234 
“« Let the per-centages of alumina and peroxide of iron be 
divided by the atomic weights of alumina and peroxide of iron 
respectively, and let the sum of the quotients so found be de- 
noted by 8. 
“* Let the per-centages of lime, magnesia, potash, and soda, 
be divided by the atomic weights of these elements, and the 
sum of the quotients called c. 
“< Then, on the hypothesis that the granite is composed ex- 
clusively of quartz, feldspar, and mica (margarodite), since 
Quartz = $1 0,, 
Feldspar = RO, SiO, + R, O,, 3 Si O,, 
Margarodite = RO, SiO; + 2(R, O;, SiO,) + 2HO; 
we find, if Q, , M denote the number of atoms of quartz, 
feldspar, and margarodite present in the granite, the following 
relations, 
a= Q+4F:3M, 
b=F'+2M, (1) 
c=FiM. 
In these equations, a, b, c are given by the analysis, and from 
them Q, F, M may be found. 
‘* Having determined Q, F, M, we can obtain the per-cen- 
tages corresponding to them, by multiplying Q, /, M by their 
respective atomic weights. ‘The atomic weight of quartz is 
known, and is 46; but the atomic weights of feldspar and 
mica vary with the relative proportions of the ingredients 
composing these minerals. Assuming the average of the ana- 
lyses of micas from this granite range, already given by me 
(Proceedings of Royal Irish Academy, vol. vi. part ii.), it is 
easy to infer from it an atomic weight of mica equal to 305. 
This atomic weight of mica has been used by me in the calcu- 
lations made in this Paper, and the per-centages of feldspar 
found by difference. 
«The calculations just mentioned do not prove that the 
granites to which they are applied are composed of quartz, 
