CLARKE.] 
AVERAGE COMPOSITION OF ROCKS. 
19 
Si0 2 .. 
A1 2 3 - 
Fe 2 3 
FeO.. 
MgO. 
CaO.. 
Na 2 0. 
K 2 0.. 
H 2 0- 
H 2 + 
Ti0 2 . 
Zr0 2 . 
co 2 .. 
P 2 5 - 
s .... 
Cl-.-- 
F .... 
BaO.. 
SrO .. 
MnO. 
NiO.. 
Cr 2 3 . 
v 2 o 3 . 
Li„0 . 
Found. 
Reduced to 
100. 
60. 91 
59. 87 
15. 28 
15.02 
2.63+ 
2. 58+ 
3.46+ 
3. 40+ 
4.13 
4.06 
4.88 
4.79 
3. 45 
3.39 
2.98 
.41 
2. 93 
% .40 
1.49 
1.46 
.73 
.72 
.03- 
.03- 
.53- 
.52 
.26 
.26 
.11 
.11 
.07 
.07 
. 02+ 
.02+ 
.11 
.11 
.04 
.04 
.10 
.10 
.03— 
.03— 
.05- 
.05- 
.03- 
.03- 
.01 
.01 
101. 74 
100. 00 
In elementary form. 
In the elementary column the iron reported: in iron pyrites is 
included, but hygroscopic water is thrown out. The elements not 
included in the calculation represent minor corrections, to be applied 
whenever the necessity for doing so may arise. For estimates of 
their probable amounts Vogt's papers may be consulted^ The per- 
centages assigned to C, Zr, CI, F, Ni, Or, and V are nothing more 
than very rough approximations. 
By a similar statistical process 1 have tried to ascertain something 
with regard to the relative abundance of the more important rock- 
forming minerals. Nearly 700 analyses of igneous rocks were studied, 
and the foregoing table of averages was also taken into account. 
For apatite, and for the titanium minerals titanite, ilmenite, and 
aZeitschr. prakt. Geologie, 1898, pp. 225, 314, 377, 413, and 1899, pp. 10, 274. 
