14 
ANALYSES OF EOCKS, U. S. GEOLOGICAL SURVEY. 
[BULL. 168. 
better than that which was formerly used, and an average based upon 
it may not be out of place here. 
In the bulletin now presented there are 830 complete analyses of 
rocks which are suitable for my purpose. I have also taken from the 
partial analyses given in the laboratory records 180 additional deter- 
minations of silica, 90 of lime, and 130 of alkalies. In 490 of the analyses 
there is discrimination between the water lost below 110° and that 
which is essential to the composition of the rocks; and this amounts to 
0.40 per cent. Omitting this water, the average found may fairly 
represent the composition of the older crust of the earth, as deduced 
from a mass of data which are reasonably uniform in character and 
entitled to a high degree of credence. The mean for the more impor- 
tant constituents is as follows, with the old average given in a parallel 
column for comparison: 
Si0 2 .. 
A1A- 
Fe 2 3 
FeO.. 
CaO.. 
MgO . 
K 2 0.. 
Na 2 0. 
H 2 0._ 
Ti0 2 .. 
P 2 5 . 
New mean. 
Old mean. 
59. 71 
58.59 
15.41 
15. 04 
2.63 
3.94 
3. 52 
3.48 
4.90 
5.29 
4.36 
4.49 
2.80 
2. 90 
3. 55 
3. 20 
1 . 52 
"1.96 
.60 
'.55 
.22 
.22 
99.21 
99.66 
a Including hygroscopic water ; probably 0.40 per cent. 
