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 to the one at the base of the Salina. Above the Eurypterus beds follows the Cobleskill limestone, and here again are representatives of the Niagara-Guelph fauna.

If the cycle just described is considered as produced by the desiccation of a closed basin, in which extreme salt pan conditions prevailed at its climax when the thick beds of rock salt were deposited, it follows as a manifest corollary that the eurypterids of Salina age had their biologic optimum in a sea of greater salinity than the typical mollusks and trilobites of the Upper Siluric could endure.

In view of the hypotheses before us and the evidence that the eurypterids flourished in brackish and fresh water in the Devonic and Carbonic this corollary requires a closer study.

The English geologists, notably Hugh Miller and Lyell, in the middle of the last century, explained the origin of salt deposits by the evaporation of sea water in basins so separated from the ocean by shallow bars that the evaporated water could be replaced by new marine water while the corresponding more saline water could not flow out on the bottom. This bar theory has been based on safe physical and chemical data by Ochsenius, while von Koenen and others have shown that this theory on the whole explains the complex composition of the German salt deposits.

A close analysis of the Salina sections and of the character of the Salina rocks also suggests this conception as fully competent to explain the conditions surrounding their deposition.

The facts which we consider as of especial importance to a correct view of the physical conditions of the Salina sea are: (1) the continuous alternations of gypseous and dolomite beds, (2) the great thickness of the salt beds.