ONONDAGA SALT GROUP. 
133 
this group, became gradually more clear, the supply ceasing, and at the same time the calca¬ 
reous matter was furnished from another source, at first mingling with this, and finally giving 
rise to an entire distinct formation, contrasting in the strongest degree both in lithological 
characters and in the production of an immense number of organic forms. 
While the deposit we have last considered forms a link, showing the gradual progress from 
one to the other of these groups, the passage from the Niagara group to the Onondaga salt 
group is abrupt, offering no gradation in character of products or in the continuation of fossil 
species. So far as we know at present, throughout the whole western part of the State, no 
trilobite and but two shells of the Niagara group reappear in any higher rock. The inter¬ 
vention of the salt group seems to have exterminated them, and though limestones succeeded, 
yet the same forms did not reappear, notwithstanding the circumstances appear to have been 
favorable to the existence of many similar ones. 
From the sudden change in the nature of the deposit, and the great accumulation of mud, 
it would appear that the Niagara limestone, which, from its abundance of corals, must have 
been formed in a comparatively shallow sea, sunk down to a great depth, allowing this accu¬ 
mulation above, of one thousand feet in thickness. Such a change, with the immediate repeti¬ 
tion of a similar rock, could scarcely have happened, without some violent influence like the 
breaking out of a mud volcano at the bottom of the ocean, by which this product was spread 
rapidly and widely over its bed. The elevation of one point, attendant on such an eruption, 
would naturally be accompanied by a corresponding depression of another, and this portion is 
probably that upon which we have been making our investigations. This deposit, forming 
the greater part of the salt group, is moreover unlike any other formation in the State, except 
the marly portion of the Medina sandstone. The presence of large quantities of common salt, 
sulphate of lime, and even free sulphuric acid, indicates an origin different from the geological 
deposits forming the greater portion of the groups of the New-York System. The rarity of 
fossils is another circumstance indicative of a different condition from that attending the other 
groups. The great amount of finely levigated mud might, however, have rendered the ocean 
too turbid for their existence; and it is very evident that in an ocean where free sulphuric 
acid existed, organic forms would soon be destroyed; and though other circumstances were 
favorable, this alone would prevent their existence. 
Shrinkage cracks. — These phenomena, which were illustrated under Medina sandstone, 
appear in the upper division of this group. The more argillaceous strata at West-Mendon, 
where the surfaces are exposed, present all the appearance of the clayey bed of a shallow 
pond which has been dried by the sun, the whole surface being divided into irregular polygonal 
figures. The cracks are filled with_the succeeding deposition, which differs a little in charac¬ 
ter, and the lower one having previously become partially indurated, very clearly marks the 
cause. These appearances are less abundant in this district than farther east. The same 
strata at Schoharie, and in the Helderberg, are strongly marked by these shrinkage cracks, 
which, in the slabs used for flagging stones, are as clearly defined as in a recent clay bed. 
