THE ORIGIN AND DESCENT OF ROCKS. 451 



Neiv System of Classification and Nomenclature. 



The present systems of classifying and naming rocks have grown up 

 gradually out of earlier and cruder methods, many of which were inherited 

 from popular usage. Most of the names and definitions came into use 

 before microscopical and other modern means of study were adopted. 

 These systems, therefore, retain many inherited crudities and incon- 

 sistencies, and lack adaptation to present needs. They are too com- 

 plex and difficult for field use and for general discussions, while not 

 sufficiently exact and systematic for the more rigorous petrological 

 discussions. A more adaptive and consistent practice has been earnestly 

 sought by petrologists, and a new system of classification of igneous 

 rocks has been offered by a group of leading American petrologists, an 

 outhne of which is here given. ^ To some extent this may be extended to 

 the met amorphic crystaUine rocks with necessary modifications and 

 additions. The classifica' ion and nomenclature of the secondary rocks 

 must probably always remain variable and plastic to express the various 

 points of view which it is desirable to take. During the transition to 

 this or some other new system, which seems inevitable, the appended 

 alphabetical reference hsts of the most common minerals and rocks, ^^ith 

 brief definitions in accordance with current usage, will be found ser- 

 viceable. The proposed system includes two parts, a field system and 

 a quantitative system, the one apphcable to rocks on casual inspection, 

 and the other, only after detailed study. 



The proposed field system. 



The proposed field names are based largely on texture and color. The 

 mineral constituents are used for subdivisions when they can be deter- 

 mined; other\\dse they are neglected. 



Classifying chiefly on the basis of texture and crystaUine state, there 

 are three groups : Phcmerites, in which all the leacUng mineral constituents 

 can be seen megascopically; aphanites, in which all, or at least an appre- 

 ciable part, of the constituent minerals cannot be distinguished mega- 

 scopically; and glasses, in which the material is whoUy or largely vdtreous. 



I. The Phanerites may be further classified by their chief mineral 

 constituents as follows: 



^ Cross, Iddings, Pirsson, and Washington. Quantitative Classification of Igneous 

 Rocks. 



