602 CROSS, IDDINGS, PIRSSON, WASHINGTON 



scheme of subdivision that should demands be made for greater 

 chemical discrimination by future petrographers, further subdi- 

 vision may be carried on as an elaboration of the classification 

 here suggested. 



But the problem before us being the classification of igneous 

 rocks, there remains the method of treatment of the rocks 

 themselves. This involves the consideration of the minerals 

 actually present, including the aluminous ferromagnesian min- 

 erals, which occur in the great majority of igneous rocks and 

 often form the preponderant constituents. It also involves the 

 consideration of the texture of the rock, which is perhaps the 

 most obvious feature, as well as the legend of its physical his- 

 tory. The elimination of the aluminous ferromagnesian minerals 

 from the system of magmatic classification just described is 

 frankly for its simplification. The exact expression of the 

 chemical magmatic units which would be required should these 

 minerals be introduced would be so intricate as to be practically 

 useless. It is conceivable that an exact mathematical expression 

 involving all of the variable factors or interchangeable combina- 

 tions, such as are represented by the compositions of biotites, 

 amphiboles, and augites, is possible, but it would be beyond the 

 reach of petrographers generally. 



The actual mineral development of an igneous rock is to a 

 great extent so related to the texture that the two are interde- 

 pendent, which naturally follows from their both being controlled 

 to a large degree by the physical conditions attending eruption 

 and solidification. But since these relations as well as the phys- 

 ical conditions are far from simple, it is therefore necessary, for 

 the present at least, to treat the mineral and textural develop- 

 ment of igneous rocks separately. 



In petrographical classifications in present use either the 

 actual composition or the texture is made the basis of subdi- 

 vision, and the other is used for further subdivision. And it is 

 recognized that there is variation in either case within the limits 

 of petrographical divisions, however constituted. 



This may be illustrated by the following examples : 



