186 Rev. SAMUEL HAUGHTON, M.D., 
the following simultaneous equations, to determine the per-cent- 
ages of the constituent minerals in the average granite :— 
Let Q = the per-centage of Quartz. 
Oiveg Orthoclase. 
PA thes - Albite. 
Wane, ree White Mica. 
DS tas _ Black Mica. 
(1) Silica, . . - 7207 =100 Q464:95 0464:70 A+44:58 W+35°55 B. 
42) Alumina, % . 1481 = —~ 1831 0+21°80 A+32:138 W+17-08 B. 
(3) Iron peroxide, . ; B20 ee ee — — 4:57 W+ 23-7 B. 
G) Lim, . 9. 6 1638 = — 0250 — .078W+ O6LB. 
(5) Magnesia, . , 53 8 08S 0. ae ae OTE W ese 
(6) Potash, . ; a 5ll= — 12:23 O+ 2:84 A+1067 W+ 9°45 B. 
(7) Soda, . , «219 = Ss 2-78 04-987 A+ 0-95 W+ 0-35 B. 
(8) Loss by ignition, < 109 = — = 534 W+ 4:30 B. 
If we select the four equations containing the largest per-cent- 
ages, viz—The alumina, potash, soda, and iron peroxide equations, 
we find after several reductions— 
A=180 + 0-156 W + 0-191 B. 
O = 37°65 — 0:909 W — 0°819 B. 
These equations show the manner in. which the two feldspars 
are related to the two micas. 
We find finally— 
>= ool percent, 
W=1916  ,, 
O =o 4ieee 
Aes 5 BOL) Xf 
Inserting these values into the last seven equations, we ob- 
tain— 


—_— Observed. Calculated. | Diff, 
Alumina, : : | 1481 1476-06 +4:04 
Tron peroxide, . . | 225 224-99 +0-01 
Lime, . “ . 163 22°28 +140-72 
Magnesia, “ ; st. OO 41°27 —8:27 
Potash; ss erase ede TL 51145 —0°45 
Soda, . ° . ; 279 280-64 —1°64 
Loss by ignition, | 109 126°89 —17°89 
The agreement between calculation and observation is as close 
as could be expected ; and the errors in the magnesia and loss by 
ignition, are, doubtless, errors of observation, due to the small 
inagnitudes to be ascertained. The excess in the limeis real, and 
must be accounted for by the existence of a small quantity of 
paste, in the form of a silicate of lime, 
