MINERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS 81 



compares this sum with the remaining plagioclase; that is, he has 

 the ratio 



Kf +Ab in albite 



Ab in soda-lime feldspar +all An ' 



His divisions are, 1 as mentioned under "Orders," above, 100-875— 

 622-37I-12I-0. 



Fig. 17 Fig. 18 



Fig. 17. — Subdivisions of the tetrahedron of Class 4 into orders 

 Fig. 18. — Subdivisions of Orders 1 and 2, Class 4, into 15 families. Order 3 is 

 subdivided similarly, but the corners represent olivine, biotite and amphiboles, 

 pyroxenes, and the "ores." Order 4 has the various "ores" for corners. 



Amphiboly 

 Biotite\" ig 



Amphibole, 

 18 ""-'Biotite 



Monoolv' 8 

 pyroxene^ 



10 \,6rthorh. 

 pyroxene 



\12 / 



Amphibole , 

 Biotite 



Fig. 19. — Family numbers in Class 4 



The quartz- (or f eldspathoid-) feldspar relations given by Lincoln 

 are 100-96-67-33-4-0, and by Iddings 2 100-625—122— o. Lincoln's 

 division at 33 does not fit at all well into Fig. 15 . Idding's divisions 



1 J. P. Iddings, op. tit., II, 38, 40-41, 42, 44. 2 Ibid., pp. 32, 38, 147, 228, 292. 



