CLA SSIFICA TION OF IGNE OUS RO CKS 6 I 3 



grain of rocks have never been sharply defined, because the 

 crystals in an igneous rock are never of uniform size. They 

 vary among themselves considerably in most cases, and it is the 

 average size which is in mind when reference is made to the 

 grain of a rock. 



The most important distinction made on this basis is for 

 practical purposes, namely, that between phanerocrystalline 

 (phaneric) rocks, whose crystals may be seen by the unaided 

 eye, and apliairitic or cryptocrystalline rocks, whose crystals are not 

 apparent megascopically. The term aplianitic is the better one 

 to use in most cases, because the term cryptocrystalline implies 

 the holocrystalline character of the rock, which may not have 

 been determined. This fact is well expressed by the negative 

 term aphanitic. 



Phaneric (phanerocrystalline) rocks are commonly divided 

 into coarse •-grai?ied, medium- grained, and fitie- grained. But for 

 these terms no limits have been set except those suggested by 

 Zirkel, 1 namelv, that the first include all those whose average 

 grain is larger than the size of peas ; that the second range from 

 the size of peas to that of millet seeds ; and that fine-grain 

 include those of the size of millet seeds and smaller. Translat- 

 ing this into millimeters we would suggest the following scale : 



Fine-grained rocks, average grain 1 millimeter or less. 



Medium-grained rocks, average grain I to 5 millimeters. 



Coarse-grained rocks, grain more than 5 millimeters. 



Aphanitic rocks have never been systematically divided on a 

 basis of the size of the microscopic crystals, though terms bor- 

 rowed from the megascopical nomenclature have been adopted 

 to fit the case. These are: ?nicrocrystalline, which is equivalent 

 to microphanerocrystalline, and applies to textures whose grains 

 can be seen with a microscope; and microcryptocrystalline, which 

 includes textures whose grains cannot be seen with a microscope, 

 but which are recognized as present by the exhibition of aggre- 

 gate polarization between crossed nicols. 



In place of distinctions corresponding to coarse, medium, 



1 Lehrbuch der Pelrografihie, Vol. I, p. 456, 1893. 



