70 The American Geologist August, ioos 
been studied in detail ; but it is known to represent a more 
or less extensive series, the acid andesite not having been 
distinguished heretofore from basic andesite and green- 
stone diabase. This dike. begins in the fine granite on 
Heron street near Washington street and, with a normal 
breadth of 20 to 25 feet, has been traced northwesterly, 
parallel with. Cottage avenue, into the normal granite and 
for o total distance of nearly half a mile. . It is clearly cut 
by typical dikes of quartz porphyry and felsite; and the acid 
andesite occurs as angular inclusions in a great dike of 
granite porphyry. These relations give it at once a unique 
position among the basic intrusives of this area and, so far 
as known, of the Boston basin, by definitely fixing it, chron- 
ologically, between the batholite and all of the acid intru- 
sives. Comparison is suggested at once with the more 
basic, pre-granitic dikes (diabase) in the Cambrian slates 
of the Blue Hills.* The latter are the only basic dikes 
in the Boston basin known to be older than the granitic 
series of the Complex; and the West Roxbury dike just de- 
scribed is the first relatively basic dike in the Boston basin 
known to be intermediate in age between the sedentary 
zones and the acid intrusions of the complex. 
Granite Porphyry and Quartz Porphyry Dikes. — -In 
this instance, also, the detailed study has been limited to 
what is, virtually, one large and complex example ; -but ;the 
general conclusions thus reached are definitely known, to 
be applicable to an extensive series of dikes. The fine 
granite of Bearberry hill in the northeast part of the Stony 
Brook reservation is traversed in a general northwest-south- 
east direction by a vertical dike of quartz porphyry 100 feet 
wide. The greenish gray, aphanitic groundmass of the 
porphyry is crowded with conspicuous phenocrysts of feld- 
spar up to one-half, and even three-fourths, of an inch in 
length, while the more scattering, rounded blebs of quartz 
are commonly one-fourth to one-half inch in diameter. Al- 
together it is a striking rock and one readily recognized and 
traced. The quartz porphyry is densely felsitic near the 
contacts, which are firmly welded, irregular in detail, and 
further characterized bv occasional inclusions of the' fine 
* Occas. Papers, B. S. N. H.. iv. 388. 
