"Four divisions have been distinguished in the description of the Onondaga Salt Group, though the lines of separation are by no means well defined. "1. Red and greenish shales below. "2. Green and red marl, shale, and shaly limestone with some veins of gypsum. "3. Shaly, compact, impure limestone, with shale and marl, embracing two ranges of plaster beds with hopper-shaped cavities between. "4. Drab-colored, impure limestone with fibrous cavities; the 'magnesian deposit of Vanuxem.' Of these, the third is the only one that has yielded gypsum in profitable quantities. The included masses of gypsum, though, for the most part, even-bedded at their base, are usually very irregular at their upper surface, often conical. The plaster beds are supposed to be separations by molecular attraction from the marl. "This third division contains not only the gypseous beds, but is most probably the source of all the salt so extensively manufactured at Onondaga, Cayuga, and Madison; at least Vanuxem informs us that, except in these gypseous beds, there is no evidence of salt existing in the solid state in any of the other divisions of the Onondaga Salt Group. "The fourth division is remarkable for a fine columnar structure, or needle-formed cavities, dispersed through the mass. "In the middle counties of New York, the entire thickness of the Onondaga Salt Group must be from six hundred to a thousand feet. Notwithstanding its great thickness, this formation is very barren in fossils. The corals and shells of the Niagara group suddenly ceased to exist, perhaps, as Hall suggests, being overwhelmed by a sudden outbreak of a buried vulcano at the bottom of the ocean, by which the waters became surcharged not only with argillaceous sediment, but became contaminated, either with free sulphuric acid, or sulphate of magnesia and soda. "The country through which the Onondaga Salt Group extends, is usually marked by a series of low, gravelly hills, and clayey valleys, on which a stunted growth of timber prevails, known by the name of 'Oak Openings.' Small portions of sulphate of strontia, galena, and blende, with rhomb spar, occur in the upper portion of the group. Gypsum and salt are, however, the only minerals of economical value: of the former many thousand tons are excavated. Several acidulous springs issuing from these deposits, have been found to contain free sulphuric acid."--_D. D. Owen's Review of the N. Y. Geological Reports._ Jules Marcou, in his Geology of the United States, places the northern portion of the southern peninsula of Michigan in the Terrain Devonian. Report of J. S. Dixon and others, on Grand Traverse Bay, p. 523, in Michigan Agricultural Reports for 1834, says: