330 EF. Hitchcock on the Coal of Bristol Co., and R. Island. 
By this argument I intend merely to show that there is no im- 
probability in the supposition that this basin of five hundred 
square miles, may be a coal field; just because it looks like one. 
But the argument has no great force ; because other deposits may 
exhibit a similar surface. Strictly speaking, it merely shows that 
the surface is underlaid by one formation, or by closely allied for- 
mations. 
Il. The rocks correspond essentially to those of the coal mea- 
sures. The predominant varieties are four: 1. A dark colored 
slate, or slaty clay, often much indurated, and more or less charge 
with carbon, lying in immediate contact with the coal, especially 
beneath it. Its surface is sometimes highly glazed, as if by heat, 
or friction. I noticed fine specimens at the Roger Williams mine, 
a mile or two north of Valley Falls, Rhode Island. 2. A coarse 
light gray grit, or sandstone, lying, so far as I have observed, im- 
mediately above the coal beds, and easily disintegrating at the 
surface. 3. A dark gray, hard grit, or sandstone: a much more 
extensive rock, forming in fact the principal one between the beds 
of coal, and in some places embracing coal without the interven- 
tion of shale. 4, A coarse gray conglomerate, which probably 
underlies the other rocks above described, and may be the equiva- 
lent of the millstone grit that forms the basis of other coal fields. 
I do not feel satisfied, however, that such is always the position 
of this rock. Further examination is needed. This rock occurs 
in various places along the eastern part of Massachusetts ; but 
nowhere, that I know of, associated with coal, save in the Bristol 
coal field. 
Several other varieties of rocks exist on the borders of this coal 
field, but whether they are the coal measures metamorphosed, or 
older rocks, such as the Devonian and Silurian, is not certainly 
known. They consist of gray and red slates, and red sandstones 
and conglomerates. As to the red varieties, I have little doubt but 
that they belong to the Devonian or old red sandstone system, and 
have accordingly so represented them where they are most fully 
developed, viz.: in Wrentham and Attleborough. In the south 
part of the former town, this red rock forms a striking feature in 
the landscape. I found it there, by the Aneroid barometer, to rise 
about four hundred feet above the general surface at the Mansfield 
coal mines, and three hundred feet above the excavation once 
made in Wrentham for coal. On this hiil I found the red rock to 
have a strike K. 10° N., anda dip 70° southerly ; so that it must 
pass beneath the coal field, as we should expect if it were the old 
red sandstone. A mile south of the meeting-house in North At- 
tleborough, however, where this red rock (here mostly a con- 
glomerate ) shows itself, the strike is E. 15° N., and the dip 35° 
northerly. This fact looks as if we might here be on the south- 
ern border of a coal basin, and at Wrentham on thé northern ber- 
der, and that there may be an axis of the older s running 
