191 1.] IDDINGS— PROBLEMS IN PETROLOGY. 299 



that the dehmitation of the regions, or provinces, may be pronounced 

 in some instances, and ill-defined in others. 



The character of the rocks in different provinces, and the distri- 

 bution of provinces throughout the earth, together with their rela- 

 tions to the geological structure and dynamical history of the region 

 in which they occur, furnish problems of the first magnitude in 

 petrology. 



One of the questions to be answered is : the relation of the com- 

 position of igneous rocks of dififerent parts of the earth to its isostasy. 

 The configuration of the earth's surface demands the presence of 

 material of dififerent densities beneath the surface. Does this show 

 itself in the character of the material erupted in dififerent regions. 

 An answer to this can not be given ofi:'hand. The requirements in 

 density are relatively so slight when great volumes are concerned, 

 as pointed out recently by Hayford;* the series of igneous magmas 

 of any region is so diversified in composition and density ; and the 

 estimation of their several volumes is so hazardous an undertaking 

 that a reasonable solution of the problem can only be expected after 

 the accumulation of a great amount of exact data. 



Whether there is any relation between the kinds of magma erupted 

 in a particular region and the dynamical events wuthin the region is 

 another problem yet to be solved. Assertions to the effect that there 

 is a definite relationship have been made, but they are in the nature 

 of broad generalizations upon questionable premises, producing the 

 results already discussed in connection with the terms "Atlantic " 

 and " Pacific." 



It is possible that dififerences in the sequence of dynamic events 

 in various regions, or in one region at various periods of its history, 

 may be accompanied by differences in the processes and results of 

 dififerentiation of chemically similar magmas; that is, in series of 

 erupted rocks, but the existence of such relationships has yet to be 

 clearly established. For it is also possible that the material of the 

 earth may be heterogeneous in composition, dififering somewhat from 

 place to place, and yielding dififerent kinds of magmas in different 



* Hayford, J. F.. " The Relations of Isostasy to Geodesy, Geophysics and 

 Geology," Science, N. S.. Vol. 33, No. 841, 191 1, pp. 199-208. 



