54 ONONDAGA GROUP. 



CHAPTER XVI. 



ONONDAGA GROUP*. 



108. THIS Group was named the Onondaga Salt Group, by the New York 

 Geologists, from Onondaga County, New York, in 1839, and re-defined by Vanuxem 

 in 1842, and by Hall in 1843. The Canadian Geologists very properly dropped 

 the word "salt" from the name. It consists, on Oneida Creek and Cayuga Lake, 

 in the lower part, of clayey deposits and red shale, showing green spots, followed 

 by gypseous shales and impure limestones, which at the commencement alternate with 

 the red shale, and this is followed by the gypseous deposit, which embraces the 

 great lenticular masses quarried for plaster, and this by a magnesian rock having 

 groups of needle-form cavities caused by the crystallization of sulphate of magnesia, 

 and the upper member is the Waterlime. It rests upon the Niagara from the west- 

 ern line of New York, east to the middle part of Herkimer County, where the 

 Niagara thins out ; it then rests upon the Clinton until it disappears, and then upon 

 older rocks until it reaches the Hudson River. It is therefore unconformable with 

 the underlying rocks in middle and Eastern New York. The red shale loses its 

 color west of the Genesee, becomes a bluish green, and gradually thins out, showing 

 the unconformability in Western New York. The passage from the Niagara to the 

 Onondaga is abrupt, offering no gradation in character of products or in continua- 

 tion of fossil species. The great mass of gypseous deposits consists of yellowish or 

 drab, and brownish colored argillaceous, and calcareous shale and slate, or of hard 

 and compact slate, which weathers as if hacked by an instrument. The dark color 

 of the gypsum, and brownish color of other rocks, is due to carbonaceous matter. 

 An important member is called the vermicular limerock, which is gray or blue, 

 and perforated with holes and cells, once filled with soluble saline material, which 

 subsequently dissolved, leaving the cavities, some of which are hopper-shaped, and 

 were produced by common salt, as no other common soluble mineral presents 

 similar ones. The sulphate of magnesia cavities are lined with carbon, showing the 

 liquid that held the salt in solution, contained bituminous matter, the salt ejecting 

 its particles in the act of assuming form, as occurs in the purification of acetic 

 acid when obtained from the distillation of wood. This Group is celebrated for its 

 salines, and formerly furnished nearly all the salt consumed in New York; for this 

 reason it has been called the Salina and Saliferous Group. Sulphate of Stron- 

 tian and sulphurets of lead and zinc occur in small quantities. Sulphuric acid 

 escapes with the water from the earth in many localities, giving rise to acid springs, 

 and sometimes destroying the water in wells for culinary purposes, as the sulphuric 

 acid becomes strong enough to coagulate milk. 



109. The Group attains its greatest thickness at about 1,000 feet in Wayne 

 County, and gradually diminishes westerly, so that on Grand River, Canada, it 

 does not exceed 300 feet, which belongs chiefly to the upper portions, from the 

 summit to a little below the gypsum-beds. The beds of gypsum are never contin- 

 uous for long distances, but appear as detached lenticular or dome-like masses ; the 

 strata above them being arched over and often broken, while those below consti- 

 tute an even, undisturbed floor. The Group is continued through Lake Huron to 



