118 
CHAMPLAIN DIVISION. 
How to distinguish the Potsdam sandstone. To distinguish this rock from other sand¬ 
stones, its position must first be noted. Traced downwards, we are led directly to the 
primary mass, as gneiss and granite ; traced upwards, we find it terminating in a sandy 
limestone. The exception to this rule is only found in the interposition of a mass of black 
siliceous slate, with obscure vegetable fossils, as at Whitehall and Chazy. The Medina 
sandstone, some parts of which resemble the Potsdam, is connected below with another 
gray sandstone, and above with green fragile shales. If any doubt exists, look for fossils. 
Of fossils, a single species, a lingula, is common at the High bridge at Keeseville, but 
small and obscure. At French creek, it is larger, but still obscure, in a sandy variety of 
the rock one and a half or two miles east of the village. 
To learn the geographical position of this or any of the New-York rocks, study the map. 
It will be seen that this rock encircles very nearly the Great Primary region north of the 
Mohawk. Let it be observed, however, that it is wanting from near Fort-Ann in Wash¬ 
ington county, south to the Highlands. It is also wanting in the valley of the Mohawk. 
When this is the case, the next rock, the Calciferous, rests upon the Primary, as at Little- 
falls, and numerous other places in the valley of the Hudson. 
Before dismissing the Potsdam sandstone, it is necessary to call the attention of the 
reader to a variety of it which occasionally appears in Washington county. It is a tough 
black or brown mass beneath the calciferous, and varying in thickness from six or ten 
feet to more than one hundred feet. It is difficult to describe it: it is sometimes compact 
and irregularly striped, and unlike any other rock in the New-York system. It some¬ 
times resembles hornstone, and breaks like it into irregular uncouth lumps with sharp 
angles : hence it is of no value as a flagging or building stone, except for the coarsest 
stone fences. This mass may be examined at Bald and Toby mountains : I have spoken 
of it as equivalent to Potsdam sandstone : it may probably with equal propriety be con¬ 
sidered as a subordinate bed of the Calciferous sandstone, inasmuch as it is associated with 
it at the places just mentioned, and is not known to be associated with the potsdam. It 
is sufficient to consider it as an intermediate mass ; but it is of no consequence, any farther 
than as it is necessary to be noticed to complete the description of the entire series. 
§ 2. Calciferous sandstone. 
Considered in its totality, this is one of the most heterogeneous rocks in the New-York 
system. That part which has furnished the name (meaning a sandstone bearing carbonate 
of lime, or a mixed rock consisting of siliceous or sandy particles and limestone), is well 
designated under the descriptive term, and is easily recognized. But there are several 
singular compounds embraced under this term ; and without a brief notice of them, our 
descriptive geology would be incomplete. So heterogeneous is this rock, that Mr. Vanuxem, 
of the Second district, applied the term Calciferous group. 
The typical rock under this name, is a gray mass with sparkling grains of lime, in which 
distinct masses of calcareous spar are always imbedded. It is an impure limestone, being 
