152 
GEOLOGY OF THE FOURTH DISTRICT. 
By reference to the geological map accompanying the Reports, the course of this rock will 
be readily traced through the District from Cayuga lake to the Niagara river, being indicated 
by the blue band following the ochre color. The great depression southward in the line of 
outcrop at Cayuga lake, the Genesee and Niagara rivers indicate the great amount of denu¬ 
dation it has suffered at these points which are the ranges of ancient valleys, formed at the 
same period in which nearly all the large lake valleys and river channels of western New- 
York were produced. The same cause and, in some places, more recent operations have 
produced minor indentations in the outline, many of which are too small to be noticed in 
the map. 
This rock is subordinate in thickness and continuation to the next succeeding mass, and 
was not separated from that by Prof. Eaton. Indeed for all practical purposes they may be 
regarded as one formation; the lower part, where fully developed, being marked by an as¬ 
semblage of fossils which sufficiently distinguish it. 
Its usual characters in the Fourth District are a light grey color often approaching to white, 
more or less crystalline in structure, and containing numerous fossils. In many instances 
this mass, like the encrinal limestone at Lockport, seems almost entirely composed of broken 
and comminuted fragments of crinoidea and corals, sometimes extremely attenuated, and at 
other times fragments of large size are preserved. These fragments of crinoidal columns, 
with some of the other fossils, are frequently of a pink or reddish color, and give a beautiful 
variegated appearance to the mass, particularly when polished. The more comminuted por¬ 
tions containing some earthy matter of a dark color, frequently embrace large fragments of 
Favosites, a perfect Cyathophyllum, or some other fossil of a light color, which forms an 
agreeable contrast with ihe surrounding mass. This character may be seen in the stone of 
which the Court House at Batavia is constructed, and which is from the town of Le Roy. 
Sometimes the mass is fine grained, more compact in texture, and of a darker color; when 
it has this character, few fossil remains are detected in it. The layers are usually separated 
by thin seams of greenish shale, which often divide blocks of the stone into wedge-form and 
irregular laminae. These seams, barely colored with the greenish deposit, often exist in the 
stone, where they are scarcely visible, though a blow with the hammer separates the block. 
In selecting specimens where much vertical depth is required, this is frequently a great in¬ 
convenience. 
Where thinly laminated by these seams of shale, and the surface covered with encrinital 
columns and plates as it usually is, the rock bears a most striking resemblance to the Wen- 
lock limestone of England, as seen in some specimens from Wenlock which were presented 
to me by Mr. Lyell. So complete is the resemblance in some instances, that one might al¬ 
most be mistaken for the other. Judging, however, from the general character of specimens, 
and the description of Mr. Murchison, there is a greater amount of shaly matter intermixed 
with the Wenlock limestone, than with our rocks of the same period. 
The similarity or even identity of specimens from Dudley in England with those from the 
Niagara group has been remarked ; and if this inference be correct, of which there seems no 
doubt, then we find a wide separation here between rocks which in England constitute one 
