No. 50.] 
363 
Near to the bridge Mr. Williard has excavated for copper as I un- 
derstand, but I saw no other metallic mineral there, but some beautiful- 
ly tarnished pyrites. 
Since the discovery of the Rossie lead vein, innumerable excavations 
have been made for ore ; and profitless in many instances have been the 
expenditure of time and money in its pursuit. In one place we visited 
a large excavation in limestone made for lead ore, the rock showing 
none ; but it contained plumbago in scales. The cause of the excava- 
tion was owing to the common name of this mineral, hlack lead, mis- 
leading its pursuers, they supposing that from similarity of name, there 
was a connection of composition and origin. They were not aware that 
the composition of black lead was carbon and iron, and that not one 
particle of lead entered into its composition, or was it in any wise con- 
nected with lead, else their time and money would have been saved. 
Potsdam sandstone. 
This rock appears in the northeast part of Lewis county, in low 
ridges, and with all the characters given by Prof. Emmons, in whose dis- 
trict it is an extensive rock. It is well suited for building, and for the 
the lining and the hearth stones of furnaces. 
Fucoidal layers. These are the layers which are interposed between 
the "calciferous sandrock" and the Mohawk hmestone, and which are 
so abundant in the valley of the Mohawk. They appear to be almost 
horizontal to the eye, and may be seen on the road from Lewisburg 
furnace to the natural bridge, extending thence north and east into the 
second district. 
With these notices and remarks, we terminate for this report all that 
we have to say of the primary rocks and their overlaying masses of the 
region to the east of the Black river, and proceed to those masses which 
follow them in age, and which extend west from the river. The first 
of these is the Mohawk limestone, no other rock being visible between 
it and the primary, along the whole line of its course in the county. 
Mohawk limestone. This rock in our former reports was connected 
with the "bird's eye," but in extending our researches from county to 
county, we found that a separation seemed necessary, and we adopted 
the division set forth in Mr. Conrad's last report, correcting the error 
which had crept into it, of placing this limestone upon the bird's eye," 
in lieu of making the latter the upper or superior mass. 
