G. W. Hawes—Trap Rocks of the Connecticut Valley. 189 
zontal, while the columns of the main ridge are nearly vertical, 
which shows that the main ridge must have cooled sufficiently 
to determine the position of the columns of the buttress, since 
the columns are always at right angles to the cooling surfaces. 
This dike is characterized by large grains (} to 4 inch across) of 
a clear whitish cleavable mineral, which render it sparsely por- 
phyritic. Even at the extremity, where the dike becomes very 
small, and where in consequence the rock is very fine in texture 
owing to the more rapid cooling, this porphyritic character is 
retained. A sufficient quantity of the pure material was ex- 
tracted for an analysis; its composition was 
ANORTHITE. = 
fe eS Sate 4 45°95 24°50 
Miiibe ook as 34°70 : 
Ferrous oxide "64 16°31 
te eS eee 15°82 
Magnesia trace 
Potash? 4.05 45 O06 
OUR: So ta cee 1°80 
ignition 1 iu. ues 96 
100°32 
It is therefore a lime feldspar; and the oxygen ratio for RO, 
R,O, and SiO, is 1:3-2:4:8, which shows the feldspar to be 
anorthite. Mr. Dana informs me that under the microscope it 
has a different appearance from the other feldspar, and although 
triclinic it shows less tendency toward twinning. An analysis 
of a sample of the trap of the dike in which the anorthite occurs 
shows it to be a dolerite of the same composition as the others ; 
which is proof of the uniformity of the ejections at different 
times in this period. 
DOLERITE FROM WINTERGREEN LAKE. Sp. gr. = 3:00. 
L 
Sifies CM ee 52°38 §2°45 52°42 
Alumina 22.502 j4°59 14°50 14°54 
Ferrous oxide... 9°89 9°7 9°84 
Ferric oxide __- 327 1°22 1°25 
Manganous oxide ‘50 53 ‘51 
We 3 a 10°63 10°54 10°59 
Magnesia_-__.--- 7°36 7°31 7°33 
OULD oie oe 2°20 2°37 2°23 
task = oc ey i | "48 49 
Ignition -.. . 0: "BB “BB 55 
99°88 99°64 99°75 
The rock is therefore a mixture of pyroxene and labradorite, 
from which the anorthite crystallized out on account of its dif. 
