ONONDAOA-SALT GROUP. 157 



of the lower part of this mass were thrown out from a well, such as fibrous gypsum and 

 Selenite embraced in fragments of green shale. From this extreme eastern point of the 

 rock, it widens and deepens westward, but probably attains its fullest development in 

 Onondaga county, in the vicinity of the principal salt wells and springs. But few of the 

 characteristic marks of this rock appear in Monroe and Erie counties ; and probably it 

 thins out to such an extent, that it is but feebly represented in the extreme western coun- 

 ties. The belt which it forms, then, though not confined to the central part of the State, 

 is still wider and deeper there than elsewhere, especially the lower part so well known by 

 the hopper-form cavities. At a low stage of the Niagara river, it appears in the banks at. 

 Grand island. Mr. Hall speaks of this mass as not uncommon in Monroe and Wayne 

 counties, but as hardly known in Genesee or Erie. 



Interesti/ig feature in the region of the plaster beds and green shales. That these rocks 

 are easily worn down, and are liable to be cut deeply by running water, has been stated 

 already ; but the combined action of running water and of atmospheric agencies generally, 

 has not been sufficiently explained. Perhaps I ought also to include diluvial agency, as 

 particularly active in giving shape or contour to this part of Onondaga county. The fea- 

 ture which I propose to speak of, is exhibited in the numerous high and round eminences 

 that occur upon the plains, like immense mounds. They are quite round, sloping steeply 

 and equally in all directions, with summits almost perfectly oval or round : they are sixty 

 or seventy feet high, and from their summits many others are seen ; indeed they command 

 fine views of the surrounding country. The rock sometimes crops out from a point, showing 

 by this that they are not entirely drift hills. Section 26, though not designed to illustrate 

 the form of the hills, will give an idea of some of the facts which stand connected with 

 them. 



Fig. 26. 



1. Round hill, the highest points of the immediate region. 2. Gypseous rock. 3. Tufa deposits. 4. Drift, made 

 up of fine and coarse cobblestones. 



As I have intimated that these hills, and the peculiar shape of the country, are not en- 

 tirely due to diluvial action, nor to present atmospheric agency, it may be expected that 

 some other cause shall be assigned for the phenomena under consideration. In searching 

 for something explanatory of these changes, I found that near the outcrop of the Onon- 

 daga limestone, circular gorges, or what might be called very appropriately roadways, 

 occur, which encompass, in the instance before us, an area of two hundred and fifty 

 acres. This area presents nearly a semicircle of perpendicular ledges on both sides, whose 

 height is not far from one hundred feet. By some means a cleft of a circular form has 



