﻿Vol. 66.^\ CLASSIFICATION OF IGNEOUS KOCKS. 481 



Vitally connected with this is the asserted existence of two great 

 magma series. Let us examine the latter point first. 



The magmatic series of Rosenbusch are unnatural. — At 

 the outset I must deny that the ' granito-dioritic and gabbro- 

 peridotitic ' and the ' foyaitic-theralitic ' series are natural. They 

 are both arbitrary and ill-defined. Magmas of corresponding silica- 

 contents in the two series are connected by intermediate ones, just 

 as are the parts of each series. An examination of Washington's 

 tables of rock-analyses, particularly if they are studied and com- 

 pared by use of the norms there given, will show this gradation. 

 From the theoretical side, viewing magmas as mixed solutions of 

 rock-mineral molecules, it is difficult to see any reason why the two 

 series of Rosenbusch should not be connected at all points. The 

 fact that the distinction is both arbitrary and indefinite makes it 

 easy for those who wish to do so to assign magmas one way or the 

 other, just as syenite and diorite were easily and conveniently 

 separable until the uncomfortably logical Brogger demanded recog- 

 nition of the equally important intermediate monzonite. 



Rosenbusch is forced to recognize that monzonite occupies in 

 some respects what he regards as a rare or peculiar (eigenartige) 

 position between his two great magmatic series. But he claims 

 that those who (with Brogger) put monzonite in a series monzonite- 

 banatite-adamellite miss the natural relationships. 1 It is difficult 

 for me to understand why the position of monzonite and essexite 

 as intermediate in more than one direction should not be recognized. 

 The existing chemical data clearly show this broader relationship. 



As we are discussing natural classification as necessarily based 

 on the facts of nature, it is here an appropriate place to point out 

 that the 'dyke rock' division of Rosenbusch loses one 

 of its most strongly emphasized peculiarities if the 

 dual series just discussed are arbitrary. 



The restricted association of 'dyke rocks.' — As to the 

 sharply restricted association of ' dyke rock ' types, asserted by 

 Rosenbusch and made a part of their definition in many cases, no 

 one of wide experience can fail to recognize the kernel of significant 

 truth in this generalization. Some rock-types are much 

 more abundant in certain associations than in others. 

 The more rare and peculiar it is in composition, the more likely is 

 a rock to be restricted as a magmatic differentiate; and, conversely, 

 certain parent magmas must inevitably transmit certain prevalent 

 characters to their descendant parts. But with differences in parent 

 magmas must come variations in the prevalence of derived characters 

 and in associations of descendant forms. 



It is difficult to present the evidence that many ' dyke rocks ' 

 have not the restricted genesis assigned to them by Rosenbusch 



1 ' Mikroskopische Physiographie : Massige Q-esteine' 4th ed. vol. ii. pt. 

 (1908) p. 166. 



