The. American Geologist. 
August, 1905 
the felsite are unquestionably igneous, being firmly welded 
and the sediment well baked for a breadth of one to several 
inches. Even closely adjacent masses do not usually agree 
in dip and strike ; but the relations are what might be ex- 
pected in the case of a bed or beds of imperfectly consoli- 
dated ash disturbed by later eruptions, largely of an ex- 
plosive character. The close relation of the consolidated 
ash or slaty tuff to the felsite agglomerate is clearly indi- 
cated in some instances by the intercalation of visibly clas- 
tic layers. Finally, we may regard the ash as the finest 
product of an explosive eruption which was in some sense 
subaqueous, closely followed by eruptions which were only 
in part of an explosive character, yielding, besides the true 
.agglomerates, compact, fluidal and autoclastic felsites in- 
tersecting and enclosing, alike, the agglomerate and the ash. 
Accepting the West Roxbury neck as a true volcanic 
vent, as apparently we must, it may be assumed to have 
originated in a more or less complex and branching fault 
fissure ; and the local widening of such a fissured zone to 
the present breadth of the neck when it finally became the 
locus of vulcanism would be a natural consequence of the 
•explosive action of which we have such ample evidence. 
In other words, we need not assume that the granite was 
melted away, or forced out en masse, Pelee fashion ; but it 
is sufficient to assume a splintering and shattering of the 
granite walls under the influence of shock and heat. The 
resulting granite debris would be borne upward by the 
constantly increasing volume of viscous lava made possible 
"by the widening of the vent, and finally discharged, largely 
through the agency of violent explosions. This explana- 
tion, the essence of which is a gradual crumbling and ex- 
foliation, accompanied by cracking and rending of the 
granite walls of the primordial fault fissure, accounts for the 
general diffusion of granitic detritus through the clastic 
lavas, as well as for larger, isolated masses of granite which 
now add to the diversity of the neck. 
The Hyde Park Neck. — This large and composite neck 
occupies a more central position than the West Roxbury 
neck in relation to the complex; and presents some other 
•distinguishing features. It is, probably, best regarded, in 
