IIG 



GEOLOGY OF THE SECOND DISTRICT. 



The greatest tliickness which I have liecn able to give to the Trenton limestone, is four 

 hundred feet. At Cliazy, where it is made ujj of alternating beds of limestone and shale, 

 this, according to the best estimate I can make, is the thickness of this rock. Tlie grey 

 variety is, however, wholly wanting at this locality ; if that is to be considered as a distinct 

 mass, the whole thickness may be greater than I have given it. But at Watcrtown, where 

 both varieties exist, the thickness cannot much exceed the above estimate. At Glen's Falls, 

 it is much less ; for, taking in a part of the calciferous and the black marble stratum, with 



iXl. 



the Trenton, the whole is not over sixty feet, the three masses of which form the bank of the 

 Hudson river below the bridge, as is exhibited in the diagram above, No. 41 : a, river; h, 

 calciferous sandrock ; c, drab-colored layers, with fucoides ; d, beds of black marble ; e, 

 Trenton limestone, intcrlaminated with black shale. The upper layers of limestone have, 

 however, been carried away ; and hence the thickness is less than usual in the Mohawk and 

 Hudson valleys. 



6. Utica Slate. 



Lithological differences between the Utica slate and the Calcareous shales of the Trenton 

 litnestone. — Differences of opinion in regard to the jwsition of the rocks above this lime- 

 stone. — Thickness. 



The Trenton limestone terminates usually in a black shaly mass, variable in thickness, and 

 in which we find many of the characteristic bivalves of the calcareous rock beneath ; but the 

 univalves have entirely disappeared, and do not pass into the shale. This is the first change 

 which appears to have taken place towards the close of the period of the Trenton limestone. 



The slate which bears the name at the head of this article, succeeds the Trenton shale, but 

 scarcely differs lithologically from it : it is, however, firmer ; exhibits a double system of 

 natural joints, and is very constantly traversed by seams of satin or calcareous spar. It is, 

 however, more certainly distinguished by attention to its fossils, in which there may be said 

 to be an entire change ; of the bivalves, scarcely an Orthis, Leptsna, or Atrypa is to be 

 found. The orthoceratites of the Utica slate are unknown in the lower rocks, and so are the 

 Crustacea. The Isotelus and Calymene, so abundant in the Trenton limestone and slate, do 



