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traced much farther, the extreme distance being опе and а half 
miles. It is a singular fact that the number of observed dikes 
is exactly the same as in the coastal belt. 
The most northerly dikes of the first system (20 and 21) on 
the northern extremity of Rocky Neck are of similar character, 
and, converging eastward, may be regarded as, possibly, 
branches of the same dike. The next dike of this system (22) 
first appears on the western shore of Rocky Neck with a width of 
from 8 to 10 feet ; and only 20 feet south of, and parallel with, it 
is the very similar companion dike (28), enclosing a large 
wedge of granite. These two dikes can be traced in the same 
direct line only 400 or 500 feet at the farthest ; but it is quite 
certain that they are shifted to the eastward by the fault between 
the 
This is clearly the case with the northern one, which may be 
granite and conglomerate and continue to the east shore. 
safely correlated with the dike of the same number in the 
western area, although it is 100 feet too far north to be just in 
line with it. Crossing the bay, we find this dike, with a width 
of 12 feet, dividing the half-tide ledge of melaphyr off Granite 
Point; and striking the shore on the north side of the point, 
with a trend N. 80? E., it takes a perfectly straight course to 
the southwest corner of East Porphyrite Hill, where it is jogged 
to the south its own width and, skirting the edge of the con- 
glomerate, is lost beneath the grass. It is probably either 
broken here by a fault not marked on the map, or dies out and 
is resumed along a parallel line; for what appears to be essen- 
tially the same dike begins a little to the northward and, though 
cut and shifted by three dikes of the younger system (27, 29, 
31), can be traced across the hill to the shore, where it is well 
exposed with a breadth of 9 feet. Closely parallel with this 
dike, on East Porphyrite Hill, and sharing the same accidents, 
but with the intersections less clearly exposed, is 24 ; and this 
dike, which is distinctly porphyritic, might safely be correlated 
with the dike of the same general character (25) east of the 
bay and the railroad, but for the fact that it lies so far to the 
2 
