Page:The Habitat of the Eurypterida.djvu/235

Rh 2. Three genera, Stylonurus, Echinognathus, and Hughmilleria were restricted to the rivers of Appalachia.

A far greater interest must attach to the vast northeastern continent of Atlantica which stretched across the north Atlantic and formed a land bridge of vital importance in the migration of the eurypterids. The organisms living in the rivers of this continent were not geographically restricted like those in the rivers of Appalachia, whose remains were washed out occasionally into the surrounding ocean waters, but which were prevented from migration to European fresh waters by the broad expanse of the Palæozoic Atlantic; more fortunate by far were the fluviatile inhabitants of Atlantica, for this continent, we may feel sure, was fairly permanent throughout the Palæozoic, even though the ocean at times encroached over much of the southern part; it was the northern portion that would be vital for the interlocking headwaters of different river systems, and as we shall see there is overwhelmingly convincing evidence pointing to such an intimate relation between the river systems of the periods from the Upper Siluric through the Devonic. Not only were the geographical position and extent of Atlantica more favorable for the widespread dispersion of the eurypterids than were the same physical features of Appalachia, but the sediments derived from the former continent were for the most part of the particular lithological character most favorable to the preservation of organic remains, while those from Appalachia were quite often coarser, being prevailingly sandstones and conglomerates, with only thin beds of intercalated muds. The early differentiation in the character of the clastic deposits from these two continents reflects the still earlier difference which had existed between them in the matter of elevation, for, whereas during Ordovicic and Lower Siluric (Niagaran) time the Canadian area, already peneplaned, had been largely covered by the sea, as indicated by the remnants of Niagaran limestones, and whereas during the same period the Baltic region and that area now forming the southern shore of the Gulf of Finland had likewise been covered by a shallow sea in which coral reefs flourished, the continent of Appalachia on the contrary, had jutted up from the Atlantic with lofty mountain ranges of crystalline rocks. Thus it came about that the rivers in their slow but efficient work of denudation brought into the waters bordering the continent of Atlantica sediments that were calcareous and usually fine-grained (waterlimes) while the rivers of Appalachia carried highly siliceous materials of medium or coarse grain (sandstones and conglomerates) and the winds transported siliceous sands.