Dikes in the Vicinity of Johns Bay, Maine. — Bascoin. 277 
lie the gneiss. Genuine aqueous veins are represented by the 
purely quartzose bands. 
G?ieiss. — The strike of the gneiss is north and south, vary- 
ing from 5° to 17° east of north. The general north and south 
structure of the land is pronounced and the eastern coast-line 
closely parallels the strike of the rock. The gneiss possesses 
two prominent sets of transverse joints, of which one strikes N. 
55°-8o° W., and the other N. 50° E. On the west shore, the sea 
has in several instances entered the land along the northeast 
joint planes, forming narrow passages in which the tide rever- 
berates at high water. A notable instance of this on Squirrel 
island is known as the "devils hallway." The passage has since 
its formation been elevated above the reach of the sea. The 
gneiss pitches gently to the southwest and is compressed into 
folds in an east and west direction. 
The pronounced planes are those of cleavage. The strike 
of the cleavage is usually parallel to the strike of the stratifica- 
tion and the dip of the cleavage is 45° N. W. This is some- 
times coincident with and sometimes transverse to the strati- 
fication dip. The stratification planes are obscure but at various 
localities on the eastern border of the island can be seen gently 
(40°) dipping N. W. and on the western side dipping S. E.with 
many minor folds. 
Age of Gneiss. — Hitchcock places all the coastal gneiss, 
lying between Portland and the Penobscot river, in the middle 
Laurentian. The sedimentary character of the gneiss of Ruth- 
erford's island and the adjacent mainland can scarcely be ques- 
tioned. Under the more recent classification of Pre-Cam- 
biian rocks, this formation will probably fall into the Algon- 
kian division of the geological column. 
Basic Dikes. — The basic dike on Rutherford's island, of 
which a petrographical study was made, outcrops at the head 
of Christmas cove. Striking N. 85° E., it crosses the island at 
Foster's point and comes into veiw again on the west bank of 
the Damariscotta river. At this point it disappears beneath the 
gneiss. It possesses a uniform width of approximately two 
hundred feet. 
There are two sets of joints of which one strikes N. 25° W. 
and the other N. 75° E. The material of this dike is a medium 
