800 IDDINGS— PROBLEMS IN PETROLOGY. [April 21, 



regions, each of which may undergo local differentiation according 

 to conditions of its eruption. The apparent persistency of the major 

 features of reHef on the earth's surface and the demands of isostasy 

 suggest an absence of homogeneity within the material of which it 

 is composed. The solution of these fundamental problems in geology 

 must rest on petrological research along the lines here indicated. 



Such are some of the more obvious problems of petrology, the 

 solution of which involves the cooperation of petrographers with the 

 chemist, the geophysicist and the geologist. 



