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59 
a striping or flow-structure which is wanting in the dikes ; the 
superficial resemblance is certainly very obvious, suggesting at 
once that the dikes may be the channels or vents through which 
the effusive porphyrite reached the surface, or at least referred 
to the same eruptions— the same period of voleanie activity. 
Mr. Merrill has found (page 88) that, although somewhat 
more crystalline, these dikes are essentially similar in micro- 
scopic characters to the Black Rock poryhyrite ; and in order to 
further test the validity of this hypothesis a typical example 
from one of these dikes was submitted, through the kindness of 
Dr. T. M. Drown and Mr. G. F. Eldridge of the Massachusetts 
Institute of Technology, to partial chemieal analysis, with the 
following result, the mean of four accordant determinations : 
silica, 60.84 per cent. ; alumina and ferric oxide, 20.12 per 
cent.; with which may be compared the percentage of silica 
— 58.25 per cent.—-afforded by the Black Rock porphyrite. As 
in the case of the melaphyr (page 38), we find that the dike is 
slightly more acid than the effusive portion of the eruption ; 
its more erystalline charaeter causing it, apparently, to yield 
less completely to the deep-seated alteration, thus reversing the 
normal relative proportions of silica observed in recent erup- 
tive rocks. 
That the classification of these dikes by the superficial char- 
acters is unsafe is shown by a second analysis, by Mr. Eldridge, 
from another dike of the series. This afforded, as the mean of 
four accordant determinations, silica, 48.47 per cent. ; alumina 
and ferric oxide 31.53 per cent.; and if not a true diabase, it 
should, apparently, be associated with the more basic melaphyrs 
of Nantasket. Obviously, then, a systematic microscopic and 
chemical examination of these dikes will be required for their 
accurate classification; and we can now only assume that, as 
appearances indicate, they are chiefly porphyrite. 
A more detailed description of these dikes will be presented 
later, in the systematic account of the dikes of this region ; and 
all that I desire now is simply to call attention to the following 
points ; — (1) That they are probably of the same age as the 
