118 CHAMFLAIN DIVISION. 



How to distinguish the Potsdam sandstone. To distingiiisli 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, witli ol)scure 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 tiie 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 lirown 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 l)reaks 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 lied 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 vvhicii has furnished tlie name (meaning a sandstone bearing carbonate 

 of lime, or a mixed rock consisting of siliceous or sandy particles and limestone) , is well 

 designated imder the descriptive term, and is easily recognized. But there are several 

 singidar 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 inqjure limestone, being 



