282 L. V. Pirsson — Classification of Igneous Rocks. 



Texture in Classification. The factor of texture has 

 so far never been given any very precise definition in 

 classification. Different varieties of texture themselves 

 have been minutely described, and, since textures merge 

 into one another gradually in various directions and 

 division lines between them must be arbitrary, quanti- 

 tative definitions and limits to textures have been sug- 

 gested, as set forth in the preceding chapter. But so far 

 as textures have been used in systematic classification 

 this has been done in a purely megascopic manner, and 

 mostly with very vague limitations. Only the broadest 

 distinctions are employed; divisions of granular, dense, 

 and glassy are used, and whether the rock is porphyritic 

 in fabric or not. No quantitative values as to the use of 

 these distinctions are suggested. The greatest stress is 

 laid upon the character of whether a rock is porphyritic 

 or not; this shows itself in the terminology in that the 

 term, as one of texture, is embodied in the name, either 

 complete, as in syenite porphyry, rliomhic porphyry, etc., 

 or in the contractional suffix phyre, as in orthophyre, 

 keratophyre, etc. 



All that ZirkeP° remarks as to the relation between texture 

 (structure) and classification is that lie lays stress on the con- 

 trast between porphyritic and nonporphyritic rocks. He notes 

 also that this gives rise to some inconsistencies, since some basalts 

 are not porphyritic, but granular. Rosenbusch does not use tex- 

 ture as a primary factor in classification, but only in a secondary 

 sense as connected with mode of occurrence ; when the latter is 

 unknown he then falls back on texture, and the porphyritic 

 quality plays the chief role. Brogger essentially follows Rosen- 

 busch but has only offered an outline of his suggested classifica- 

 tion. Iddings-^ merely remarks that on the basis of texture the 

 rocks are divided into the phanerocrystalline (grained) and 

 aphanitic (dense) groups ; the latter is subdivided into those with 

 paleotypal, and cenotypal habits, this distinction being based on 

 the appearance of the rock due to more or less alteration from, in 

 general, greater age,-^ as suggested by Brogger. Harker^^ does 

 not use texture in primary classification but, in general, follows 

 Rosenbusch and Brogger in relegating it to a secondary position. 



These examples will serve to show that no general 

 agreement either as to the use of texture in classification, 

 or if used, as to its limits, obtains among petrographers. 



^"Lehrbueh der PetrograpMe, Vol. I, p. 837. 



^^ Igneous Eocks, Vol. I, p. 350. 



^- Op. cit. p. 353. 



^' Petrology for Students, 3d Ed. 1902. 



