KOCKS : THEIR CONSTITUENTS AND KINDS. 77 



more than one, the prominent distinctions are usually based on the two most 

 characteristic ; and the others are considered as accessory minerals, and are 

 made to distinguish varieties. 



The following are distinctions among crystalline rocks, based on texture 

 and structure : — 



1. Granitoid. — Granular-crystalline, like ordinary granite. 



2. Micro-granitic. — Like granite, but very fine in grain. 



3. Micro-crystalline. — Compact, and so fine in texture as barely to glisten 

 over a surface of fracture. 



4. PorjyJiyritic. — Having one of the minerals of the rock in distinct 

 crystals (Fig. 58). The original porphyry of geology included a red por- 

 phyry (from Egypt), a compact red rock, finely 



spotted with pale feldspar (orthoclase) crystals ; 

 and a green porphyry — the Oriental verd-antique 

 — with rather large crystals of whitish labradorite, 

 from western Greece. The rocks, although alike 

 in being porphyritic, are not of the same species, 

 but are porphyritic varieties of different species, as 

 described beyond. 



The mineral in crystals in a porphyritic rock 

 may be any feldspar, or it may be augite, leucite, quartz, or some other 

 species ; and whatever the mineral, the crystals are called phenocrysts, from 

 the Greek for " visible crystals," a term proposed by J. P. Iddings. The kind 

 of mineral is indicated by the terms orthoj^hy^Hc, if orthoclase ; labradophyric, 

 if labradorite ; aiigitopliyric, if augite ; leucitophyric, if leucite ; quartzophyric, 

 if quartz ; sp>herop}hyric, if containing spherical concretions, etc. 



5. Foliated. — Having the cleavage-structure of slate, as in extreme cases 

 of foliation ; or having an arrangement of the minerals, especially of any 

 foliated mineral like mica, approximately in planes, so that the rock has 

 the appearance of being stratified, and often breaks easily into slates or 

 sheets. The slaty, and all schistose, structure, to the faintest, is here in- 

 cluded. The planes of foliation are either pressure-made planes, or corre- 

 spond to planes of bedding or stratification. 



6. Fluidal. — In igneous rocks, having the material of the rock or of 

 portions of it in parallel lines or bands and looking as if due to the flow of 

 the rock while melted. 



7. Glassy, glass-bearing. — Melted rocks, when cooled rapidly, often 

 become glass at surface instead of rock ; and in some cases all gradations 

 occur in the mass of an igneous rock between glass with microscopic stony 

 points, or microlites, and stone with microscopic glassy particles. Lavas 

 have usually particles in a glassy state among the stony particles, which a 

 microscopic study of the rock will detect. 



