702 CROSS, IDDINGS, PIRSSON, AND WASHINGTON 



Microscopic phenocrysts, or microphenocrysts. 

 In the first case the rock may be said to be — 



Megaphyric, having megascopic phenocrysts; 

 and in the second case it may be called — 



Microphyric, having microscopic phenocrysts. 



Considering the megaphyric rocks, the phenocrysts may be large 



or small, of any size. Following the custom in describing the grain 



of equigranular rocks, we might establish three common distinctions : 



Magnophyric — coarsely porphyritic, the phenocrysts greater than 



^mm y^ longest diameter. 

 Mediophyric — moderately porphyritic, the phenocrysts between 



^mm g^j^(^ J mm jj^ longest diamctcr. 

 Minophyric — minutely porphyritic, the phenocrysts from i™"^ to 



0.2"^™ in longest diameter. 

 In case corresponding distinctions in the size of microscopic 

 phenocrysts are desirable — that is, in microphyric rocks — the same 

 terms may be used with the change of the letter to i, and a corres- 

 ponding decrease in the value for the diameters. 

 Magniphyric — coarsely microphyric, the phenocrysts having longest 



diameters from 0.2 to 0.04"^*". , 



Mediiphyric — moderately microphyric, phenocrysts having longest 



diameters from 0.04 to 0.008"^™. 

 Miniphyric — minutely microphyric, phenocrysts having longest diam- 

 eters less than 0.008"^*". 

 Such minute phenocrysts are microlites in a glass base. 



There is further the distinction as to whether all the phenocrysts are : 

 Of like magnitude^ or 

 Of differ ent magnitudes. 



Shape. — The shape or form of phenocrysts is an important 

 factor in determining fabric. While the specific shape of a pheno- 

 cryst may be a crystallographically complex form requiring some- 

 what elaborate description, there are certain terms which serve to 

 describe the crystals without reference to crystallographic detail. 

 Some of these are as follows: 

 Equant — with surfaces about equidistant from the center, sometimes 



cuboidal or spheroidal. 

 Tabular — flattened in one plane. 



