92 ALBERT JOHANNSEN 



containing both anorthoclase and soda-lime feldspars, the three 

 molecules Kf, Naf, and Caf are to be separated, and the orders 

 determined by the total Ab-An ratio. (See above, under the 

 heading "Orders," for an example of ciminite so separated.) In 

 Class 4 the orders are determined by the percentage of "ores" and 

 other dark minerals in the rock, the division points also being 



3. Determine the family. In Classes 1, 2, and 3 first recalculate 

 the quarfeloids to 100. The amount of quartz (or feldspathoid) 

 immediately determines the distance from the feldspar line. The 

 separation points are 0-5-50-95-100. Now recalculate 1 the Kf 

 plus plagioclase to 100, and determine the proper point on the Kf- 

 Plag line. (If plotted graphically, the family is directly determined 

 by the position of the intersection of the three lines. If the 

 point falls very close to a division line, it may be necessary to 

 compute its position accurately.) The separation points for Kf- 

 Plag are 0-5-35-65-95-100. 



In Class 4, Orders 1 and 2, recalculate the olivine, pyroxenes, 

 biotite, and amphiboles to 100 and find the proper positions graphi- 

 cally, or find the position analytically by taking the ratio of the 

 minerals of one corner to each of the others; thus augite to olivine, 

 augite to hypersthene, and augite to biotite or amphibole. The 

 division points are 0-25-75-100. In Class 4, Order 3, the corners 

 represent olivine, amphibole and biotite, all pyroxenes, and the 

 "ores" and other dark constituents. In Class 4, Order 4, the 

 writer groups all the ores in a single family, but classifies the vari- 

 ous hematite, ilmenite, magnetite, etc., ores as subfamilies. If 

 desired they may be further separated. If accessory dark min- 

 erals, not used in the computation, are abundant, they determine 

 subfamilies and may be mentioned in the rock name. 



A few points to be observed. — Any percentage value falling 

 exactly on a line should be moved in the direction of the center of 

 the triangle. Thus a syenite with 5 per cent quartz is classified 

 with granite, a rock with 95 per cent mantes belongs to Class 3, 



1 It is immaterial whether the orthoclase-plagioclase ratio is taken from the original 

 values or from those reduced as quarfeloids to 100. The results are naturally the 

 same. 



