Relations of the Igneous Rocks — Crosby. 8r 
ary species. The microstructure is commonly trachytic and 
inconspicuously fluidal and porphyritic; and the effusive 
phases are often amygdaloidal or clastic. The distinguish- 
ing feature, chemically, as for the trachyte, is found in the 
high percentage of soda; and the extensive mineralogic 
alteration, without obliteration of original structures, makes 
this, therefore, a normal soda apoandesite. 
In areal extent and structural value the Carboniferous 
andesite is comparable with the pre-Carboniferous felsite;. 
and like the felsite, it is found in the three general modes of 
occurrence— necks, dikes and flows. The bedded lavas, 
both acid and basic, of the Neponset valley are believed to 
be exclusively contemporaneous. 
The andesite dikes are very numerous ; and they are 
found in all parts of the complex— cutting the successive 
zones of the batholite and its cover of acid lava (felsite), 
and cutting also the acid dikes of various types, including 
the porphyry dikes, and the necks, stocks and dikes of fel- 
site. In distribution, trend, form and size they are com- 
parable with the felsite dikes ; and the profusion of the basic 
dikes clearly indicates a very general and extensive Assur- 
ing of the subcrust during the subsidence which permitted 
the deposition of the Carboniferous conglomerate. 
It is not improbable that some of the andesite dikes 
have formed effective vents. But of unequivocal or nor- 
mal necks there are no indications in the sedentary zones 
of the batholite or in the vicinity of the felsite necks ; but 
they are to be found farther east, in the effusive felsites, 
the clearest examples occurring on either side of the Nepon- 
set, in the Mattapan district of Dorchester and the Colum- 
bine district of Milton. These vents are decidedly elon- 
gated or fissure-like ; but they are readily distinguished from 
the andesite dikes by even greater irregularity of outline 
and especially by the heterogeneity of structure and the 
prevalence of coarsely clastic or agglomeratic lava. The 
evidence is quite as clear as for the dikes that the andesite 
is younger than the effusive felsites. 
The andesite flows, like the dikes, are chiefly aphanitic, 
but embrace, also, amygdaloidal and scoriaceous forms ; and 
in the western part of the field, especially, bedded tuff and 
