Relations ot the Igneous Rocks — Crosby. 43 
quartz porphyry and a great thickness of volcanic and sedi- 
mentary formations. In the direction of its disappearance 
the normal granite does not reappear north of the Blue 
hills ; and undoubtedly its disposition, especially in relation, 
to the unaltered sedimentary formations which meet it ab- 
ruptly on the north, in the West Roxbury district, finds its 
readiest explanation in a profound displacement along the 
northern border of the complex, with the downthrow, of 
course, to the north. 
The normal granite is observed at many points tO' 
grade upward into the fine granite by which it is bordered; 
and its surface continuity is frequently interrupted by 
island-like outliers of the fine granite. These relations are 
particularly well-exhibited in the broad and massive ledges 
in the -area bounded by Washington, Grove and Center 
streets and Cottage avenue ; and nowhere more favorably 
than in the vicinity of the large quarry on Cottage avenue, 
northwest of Washington street. 
CONTACT ZONE OF THE BATHOLITE. 
Fine Granite — The chief difference between this type 
and the normal granite is textural. The essential minerals, 
according to Dr. Bascom, are the same, with the addition 
of a little microcline and oligoclase to the feldspars. 
Quartz is reported as more abundant, and the chloritized 
ferromagnesian constituent as less so, and these distinctions 
are confirmed by the analysis, which shows higher silica 
and lower lime, magnesia and iron. 
The fine granite belongs to the contact zone and hence 
overlies the normal granite. It might, therefore, where 
not removed by erosion, be expected to exhibit a broad 
areal development, but for the fact that it is, in turn, cov-' 
ered by the quartz porphyry phase of the contact zone. In 
harmony with this general relation and the shallow syn- 
clinal structure of this part of the batholite, the principal 
area of the fine granite takes the form of an irregular V- 
shaped belt, 1000 to 3000 feet wide, separating the normal 
granite on the north and southwest, from the quartz por- 
phyry on the south and northeast, respectively. 
As to the thickness of the fine granite, we have no very 
definite clue. No approximately vertical or continuous 
