694 CROSS, IDDINGS, PIRSSON, AND WASHINGTON 



this case it is suggested that five degrees of comparison be adopted, 

 corresponding to the five divisions used in magmatic classification. 

 These would be, in addition to the absolute terms already in use : 



Holocrystalline, 



extremely crystalline with 

 some glass. 



dominantly crystalline. 



crystals and glass equal or 

 nearly so. 



dominantly glassy, 

 extremely glassy. 



Hypocrystalline 

 or hypoliyaUne 



Holohyaline, 



wholly crystalHne. 

 percrystalline, 



docrystalline, 



hyalocrystalline, 



doliyaline, 



perhyaline_ 

 wholly glassy. 



crystals 7 



glass I ' 



crystals 7^ =; 



-^, <^>^ 



glass I 3 



crystals _^5^ 3 

 glass 3 5 '■ 



crystals t. 1 

 glass 5 7 



crystals i 



glass 



7 



II. Granularity 



The magnitude of crystals composing rocks is considered in 

 two ways: first, as regards the absolute size; second, with respect 

 to relative sizes of associated crystals. The second is a factor enter- 

 ing into the fabric of a rock. Granularity may properly be limited 

 to the first character, that is, the absolute size of the crystals 0} a rock. 

 h When a rock is evenly or uniformly granular, or nearly so, the 

 granularity — the grain — of it may be considered to be the size of 

 the average- sized crystals. But when a rock is porphyritic — that 

 is, consists of a matrix, or groundmass, with phenocrysts — ^it is 

 clear that there may be two or more expressions for the size of crystals ; 

 one relating to the components of the groundmass, another to the 

 phenocrysts. For reasons which will be explained in the discussion 

 of porphyritic textures, it is advisable in these cases to apply terms 

 of granularity primarily to the groundmass. 



The size of crystals in rocks ranges through extremely wide 

 limits — from dimensions that are submicroscopic, and are only rec- 

 ognized by the exhibition of aggregate polarization, to those that 

 may be expressed in meters; a range of more than 1:1,000,000, 

 In the great majority of rocks, however, the range is more nearly 

 1 : 1,000. Up to the present time no effort has been made to describe 

 the grain of rocks with anything like mathematical definiteness, 

 even to stating an approximate average size of the crystals. The 



