I9I1.] IDDINGS— PROBLEMS IN PETROLOGY. 297 



of rocks, minerals of " first and second generation," and the like, 

 tends to convey by implication the idea that there exists among 

 igneous rocks genetic relationships analogous to those sustained by 

 living organisms. In fact, this idea has been clearly formulated 

 by Marker- in stating that the mutual relationships of igneous rocks 

 will furnish a " fundamental principle analogous with that of descent, 

 which lies at the root of natural classification in the organic world." 

 The significance of the term " natural " when applied by some 

 petrographers to petrographic classification appears to be pregnant 

 with biological conceptions. But what is proper and natural in the 

 treatment of assemblages of organisms is not for that reason, neces- 

 sarily, proper, or natural, in the treatment of a series of chemical 

 solutions and their solidified phases, however much the various solu- 

 tions may be related to one another by reason of differential dififusion 

 or fractional crystallization. 



3. Petrographical Provinces. 



Although the fact has been recognized for twenty-five years that 

 there are regions within which the rocks erupted during any par- 

 ticular period exhibit certain peculiarities of mineral composition and 

 texture that distinguish them from rocks belonging to the same gen- 

 eral group, erupted simultaneously in other regions,' little or no 

 attempt has been made to define more precisely what constitutes the 

 characteristics of any so-called petrographical province. 



It has been pointed out that in some regions many of the igneous 

 rocks are especially rich in alkalies ; in some sodium being promi- 

 nent ; in others potassium. But nothing approaching completeness 

 of definition, either as to composition of the rocks, or extent and 

 limit of the region of occurrence, has ever been attempted. 



And yet some very general and far-reaching speculations have 

 been indulged in on the basis of hastily formed impressions, both as 

 to the character of such groups of rocks and their relationship to 

 assumed structural features of the earth. As a result certain petrog- 

 raphers have grouped all igneous rocks into two contrasted cate- 



'Ibid., p. 362. 



^Judd, J. W., Quar. Jour. Geol. Society, London, 1886, Vol. 42, p. 54. 



