CLASSIFICA TION OF IGNEOUS ROCKS 6 1 I 



descriptive value, or that the definitions be so restricted that they 

 are not appropriate to many of the rocks to be classified. 



It has been common to assume a fictitious dependence of 

 texture upon geological occurrence, ignoring other conditions 

 which are plainly of much influence ; as, for example, the 

 assumption that granular rocks are plutonic or deep-seated, or, 

 from the converse point of view, that effusive rocks must be 

 partially glassy, or trachytic, or porphyritic. 



Less often definitions of texture have involved assumptions 

 of their origin only partially in accord with the facts. 



In some cases rocks have been classified on a basis of only 

 part of their texture, as when in certain porphyritic rocks the 

 texture of the groundmass alone has been taken into considera- 

 tion. 



It appear to us that the use of texture in previous petro- 

 graphic classifications has, on the one hand, weakened the 

 systems by rendering them unnecessarily artificial and illogical, 

 and, on the other hand, has prevented the natural application of 

 certain textural terms. The classificatory role assigned to texture 

 in this system is that of a qualifier of any of the various 

 divisions based upon chemical and mineral characters. We 

 present certain general considerations of rock textures which 

 may aid in securing for the various factors which enter into 

 texture a recognition which it seems to us they have not thus 

 far commonly secured. The discussion does not profess to be 

 complete as to all rock textures but simply takes up some of the 

 more prominent textures and discusses their essential properties 

 and relations. 



When we consider the texture of igneous rocks we find that 

 it may be separated into three factors, which though not in all 

 cases absolutely distinct from one another are so to a very con- 

 siderable extent. They are : 



I. Crystallinity. The degree of crystallization. 

 II. Granularity. The magnitude of the crystals. 

 III. Fabric. The shape and arrangement of the crystalline and non- 

 crystalline parts. 



