82 SCIENCE PROGRESS 



is that Lincoln prefers a three-fold to a five-fold subdivision. 



1 1 is main divisions are as follows : 



Felsic v 2 

 Division A. Leucocratic . . jyf~ fic / y 



Felsic / 2 v i 

 „ B. Mesocratic . . ^^ < y > y 



Felsic / i 

 „ C. Melanocratic . ^^ <^ y 



Divisions A and B are further subdivided on the basis of 

 the ratio of quartz or felspathoid to felspar, and Division C on 

 ratios subsisting between ferromagnesian silicates and ores. It 

 is only possible here to cite the treatment of Division A as a 

 sample of the method. Division A is subdivided as follows : 



Q V 2 



(a) Quartz group . . . -pr / y 



Q / 2 x I 



{b) Quartz-felspar group . . -pr \ — / y 



Q or L / i 

 (V) Felspar group . . . — p — \ y 



L \ I / 2 



id) Felspar-felspathoid group . v / y \ y 



L v 2 



(i?) Felspathoid group . . p / — 



Further subdivision is made in groups (b) and (c) on the 

 basis of the percentage of orthoclase to total felspars present, 

 and in groups (d) and (e) on the percentage of leucite to total 

 felspathoids, giving rise to thirteen series. 



This mode of arrangement may be subjected to precisely the 

 same criticisms as have been applied to that of Iddings. A 

 further criticism is that the ultimate compartments are far too 

 comprehensive. Take, for example, group (c), in which the 

 ratio Q or L to F is less than one to two. This is subdivided 

 into three series on the basis of the percentage of orthoclase in 

 total felspar present as follows : 



Series V. Syenite-Trachyte . Percentage of orthoclase 100-67 

 „ VI. Monzonite-Vulsinite . ,, „ „ 67-33 



„ VII. Diorite-Andesite . „ „ „ 33-0 



Taking the extreme variations, the syenite-trachyte series 



may contain rocks of the following composition : 



1. n. 



Quartz 33 o 



Orthoclase 67 30 



Lime-soda felspars . . . . .0 15 



Felspathoids o 22 



Mafic minerals ...... o 33 



