80 The American Geologist. August, 1905 
posures are numerous and instructive, by far the most com- 
plete section of the felsites was that afforded temporarily 
during the construction of the Stony Brook-Neponset tun- 
nel, nearly a mile long, of the High-level sewer. This shows 
the felsites resting upon both the normal and fine granites 
at points remote from the nearest surface exposures of these 
rocks. The flow structure of the felsites, originally hori- 
zontal, is now everywhere highly inclined and chiefly ver- 
tical, showing that the plication of the Carboniferous sedi- 
ments was shared by their volcanic floor. As to the orig- 
inal or normal thickness of the acid effusives of the Nepon- 
set valley, we have no reliable data ; but it was quite cer- 
tainly to be measured by hundreds and probably not by 
thousands of feet. 
CARBONIFEROUS VOLCANICS. 
The volcanic rocks definitely known to be contempo- 
raneous with the Carboniferous conglomerate of the Nepon- 
set valley include a moderately acid type — trachyte, and a 
moderately basic type — andesite. The andesite largely pre- 
dominates; but the trachyte is, in the main at least, the 
older and may, perhaps, be regarded as in some sense a 
transition type between the felsite and andesite. 
Apotraehyte. — As described by Dr. Bascom, this is a 
coarsely and profusely porphyritic rock of highly feldspathic 
composition, with albite as the predominating feldspar. Dr. 
Bascom shows that the chemical analysis of this rock con- 
firms its classification as a soda-trachyte in which diopside 
must have been an original constituent ; or, having regard 
for its present altered condition, it may be more precisely 
defined as a soda-apotrachyte. This rock has been recog- 
nized as forming one small flow* conformably interbedded 
with the conglomerates of the Central Avenue district in 
Milton, and a probable vent, in part of agglomeratic struc- 
ture, on the New England railroad north of River street, 
Hyde Park. The latter occurrence was intersected and 
more fully exposed by the Stony Brook-Neponset tunnel. 
Apoandesite. — This important volcanic is described by 
Dr. Bascom as an aphanitic rock of dark purplish and 
greenish tints in which the original constituents are mainly 
altered to calcite, chlorite, epidote, quartz and other second- 
