Dikes near Portland, Maine. — Lord. 337 
schists and dikes alike, and consists of two systems of ver- 
tical fractures intersecting each other at an angle of about 30°. 
and causes the rock to break down in rhoniboidal blocks 
when exposed to the direct action of the waves. The schists 
are overlain at various points by glacial deposits, and fre- 
quentlv along the shore considerable stretches of beach sand 
are met with. This sand is ordinarily of the common white 
variety, consisting principally of quartz with some feldspar 
fragments and flakes of mica, but occasionally it is colored 
brownish red, owing to local accumulation of rounded garnet 
crystals. 
The glacial deposits are especially well developed at Port- 
land and a point on cape Elizabeth, about two miles south of 
the city. They consist of basal, or lower boulder clay, com- 
posed chiefly of large, highly striated rock-fragments, and of 
an upper, looser, brownish-red till, which at Portland merges 
at the top into gravel beds. 
This glacial debris has been removed to a great extent 
from the islands in the bay, especially from those immediately 
facing the ocean. Even great Chebeag island, having a 
somewhat protected position, is but lightly covered with 
gravel and boulders. 
The low, flat islands of the inner bay (Mackey, Clapboard 
and Basket islands) are composed of reassorted glacial ma- 
terial similar to that forming the marine terrace which fringes 
the adjoining coast. 
The study of the basic dikes about Portland was taken up 
to discover what relations, if any, exist between them and the 
diabasic intrusions of Kennebunkport and Bald ClifT, and, if 
possible, to ascertain the nature of the parental mass from 
which they originated. The prosecution of the work devel- 
oped many features of special petrographical interest.* 
These basic dikes occur on the islands of Casco bay, and 
the eastern shore of cape Elizabeth, and can be followed as 
far south as Front's neck, some three miles beyond the limit 
*The work was carried on in the petrographical laboratory of the 
U. S. National Museum in Washington. D. C, and the writer wishes 
to acknowledge his thanks for facilities oflTcred. and the privilege of 
studying the collections. He is especially indebted to professor G. P. 
Merrill, head curator, Dept. of Geology, in the museum, for many 
valuable suggestions, and to Dr. Thomas L. Watson for advice and 
aid. 
