A REVIEW OF IGNEOUS ROCK CLASSIFICATION 83 



Both these rocks are leucocratic. In I. quartz forms 33 per 

 cent, of the whole, and in II. felspars make up 67. per cent, of 

 the leucocratic minerals. Hence both rocks fall into group (c). 

 In I., moreover, orthoclase forms 100 per cent., and in II. 

 67 per cent., of the felspar present. Hence they both fall into 

 Series V. On comparison of the mineral composition, however, 

 these rocks are seen to be widely different. One is a holo- 

 leucocratic granite ; the other is a nearly mesocratic felspathoidal 

 monzonite. Hence it must be agreed that the three-fold method 

 of partition results in far too comprehensive compartments. 



A. N. Winchell has recently devised a useful modification of 

 Rosenbusch's classification. 1 His main classes are peralkalic, 

 alkalic, and alkalcic (= alkali-calcic). Symmetry would seem 

 here to demand a percalcic class also. The next co-ordinate 

 used is that of occurrence and incidentally texture. The rocks 

 are divided into plutonic, hypabyssal, and volcanic groups. 

 The hypabyssal is further subdivided into the aschistic and 

 diaschistic types (the latter with both felsic and mafic varieties) ; 

 and the volcanic into felsitic and glassy types. The third factor 

 in the classification is mineralogical, and subdivides the rocks 

 according]to whether they have alkali-felspars, soda-lime felspars, 

 or are devoid of felspars. A further mineralogical division is 

 employed which, however, differs in detail in each of the three 

 main classes. 



No quantitative relations whatever are formulated to regulate 

 the application of the various factors employed ; but tables 

 showing the average chemical composition of the principal 

 rock-types are appended to the paper. This is an admirable 

 rearrangement of Rosenbusch's plastic qualitative classification. 

 With one or two further modifications (as, for example, the 

 institution of a percalcic class, and the recognition of the felsic- 

 mafic ratio throughout), and with a certain amount of quantita- 

 tive stiffening, it would form an excellent classification to 

 present to students in geology classes, from which, in a more 

 advanced stage, they could pass to the more elaborate quantita- 

 tive classification based on the mode. 



1 Rock Classification on Three Co-ordinates, Journ. Geol. xxi. 1913, pp. 208- 

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