72 The American Geologist. August, 1905 
The next outcrop of this dike, where it crosses Hleron 
street, is also granite porphyry ; but beyond this the quartz 
porphyry phase recurs, and the dike, now 75 feet wide, is 
lithologically indistinguishable from the Bearberry hill sec- 
tion. From this point on, it is near to and approximately 
parallel with the dike of acid andesite previously described; 
and the latter is cut by minor dikes or apophyses of quartz 
porphyry. The outcrops of both dikes are now interrupted 
by a swamp, beyond which the dike of acid andesite con- 
tinues unchanged, while the porphyry dike, now 80 feet 
wide, is once more a typical granite porphyry, apparently 
repeated by an oblique strike fault and, as previously 
noted, enclosing angular fragments of the acid andesite. 
Thus twice in a total distance of a little more than a mile, 
the quartz porphyry gives way to granite porphyry. In 
neither case, unfortunately, can the transition be fully 
traced ; and yet we may not reasonably doubt its reality. 
This remarkable and rhythmic textural variation in the con- 
tents of one and the same continuous fissure may, per- 
haps, be regarded tentatively as finding its most natural 
explanation in varying original depths of solidification. 
That is, if we may assume a natural gradation upward in 
the dike from granite porphyry to quartz porphyry, then a 
moderate afnount of subsequent displacement, or even of 
unequal erosion, might suffice to give the alternations of 
texture which the outcrops now show. • In this connection 
it may be noted that in the lowest outcrop of the main dike, 
at the western base of Bearberry hill, the quartz porphyry 
is well advanced in the change to granite porphyry. It is a 
fair corollary of this explanation that the granite porphyry 
should be found chiefly in the normal granite, and the 
quartz porphyry in the fine granite. This relation is 
clearly realized in part; and would, perhaps, be more fully 
realized, but for the fact that, as we suppose, the batholite 
suffered strong and unequal erosion before its injection by 
these acid intrusives. 
Felsite Necks. — Besides its abundant occurrence in 
effusive forms or surface flows, the devitrified rhyolite 
(aporhyolite) or felsite has an important development in 
necks and dikes, which are undoubtedly contemporaneous 
