576 CROSS, IDDINGS, PIRSSON, WASHINGTON 



well-known chemico-mineralogical relations affecting the salic 

 and femic minerals, both as regards the proportions of their 

 chemical constituents, the relative affinities of the bases for 

 silica, and the frequent associations of certain of the rock-making 

 minerals. 



The method has been developed by considering, first, the 

 chemical composition of igneous rocks, and by devising a plan 

 for the calculation of standard minerals from it. Then, the 

 actual mineral composition of noncrystalline rocks has been 

 taken into account, and a plan devised, after reckoning the pro- 

 portions of these minerals, for estimating their approximate 

 chemical composition and from these data calculating the stand- 

 ard mineral composition of the rock. 



It is evident that rocks that are not holocrystalline, or those 

 in which the proportions of the actual minerals cannot be 

 determined, must be classified in the first instance by means of 

 chemical analysis. Subsequently similar rocks may be classified 

 with greater or less precision by comparison with rocks having 

 similar textures and the same actual mineral compositions, 

 which have been analyzed chemically. 



In comparing the relative quantities of different minerals in 

 rocks, either their mass or their volume may be made the basis 

 of comparison. In calculating the mineral composition from 

 the chemical composition of the rock, the mass is the natural 

 unit of comparison. The same is true if mechanical separation 

 of the mineral constituents is undertaken. When a comparison 

 is made by the eye, megascopically or microscopically, the basis 

 of comparison is volume. This may be transformed into mass 

 by multiplying the volume by the specific gravity of the mineral. 

 The optical methods of estimation being less exact than those 

 first mentioned, the basis of comparison should be mass. 



CONSTRUCTION OF THE SYSTEM. 



It is proposed to arrange igneous rocks by a system which 

 shall express their quantitative chemical and mineral constitu- 

 tions. With the known range and degree of variation in these 



