Dikes in the Vichiity ofJoJins Bay, Maine. — Basconi. 279 
breadths, and as phenocrysts broadly rectangular or lath- 
shaped and sometimes showing zonal growth. Both genera- 
tions are remarkably clear and fresh. Albite twinning is in- 
variable, Carlsbad twinning is common and Pericline twinning 
somewhat rare. The alteration product, where there is altera- 
tion, appears as brightly polarizing scales of what seems to be 
kaolin. 
By the statistical method, which was made use of in the in- 
vestigation of the feldspar in every section — the feldspar was 
determined to be labradorite of about the composition Abs An^. 
Where extinction on the face 010 could also be determined, it 
varied from 23° to 2i7° • 
Magnetite is present in about the usual amount and is al- 
lotriomorphic with reference to the labradorite. Apatite is very 
rare. 
The structure of the dike is very uniform but the grain of 
crystallization is coarser in the centre of the mass than it is on 
the selvage, where the augite. olivine and some of the feldspar 
take on the characters of phenocrysts though not altering in 
size, while the feldspar net work becomes finer. 
The gneiss in immediate contact with this dike, on the west 
bank of the Damariscotta river, is altered to a hard compact, 
tine-grained, slaty green rock. It possesses as constituents, 
granular quartz, augite, plagioclase, orthoclase,titanite, magne- 
tite and scanty hornblende. The augite occurs in large irregu- 
lar areas with round quartz inclusions, such as are quite char- 
acteristic of recrystallized elastics. 
Upon the east shore of Thrumbcap island are numerous 
dikes varying in width from four feet to four inches. (PlatesIX, 
X, XI). A horizontal columnar structure is very pronounced 
in these dikes (Plate XI). The gneiss in proximity with the 
dikes is thoroughly crystalline, compact, and garnetiferous. 
The dikes are transverse to the strike of the gneiss and to the 
I)egmatyte veins. The dike rock is dark green and aphanitic 
with inconspicuous phenocrysts. 
The constituents are the same as those of the dike just de- 
scribed and the structure is very similar. Phenocrysts of lab- 
radorite, augite, and olivine abound in a vcr\- fine-grained 
ophitic groundmass, composed of plagioclase, augite, and 
