Page:The Habitat of the Eurypterida.djvu/81

 remarkable distribution of marine remains far up the estuaries is due to the strong tidal current which brings the debis of organisms living along shore in the more open waters. This current rushes up the Severn at the rate of 6 to 12 miles an hour and it distributes the muds and microscopic organic remains as far north as Gloucester and to every estuary opening into the Severn. "On these shores, so remote from their source, some of these organic fragments find a permanent resting place, and thus far inland we discover along a river bank deposits, containing marine remains. But those which stay are few compared to those which are washed away again and carried out to sea, there to be deposited in marine mud-banks, probably not far from their original home" (264, 620). Just a few miles above the points at which the marine organic remains were found, the muds were examined and every sample showed abundant sponge-spicules, but these all proved to be of the fluviatile species, Spongilla fluviatilis, and none of the marine forms were found. This is the fauna of the recent muds, but a section through the older alluvial deposits which have a maximum thickness of 50 feet, shows that conditions have been much the same for a long time, and that there has been a constant alternation of conditions, first marine, then terrestrial, with the formation of peat beds. Wherever the estuarine sands and muds are washed over the peat beds a similar fauna, dominantly marine, though with fresh-water forms intermixed is found. The section is as follows in descending order:

The deposits in the Severn estuary indicate a gradual subsidence of the land or advance of the sea. Both the upper and lower clay are blue and usually highly fossiliferous. In some sections a few feet will show an abundance of Foraminifera followed by several feet containing vegetal matter. In one section in which no peat was