Geology and Mineralogy of a part of Massachusetts, &c. 23 
ry fragments, whose surfaces are uniformly glazed, as if 
by ajet-black varnish, and then polished. The glazing 
is probably carburet of iron, or black oxide of manganese. 
I examined this mineral at the north part of Troy, where 
great quantities were thrown out for the short canal con- 
structed for sloop navigation. It contains flattened masses 
of quartz, sometimes crystallized, whose surfaces or the 
surface of the argillite in contact, are beautifully marked 
with longitudinal strie. Sulphuret of iron is found in it, 
which Professor Eaton supposes, with much probability, to 
be the cause of the hepatic springs along the Hudson. 
This slate is abundant on the opposite side of this river. 
9, GRAY WACKE. 
Colored bistre brown. 
This isan abundant rock. It is obviously a mechanical 
deposite, consisting more commonly of rounded masses of 
quartz, or quartz and feldspar, held together by an argilla- 
ceous cement, which may be claystone. The cement is 
sometimes the larger part of the rock, and sometimes the 
reverse. The cemented masses, not always rounded but 
sometimes angular, differ very much in magnitude as well 
as quantity. Some masses resemble a coarse pudding- 
stune; others appear porphyritic, till closely examined ; 
and others have a homogeneous slaty appearance, so fine are 
the mingled materials. The colours too are very dif- 
ferent; generally some shade of green, often gray, bluish, 
or dark brown. The surface of the rock often changes 
from the natural colour of the rock, like greenstone trap, 
by the action of the weather, the surface becoming brown 
from the higher oxidation of the iron. ‘The general varie- 
ties are the common gray wacke, rubble stone, and gray 
wacke slate. There is another variety wrought for build- 
ing stones, in which the materials are so fine that the rock 
may be polished, but the structure is not slaty. It might 
be called compact gray wacke. The grains are as fine as 
those of many compact limestones. The common variety 
and rubble stone, ofien contain seams of quartz, dividing 
them into frappose or prismatic forms. The action of the 
