THE MINERAL COXSTITUEXTS OF ROCKS. 25 



Section III. — The Origin and Ilclations of the Mineral Confiiituents of Bocks. 



Taking the consolidation of any rock as the initial point, particularly 

 those of an eruptive nature, the constituents fall into one of three classes: 

 I. Those of prior origin ; II. Those formed at that time ; III. Those of later 

 origin.* 



The minerals of the first class naturally fall into two divisions, in the 

 eruptive rocks. 



1. Those that are characteristic of the rock species. 



2. Those that are accidental, being probably caught up in the passage 

 upward or during the outflow. Similar divisions are found to a greater or 

 less extent in the sedimentary rocks, according as they -were derived from 

 one or more rocks, and also according to the preponderance of different 

 rock fragments and minerals in them. 



The minei'als, and fragments of minerals and rocks, occurring in rock 

 masses that belong to the first class, have an important bearing iipon the 

 questions of the origin and relations of rocks — so much so that more atten- 

 tion will be given to them in the future than has been the case in the past. 

 These are in a great measure characteristic of the rock species, and should 

 have a very great weight in the nomenclatui'e of sedimentary rocks ; for one 

 of the most important questions regarding these is, what was the original 

 material from which they were derived? In the volcanic rocks these minerals 

 are distinguished generally by the effect that the magma has produced upon 

 them — the blackening, breaking, tearing, and dissolving action which is so 

 conspicuous in the case of olivine and horidjlende ; while in quartz it is shown 

 in the fractures, the rounding of the grains, and the interpenetration of the 

 magma. Frequently these foreign materials, especiallj^ quartz, have radiating 

 rings of the groundmass surrounding them, these rings being largely composed 

 of crj'stals standiug perpendicular to the surface of the inclosed piece. In all 

 rocks of an eruptive nature, the fragments are apparently either inclusions 

 caught in the passage upward, or during the surface flow of the lava, or else 

 derived from the remelting of the more crystalline portions of these or other 

 rocks at the time of, or prior to the eruption ; especially when the eruption 



* A. Michel Le\7, Bull. Soc Geol. France, 187-t (3), iii. 199-236 ; Aim. Mines, 1875 (7), vill. 3il- 

 346; Wadsworth, Bull. Mus. Comp. Zoul., 1S79, v. 277, 27S. 



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