SYSTEM A TIC PETROGRAPHY 47 S 



able. In detail Kalkowsky's scheme is similar in its results to 

 that of Zirkel, but, as he does not define his smaller rock divi- 

 sions, a further discussion of his arrangement seems unnecessary. 

 The definitions are omitted, according to the author, because the 

 student must learn to know the rocks by the study of named 

 hand specimens and will, therefore, find out what they are with- 

 out definitions. 



J.J. Harris Teall, 1886, 1888. — The most extensive treatise 

 on rocks thus far published in England is the descriptive work 

 British Petrograpliy , by J. J. Harris Teall, issued almost simul- 

 taneously with the second edition of Rosenbusch's Massige 

 Gesteine. This work lays no claim to being a systematic petrog- 

 raphy, and describes almost exclusively the igneous class ; but 

 from its scope a discussion of principles of rock classification 

 was necessary, as explanatory of the arrangement actually used. 

 Teall considers rocks so complex and indefinite in character 

 that in the existing state of knowledge no true systematic 

 arrangement is possible. His order of presentation is, in fact, 

 one of convenience, and does not express his own views of the 

 most natural basis of classification. 



In discussion of principles, Teall points out that chemical 

 composition, as the constant and primary character of igneous 

 rocks, is the natural basis of classification and in accordance 

 with the Bunsen law of two magmas. He, however, does not 

 work out any new proposition to use chemical composition. 

 The arrangement under which rocks are described is a mixture 

 of the methods of Rosenbusch and Michel-Levy. All igneous 

 rocks are placed in seven groups, as follows : 



A. Rocks composed of the ferro-magnesian minerals : olivine, enstatite r 

 augite, hornblende, and biotite. Feldspar absent ; or, if present, occurring 

 only as an accessory constituent. 



B. Rocks in which plagioclase is the dominating feldspathic constituent.. 

 Nepheline and leucite absent. Orthoclase is frequently present. 



C. Rocks in which orthoclase is abundant. Plagioclase usually present. 

 Nepheline and leucite absent. 



D. Rocks containing nepheline or leucite ; sometimes nepheline and 

 leucite. 



