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, and the 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 Eastern 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. 

 Either 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 

 fitst volume facts have been observed which prove conclusively that the 

 gypsum 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- 



