196 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY 
3. Blue slates. 
4. Gray sandstone and conglomerate (fossiliferous). 
Granites :—These form the door on which the Carboniferous 
series was laid. The most common variety is a medium-grained 
pinkish granite, but patches of a coarsely porphyritie variety occur 
both on the western and northern borders of the Basin at this 
point. The relation of the two granites is nowhere shown in the 
vicinity, but the change from one to the other is in all probability an 
abrupt one, neighboring outcrops of the two presenting in every 
case their own peculiar features, no gradation of one type into the 
other being observed. Diorite, the usual associate of the granites 
of this part of the State, is not found in the immediate vicinity of 
the stratified group, a belt of granite half a mile or more in width 
and entirely free from diorite intervening between the two. Igneous 
rocks later than the granite are not at all common, though small 
dikes of felsitic material may be observed in several places. The 
diabase dikes, so common a feature of the Boston Basin, are of 
extremely rare occurrence, only two, and these under six inches in 
diameter, being noted. They are of an east and west system. 
Basal arkose :—This term is applied to the fine-grained con¬ 
glomerate lying directly on, and composed of material derived 
from, the granite. In color it is a delicate pink, interspersed, how¬ 
ever, with numerous bands of a more ferruginous sandstone, but the 
deep red tints so prominent in the Norfolk Basin are wanting. The 
pebbles are mostly granite, but diorite, felsite, and porphyry are by 
no means rare. Certain parts of this conglomerate resemble closely 
the granite from which they were derived, the resemblance becom¬ 
ing especially noticeable on weathering. No fossils are known to 
occur in the arkose of Brockton. 
Blue slate :—These are of a deep blue color and are very homoge¬ 
neous in composition, all indication of stratification being in most 
cases wanting, though the cleavage, which is highly developed, 
might easily be mistaken for it. In some cases the slates are 
mottled with pink owing to the oxidation of the iron from the 
ferrous to the ferric state. The parts so changed are especially 
liable to decay and weather rapidly away leaving cavities in the 
otherwise unaltered slate. 
Gray sandstone and conglomerates :—These are characterized 
by the presence, in addition to the pebbles of granite, quartzite, etc., 
of numerous dark, almost black, angular fragments probably derived 
