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 i iron, or black oxide of manganese. 
I examined this minere] 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 —— are somctaeee marked 
with longitudinal strie uret 
which Professor Eaton sanpoens; with =e ; 
be the cause of the hepatic springs along 
This slate is abundant on the opposite side of this river, 
9. GRAY WACKE. 
Colored bistre brown. 
This is an abundant rock. It is ——— a mechanical 
deposite, consisting more commonly of r e 
quartz, or quartz and feldspar, held Reha 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 = 
as — Some masses resemble a coarse 
s 
wacke slate. There is another variety wrought for = 
ing stones, in which the materials are so fine that the rock 
may be polished, but the sage is not slaty. It might 
be called compact gray wacke. rains are as fine as 
those of many compact sesame ate he common variety 
and rubble stone, often contain seams of quartz, dividing 
them into ‘rappose or prismatic forms. The action of the 
