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



rocks which has been so prominent in current system 

 is not only unnatural, but is in the highest degree 

 arbitrary. There is no logical consistency displayed in charac- 

 terizing and treating as felspathic a rock whose predominant 

 characters of all sorts are due to the greater abundance of non- 

 felspathic constituents. It is done purely for the convenience of 

 the systematise 



VI. Factors of Kock-Texture. 



The texture of igneous rocks is very commonly used in their 

 classification. Let us consider whether this factor is or can be 

 applied for that purpose, in accordance with the facts of nature. 



Nature of rock-textures. — Rock-texture may be considered 

 to embrace all the material features exhibited by the rock con- 

 stituents, whether mineral particles or glass, and whether mega- 

 scopic or microscopic. It thus includes a great variety of allied 

 characters which may be conveniently brought into three cate- 

 gories 1 : — 



(1) Crystallinity — the degree of crystallization. 



(2) Granularity — the magnitude of the crystals. 



(3) Fabric — the shape and arrangement of the crystalline and non- 



crystalline parts. 



Texture is a physical feature, largely independent of chemical or 

 mineral composition, produced during the consolidation of the 

 magma into rock. The range of crystallinity is from holohyaline 

 to holocrystalline ; of granularity, from very coarse to submicro- 

 scopic ; of fabric, an almost infinite variety of design with many 

 pronounced forms and often a mingling of them in a single rock. 

 The common grouping of textures under the heads granular, 

 porphyritic, fluidal, or glassy, is a crude approximation to the 

 truth, useful when applied in accordance with the facts, but often 

 serving to obscure or conceal actual relations. A granular rock 

 answering to the definition is rare, as compared with the aberrant 

 forms called granular for convenience; and the porphyritic 

 fabric is one of limitless transitions to the granular, the felsitic, 

 the fluidal, and the hyaline. 



Origin of rock-textures. — As already remarked, texture 

 depends upon the conditions controlling consolidation. Each 

 magma has an infinite range of possible textures representing 



1 This classification of textural features, and the general view of the subject 

 here adopted, is presented more fully in a paper by the writer and his col- 

 leagues Iddings, Pirsson, & Washington, published a few years ago ('The 

 Texture of Igneous Bocks ' Journ. Greol. Chicago, vol. xiv ; 1906, pp. 692-707). 

 In this paper the great variety of textural features is emphasized, the paucity 

 of adequate terms for descriptive purposes is pointed out, and a large number 

 of new terms proposed. 



