178 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



Mineralogical Composition of the Granites. 



Various methods have been suggested for ascertaining the relative 

 proportions of simple minerals, entering into the composition of com- 

 pound rocks, such as granites. In Table III. I have given the 

 ultimate chemical analysis of the rock ; in this section I propose to 

 determine the physical or mineralogical analysis of the granites under 

 consideration. 



Of the different methods in which the per-centage of elementary 

 minerals in a given rock may be found, the following appears to be 

 more simple and accurate than the methods hitherto in use, which 

 are based either on specific gravities or on measurements of surfaces 

 of crystals in polished specimens of the rock. 



Let the rock be assumed to be composed of a number of minerals 

 whose mineralogical formula is known, and also their per-centage 

 composition, and let the atoms of silica, peroxides, and protoxides in 

 the rock be ascertained from the ultimate analysis of the rock, and 

 denoted by a, /3, -y ; it is possible thus to obtain three equations 

 from which the per-centage of elementary minerals, supposed to be 

 not greater in number than three, may be ascertained. 



If the elementary minerals exceed three in number, this method 

 will fail to give a determinate solution, and will lead to an equation 

 of condition, from which certain properties of the rock may be 

 deduced, although its exact composition cannot be ascertained. 



This reasoning will be made quite clear by its application to the 

 granites under discussion. 



Dividing the numbers in Table III. by their respective atomic 

 weights, and adding together the isomorphous protoxides and per- 

 oxides, I find 



Table IV. — Atoms of Granitic Constituents. 





1. 



2. 



3. 



4. 



5. 



Silica 



1-530 

 0-286 

 0-353 



1-587 



0-296 

 0-274 



1-528 

 0-352 



0-287 



1-529 

 0-353 

 0-256 



1-613 



0-282 

 0-226 



Peroxides 



Protoxides 





1 





6. 



7. 



8. 



9. 



Average. 



Silica 



1-540 

 0-317 



0-282 



1-592 

 0-321 

 0-232 



1-587 

 0-322 

 0-238 



1-593 

 0-271 

 0-257 



1-567 

 0-311 

 0-267 



Peroxides 



Protoxides 





Assuming the granite to be a ternary compound of quartz, marga- 

 rodite-mica, and a feldspar rich in silica, we know that the minera- 

 logical formulse of these minerals are 



Quartz = SiOg 



Margarodite = RO, SiOg -H 2{R20 



Feldspar = RO, SiOg -h 



SiO. 



3, ^.^,\ + 2H0 

 R,03, 3 SiOg. 



