50 Tlie American Geologist. juiy, 1892 
away for foundation material so that the dikes in the face of the 
quarry are easil}' accessible, but are visible nowhere else in the 
immediate vicinity, being covered by drift sands and clays. 
These dikes, of which there are nine exposed within a distance 
of 250 to 300 yards vary from fifteen inches to five feet in diam- 
eter, and have apparently a nearly east and west trend, though as 
they are here almost vertical and exposed only in the face of the 
quarry, this could not be told with certainty at the time of my 
visit. The rocks of the dikes are, as a rule, very fine grained, 
compact and nearly black in color, containing only olivines, an 
occasional augite, or rarely a feldspar in macroscopic forms. The 
olivines are in some of the dikes peculiarly large and abundant, 
comprising at times fully one-fourth the entire mass of the rock. 
In several instances on a surface five cm. square not less than 
fifty olivines could be counted, varying in size from a pin's head 
to five and even ten and fifteen mm. in diameter, all fresh and 
glassy, or with but a narrow well defined greenish white border of 
serpentine. Such large and fresh olivines are very rare except- 
ing in rocks of Tertiary or Post Tertiar}' age. The dikes are 
often beautifully banded parallel to their sides, the bands varying 
from two to five mm. in width and of light purplish gray and 
nearly black colors alternating. This feature is especially notice- 
able in the smaller and more compact dikes. On weathering the 
rock assumes a schistose structure cleaving parallel with the 
bandings, l. e. with the walls of the dikes, into scales of all thick- 
nesses up to several inches, and often some feet in diameter. 
Submitted to microscopic examination the rocks from the vari- 
ous dikes present close textural and mineralogical resemblances, 
which may l)e generalized as follows: — A groundmass of stout 
acicular idiomorphic l)rown basaltic hornblendes, augites with a 
faint violet brown or purple tinge, elongated plagioclases and the 
usual scattering of iron ores, carrying abundant phenocrysts of 
olivine, more rarely of augite and still more rarel}- of plagioclase. 
A glassy base, though not identified beyond a reasonable doul)t,^ 
is strongly suggested by almost amorphous interstitial areas filled 
with chloritic needles and ferruginous decomposition products. 
The olivines in some of the dikes are highly altered, but show 
beautifully perfect crystal outlines. In others, as above noted, 
they are almost as fresh and glassy as on the day of their erup- 
tion. The brown hornblendes occur throughout onlv in the 
