﻿492 DR. WHITMAN CROSS ON THE NATURAL [A.Ug. I9IO, 



granular texture made it appear possible to express two important 

 genetic relationships at once, in the distinction between Plutonic 

 and Volcanic rocks. The former might, with a few known ex- 

 ceptions, be called granular, and they thus stood in marked con- 

 trast to surface volcanics, which are glassy or highly fluidal in their 

 typical occurrences about volcanoes. 



Plutonic and volcanic rocks are certainly different in texture, to 

 a degree rendering their separation and the distinct names which 

 they bear eminently desirable — if that were the whole story. But this 

 procedure ignores the fact that the earth's crust for a few thousand 

 feet downwards is, in many great mountain-tracts and in other 

 places, characterized by a ccmplex of igneous masses of all shapes 

 and sizes, and representing the entire range of chemical variation. 

 And with our present understanding of rock-texture it is easy to 

 see that here must occur both the textures of the depths and of the 

 surface, with others less commonly produced above or below. With 

 this recognition of the zone of intrusion, as it is sometimes called, 

 came as the first proposition, to make three classes : I, Abyssal 

 (Plutonic) ; II, Hypabyssal (Intrusive) ; III, Volcanic (Effusive) 

 rocks. This is nominally on a basis of geological occurrence. It is, 

 however, not a natural division, as such, because there must be 

 arbitrary lines between the Hypabyssal zone and both the Abyssal 

 and Volcanic. If these divisions are equivalent to Granular, 

 Porphyritic, and Fluidal or Glassy, the scheme is arbitrary, since 

 those textures grade most insensibly into each other. It is 

 unnatural, because the textures have no such restric- 

 tion in occurrence. It is certainly illogical to make in fact 

 a textural division while expressing one of occurrence, and vice 

 versa. 



Brogger frankly states that his principal object in distinguishing 

 the Hypabyssal rocks is to make it possible to give special names to 

 the porphyritic rocks. This he illustrates by asserting that 

 laurvikite, rhomb-porphyry, and trachyte are three essential names 

 for textural phases from the same magma. 1 



Conclusion as to textural classification. — The plain 

 fact is that classification by occurrence, as determin- 

 ing texture, or by texture, as expressing the broad 

 phases of occurrence, is based on long disproved gener- 

 alizations made from limited observation. But, like the 

 age distinction, the unnatural connexion of texture with occurrence 

 has so many expressions in system or nomenclature that recogni- 

 tion of the truth is embarrassing. Witness the treatment of many 

 diabasic and allied rocks by Hosenbusch, classifying them at first 

 with the plutonics, now with the effusives, yet a host of them 

 arc neither, being hypab}'ssal in actual occurrence. The known 



1 ' Die Eniptivgesteine cles Krislianiagebietes, I : Die Gesteine der Grorudit- 

 Tinguait Serie ' 1894, p. 124. 



