S YS TEMA TIC PE TR O GRA PHY 497 



the system of Rosenbusch, and is also seen in the desire to 

 recognize consanguinity of the magmas of petrographic provinces, 

 as partially worked out by Iddings and Brogger. The avail- 

 ability of such factors in petrographic system is doubted by 

 many authorities. 



b. While the distinction of older and younger series of rocks 

 through different sets of names is still found in the German and 

 French systems there is practical unanimity of opinion that the 

 real differences between the rocks are much less fundamental 

 than was supposed. In America, Great Britain, and elsewhere, 

 this distinction is held to be unwarranted. 



c. The chemical and mineral composition of igneous rocks 

 and their textures are characters used as means of classification 

 in present systems. Chemical composition per se is used, but 

 only by considering a portion of some rocks, and hence fails to 

 provide an adequate system. The broad chemical divisions used 

 by some authors are vague and overlapping. 



Mineral composition is commonly assumed to represent the 

 fundamental chemical constitution and to be, therefore, a con- 

 venient and practicable means of expressing the latter. In 

 practice the qualitative method of applying mineral composition 

 in existing systems destroys its effectiveness as expressing 

 chemical composition. 



Structure is variously used in present systems. It is acknowl- 

 edged to be the product of conditions, and not dependent in 

 marked degree upon mineral development. When applied as a 

 primary factor in classification (as by Rosenbusch) it separates 

 things which are similar in more fundamental characters, and on 

 this ground some authorities believe that structure should be 

 applied in classification after the other characters named. 



3. The rocks which have formed upon the surface of the 

 earth by the destruction of older rocks may be viewed from so 

 many standpoints, as regards the origin of materials, agencies of 

 transportation, relations to the earth or to other rocks, charac- 

 ters of materials, and processes of induration, that no consistent 

 arrangement of these objects, deserving the name of a petro- 



