— 
27 
do some of the dikes coincide in position, but they all coincide 
in direction, with the fault-lines of the district. This general 
coincidence in trend of the faults and dikes is a very significant 
fact, indicating that the dikes date from the period of general 
disturbance, when the rocks were tilted and faulted. The map 
also brings out the fact that the east-west dikes occur in three 
distinct zones or belts, marking the three principal dislocations 
of the Nantasket area. 
LITHOLOGY. 
To avoid unnecessary repetition in the detailed descriptions 
of the Nantasket ledges we may advantageously notice in 
advance the general lithological characters of the principal 
types of rocks. hese fall naturally into three classes, ав 
follows :— 
(1). Sedimentary rocks, consisting chiefly of the conglom- 
erates. 
(2). The older eruptive rocks, embracing the diorites, 
granites and felsites. 
(3). The newer eruptive rocks, including both dikes and 
lava-flows. 
The third division is by far the most important; and it is a 
matter for sincere congratulation that it has been worked up 
by a competent specialist. 
The Sedimentary Rocks. 
The Nantasket conglomerate is, with unimportant exceptions, 
chiefly composed of well rounded or water worn fragments of 
granite, felsite, melaphyr and porphyrite. On account of tho 
predominance of the granite and felsite débris, the conglomer- 
ate is, in general, light-colored, pinkish or reddish, It has in 
