Weathering of Diabase i?i Virgi/iia. — Watso?i. 
357 
Micrographic intergrowths of feldspar and quartz are rather 
common in the thin section. 
Optical properties indicate the presence of two feldspars, 
the principal one of which gave an average extinction of 27° 
with M= Oc Poo (010), corresponding to AbiAn.j. A still more 
basic feldspar than this is indicated, corresponding to about 
Ab. Ani=.' 
In 1 88 1, Dr. Geo. Hawes* published on the mineralogical 
composition of the Atlantic border Mesozoic diabase, and 
pointed out the fact that the feldspathic element in the normal 
type of this rock does not consist of any single feldspar. The 
following table of analyses indicates the kind of feldspar oc- 
curring in this type of rock in various localities: 
I II III 
Sp. CJr. over 2.6y Sj). Gr. under 2.6q 
SiO. 
.-V1203 
Fe^Oa 
MnO 
CaO 
-MgO 
K.O 
Na=0 
Total 
52.84 
880 
60.54 
1. 010 
45-95 
28.62 
278 
24.11 
■234 
34.70 
1-52 
009 
210 
1. 14 
9-15 
.007 
.163 
0.64 
II.8I 
15-82 
0.46 
010 
0.27 
.107 
1.80 
0.86 
009 
1.06 
.011 
/ 
2.38 
038 
4.11 
.066 
^ U.45 
1.06 
0.59 
0.96 
99-55 
100.97 
100.32 
IV 
V 
VI 
VII 
SiO. 
57.820 .973 55-345 -922 51.40 .922 SI. 18 
AUOi 28.160 .276 27.299 .267 30.98 .267 27.00 
Fe.Os 
MnO 
CaO 7-720 
MgO 
Na=0 5-352 
K.O 
H2O loss .' 0.720 
.139 12.284 -219 
.087 5.072 .o8r 
13-40 
0.45 
2.85 / 
0.39 ^ 
219 
0.81 
3-19 
0.17 
11.70 
1.92 
0.41 
3-48 
1. 19 
Total 99.772 
99-69 
100.24 
*Ha\ves. G. W., on The Mineralogical Composition of the Normal 
Mesozoic Diabase upon the Atlantic Border. Proc. U. S. Nat'l Mu- 
seum. 1881, 129-134. 
