180 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



But the values of x and y are found by the relations 



36-18 = 0-667 X x 

 13-65 = 0-338 X y 



:} 



(4). 



The left-hand numbers being the per-centages of peroxides and 

 protoxides respectively ; and the right-hand numerical coefficients 

 denoting the atomic quotients of peroxides and protoxides respect- 

 ively ; from these equations we find the values of x and y to be 54 

 and 40. And finally 



Atomic weight of mica = 304. 



Assuming this atomic weight for the grey mica, we find from 

 equation (2) the quartz and mica per-centages directly, and the per- 

 centage of feldspar from equation (3) by diiference. 



HeHce we obtain : 



Mineralogical Analysis of Granite. 



Mica 13-37 



Feldspar 61-18 



Quartz 24-98 



99-53 



Having ascertained the proportions of quartz, margarodite, mica, 

 and feldspar which form this granite, we can ascertain with precision 

 the composition of the feldspar of the granite, and at the same time 

 verify the entire calculation as follows. Adopting the mineralogical 

 analysis of the granite already found, and the average composition of 

 the mica, we can form the following Table, which assigns to each 

 mineral its own share of each constituent of the granite ; the feld- 

 spar column being found by difference. 



Table V. 



Silica 



Alumina ... 

 Peroxide of iron 



Lime 



Magnesia . 

 Potash .... 



Soda 



Loss by ignition 



Totals .... 



Quartz. 



Mica. 



Feldspar. 



Granite. 



24-98 



5-93 



41-18 



7209 



— 



4-14 



10-32 



14-46 



— 



0-70 



1-70 



2-40 



— 



0-16 



1-60 



1-76 



— 



0-10 



— 



010 



— 



1-47 



3-33 



4-80 



— 



0-12 



2-89 



301 



— 



0-75 



016 



091 



24-98 



13-37 



61-18 



99-53 



If the preceding calculations be correct in theory, the feldspar 

 column must give us a true feldspar rich in silica ; which is verified 

 as follows. Raising the proportions of the feldspar column to per- 

 centages, we find — 



