470 WHITMAN CROSS 



tion as all important, Rosenbusch retains the group term massive, 

 yet in both works it is clearly the igneous origin which is first in 

 mind and which is recognized as of prime importance in pro- 

 ducing rock structure, chosen as the leading factor in construct- 

 ing the new system. 



After having stated his belief that rock structure is the best 

 basis of classification of "massive rocks" Rosenbusch proposed 

 to divide them into three groups: (i) Deep-seated rocks 

 ("Tiefengesteine"), (2) Dike rocks (" Ganggesteine"), and (3) 

 Effusive rocks (" Ergussgesteine "). The critic is obliged to 

 point out that this proposition is inconsequent, for not only is 

 structure not expressed in the terms chosen, but another distinct 

 factor is expressed, namely, mode of occurrence. The further 

 development of Rosenbusch's scheme makes it clear that he did 

 not intend to emphasize the actual facts of geological occurrence, 

 plainly as he stated them, but rather to express in this way his 

 conception of the genesis of structure. Recognizing that dif- 

 ferent structures result from the consolidation of a given magma 

 according to the attendant conditions, Rosenbusch selected the 

 geological factor appearing to him of greatest influence among 

 many conditions and made that the expressed basis of structural 

 classification. Since simplicity and logical directness are surely 

 of utmost importance in systematic constructions the unnecessary 

 indirectness of this proposition may be designated a fatal weak- 

 ness. Furthermore, the geologist is warranted in objecting to it 

 because the expressed division of igneous rocks is one which he 

 has used in the past and must use in the future, in its literal and 

 appropriate sense, quite apart from the idea hidden in the terms 

 of Rosenbusch's system. 



To the above noted criticisms of Rosenbusch's first applica- 

 tion of structure in classification must be added another, based 

 upon the fact that the division of Dike rocks was not in reality 

 provided for rocks occurring in dikes, but for a group of rocks 

 for which Rosenbusch assumed a certain genesis. An hypothesis 

 of magmatic differentiation and assumptions of limited occur- 

 rence and of characteristic structure are all involved in the dis- 



