 

 rocks, though by no means the only ones. Rocks which have the same 

 composition often differ greatly in texture or structure, omng to the 

 varying conditions under which they were formed. In the sohdifica- 

 tion of rocks from the molten state, the rate of cooling causes many differ- 

 ences. A means of studying this is afforded by the various lava flows 

 that are now being poured out on the surface under different conditions; 

 but a more important means is afforded by extinct volcanoes, especially 

 by those which have been deeply cut open by erosion. In certain very 

 ancient volcanoes, not only have the solidified lava streams of the sur- 

 face been cut across by erosion, but the lava that remained in the crater, 

 or in the neck that led up from below, is laid bare for inspection. Ex- 

 posures of even more profound nature have been made by the great 



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