SILURIC SALTS OF AMERICA 377 



tinuous and only locally developed. The beds are all horizontal and 

 in rare cases are any of the layers distorted as is so commonly the 

 case in the North German salt deposits. Pieces of rock, either shale 

 or limestone, are not uncommonly embedded in the salt ; sometimes 

 these pieces are large, and seem to represent a broken-up cover of 

 clay or lime, which subsequently sank into the upper salt layers, or 

 was buried by the new formed strata. 



The Pittsford shale at the base of the series represents a change 

 from the preceding Niagaran (Guelph) sea to the continental con- 

 ditions under which the salt was deposited. The shale alternates 

 with dolomitic layers which contain the last survivors of the Guelph 

 fauna. Near the middle of the mass occur a few black shale layers 

 crowded with fragments and molts of entire individuals of six spe- 

 cies of Eurypterida, including the genera Eurypterus, Pterigotus, 

 and Hughmilleria, and several species of Phyllocarida, and at least 

 one Synxiphosuran (Pseudotiiscns roosevelti). Followed eastward 

 for two hundred miles, this fauna is found in black shale layers 

 in the Shawangunk conglomerate of eastern New York and Penn- 

 sylvania, which is interpreted as a great continental fan formed 

 along the rising Appalachians during the same period, and form- 

 ing part of the upward movement of the North American continent 

 of that time, which resulted in the complete withdrawal of the 

 Niagaran sea of that period. (Grabau-20.) 



The close of the Pittsford shale epoch marks the period of de- 

 struction of the animal life of the Siluric sea in North America, the 

 red Vernon shales, a distinct continental type of deposit 400 feet or 

 more in thickness, succeeding in central New York. These can like- 

 wise be traced eastward, where they swell enormously, forming the 

 Longwood shales of New Jersey some 2,000 feet thick, and through- 

 out of continental origin. While the red Vernon shales of central 

 New York were depositing, salt formed in the deeper basin of south- 

 ern Michigan. Some of this may have resulted from the final evap- 

 oration of the waters of the Niagaran sea, for the salt there lies fre- 

 quently directly upon the limestone. No gypsum or anhydrite has 

 been recorded, but, as before noted, it is not improbable that some 

 of the limestone beds recorded in the well sections are in reality 

 anhydrite. The salt of central New York was probably derived 

 altogether from connate waters enclosed in the recently formed 

 marine Niagaran and earlier strata which underlay and surrounded 

 the salt basins and which suffered extensive erosion during this pe- 

 riod. While these salt deposits were forming, red sedimentation 

 was still going on in eastern New York, New Jersey, and Pennsyl- 

 vania. The centra) New York salt basin extends from the Oatka 



