8o2 WHITMAN CROSS 



later editions, is very nearly the same, in effect, as other forms of 

 the mineralogical system which have been in use for many years 

 past. But he introduces certain new factors, intended to give the 

 system greater precision and a quantitative character hitherto want- 

 ing. Because of the frankly expressed object of the author to 

 formulate a classification free from the, to him, objectionable fea- 

 tures of the Quantitative System, it seems appropriate to review 

 some features of the revised system. I wish to emphasize the fact, 

 however, that the critical comments to be made are, almost without 

 exception, of general application to many earlier propositions and 

 are not personal to Hatch. 



The attitude of Hatch toward the Quantitative System and the 

 claims made for his own scheme are in part expressed in the follow- 

 ing quotations: 



In the quantitative classification of igneous rocks devised by Messrs. 

 Whitman Cross, Iddings, Pirsson, and H. S. Washington, the hitherto existing 

 nomenclature of rock types is entirely discarded, and a new nomenclature 

 introduced which is based on purely chemical considerations without regard 

 to mode of origm. .... 



I desire in this paper to show that it is not necessary to throw over the 

 existing rock nomenclature, nor to disregard mode of origin, in basing a natural 

 system of classification on chemical considerations.^ 



The element in Hatch's system which underlies the claim that 

 it is a ''natural system" is the primary division by "mode of 

 occurrence" into plutonic, hypabyssal, and volcanic rocks. Mode 

 of occurrence is, indeed, a criterion for the classification of igneous 

 rock masses, and Hatch devotes a chapter to this subject. Where 

 this primary division is made, Hatch points out that "volcanic- 

 rocks are, however, connected with their deep-seated or plutonic 

 roots by necks and pipes, or by dikes. A third division of igneous 

 rocks is therefore necessary to embrace these connecting hnks 

 between plutonic and volcanic rocks. For this division the term 

 hypabyssal is used."^ 



But neither Hatch nor any other systematist using plutonic 

 in this sense necessarily means deep-seated or abyssal. He means 

 granular, and yet it is known that certain granular rocks, treated 



' Science Progress, p. i. Italics by W. C. 



^ Textbook of Petrology, 5th ed., pp. 7 and 8; 7th ed., pp. 5 and 6. 



