7 6 SCIENCE PROGRESS 



The fact that certain mineral species may vary somewhat in 

 chemical composition ceases to be an objection to modal 

 classification when chemical composition is deposed from the 

 position of chief factor in quantitative classification. For a 

 purely utilitarian purpose of classification the petrographer 

 after all is not so much concerned with comparing the chemical 

 composition of rocks as in comparing their modes. If the rocks 

 be treated simply as classifiable objects, the differences in the 

 chemical composition of certain mineral species become as 

 immaterial as the individual differences between animals of the 

 same species ; or if a higher grade of importance be assigned to 

 these variations they may be compared to varietal differences in 

 the same species. 



To take a classic example, the hornblende-rock of Gran 

 is as well, or perhaps better, classified with other hornblende- 

 rocks than with the camptonite associated with it in the field, 

 although it is nearly identical in chemical composition with the 

 latter rock. It is better to classify a rock consisting of horn- 

 blende with other hornblende-rocks, even though they differ 

 somewhat chemically, than to classify it with a rock consisting 

 essentially of plagioclase and hornblende. 



That magmas of identical chemical character may and do 

 crystallise into two or more different mineral aggregates in 

 response to differing conditions is an argument rather for separat- 

 ing the various products in classification than for bringing them 

 together. A chemical classification only takes into consideration 

 the chemical composition of the magma from which the rocks 

 have been derived. A classification taking cognisance of the 

 modal habit of the rock also takes into consideration the con- 

 ditions under which solidification occurred, and therefore has a 

 more genetic character than the chemical classification. A 

 modal classification treats the rocks as mineral aggregates 

 with a history ; a chemical classification treats them merely 

 as magmas. 1 



Having obtained the mode or actual mineral composition of 



1 C. P. Berkey, speaking of the Quantitative Classification, says : " It is a 

 splendid magmatic classification scheme. But in real life we are dealing not 

 with magmas so much as the products of magmas which, because of differing 

 conditions, have given rocks that do not usually agree wholly with their 

 theoretical behaviour." — "Objects and Methods of Petrographic Description," 

 Economic Geology^ viii. 1913, p. 701. 



