CLARKE.] 
AVERAGE COMPOSITION OF ROCKS. 
15 
British rocks, and recently Washington a has worked out the average 
of 1,811 analyses given in his compilation. 
For the more important constituents of igneous rocks the five esti- 
mates mentioned above agree remarkably well, and yet they are not 
thoroughly comparable. The 397 analyses discussed by Harker were 
in most cases incomplete, at least when considered from a modern 
standpoint. In only 3i of them was titanium taken into account, and 
in only 55 is phosphorus mentioned. These omissions affect the per- 
centages of other things and lessen the value of the computation very 
materially. Excluding manganese the five averages may be tabulated 
as follows. Minor constituents will be considered later: 
Si0 2 
A1A 
Fe 2 3 
FeO 
MgO 
CaO 
Na 2 
K 2 
H 2 0at 100° 
H 2 above 100° 
Ti0 2 
PA 
Clarke. 
First, 
Second. 
Third. 
58.59 
59.77 
59. 71 
15.04 
15.38 
15. 41 
3.94 
2.65 
2.63 
3.48 
3.44 
3.52 
4.49 
4.40 
4.36 
5.29 
4.81 
4.90 
3.20 
3.61 
3. 55 
2.90 
| 1.96 
2.83 
2.80 
1.51 
1 . 52 
.55 
.53 
. 60 
.22 
.21 
.22 
99.66 
99.14 
99. 22 
Harker. 
58.75 
15.64 
5.34 
2.40 
4.09 
4.98 
3.25 
2.74 
2.23 
.12 
.02 
99. 56 
Washing- 
ton. 
58. 239 
15. 796 
3.334 
3.874 
3.843 
5. 221 
3.912 
3. 161 
.363 
1.428 
1.039 
.373 
100. 583 
Professional Paper U. S. Geol. Survey No. 14, 1903, p. 106. 
