00 THE GRANITES OF MAINE. 
gabbro is a very dark olive. The Hermon Hill rock is pprphyritic 
and dark green. A little less dark than these, and without any green- 
ish tinge, are the mica-quartz diorites of Beaver Lake (Calais) and 
the gabbros from Meddybemps Lake and Mingo Bailey's quarry, in 
Calais, while the quartz and mica-quartz diorites of Round Pond, 
East Sullivan, and Calais (Gardner) are all still lighter, and would 
pass for dark gray. Most of these stones take a beautiful polish, and 
all of them show very marked contrasts between the polished and cut 
surfaces. That contrast naturally is still more marked in the darker 
ones. The polished surfaces of most of these rocks show minute par- 
ticles of magnetite. Large Mocks of the Meddybemps Lake gabbro 
deflect the magnetic needle, and it is reported that the rock contains 
a very small amount of gold by assay, while platinum is reported 
from the Hermon Hill rock. 
GENERAL STRUCTURE. 
Rift. — The course of the rift at the black granite quarries is given 
in the quarry descriptions. No other Maine black granite has such a 
marked rift as that of the Meddybemps Lake gabbro. 
Sheets. — The sheet structure is not so well marked in the black 
granites as in ordinary granites, and herein lies the chief difficulty in 
quarrying them. Pis. X. //, and XI. />, show the character of the 
sheets in the Round Pond quartz diorite. PI. X. J, shows it in the 
Addison gabbro. The sheets there range from 3 to IT feet in thick- 
ness. In the Meddybemps Lake gabbro the sheets are well developed, 
being parallel to the banding and the rift, and are spaced from 1 to G 
feet.' 
Joint*. — The courses of the joints are shown in the quarry dia- 
grams. Jointing in the Addison gabbro i^ shown in PI. X, A, and in 
the Round Pond diorite its relation to a diabase dike is shown in 
PL XI, A. Generally the spacing of the joints in the black granites 
is small, which prevents the quarrying of blocks of very large 
dimensions. 
VARIATIONS IN THE ROCK. 
Banding. — The gabbro of Meddybemps Lake is traversed, in at 
least its upper part, by light-gray bands that range in thickness from 
one-fourth inch to 2 inches. Their lighter shade is the result of a 
greater proportion of feldspar. These bands dip at an angle of 15°, 
and run parallel to both sheet and rift structure. PI. X. A , shows a 
similar but less pronounced banding in the Addison gabbro. 
This banding represents not only the flow of the eruptive, but also 
different segregations of the principal minerals of the rock, alternat- 
ing with one another. This structure resembles that observed in 
