176 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 
description of any of these, since what has been said in regard to the 
granites will apply equally well to the gneisses. At present they seem 
to be quarried only to a limited extent and in but few localities. At 
Turner a biotitic gneiss of medium texture is quarried, in which a few 
grains of sphene were noticed. At Jefferson a fine gray gneiss is quar- 
ried which contains both biotite and muscovite, the latter occurring fre- 
quently as slender prisms inclosed in the orthoclase. Epidote is also 
present in irregular grains as well as a little apatite. 
At Chesterville a fine gray gneiss is quarried, which somewhat closely 
resembles the Jefferson stone both in appearance and mineral composi- 
tion. 
On account of its laminated structure gneiss splits readily from the 
quarry in slabs of any desirable thickness. Its distinct grain, however, 
causes it to work unevenly in different directions, and it is not so well 
suited for purposes of general construction as granite. It is best suited 
for curbing and the rough work of foundations. 
DIABASE. 
Under the name of black granite, diabase is quarried at three locali- 
ties in the State—Indian River, in the town of Addison, Addison Point, 
and Vinalhaven. 
The Indian River rock is nearly black in color and of a texture too 
fine to allow an accurate determination of its true character by the 
unaided eye. Under the microscope it is found to consist essentially 
of plagioclase and augite, although considerable hornblende and mica 
are present. The hornblende, without doubt, results from the alter- 
ation of the augite, since abundant examples are to be seen in which 
the centers of crystals are nearly colorless, unaltered augite, while 
the borders are plainly hornblendic. The mica is sufficiently abundant 
to be recognizable to the naked eye, and is of a smoky-yellow, or at 
times copper-red, color, in thin sections. Apatite is abundant in the 
usual colorless prisms. 
The Addison Point rock closely resembles that of Indian River in 
external appearance, but shows a marked difference in structure un- 
der the microscope. Olivine is present in abundant, rounded, colorless 
grains, traversed by numerous irregular curvilinear lines, and fre- 
quently much altered into serpentine. Hornblende and chlorite are 
both present as in the Addison Point rock, and-evidently derived from 
the augite. Titanic iron isabundantand often much altered, taking on 
fan-shaped and other fantastic forms. 
An olivine-bearing diabase is also quarried at Vinalhaven. It is a 
hard, compact rock, quite fresh-appearing in thin sections, although con- 
taining a little of both hornblende and chlorite as alteration products. 
Some mica is also present. rl 
Magnetite is so abundant in all these diabases as to be very notice- 
able as black, lusterless grains on a polished surface of the rock. 
