TEXTURE OF IGNEOUS ROCKS 695 



terms in use express certain general ideas of magnitude based on 



the limits of vision, as follows: 



Phanerocrystalline — All sizes large enough to be seen with the 



unaided eye, that is, megascopically. 

 Cryptocrystalline (aphanitic) — all sizes too small to be seen with 

 the unaided eye. Aphanitic is a purely negative term which 

 does not state whether the rock is crystalhne or glassy, and is 

 very convenient when it is desirable to express simply the fact 

 that the matter is not determinable megascopically, Cryptocrys- 

 talline states that the rock is crystalline, and that the crystals 

 are too small to be seen megascopically. 

 Microcrystalline — all sizes recognizable only with a microscope, 

 and therefore embracing most of those that are megascopically 

 cryptocrystalhne. 

 Microcryptocrystalline — sizes too small to be seen with a microscope, 

 but recognized as crystal particles by the exhibition of aggregate 

 polarization. 

 Microaphanitic — This term may very well be employed to cover those 

 doubtful cases of microscopic and submicroscopic aggregation 

 which have been sometimes described as microfelsitic. 

 Phanerocrystalline rocks have been described as coarse^ medium, 

 or fine grained, with little effort to fix absolute values for these terms. 

 Zirkel 's suggestion that they be compared with the size of peas and 

 millet seed has led to the more definite suggestion, already made 

 by us, that the average diameter of crystals in — 



Fine-grained rocks be taken as less than 1 millimeter; in 

 Medium- grained rocks, between i and 5 millimeters; and in 

 Coarse-grained rocks, greater than 5 millimeters. 

 Coarse-grained rocks are vaguely described as very coarse, or 

 extremely coarse, without any indication of the actual degree of 

 granularity. It would be more definite to express the approximate 

 size of the grain of rocks in terms of the various units of the metric 

 system. There are some extremely coarse rocks, such as pegmatites, 

 whose crystals may be measured in meters; more whose crystals 

 are several decimeters in diameter; many that may be measured 

 in centimeters. Using a decimal series of units, it would be reason- 

 able to employ the following terms for approximate descriptions 



