CONCLUSION. 
523 
After this a new source of materials is opened; and apparently from a mud volcano, in the 
midst of this ocean, is poured forth rapidly a vast deposit of mud and fine sand charged with 
ferruginous matter. No vestiges of living forms are found till towards the close of the period, 
when the mud had in a great measure given place to sand ; and here only a few appear. In 
this period are included the marls and shaly sandstones of the Medina formation. To these 
succeed finely levigated mud; but it soon alternates with arenaceous and calcareous deposits, 
and even with coarse conglomerate over limited areas, showing the changing condition of 
the ocean, and the nature of the sedimentary and chemical products. Large quantities of 
hydrate of iron are spread over extensive areas, mingled with the fragments of organic remains. 
The whole presents a group of Protean character, indicating the fluctuating nature of the 
deposits, and the disturbance at the source of the materials. Still, over a large portion of 
the country where it is exposed, the influence of those disturbing causes did not extend, and 
we have all the phenomena indicative of a quiet deposit in a moderately deep ocean. Such 
are the materials of the Clinton group. 
The Niagara group commences in New-York with a deposit of finely comminuted mud, 
while at the west it is almost entirely calcareous. Its attenuated northern and eastern extremity 
in New-York reaches nearly to the Hudson river ; while, pursuing it westwardly, it expands 
to an enormous thickness, and covers a vast area in the valley of the Mississippi. For an 
almost incalculable period of time, this part of the ocean bed could have suffered little change, 
nor have been disturbed by inundations of sedimentary matter. The whole expanse must have 
been one broad blue ocean, with its infinity of coral groves, among which lived the shells and 
crustaceans ; and here, in sheltered nooks, the crinoideans reared their beautiful and gorgeous 
heads, and above them shone the bright tropical sun. But this period of repose is suddenly 
broken ; the ocean bed sinks down, and an overwhelming inundation of mud is spread out over 
a large portion of its before limpid waters. The whole family of corals, shells, and crustaceans 
are alike exterminated. Nor is it mud alone that here operates as a destroying agent of the 
organic forms ; the deposit, doubtless ejected from a huge mud volcano, is charged with saline 
matter, and corroding acids, which would alone destroy all organisms. This vast mass of cal¬ 
careous mud formed the marls and shales of the Onondaga salt formation. 
Towards its close, calcareous matter becomes intermingled in an increasing proportion, and 
a few shells appear. At last the turbid ocean becomes clear, when again the corals commence 
their habitations, and with myriads of shells, and crustaceans, continue through an immense 
period of time, disturbed, it is true, by an inundation of sand and mud, forming the shale of 
the Cauda-galli grit and Oriskany sandstone, which, in some parts within their range, destroyed 
many of the living beings. This, of all periods, seems to have been the most prolific in its 
growth of corals; and numerous species abound, not only throughout New-York, but far 
westward to the Mississippi river, showing, over an extent of a thousand miles, a uniform 
depth and condition of the ocean ; and like the barrier reefs of modern seas, giving birth and 
shelter to myriads of living creatures, whose destruction formed the rocky strata. 
66 * 
