350 
MANGANE^ DEPOSITS OF UTDIA : GEOLOGY, [PaRT II : 
all the other oxides must be present in the protoxide portion of the garnet, 
for which there is not enough manganese protoxide (MnO) . For this 
reason it is necessary to regard the larger portion of the iron as forming 
a part of the protoxide portion of the garnet, although the iron is mostly 
returned as being in the form of ferric oxide (Fe203). The other alter- 
native (II) is to regard the alkalies as present in the form of orthoclase and 
albite, although neither felspar is visible under the microscope. For 
it does not follow that there is not present in the rock the very small 
proportion of felspar to which the alkalies present correspond ; for such a 
small quantity of felspar might easily be overlooked, especially if not 
twinned. After setting aside the amounts of alumina and sihca required 
for the felspar, the remaining constituents can be combined as garnet in 
the same way as in the former case ; but the analysis does not then 
work out so well, because there is then a larger amoimt of ferric oxide 
and alumina left over unaccounted for. It would of course be possible 
to calculate a portion of this residue as a further amoimt of garnet by 
taking a little of the sUica that has been entered up in the analysis as 
free silica. The following shows the mineralogical composition of the 
rock according to these two methods of calculation : — 
I 
II 
Rutile . 
. 0-46 
RutOe 
Apatite , 
. 1-58 
Apatite , 
• ■ 
Spessartite : — 
Spessartite : — 
MnO 
13-37 
5InO . 
. 13-37 
FeO 
5-43 
FeO 
. 3-97 
CaO 
1-70 
CaO 
1-70 
MgO 
1-90 
MgO . 
1-90 
AI0O3 
11-89 
AI2O3 . 
, 10-91 
Si02 
20-60 
SiOa . 
. 19-38 
54-89 
54-89 
51-23 
FezOs . . . . 1-58 
AI2O3 .... 0-47 
Surplus oxygen . . . 0-59 
K2O 0-03 
Xa20 . . . . 0-19 
Quartz . . . .39-85 
CuO .... 0-02 
Combined water . . 0-20 
Moisture at 100°C. . . 0-10 
99-96 
FeoOa 3-20 
AI2O3 1-11 
Surplus oxygen . .0-43 
Orthoclase . . . . 0-17 
Albite 1-61 
Quartz 39-85 
CuO 0-02 
Combined water . . . 0-20 
Moisture at 100<=C. . . .0-10 
99-96 
Taking the mineralogical composition of the rock as expressed in 
column I, and knowing the specific gravities of quartz and of the rock to 
