464 Professor S. J. Sltand — A System of Petrography. 



In the classification adopted by Iddings 1 there are also five 

 primary groups, characterized respectively by (1) quartz in excess, 

 (2) quartz and felspar, (3) felspar with little or no quartz, (4) felspar 

 and felspathoid, (5) felspatboid without felspar. Within each of 

 these groups Iddings employs a cross-classification in which the 

 colour ratio (expressed simply by the terms leucocratic and melano- 

 cratic) is put against the ratio of alkali- to lime-bearing felspars. 

 The next subdivision is based on crystallinity, only two groups, 

 phanerocrystalline and aphanitic, being recognized ; and further 

 splitting is effected on grounds of texture, or preponderance of 

 a particular mineral, or for other trivial reasons. 



Now I am with Holmes in adopting the degree of saturation as 

 the basis of classification ; with Iddings in desiring to give increased 

 prominence to the colour ratio; with Iddings also in preferring 

 a twofold to a threefold division with regard to crystallinity ; and 

 with both of them in recognizing the necessity of yielding to the 

 old-established practice of distinguishing potassic from sodic and 

 calcic rocks on the basis of the felspars which they contain. I differ 

 from both Holmes and Iddings regarding the treatment of felspathoids 

 and in certain other minor matters. 



I propose to apply the various taxonomic factors in the following 

 order : — 



1. Degree of saturation, giving five divisions. 



2. The double ratio of Or-Ab-An, giving about eight families 

 within each division. 



3. The colour ratio, 2 giving from two to ten, but preferably four 

 groups in each family. 



4. Crystallinity, giving two sub-groups within each group. 



5. Ratios of specific minerals or groups of minerals, giving the 

 types to which " specific " names will be attached. 



6. Trivial characters of mineralogy and texture, giving varieties. 

 I shall discuss the application of these factors very briefly. 



1. Regarding the employment of the degree of saturation as the 

 basis of the system I have nothing to add to what I have already 

 written. It furnishes five sharply distinguished divisions, as 

 follows : — 



(1) Oversaturated rocks [0]. 



(2) Saturated rocks [S]. 



(3) Undersaturated rocks. 



(a) Dyad and triad metals unsaturated [u]. 



(b) Monad metals unsaturated [U]. 



(c) Both monads and dyads unsaturated [W]. 



2. The distinction of potassic from sodipotassic and sodic rocks, 

 judged by the ratio of orthoclase to albite, is deeply ingrained in 

 petrography. Iddings has tried to make the distinction quantitative 

 by introducing the limiting ratios of 5 : 3 and 3 : 5, or 37*5 per cent 



1 Igneous Rocks, vol. i, 1910. 



2 Journ. Geol., 1916, p. 400. 



