96 Geology of the Monimorenci — Emmons. 
To be more particular as it regards the designation of rocks, I remark, 
that the first is very clearly the Trenton limestone, so frequently spoken 
of in the New York Geological Reports, containing abundance of the 
Strophomenaalternata, and Orthis testudinaria, together with many other 
fossils, which are known to occur in this rock at various places in the 
state. 
It is equally clear, that the slate is the Hudson river slate, and which 
has been shown in the reports referred to, to be geologically above the 
Trenton limestone, though here it appears resting against it. 
We now proceed to speak of the rocks at the fall, and here it is neces- 
sary to remark, that the strata both above and below will require exami- 
nation. First, then, above the fall, the rock of the bed of the river is 
gneiss, and reposing upon the edges of the strata composing the bed of 
the river is, first, a sandstone, in a horizontal position, loose in texture, 
large portions of which are stained green by carbonate of copper. It is 
not over ten feet thick, and is of course unconformable with the gneiss 
on which it rests. This is the Potsdam sandstone, so largely developed 
in the northern counties of this state. 
The next mass, as we ascend in the series, is made up of boulders, some 
of which are six or eight feet in length; it corresponds to a mass which 
occurs at Chazy, in Clinton county, and which has been considered as 
the upper portion of the Potsdam. It is one of the rare instances in 
which boulders of this size appear as the constituent parts of the regul- 
arly stratified rocks. 
Upon these boulders is a compact limestone, with its layers conform- 
ing to their regular surface on which it rests. It contains obscure or- 
ganic remains, principally of encrinites. This mass graduates into a 
gray crystalline limestone, which is composed mostly of broken encrin- 
ites; it is about fifteen feet thick. Succeeding to this gray limestone, is 
the Trenton, a mass very well developed, and abounding in characteristic 
fossils. It is at least sixty or seventy feet thick, and presents as a whole 
the characters of this limestone, in as great perfection as at any locality 
on the Champlain, or in the Mohawk valley. It is worthy of remark 
here, that this limestone is an excellent;guide in determining the relative 
position of the lower rocks: it forms an excellent starting point from 
which to trace the ascending or descending series. 
It will be seen from the remarks above, particularly by those acquainted 
with the lower rocks, that two important ones are absent, viz. the Calci- 
ferous sandrock and the Birdseye. The first is sometimes two hundred 
feet thick — the latter thirty. 
This omission is not, however, to be considered as a very remarkable 
case, though'it mvist be confessed that the Calciferous is one of the most 
constant rocks in the Mohawk, and along the Champlain and St. Law- 
rence valleys. 
The thinning out and final disappearance of a particular rock is, in fact,, 
one of the common changes occurring in the rocky strata. 
Having briefly enumerated the rocks above the fall, it is time to ex- 
