7 6 



ALBERT JOHANNSEN 



thus grouping albite (allowing up to Ab 9S An s for latitude), oligoclase 

 and andesine, labradorite and bytownite, and anorthite (with 

 Ab s An 95 for latitude), and conforming to the present lines of separa- 

 tion between the alkali rocks, the acid plagioclase (dioritic) rocks, 

 the basic plagioclase (gabbroic) rocks, and the anorthite rocks. 



Each of the first three classes of rocks may be divided in this 

 manner into four orders, making twelve orders in all. The fourth 

 class, that is, the one in which the dark constituents form over 95 per 

 cent of the rock and the light constituents, including the feldspars, 

 only 5 per cent, naturally cannot be divided on the basis of the 

 feldspars; consequently its orders are differently formed. 



Fig. 13. — Number of rocks with various plagioclases, all families from o to 31 

 included. 



Lincoln does not divide his rocks on the kind of plagioclase, but 

 separates his gabbro from diorite, for example, simply on the basis 

 of its leucocratic or mesocratic character, which is not according to 

 common usage. 



Iddings 1 unites his orthoclase with albite and uses the ratio of 

 orthoclase plus albite to other plagioclases, and makes his divisions 2 

 at the points o-^-- §— f- y-100; that is, at 0-123— 3 7§— 622-870-100 

 per cent. These divisions are not quite comparable to the present 

 writer's triangular divisions into the Kf, Naf, and Caf ratios. 

 Owing to the fact that soda is of more importance in connection with 

 the lime of the plagioclases than it is in connection with the potash 

 of cryptoperthite, it seems more reasonable to separate Kf from 

 Naf + Caf than to separate Kf+Ab from the plagioclase minus 

 albite. The latter would be simpler in placing microperthite, but 



1 J. P. Iddings, op. cit., II, 41. 2 Ibid., pp. 38, 40-41, 42, 44. 



