CLINTON GROUP. 
63 
to a small portion of the rock, and seems gradually to disappear farther westward. In Orleans 
county the same limestone is entirely destitute of this fossil, but is readily recognized by posi¬ 
tion and other characters. In many places in Wayne and Monroe counties it contains nodules 
of hornstone, which sometimes assume the form of calcedony. This matter increases so 
much in Orleans and Niagara counties, that it forms thin layers alternating with the lime¬ 
stone. Associated with this chert are commonly found silicified fragments of shells and cri- 
noidal joints. At Rochester, distinct layers are entirely composed of the silicified shells of a 
species of Strophomena. 
One mile south of Medina, in the bed of the stream, the limestone is seen separated from 
the upper member of the Medina sandstone by only three feet of shale. At that locality, the 
rock is composed of thin irregular layers of impure limestone with much hornstone; the 
courses are separated by shale, which in the exposed banks of the stream has dissolved out, 
and left the solid parts piled loosely together like a stone wall. The Pcntamerus is not here 
found, but the rock is abundantly charged with other fossils, particularly the Atrypa congesta, 
which is rare on the Genesee. About eight miles east of Lockport it is excavated from the 
bed of the canal for some distance, where it is composed of thin irregular layers of impure 
limestone and greenish shale, with much hornstone. When we arrive at Lockport, it becomes 
evident, that from the thinning of the Second green shale of the Rochester section, the two 
limestones have come together ; the Pentamerus division forming but a few feet of the cherty 
layers in the lower part of the mass. This might have been expected from the thinning of 
the lower shale at Medina ; and at that place there is no evidence of the existence of the upper 
shale, though the absolute contact of the two limestones cannot be determined. From Lock- 
port, the Pentamerus portion continues to form the lower layers of what is thence one mass 
of limestone ; and it appears in like manner and situation on the Niagara river. At Lockport 
it has become nearly destitute of fossils, and on the Niagara river it is entirely so, with the 
exception of crinoidal joints. The lower part of the limestone, as it appears on the Niagara 
river, is highly magnesian, and from the presence of iron pyrites rapidly decomposes, giving 
rise to the production of sulphate of magnesia, which at favorable points, along the overhanging 
mass upon the river bank, may be collected in quantities of several pounds. 
What changes this rock may assume beyond the district is uncertain; so far as traced in 
Canada, it maintains the same characters as on the Niagara river. We know, however, that 
in the southwest part of Ohio, and in Indiana near the junction of the Blue and Cliff forma¬ 
tions (of the Ohio reports), there is a limestone charged with Pentamerus oblongus. The 
same continues westward, and is seen in Wisconsin and Iowa. From this it is evident, that 
while this fossil and the mass to which it is confined nearly disappear within New-York, there 
is a reappearance of the same at the southwest and west; and that whatever may have been 
the cause of its non-existence in the western counties and in Canada, it still was living in a 
remote part of the ocean in great profusion, and perhaps undisturbed by the causes which put 
an end to its existence here. It was before remarked that the shells of this fossil are nearly all 
broken, and the valves packed together as if drifted or washed by the waves, few perfect ones 
being found. At the west, on the contrary, the larger number appear to be perfect; and 
