TEXTURE OF IGNEOUS ROCKS 703 



Prismoid — elongated in one direction. 



All of the phenocrysts may have the same kind of form — that is, 

 they may be equiform; or they may possess several forms, they may 

 be multijorm. 



Specific shapes of phenocrysts, when sufficiently distinctive, may 

 give rise to special terms, such as rhombenporphyry, in v^hich the 

 large feldspar phenocrysts have rhombic cross-sections. Another 

 instance is spherophyre, in which phenocrysts are composite or 

 simple aggregations of crystals in the form of spherulites. 



Arrangement. — The arrangement or distribution of phenocrysts 

 through the groundmass is another factor in the composition of 

 fabric. Phenocrysts may be — 



1. Scattered more or less uniformly through the groundmass. 

 Such a porphyry may be called a skedophyre^ and the texture 

 is skedophyric. This is the commonest case. 



2. Grouped together in various ways, as in — 



{a) Clusters — or irregular groups of phenocrysts; such a por- 

 phyry may be called a cumulophyre, and the texture cumulo- 

 phyric. When the phenocrysts are equant, the texture is 

 glomerophyric (glomeroporphyritic, Judd, 1886). 

 (&) Layers or laminae — constituting a planophyre, or planophyric 



texture. 

 (c) Lines or streaks — producing a linophyre, or linophyric texture. 

 (c) Character 0} the groundmass. — The groundmass of a porphyry 

 may have any texture. It may exhibit any degree of crystallinity. 

 It may possess any degree of granularity, from microscopic to mega- 

 scopically -coarse-grained, and may have any fabric. 



Crystallinity. — All the terms denoting degrees of crystallinity or 

 glassiness may be applied to the groundmass of porphyries. Certain 

 general terms have been in use, one of which (granophyre) has been 

 variously defined. 



Vitrophyric — with groundmass megascopically glassy, or vitropatic. 

 Vitriphyric — with groundmass microscopically glassy, or vitripatic. 

 Felsophyric or aphanophyric — with groundmass megascopically 



aphanitic, or felsopatic. 

 Felsiphyric or aphaniphyric — with groundmass microscopically 



aphanitic, or felsipatic. 

 Granophyric (Vogelsang) — with groundmass holocrystalline granular. 



