116 
GEOLOGY OF THE SECOND DISTRICT. 
The greatest thickness which I have been able to give to the Trenton limestone, is four 
hundred feet. At Chazy, where it is made up of alternating beds of limestone and shale, 
this, according to the best estimate I can make, is the thickness of this rock. The 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 Watertown, where 
both varieties exist, the thickness cannot much exceed the above estuuate. At Glen’s Falls, 
it is much less ; for, taking in a part of the calciferous and the black marble stratum, with 
41. 
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 ; &, 
calciferous sandrock; c, drab-colored layers, with fucoides ; d, beds of black marble ; e, 
Trenton limestone, interlaminated 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 
limestone. — Differences of opinion in regard to the position 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, Leptaena, 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 Calymenb, -so abundant in the Trenton limestone and slate, do 
