MINERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS 83 



was opened out at one corner into a secondary tetrahedron, as 

 shown in Fig. 18. The division points between families are at 0-25- 

 75-100 to make the nomenclature conform to the older systems, and 

 they are numbered, from the top and counterclockwise, from 1 to 

 15. The four corners, in Orders 1 and 2, represent respectively oli- 

 vine, biotite and amphibole, monoclinic pyroxene, and ortho- 

 rhombic pyroxene. 

 In Order 3 the corners 

 are olivine, biotite 

 and amphibole, the 

 pyroxenes, and the 

 "ores" and other 

 dark constituents. In 

 Order 4, if thought 

 desirable, they may 

 be taken to repre- 

 sent the various ores ; 

 the writer, however, 

 groups the ores in one 

 family, for, considered 

 as rocks, they are un- 

 important and hardly 

 worth while separat- 

 ing. All of the fam- 

 ilies of the whole class, 

 except Family 15, ap- 

 pear on the surface 

 of the tetrahedron, 

 Families 5 and n 

 being at the back of 

 Fig. 18, Family 14 

 underneath, and Family 15 in the center. Fig. 19 shows the 

 tetrahedron opened out, Family 15 alone not appearing. 



Kf '. 



Fig. 21. 



FOid* 



-Rocks of Order 2 falling in Classes 1 to 3 



ROCKS INCLUDED IN THE VARIOUS FAMILIES 



Computed by the rules which follow, nearly 600 rocks are 

 represented in Figs. 20 to 23. In these diagrams the rocks of the 



