CLARKE.} 
IGNEOUS AND OP.YSTALLINE BOOKS. 
57 
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. Alsbachose. Probably of sedimentary origin. 
Described by Williams. Analyses by Hillebrand, record No. 1459. 
Si0 2 
AlA 
Fe 2 3 
FeO 
MgO 
CaO 
Na 2 
K 2 0.---'- 
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 
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 
X. 
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 
