double, giving off a braneh parallel with itself. It runs 
directly toward the middle of Green Hill Rock, but no trace of 
it can be found in this bare mass of porphyrite. 
The fault bounding Melaphyr Plateau on the west is 
accompanied by several small dikes, one of whieh (64) сап be 
traced, with some interruptions and dislocations, the entire 
breadth of the plateau, crossing and probably cutting all the 
east-west dikes. The most important and decidedly the most 
interesting of all the dikes of this system is the parallel series 
(66) separating Kast Porphyrite Hill and Cliff Plateau. Near 
the shore these afford the following section from east to west :— 
(1) Diabase, 9.75 feet. Separating melaphyr, 8. feet. 
(2) s vd» Au " conglomerate, 5.75. 
(3) K 6 n i е 
(4) “ 9% “ “ 29 
(5) A ИЕЛ, n Lo 8. ۳ 
(6) » ‚100% 
89.75 feet. 74.75 feet. 
This composite dike or series of dikes is thus equivalent to 
about 40 feet of diabase and 75 feet of separating conglomerate 
and melaphyr, or 115 feet for the entire fractured zone. The 
series is actually exposed for only a few rods; but there is a 
clear path for it between Conglomerate Hill and Cliff Plateau 
to the northern end of the western marsh. Almost in the 
same direct line at the southern end of the marsh is the largest 
of all the Nantasket dikes (2), which might be regarded as 
due to the union and continuation of the composite dike. This 
view is precluded, however, by the lithological contrast, the 
southern dike being a true melaphyr and undoubtedly contem- 
poraneous with the first flow of that rock, while the northern 
or composite dike, representing the latest period of igneous 
activity at Nantasket, is very much newer. Probably both the 
melaphyr dike and the composite dike are intercepted by the 
boundary fault under the western marsh. 
