48 
ANALYSES OF EOCKS, U. S. GEOLOGICAL SURVEY. 
[BULL. 168. 
L. Typical gneiss of Washington, D. C, from quarry of Potomac 
Stone Company, 1 mile below Chain Bridge. A basic granite. 
M. Fine-grained, fissile, chloritic gneiss, from northwest of Cabin 
John Bridge. Also a basic granite. 
N. Fine-grained, hard gneiss, from the second lock at the Great Falls 
of the Potomac. Probably of sedimentary origin. 
Described by Williams. Analyses by Hillebrand, record No. 1459. 
Si0 2 
A1 2 3 
Fe 2 3 
FeO 
MgO 
CaO 
Na 2 
K 2 
H 2 OatllO°--- 
H 2 above 110 
Ti0 2 
PA 
MnO 
SrO 
BaO 
Li 2 
67.22 
15.34 
2.78 
3.41 
1.65 
1.36 
2.00 
3.26 
.29 
1.68 
.84 
.14 
.13 
trace 
.04 
trace 
100. 14 
M. 
63.43 
16.69 
' 3.36 
3.87 
2.33 
.80 
2.38 
3.22 
.23 
2.67 
.91 
.11 
.09 
trace 
.03 
trace 
100. 12 
78.28 
9.96 
1.85 
1.78 
.95 
1.68 
2.73 
1.35 
.12 
.83 
.70 
.11 
.08 
trace 
.02 
trace 
100. 44 
