Page:The Habitat of the Eurypterida.djvu/123

 quadrangle, Ontario County, the Bertie is a hard, dark, impure, hydraulic limestone, occurring in thick layers separated by thin seams of dark and apparently carbonaceous matter. The waterlime here shows a gradual transition from the Camillus. Fragments of eurypterid heads and appendages are not uncommon, and frequently ''Leperditia cf. alta, Whitfieldella laevis, and Leptostrophia varistriata'' occur. Yet the marine shells in both cases are seen to be of small specific gravity such as would easily be floated in across mud flats, and they evidently do not constitute a typical marine fauna since too few forms are represented. These occurrences of two or three species of brachiopods and of a crustacean in certain localities, far from proving that the Bertie as a whole was deposited in the littoral district of the sea, shows very clearly that the greater part of the waterlime was not deposited in any part of the sea and that only at intervals were a few marine organisms washed inland. Another significant fact that has already been referred to in connection with modern deposits is the separation of marine and fluviatile faunas in distinct layers. When river water meets with the invading tide, the current is checked and held back; this slack water is still fresh, and it deposits its load of mud and organic remains above the reach of marine waters. If marine currents later overcome the river currents and pass up the stream channel, marine organic remains may be deposited over the freshwater ones. Such lightweight structures as the exoskeletons of fluviatile crustacea and other arthropods are probably seldom carried out to sea against the opposing, denser salt water. If the eurypterids were fluviatile, the occurrence of their remains in abundance and well preserved in the regions where marine fossils are absent, and their scattered occurrence in the localities where a few brachiopods have been found is easily explained. Their entire absence from the Rosendale waterlime and the appearance of only a single specimen in the Rondout is likewise explained, since these deposits show a more marine character than does the Bertie of the Buffalo and Herkimer regions. The river portions of the Rondout and Rosendale either are not uncovered or else have been removed by erosion.

. The only available source of the lime in the Bertie is from the muds derived by the erosion of an older magnesian limestone, the Niagaran, or in some cases, perhaps, the Trenton. Where the Bertie is eurypterid-bearing, the rock was evidently deposited above sea-level, as a river flood plain and subaerial delta deposit. Southward and laterally the subaqueous part of the delta carries few