L. V. Pirsson — Classification of Igneous Rocks. 283 



One reason for this is the feeling that as a character its 

 importance is secondary to chemical-mineral composi- 

 tion, and another, that if all the various ramifications of 

 texture, strictly defined, were introduced into a primary 

 system of classification as divisions in one direction, and 

 mineral groupings as divisions in another direction, the 

 variety of rocks produced would be bewildering in num- 

 ber. Nevertheless the texture of a rock has always been 

 held to be one of its most obvious features and the recog- 

 nition of it, in the simple forms mentioned above, is seen 

 in classifications, either directly or indirectly. We pro- 

 pose to employ it in a megascopic manner, but will 

 attempt to give it a more precise definition than is usual, 

 for the benefit of the student. The scheme is as follows : 



Grained Rocks; mineral constituents megascopically 

 determinahle. 



A. Apparently even-granular in fabric ; fine to coarse ; rarely 

 subporphyritic. 



B. Distinctly porphyritic in fabric. 



1. Groundmass grained, constituents determinable. 



2. Groundmass dense, but sempatic to presemic; pheno- 



crysts determinable. 



Dense and Glassy Rocks; constituents mostly or wholly 

 indeterminable. 



C. 1. Porphyritic, but sempatic to prepatic. 

 2. Nonporphyritic. 



This is probably as far, in preciseness of definition, as 

 it is wise to go at present; to be more detailed would 

 introduce such radical differences with the existing litera- 

 ture as would confuse the mind of the student, and greatly 

 hinder his use of it. As it is, the scheme introduces a 

 number of differences, not however of major importance, 

 and these will be pointed out in proper places. 



We have then three textural divisions, which may be 

 summarized as A, granular; the names of these rocks 

 are shown in the table in bold-face t3^e ; B, porphyritic, 

 in italics, and C dense or glassy, printed in roman type. 



The fragmental volcanic rocks, the tuffs and breccias 

 composed of dust, ashes, bombs, etc., might well form a 

 fourth textural division, but for the sake of simplicity 



