TEXTURE OF IGNEOUS ROCKS 701 



In porphyries the phenocrysts may be of any size, microscopic or 

 megascopic. Groundmass and phenocrysts are the essential parts 

 of porphyritic fabric, variations in which produce different porphy- 

 ritic fabrics. The kinds of variation are as follows: 



(a) Relative amounts of groundmass and phenocrysts. 



(b) Character 0} the phenocrysts. 

 Sizes. 



Shapes. 

 Arrangement. 



(c) Character of the groundmass. 

 Texture: Crystallinity. 



Granularity. 



Fabric. 

 (A) The relative amounts of phenocrysts and groundmass. — Differ- 

 ences in the amounts of phenocrysts give a noticeable character to 

 porphyritic rocks, which up to this time has found very little expres- 

 sion in petrographic literature. At one extreme are porphyries with 

 very few phenocrysts; at the other, those crowded with pheno- 

 crysts and having very little groundmass. Employing the French 

 word for groundmass, ^ai^e = " paste, " and 5ew^ = "sown" or 

 "sprinkled," comparisons are easily described by the terms: 



e.m. 7 .... 



PerpaHc, — ^ — >- , extremely rich in groundmass. 



ph. I 



ET.m. 7 S 1 1 ■ 



Dopahc, ^ — <->-. groundmass dominant, 

 ph. 13- 



Sempatic, ——-<->—, groundmass and phenocrysts equal or nearly equal. 



^ . g.m. 31, , . 



Dosemtc, — ; — <->-, phenocrysts dominant. 

 ■ ph. 5 7 



„ . g.m. I ..... 



Persemic. ^ — <— , extremely rich m phenocrvsts. 



ph. 7 ^ - 



Having noted the relative amounts of groundmass and pheno- 

 crysts, the relative sizes, shapes, and arrangement of the phenocrysts 

 may be taken into account. 



(b) Character of phenocrysts. — Size. Phenocrysts may be large 

 enough to be seen megascopically, or they may be so small as to be 

 only visible microscopically; for these distinctions we have the 

 terms : 



Megascopic phenocrysts, or megaphenocrysts. 



