638 CX OSS, IDDINGS, PIRSSON, WASHINGTON 



priately named by the early geologists, and most of the terms 

 used by them are in use at the present time, though their appli- 

 cation has been variously modified by repeated redefinition. 



We recommend that those terms which are needed for gen- 

 eral field purposes, and are to be based on purely megascopic 

 characters, be selected from the terms originally proposed by the 

 founders of geology, and be given their original significance so 

 far as possible, with only such modifications as a somewhat more 

 systematic treatment of the matter may suggest. 



When all igneous rocks are considered with reference to their 

 megascopic characters their most generally recognizable features 

 appear to be their texture and color, and in some cases, and to 

 various degrees, their mi?ieral constituents . 



If we attempt to group them according to texture we find 

 them falling into three large divisions: 



1. Those whose mineral components can be seen with the 

 unaided eye. 



2. Those whose mineral components cannot all be seen with 

 the unaided eye, and that are composed of a greater or less 

 amount of lithoidal material not resolvable into its component 

 parts. 



3. Those with vitreous luster in the whole or a part of the 

 mass. 



The first have been called by Hauy phanerogene, and may 

 be termed phanerites. 



The second have been called by d 'Aubuisson (1819) aphani- 

 tes, a name which cannot be improved upon. 



The third have been long known as volcanic glasses — 

 obsidia?i, pitclistone, etc. 



All phanerites, whether igneous or metamorphic, massive or 

 schistose, were at an earlier time called granite. But at the 

 beginning of the last century a number of kinds of phanero- 

 crystalline (phaneric) rocks were recognized, the distinctions 

 being based upon the minerals that could be identified mega- 

 scopically. These minerals are: quartz, feldspar, leucite, 

 nephelite, mica, hornblende, augite, the iron ores, etc. It 



