S YS TEMA TIC PE TR O GRA PHY 499 



systematic propositions. Among the essays thus disregarded 

 are notable ones by Rosenbusch, Brogger, Becke, Michel-Levy, 

 Teall, Iddings, Spurr, Turner, and many others. 



The telling effect of searching investigations touching con- 

 troverted points of fundamental significance and of the judicial 

 remarks of those who have carried out such studies is often much 

 greater than either author or reader is aware. In the course of 

 time the influence exerted by a succession of investigations 

 becomes evident in some proposition for the revision of classifi- 

 cation. Petrography has thus come to its present condition by 

 a steady natural evolution, and its future growth must undoubt- 

 edly follow the same course. 



Whitman Cross. 



