CLA SSIFICA TION OF IGNEO US ROCKS 6 1 5 



those which do not occur to a notable extent and use those which 

 predominate as the means of discrimination, that is, as a basis 

 for textural subdivision. 



1. As to the shape of the constituent crystals, there are two 

 distinctions of a general nature : 



[a) Automorphic x (idiomorphic) forms, in which the crystal 

 form has been developed more or less perfectly. 



(<£) Xenomorphic 1 - (allotriomorphic) forms, in which the proper 

 crystal form has not been developed owing to interference from 

 outside influences. 



These latter forms are of two kinds : one caused by the 

 interference of adjacent crystals, the commonly recognized case 

 in holocrystalline rocks ; the other is caused by outward forces 

 acting in the liquid from which crystallization has taken place, 

 which produce irregularly shaped crystals peculiar to micro- 

 scopic growths in the more siliceous glasses, but not confined to 

 them. 



Other distinctions of shape depend on the particular forms 

 of the crystals, whether equidimensional, tabular, prismatic, or 

 otherwise. 



2. The arrangements or positions of crystals with respect to 

 one another may be grouped under several heads : 



A. Juxtaposition. — In this case, although subordinate minerals 

 may be enclosed in the preponderant ones, the preponderant 

 minerals are adjacent to one another, and the following divisions 

 based on the shape of the crystals are recognized : 



Granular, in which adjacent crystals have nearly the same 

 size, so that the rock appears to be made up of more or less 

 uniform grains. According to the form of the individual crys- 

 tals the rock is said to be : 



a) Xenomorphic granular, in which the crystals are xenomor- 

 phic and equidimensional. This is commonly called "granitic" 

 fabric. 



1 We adopt the use of aiitomorphic and xenomorphic instead of idiomorphic and 

 allotriomorphic, as they have undoubted priority, Rohrbach having used the former in 

 1886 (7. M. P. M., Vol. VII, p. 88), while Rosenbusch introduced the latter terms in 

 1887. Rohrbach's terms have the further advantage of being shorter. 



