MATERIALS OF THE EARTH 59 



elated, arenaceous, argillaceous, etc.); or pyroclastic (tufaceous, 

 agglomeratic), etc. 



As sometimes one of these characteristics and sometimes an- 

 other is most important in a given rock or in a given study, no one 

 classification is satisfactory in all cases, yet each has its advan- 

 tages in particular cases. 



A New System of Classification and Nomenclature 



The familiar systems of classifying and naming rocks, if indeed they can 

 be called systems, 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 modern methods of study were adopted. 

 These systems, therefore, retain many crudities and inconsistencies, and 

 lack adaptation to present needs and knowledge. They are too complex 

 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 system is needed, and in response to this need, 

 a new system of classification of igneous rocks has been offered by a group 

 of leading American petrologists. 1 To some extent this proposed system 

 may be extended to the metamorphic crystalline rocks, with necessary modi- 

 fications and additions. The classification 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 lists of the most common 

 minerals and rocks, with brief definitions in accordance with current usage, 

 will be found serviceable. 



The proposed system includes two parts, afield system and a quantitative 

 system, the one applicable to rocks on casual inspection, and the other, only 

 after detailed study. The field system only is here outlined. 2 



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 determined; otherwise they are neglected. 



Classifying chiefly on the basis of texture and crystalline state, there are 

 three groups: Phanerites, in which all the leading mineral constituents can 

 be seen without a lens; aphanites, in which all, or at least an appreciable part 

 of the constituent minerals cannot be distinguished by the unaided eye; 

 and glasses, in which the material is wholly or largely vitreous. 



1 Cross, Iddings, Pirsson, and Washington. Quantitative Classification of 

 Igneous Rocks. 



2 For a statement of the quantitative system reference should be made 

 to the original work mentioned in the preceding foot-note. An abbreviated 

 statement appears in the larger work of the present authors, Vol. I, p. 454, 

 et seq. 



