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ALBERT JOHANNSEN 



fit quite as well as the divisions 100-95-50-5-0 proposed in the 

 present paper, but the writer feels that a rock with 12^ per cent 

 quartz (see Fig. 7 with 10 per cent) is too rich in quartz to be called 

 a syenite. The writer would have no objection to making the 

 divisions at 100-95-65-5-0 quartz (or f eldspathoids) , that is, on 

 the basis of the 100-95-56-35-5-0 divisions with the omission of the 



35 per cent line, but 

 thinks it better to 

 leave the divisions 

 symmetrical. A rock 

 with over 50 per cent 

 quartz or feldspathoid 

 is certainly distinct 

 enough to deserve a 

 separate place. 



Class 4. — Owing to 

 the absence of light 

 constituents in Class 4 

 it was necessary to 

 make the subdivisions 

 on a different basis. 

 After numerous at- 

 tempts with different 

 figures and different 

 groupings of minerals, 

 it was found that the 

 compartments shown 

 in Fig. 17 correspond 

 most closely to the 

 present subdivisions of 

 the melanocratic rocks. 

 The tetrahedron is sub- 

 divided into four orders 

 by planes parallel to the left-hand face, each order represent- 

 ing an increasing amount of the ores. The division points for 

 these planes, as in the other classes, are 0-5-50-95-100. To 

 accommodate the rocks of the old classification, each order triangle 



Folds 

 Fig. 20. — Rocks of Order 1 falling in Classes 1 to 3. 

 Open circles are rocks of Class 1, dark circles rocks of 

 Class 2, and triangles rocks of Class 3. 



