Page:The Habitat of the Eurypterida.djvu/121

 which I offer the following suggestion. The only decomposition product in which alumina is higher than silica is laterite which might have been formed either during the Salina, to the north of the desert in which the limestones were disintegrating, or else during the Monroan when the arkoses previously formed by mechanical breaking up were subjected to decomposition. That the northeastern portion of Atlantica was of a more pluvial character than the northern part which supplied the lime mud is independently inferred from the character of the deposits formed in western Europe at this time. For here the semi-arid conditions existed on the eastern side of the highland which supplied the sediment, indicating that the moist region lay to the west, where the great southward flowing rivers of Atlantica appear to have had their source.

So far it has been shown that the Bertie waterlime is of clastic origin, and that the sediments were river-transported from the north. The fine stratification of the deposit and layers of sun-cracks in certain localities are structural features indicating that the muds were deposited in quiet waters, while the nature of the fauna has shown that the place of deposition could not have been in the sea, either far from shore, or in any protected, littoral portion; the only remaining place is on the land. In concluding this discussion, therefore, we may test the hypothesis of the flood-plain or delta origin of the Bertie by determining whether it accounts for all the facts. We are to imagine, then, two rivers flowing from the low-lying Canadian area southward until they empty into the slowly-advancing Upper Siluric sea. Marine deposition would be active to the south and if the rocks now covering the Monroan in southern New York and northern Pennsylvania were removed, we would expect to find the mixed marine and freshwater beds which marked the interfingering of the delta deposit with those that were laid down in the sea. Unfortunately, at present we know only the marine Monroan limestones from Pennsylvania, the position of that ancient strand-line being nowhere exposed. If we bear in mind the fact that the outcrop of the Bertie waterlime in New York forms only a narrow belt extending east and west, it is readily understood that the cross-sections of the two eurypterid-bearing "pools" are to be interpreted as cross-sections of the two northsouth river channels (see figs. 3 and 4). The northward extension of those river courses has been removed by subsequent erosion, the southward continuation to the strand line is covered by later strata. If the Bertie waterlime of the two "pools" represents muds really