124 A. Holmes — Classification of Igneous Rocks. 



divisions together as " leucocratic " gives too broad a meaning to the 

 term, for as a rule only perfelsic rocks could accurately — with due 

 respect to their prevalent colour — be described as leucocratic. 



(b) A. much more valuable basis for a prknary subdivision of rocks 

 is that recently proposed by Shand (1913, p. 313). In part it 

 corresponds to the orders of the C.I.P.W. Classification and to the 

 vertical divisions of the mineral classification tabulated by Iddings 

 (vol. i, pp. 348-9), but it goes a step further since it contrasts quartz- 

 bearing rocks not only with felspathoid-hearing rocks, but also with 

 rocks characterized by the presence of olivine and of other unsaturated 

 minerals. Shand's contribution to "the framework of a classifica- 

 tion" (1915, p. 340), is as follows : — 



I. Oversaturated Eocks, which contain free silica of magmatic origin. 

 II. Saturated Eocks, which contain only saturated minerals (i.e. saturated 



with regard to silica). 

 III. Undersaturated Eocks, which contain unsaturated minerals : 



(a) Dyad metals undersaturated, e.g. oZwme-bearing rocks. 



(b) Monad metals undersaturated, e.g. felspathoid-bea,vmg rocks. 



(c) Monad and dyad metals undersaturated, e.g. felspathoid and olivine 



bearing rocks. 



In the classification advocated in this paper Shand's five groups 

 are adopted as those of first importance. The principle of saturation 

 is one far superior in its significance to the old conception of igneous 

 rocks as acid, intermediate, basic, and ultrabasic. If the term acid 

 means anything at all, as applied to rocks, it suggests the presence of 

 free silica, and quartz-diorites and gabbros should, according to this 

 implication, be classed as "acid rocks". On the other hand, if the 

 term basic were logically applied, it would include nepheline-syenites 

 as well as olivine- gabbros. 



(c) The value of salic orfelsic minerals in classification is indicated 

 by the fact that in the norm of Clarke's average igneous rock, the 

 salic minerals amount to 79 per cent of the total (Cross, 1912, 

 p. 759 ; Tyrrell, p. 69). Moreover, the felspars alone constitute 

 67 per cent of the total norm. Clarke has also given a modal 

 average of igneous rocks, and according to his figures the felspars 

 constitute practically 60 per cent. The felspars are therefore well- 

 adapted for a further division of rocks within each of the five groups 

 separated according to the saturation principle. Since there are 

 three felspar molecules, orthoclase, albite, and anorthite, with albite 

 as a common associate of each of the others, the ratios of albite to 

 anorthite and of orthoclase to albite afford excellent co-ordinates for 

 a cross classification. The writer considers that it is of doubtful 

 value to use the ratio of orthoclase plus albite to anorthite, as is 

 suggested by Tyrrell (p. 79); or that of alkali felspars to soda- 

 lime felspars, as is done by Iddings {Igneous Rocks, vol. ii). In 

 the former ratio the kind of soda-lime felspar does not appear in the 

 classification (as in the case of the present method of calculating the 

 rang), and in the latter ratio the terms compared are not independent. 



It is proposed in this paper to divide felspathic igneous rocks 

 according to the kind of soda-lime felspar present — the kind being 

 conveniently expressed by the percentage of anorthite. At least 



