262 SCIENCE PROGRESS 



But they were not sufficiently consistent to carry out their 

 classification on chemical lines. 



" While the chemical composition of igneous rocks is their 

 most fundamental characteristic, it is known that there is an 

 absence of stoichiometric proportions among the chemical 

 elements or components" and "an absence of chemical division 

 lines or of groups or clusters of similar combinations of 

 elements " ; this, and the fact that " all noncrystalline, and 

 many of the partially crystalline rocks, derive their most obvious 

 characters from the mineral particles composing them," led 

 to the conclusion that " the systematic classification should be 

 constructed, as far as may be, by the use of mineralogical data 

 in one form or other," and it was finally decided " to treat 

 the chemical composition of rocks in terms of minerals, and to 

 make the basis of primary subdivisions chemico-mineralogical." 



As, however, magmas having the same chemical composition 

 may crystallise out in different mineral aggregates, or may 

 consolidate as a glass, they devised a method of calculating 

 from the chemical analysis of a rock, a standard mineral 

 composition called the " norm," which might in some cases 

 practically coincide with the actual mineral composition or 

 " mode," but, as a rule, differed more or less widely from it. 

 Finally, from the minerals of this ideal rock, and the propor- 

 tions in which they occurred, the position of the rock in the 

 quantitative classification was determined. 



Outlines of the Quantitative Classification 

 (i) The Chemical Composition 



The starting point of the new quantitative classification is 

 the chemical composition of the rock, which may be ascertained 

 either from a chemical analysis or calculated from a quantitative 

 estimation of the minerals present. The latter operation is 

 best carried out by the linear traversing method of Rosiwal, 

 which gives the volumetric proportions. 1 By multiplying these 

 by the specific gravities we get the gravimetric proportions, 

 and thence, with a knowledge of the chemical composition of 

 the different minerals, that of the rock can be obtained. Those 



1 See Rosiwal, Verh. Wien Geo/. Reic/is-Ajtst., vol. xxxii. (1898) p. 143 ; 

 Quant. Class., supra at., 1903 ed., p. 204 ; Joseph P. Iddings, Journal of Geology, 

 vol. xii. (1904) p. 225 ; Ira A. Williams, American Geologist, vol. xxxv. (1905) p. 34. 



