Relations of the Igneous Rocks — Crosby. 77 
detail, as two necks developed on fissures, approximately 
parallel with the major axis of the West Roxbury neck ; 
but converging irregularly northwestward, near the Cam- 
brian outliers ; and the united axes might be regarded as 
continued in the great dike of quartz and granite porphyry. 
Where most widely separated and most distinctly developed 
as necks, these fissures are clearly compensating displace- 
ments bounding a depressed area approximately half a mile 
wide. 
The southwestern fissure, designated the Bold Knob 
neck, shows a wall of the massive quartz porphyry of the 
contact zone bordered on the northeast by several hundred 
feet in breadth of coarse felsite agglomerate, the felsitic 
matrix of which is packed with large, angular fragments 
of both felsite and quartz porphyry. Northeastward, or 
away from the wall of quartz porphyry, the agglomerate 
becomes rapidly finer and shades off into fluidal, spheru- 
litic and other obviously effusive forms of felsite. Similar- 
ly the northeastern fissure, known as the Grew's Woods 
neck, shows an immense mass of exceptionally coarse ag- 
glomerate sharply limited on the northeast by normal gran- 
ite. The agglomerate has a maximum breadth of nearly a 
thousand feet, passing gradually, as before, into the effu- 
sive felsites, which are continuous over the area interven- 
ing between the two bodies of agglomerate. Not only has 
the effusive felsite discharged by flow and explosion from 
these fissures overspread the depressed area which they 
bound ; but from this expanding area, as from a cornucopia, 
the felsite flows have spread eastward over a large part of 
Hyde Park and into Dorchester and Milton. Unlike the 
West Roxbury neck, and on account of the general east- 
ward inclination of the geological structure of the district, 
erosion has not cut deeply enough to remove entirely the 
effusions of this most eastern of the recognized felsite 
necks. 
Felsite Stocks. — Besides the large and essentially in-' 
dubitable necks of acid lava described in the preceding 
pages, we must, as previously noted, recognize several more 
or less probable stocks or plugs of felsite in the sedentary 
zones of the batholite. These masses, which may, perhaps, 
