Weathering of Diabase ifi Virgi7iia.—]Vatsoii. 
89 
Table No. 3. 
Analysis of Feldspar. 
CONSTITUENTS. 
I 
II 
III 
IV RATIO OF II 
Silica SiOg 
49-65 
24-49 
5.98 
11.02 
*4-55 
55-345 
27.299 
12.284 
5.072 
28.60 
10.64 
5-55 
51.40 
30.98 
0.22 
13.40 
0.45 
2.85 
0.39 
.922 
.267 
.219 
.081 
Alumina AlgOg 
IronFegOg 
Lime CaO 
Magnesia MgO 
SodaNaoO 
Potash K,0 
Total 100.00 
Specific gravity 2.697 
100.00 
2.697 
99.99 
2.695. 
99-69 
R2O: RO: R0O3: Si03=i:2:3:ii. 
I. Olivine diabase from near Chatham, Virginia. 
II. No. I calculated to an iron and magnesia free basis. 
III. Theoretical calulation for a feldspar corresponding to Ab^, An,,. 
W . Analysis of feldspar in hypersthene diabase from Culpepper 
county, Virginia. See Bull. G. S. A., 1891, 2, p. 343. (Campbell & 
Brown.) 
Feldspar. A chemical analysis of the feldspar which was 
separated from the other minerals in the same way as the 
augite, indicates a basic plagioclase which is shown to cor- 
respond to labradorite, whose albite-anorthite ratio isAbf^An^ 
(See 3 in table). The optical properties are in accord with and 
confirm the chemical analysis. Extinction angle measured 
on M=oo Poo (010) gives 22°. Inclusions of magnetite and 
some augite are common in the feldspar. A quantitative 
separation of this mineral from the others shows that the entire 
rock is made up of 32.10 per cent of feldspar. 
The Olivine occurs in large crystals with fairly distinct 
crj'stal outline. Abimdant inclusions of magnetite are coiu- 
mon. This mineral has stiffered in some cases considerable 
alteration into serpentine and chlorite. 
No. 2 (See bulk analysis, page 87) represents the first stage 
in weathering of i, and contains the same minerals with a 
further development of secondary products. The olivines in- 
dicate a somewhat advanced stage of decay, shown by the iron 
sesqui-oxide covering to a more or less extent the crystals, 
*Calculated bv difference. 
