CLASSIFICATION OF IGNEOUS ROCKS 89 



criteria of form and place of occurrence for the classification of 

 igneous rocks into certain logical and natural groups, and has 

 for this purpose a consistent language and mode of expression, 

 he must surely demand of the systematic petrographer that 

 these criteria shall not be used to produce another classification 

 with other definitions of the same terms. Such a course pro- 

 duces confusion for which there can be no justification. It is to 

 me utterly incomprehensible how the appropriation and redefi- 

 nition of the geologist's terms and nomenclature can have been 

 carried through as in the Rosenbusch system under the idea that 

 thereby the science of geology would be benefited. And where 

 is the science of petrography benefited by the formation of 

 systematic groups which are confessedly unnatural and wholly 

 unnecessary ? 



The petrologist must express the facts of the relationship 

 between rock structure and geological occurrence, as between 

 structure and other factors, to the best of his knowledge at any 

 given time. It is not to his advantage to have this relation, 

 always to be a matter of interpretation or theory to some degree, 

 expressed in the systematic scheme. Nor can it be to the advan- 

 tage of the system, for it will be a cause of instability. 



The genetic interrelationships of igneous rocks, which most 

 petrologists believe to be the result of what is called magmatic 

 differentiation, are most important, but are clearly of hypo- 

 thetical nature, and must remain at best matters of theory as 

 long as the origin of the earth itself is veiled in mystery. It 

 seems to me utterly impossible to admit such factors into the 

 petrographic system. But it is the tendency of several leading 

 investigators of today, notably of Rosenbusch and Brogger, to 

 make these theoretical relationships more and more prominent 

 in systematic classification, and from the considerations above 

 presented I wish to make most earnest protest against this ten- 

 dency as really contrary to the best interests of both geology 

 and petrography. In thus protesting, I must not be understood 

 as failing to appreciate the great advance in our knowledge of 

 the origin of igneous rock varieties and of their structures, and 



