78 The American Geologist. August, 1905 
best be regarded as more deeply denuded as well as smaller 
necks, are intermediate, in size at least, between the un- 
doubted necks and the normal felsite dikes. They are 
related to the necks in form and to the dikes in lithologic 
character, lacking entirely the clastic and fluxion characters 
and the general structural heterogeneity of the necks. One 
•difficulty in regarding them as dikes is that they do not 
appear to occupy dynamic fissures, being chimney-like 
rather than dike-like in form. The clearest examples are 
roughly circular or elliptical plugs 200 to 300 feet in diam- 
eter, isolated in, and enclosing numerous fragments of, the 
normal granite. The rock, although felsitic in general as- 
pect, is, perhaps, better described as a dense, non-porphy- 
titic microgranite. It is essentially homogeneous and 
structureless, except for an indistinct peripheral zone of 
true felsite, in part fluidal. 
Felsite Dikes. — In the general view the entire area of 
the complex appears to be traversed by dikes of felsite. 
They are, however, especially characteristic of the seden- 
tary zones of the batholite and the felsite necks, and are 
observed less commonly in the effusive felsites. In spite 
of the fact that they sometimes cut both the felsite necks 
and flows, the dikes, as a whole, are undoubtedly best re- 
garded as essentially contemporaneous with the acid vol- 
canics, and we may fairly suppose that in some instances 
they have formed effective vents. We have seen that they 
intersect the great dikes of quartz porphyry and granite 
porphyry ; but here, again, an important difference of age 
is not, apparently, a necessary inference. The relation of 
the felsite dikes to the necks is in some cases distinctly 
radial ; but a broader view shows that with few exceptions 
they tend to be normal to the major axes of the felsite 
necks and the fracture zones in which the necks have been 
developed. The prevailing trend, therefore, is northeaster- 
ly and southwesterly. 
In the extension of the axial shear zone of the West 
Roxbury neck, the felsite forms a plexus of irregular intru- 
sions, which branch and coalesce in a quite remarkable 
way ; while the true dikes represent the filling of compara- 
tively simple and sharply-defined transverse fissures due, 
