-_Houghton Geological Reports of Michigan._

"Foster and Whitney, United States Geologists, in their Reports to the
Government, laid down the Onondago Salt Group of rocks as extending
over a portion of the southern part of the northern peninsula of
Michigan, not a great distance from Mackinaw, and also as existing on
the St. Martin's and Mackinaw Islands.

"ONONDAGA SALT GROUP.--As a whole, it is an immense mass of
argillo-calcareous shaly rocks, inclosing veins and beds of gypsum;
hence this has been designated by some as the 'gypseous shales.'

"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 Gr