8 GEOLOGY OF OHIO. 
~ 
series, seems to be a local accumulation of mechanical material, that 
must have been brought from the land at the east, during a period of 
shallow water, by the action of some local agent, such as a narrow tide- 
way or river current. 
Unfortunately the Niagara Group dips rapidly eastward, and is, for two 
hundred miles, buried under the Allegheny coal-fields, so that we are 
unable to trace this mechanical material to its source. That it came from 
the east or south-east is indicated by the fact that the Cincinnati arch 
could furnish no sand, and in Northern Ohio, Canada, and New York, no 
sandstone is contained in the Niagara Group. 
THE SALINA GROUP. 
The Salina Group makes little show in Ohio, andthe phenomena which 
it presents here have been already described. Additional light has, how- 
ever, been thrown upon its general character and history by observations 
made in Canada and New York. These confirm the views advanced in 
our first volume that this formation was deposited in a local basin left on 
the withdrawal of the Upper Silurian Sea; a body of salt water to be 
compared with the Caspian or Great Salt Lake. The limits of this basin 
are in a general way marked out by the thinning away and termination 
in different directions of the peculiar sediments that accumulated in it. 
It extended from Central or Hastern New York to the base of the Cincin- 
nati axis, and from the foot of the Laurentian highlands, on the north, to 
Virginia. It was filled with material washed in from the surrounding 
lands mingled with salt and gypsum, precipitated from the water which 
occupied it. These latter now form extensive beds many feet in thick- 
ness—of salt in Canada—of gypsum in Ohio and New York. Whether — 
they were precipitated from the water of rivers which flowed into the 
Salina basin, and was there evaporated so as to deposit its saline matter, 
or were derived from influxes of the sea, cannot now be determined. 
Hither cause would be adequate to produce the result. 
In the former notice of the Salina Group exception was taken to the 
theory advocated by high authority that the gypsum it contained was 
formed by the action of acid waters on limestone ; and it was argued that 
this—like all other great deposits of gypsum—was a true sediment pre- 
cipitated by evaporation from saline water. Since the publication of our 
first volume facts have been observed which prove conclusively that the 
eypsum of the Salina Group in Ohio, at least, was deposited as a sediment 
or precipitate, and is not a secondary product, for in the quarries near 
Sandusky the strata of snowy gypsum are not only too extensive and | 
regular to have been formed in this way, but they are separated by persist- 
