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conspicuous quartz veins from 2 to 3 inches wide, parallel, and 
75 feet apart. Curiously enough, however, they conform in 
direction with the second system of dikes. This is only one 
of several indications that the dikes of the two systems are not 
widely separated in time. 
The outermost dike on Gun Rock (13) is undoubtedly con- 
tinued in the Green Hill conglomerate, as mapped. In the 
eastern half of the ledge, both this dike and 14 are especially 
characterized by extreme irregularity and lack of continuity, 
the conglomerate, apparently, having been repeatedly faulted 
after they were formed. One result of this disturbance is that 
they converge eastward, so that, although 50 feet apart along 
the north-south dike (62) they are almost in contact at the east 
end of the ledge. About 50 feet from the extreme northern 
edge of the conglomerate and 22 feet north of 14 on 62, but 
diverging from it eastward, is dike 15. It is not well exposed, 
but it is clearly a dike of the second system. The chief interest 
at this point centers in the clear intersections of all these dikes 
(18-15) by the single small representative (62), in all the 
coastal area, of the third system of dikes. 
Dike 16, on the north side of Green Hill Rock, is quite 
irregular in trend, but doubtless belongs to the second system. 
If so, it is probably parallel with, and passes to the south of, 
all the dikes (17-19) on Black Rock. Dikes 17 and 18 
converge eastward, their outerops being 50 feet apart at the 
west end of the island and almost in contact at the east end. 
Dike 19 on the northeast corner of the island ends abruptly on 
the west and is much given to irregular branching, one of the 
easterly branches splitting a prominent vein of quartz. 
sy far the most prominent set of joints in the rocks of the 
coastal area trends due north and south. They form or bound 
the valley between Atlantic and Centre Hills and are very 
marked along the shore of Centre Hill and Gun Rock, and, in 
faet, along the entire Nantasket coast, producing the many 
fissures and chasms that indent the shore. It is certainly, 
acid 
