A REVIEW OF IGNEOUS ROCK CLASSIFICATION 77 



our igneous rocks, the next step is to consider the best method 

 of subdivision and classification. It is argued in this paper that 

 as the felsic constituents form more than 79 per cent, of the 

 mass of igneous rocks, the dominant factors in classification 

 should be based on them, and that the subordinate mafic con- 

 stituents should have a correspondingly subordinate place. 

 Consequently the main classes must be based on felsic ratios- 

 Fortunately we have alread}' to hand the main lines of an 

 excellent classification devised by Iddings on this basis in his 

 new book. His principal divisions are as follows : 



Division I. Rocks characterised by quartz. 



,, II. „ „ quartz and felspars. 



„ III. ,, „ felspars. 



„ IV. „ „ felspars and felspathoids. 



,, V. „ „ felspathoids. 



„ VI. ,, „ mafic minerals. 



These divisions have definite quantitative limitations. In 

 all save the last, the ratio of felsic to mafic constituents 

 must exceed three to five. Then in 



Quartz v 5 



Division I. ^— ; ■- > — 



Felspars x 3 



Quartz , 5 x j_ 



" ' Felspars ^ 3 ' 7 



Quartz or Felspathoids , 1 



" ' Felspars ^ 7 



IV 



Felspathoids 

 Felspars 



< JL > JL 



x 3 X 7 



Felspathoids v 5 



" Felspars ' 3 



Felsic minerals , 3 



" ' Mafic minerals ^ 5 



It is impossible in the limits of this paper to follow out 

 completely the further details of the classification. As a sample 

 the method of dealing with Division III. will be cited. 

 Further subdivision of III. is based on the kind of felspar 

 present. Alkali-felspars (orthoclase, microcline, anorthoclase, 

 albite) are contrasted with lime-soda felspars (oligoclase to 

 anorthite). On this basis the following divisions are made: 



. _ . Alkali-felspars , 5 

 A. Syenites . . . 7-. 3 — ri < -r 



Lime-soda felspars x 3 



Alkali-felspars , 5 



Lime-soda felspars ^ 3 5 



Alkali-felspars ^ 3 



Lime-soda felspars ' 5 



Alkali-felspars , 5 



B. Monzonites . . ,-■ ^— ^ < > , 



Lime-soda felspar 



C. Diorites and Gabbros , Alkali-felspars y ± 



