366 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY, 



The passage from the Niagara group to the Onondaga is abrupt, 

 without gradation of any kind. 



The Niagara strata seem to have sunk down to a great depth, and 

 thus allowed this accumulation of a new sediment 1000 feet thick, — 

 the new sediment brought in perhaps, as James Hall thinks, through 

 the agency of a mud-volcano, which widely spread this vast body of 

 mud over the bed of the existing ocean. 



Place. — The group now under consideration occupies the largest 

 territorial surface in Central New York ; and about Lake Cayuga 

 and in the county of Onondaga. There the denudation of the 

 newer rocks gives this formation a greater southern extension by 

 several miles than it has further west. It has been, as well as the 

 Niagara group, deeply excavated by ancient agencies throughout the 

 greater part of Western New York. It is often a mere band in pass- 

 ing from the last-mentioned district to the River Niagara, but widens 

 to the breadth of fifty miles in the Canadian counties, south-east of 

 Lake Huron, and is still broader on Lake Michigan. 



Its northern and western outcrop can be traced for 1 130 miles from 

 Schoharie, in New York, to the west side of Lake Michigan. 



The Onondaga rests upon the Niagara group from the middle of 

 Herkimer County westwards, and not only in New York, but where- 

 ever visible in its long course across Canada West to Michilimac- 

 kinac and Lake Michigan, many of the islands of which, with parts 

 of its west side, it occupies. 



From where the Niagara group ends eastwards (Little Falls, N. Y.), 

 the Onondaga rests on the Clinton, and still more easterly it reposes 

 on the Frankfort Slate of the Hudson- River-group, and so continues to 

 within a short distance of the Hudson River ; but then it thins out 

 to only few feet of thickness, begins to be absent in places, and finally 

 disappears (Mather, Report, 353). 



The grey or yellow porous (vermicular) limestone of this group 

 was met with by Vanuxem at Sharon Springs, overlying Frankfort 

 Slates, and underlying the Helderberg series. 



Position. — This group displays a series of gentle undulations, like 

 its associate strata. Sometimes they may be, in reality, excavations ; 

 but in many places, the layers of this group undeniably dip to the 

 south-west, — at Syracuse, as well as in other situations, at the rate 

 of 25 feet per mile (Vanuxem, p. 108). 



Thickness. — This varies from east to west ? thinning out in the 

 former direction. It may be estimated at 700-1200 feet. 



Fossils. — From the experiments of M. Beudant (Vanux. p. 102), 

 testaceous animals cannot live in water saturated with gypsum ; and 

 such was mostly the state of the Onondaga-salt sediment. 



Therefore, as Hall says, this group cannot be characterized by its 

 fossils ; there being so few, and these so imperfect. The few near the 

 base are a continuation of species from the Niagara ; and those near the 

 top of the Onondaga-Salt group appeared after the various salts had 

 ceased to prevail, and during the gradual change which restored the 

 Wenlock epoch. There are few or no fossils in the middle of this 

 group. In one place only, near the road from Jordan to Peru, did 



