49 6 WHITMAN CROSS 



The genetic theory has its proper field of great usefulness in the 

 department of petrology dealing with petrogenesis. Ultimately 

 we may hope and expect that genetic relations of igneous rocks 

 may be available for a more natural classification than is now 

 feasible. 



Any system of classification should be broad and thorough 

 enough to include all the objects which it professes to deal with. 

 But the authors of many systems outlined in this review have 

 been obliged to resort to the expedient of appendices to bring in 

 rocks not otherwise provided for. Such a necessity is, at once, 

 evidence of the inadequacy of the criteria guiding the authors of 

 such systems. 



Even in the class of igneous rocks, propositions for chemical 

 and mineral classifications do not fully recognize the systematic 

 importance of some of the relatively rare constituents. Chemical 

 systems which consider all igneous rocks as mixtures of silicates, 

 or reject magnetite as extraneous, are not comprehensive. 

 Similarly, the schemes which do not provide for the due recogni- 

 tion of titanium minerals, corundum, apatite, sulphides, etc., as 

 important constituents in some cases, are inadequate, even for 

 present uses, and certainly do not provide for future needs 

 which can be clearly foreseen. 



In conclusion, the status of systematic petrography at the 

 close of the nineteenth century may be summarized as follows : 



i. There is as yet no comprehensive and properly systematic 

 classification of all rocks. All so-called systems exhibit portions 

 in which the rocks are treated in an unsystematic manner, for 

 convenience of description, and discussion. The grand divisions 

 are not treated by similarly logical and definite methods. 



2. Rocks of igneous origin have been much more thoroughly 

 investigated than others and they have received correspondingly 

 more definite and systematic classification. The factors used in 

 systematic construction pertain to genesis, age, and characters. 



a. The origin of the great range in chemical composition 

 exhibited by igneous magmas, expressed in theories of magmatic 

 differentiation, is an underlying factor of much importance in 



