﻿470 DR. WHITMAN CROSS ON THE NATURAL [Aug. I9IO, 



20. The Natural Classification of Igneous Bocks. 

 By Dr. Whitman Cross, F.G.S. (Read June 15th, 1910.) 



Contents, 



Page 



I. In trod uction , „ 470 



II. Factors of Chemical Composition 473 



III. Factors of Magmatic Differentiation 478 



IV. Classification by Eutectics 485 



V. Factors of Mineral Composition 489 



VI. Factors of Bock-Texture 490 



VII. Quantitative Classification 493 



VIII. Summary 503 



I. Introduction. 



In how far may the systematic classification of igneous rocks,, 

 the basis of their petrographic description and nomenclature, be 

 constructed by the application of truly ' natural factors ' ? This 

 question has long been debated, but petrographers have not reached 

 agreement as to the answer. Meanwhile petrographic system has 

 become greatly confused through varying views as to what are 

 ' natural relations,' and by the illogical or inconsequent use of 

 arbitrary factors. It seems useless to hope for agreement in this 

 matter until the petrographers of different schools have come to 

 understand thoroughly each other's points of view, which are based 

 largely on experience in widely separated fields of work. It is 

 with the idea of promoting in some measure this mutual under- 

 standing, that I offer this discussion to the Geological Society of 

 London. 



The rocks of the earth exhibit a great variety of relationships of 

 geological importance, as to their origin, their mode, form, and place 

 of occurrence, and possess, in many groups, a complex and often 

 obscure character. The rock is usually defined with emphasis on 

 its geological importance, and it is considered as a geological unit. 

 But there has not always been the desirable clear distinction 

 between the rock-mass, of certain relations of form or position with 

 regard to the earth, and the substance of the mass, the rock 

 proper. 



From various standpoints of geology classifications of rocks 

 have been set up, more or less refined, for special or general 

 purposes. All of these classifications are of use so long as their 

 generalizations agree with experience, but manifestly they cannot 

 be all combined in the one scientific systematic classification 

 of petrography, from which the rocks derive their specific names. 

 There must be various classifications of rocks or of rock-masses, 



