PETROGRAPHIC TERMS FOR FIELD USE 319 



a) Aphaneids, properly so called. Non-porphyritic. 



b) Aphaneid porphyries. Porphyritic rocks with aphanitic 

 groundmasses. For consistency some other word should be 

 substituted for porphyry, since this term is generally applied only 

 to orthoclase rocks, and by some writers to rocks with two genera- 

 tions of minerals, with or without phenocrysts. The term, however, 

 is in such general use among miners and quarrymen, and by the 

 U.S. Geological Survey, for all rocks containing larger crystals in 

 a fine-grained or dense ground-mass, that it may be better to retain 

 it. Poikileid (ttolkiXos, "spotted," and etSo?) would be much 

 better. 



III. Glasses. Rocks which are hyaline or which have hyaline 

 ground-masses. 



On the basis of mineral composition, the rocks can be divided 

 megascopically most easily into those that are light and those 

 that are dark. Consequently the groups are divided into those 

 in which ' 



1. Femag 1 minerals form less than half the rock. 



2. Femag minerals form over half the rock. 



3. Light-colored minerals absent or nearly so. 



While it is generally impossible to separate different feldspars 

 megascopically, quartz can usually be recognized. Its presence 

 or absence is therefore made the basis of a further subdivision, as 

 is also the presence or absence of olivine in the no-feldspar rocks. 



It has been found possible to subdivide the rocks megascopically 

 into the following groups: 



I. PHANEREIDS 



Graneid. — A noncrystalline, medium- to coarse-grained igneous 

 rock, consisting of quartz and any kind of feldspar, with one or 

 more members of the biopyribole group, 2 generally biotite or 

 hornblende. The femag minerals form less than 50 per cent of 



1 The term femic has been used repeatedly by recent writers as synonymous with 

 ferro-magnesian. This is incorrect, since femic minerals include apatite, fluorite, 

 calcite, etc. The term femag is here used for ferro-magnesian. 



2 Biopyribole is suggested as a substitute for the awkward words biotite ] an 

 pyroxene \ I amphibole, and pyribole for pyroxene j a i amphibole. 



