Relations of the Igneous Rocks — Crosby. 73 
with the acid effusives ; and nowhere, apparently in the 
Boston basin, are normal effusives more clearly or typically 
developed. Some of the felsite dikes are large enough to be 
regarded as the vents of effusive masses ; and very prob- 
ably several of them are deeply denuded necks. The more 
typical and unequivocal necks, however, are less dike-like 
in outline and far more diversified in structure, consisting 
chiefly of clastic lavas — agglomerate and tuff — suggestive 
of explosive eruptions toward the end of the volcanic activ- 
ity and following more quiet liquid effusions. Three essen- 
tially distinct vents have been more or less fully worked 
out; and it is considered not improbable that others await 
recognition in the felsite areas. This appears the more 
likely in view of the fact that acid lavas of tiuidal and auto- 
clastic types, that is, lavas which were stiff enough at the 
time of their effusion to develop fluxion lines, or even to be- 
come brecciated by their own flow, would not spread far 
from the vents through which they reached the surface. 
We must recognize also the extreme probability that some 
vents are still concealed by their own sluggish effusions or 
by later sediments. The three vents referred to as more 
or less fully identified are near the heart of the complex and 
bordered either wholly or in part by the sedentary zones 
of the batholite, including the normal granite, fine granite 
and quartz porphyry ; and evidence is not wanting that they 
are, in each case, located on important displacements, one 
indication of this relation being elongation in a definite 
direction, the outlines being distinctly lenticular. 
TAe West Roxbury Neck. — This neck is the most clearly 
exposed in outline and in structural detail, and probably the 
largest, as it is certainly the most indubitable of the series. 
It occupies approximately the irregular triangular area 
bounded by Grove, Center. Stimson and Washington 
streets in West Roxbury. near the Dedham line. 
It is elongated in a general northwest-southeast direc- 
tion, the extreme dimensions being approximtaely 1200 by 
3500 feet. The major axis coincides in position and trend 
with the common boundary of the fine granite and the 
western area of normal granite, the evidence being quite 
conclusive that this line marks an important displacement. 
