298 IDDINGS— PROBLEMS IN PETROLOGY. [April 21, 



gories, without considering the probability of their being many phases 

 of combination of the variable factors of igneous rocks that must 

 characterize all petrographical provinces of the earth. 



The assumption that rocks must either belong to what have been 

 called the "Atlantic " or the " Pacific " provinces, without serious 

 definition of either of these rather comprehensive terms, has led to 

 the humorous conclusion that the igneous rocks of Great Britain 

 belonged in some periods of geological history to the "Atlantic," in 

 others to the " Pacific "' provinces ; indicating the flexible, one might 

 say caoutchouc-like, nature of these provinces. 



The igneous rocks of the Andes and of the western Cordillera 

 of North America have been referred to as representing the "Pacific" 

 province, while the more alkalic rocks of Scandinavia and of some 

 other parts of Europe are considered to represent the "Atlantic." 

 The igneous rocks of Great Britain belong to neither of these dis- 

 tinctive groups as a whole. And the rocks erupted at diflferent 

 geological periods in Great Britain, while they exhibit some varia- 

 tions in extremes of composition, which might result from different 

 degrees of differentiation of chemically similar magmas, bear some 

 of those resemblances to one another that are supposed to charac- 

 terize rocks of one petrographical province. 



The misconception underlying the generalization responsible for 

 the terms "Atlantic " and " Pacific," as applied to petrographical 

 provinces, appears from the facts brought out by Cross regarding the 

 alkalic character of some of the lavas of Hawaii, and by Lacroix 

 regarding alkalic rocks in Tahiti ; to say nothing of similar rocks in 

 Xew Zealand and elsewhere in the southern Pacific. Moreover, in 

 the midst of Europe, in Hungary, there are groups of rocks identical 

 in all respects with those of the Great Basin in western America. 



From this it is evident that one of the most important and inter- 

 esting problems before petrologists is the investigation and exact 

 definition of the districts and regions of igneous rocks in all parts of 

 the world, with the purpose of obtaining the data with which to form 

 definite conceptions of what have been termed petrographical prov- 

 inces. Enough is known already to make it evident that there are 

 many kinds of such groups of igneous eruptions and not two strongly 

 contrasted series ; that they blend into one another in composition ; 



