Page:The Habitat of the Eurypterida.djvu/119

 Temiscaming region and elsewhere. At the base of the series is the Clinton followed chiefly by shales and limestones representing the Rochester and Lockport, and finally by a dolomite. Since the sea in which these deposits accumulated was a transgressing one, it is apparent that in some sections the Niagaran deposits would overlap the late Ordovicic deposits and come to rest directly upon the crystallines of the Canadian shield. Furthermore, progressively higher members of the Niagaran would come to rest upon the old land as the Niagaran sea continued to spread. By the end of Lockport time, the greatest expansion was reached, and contraction of the sea set in, the Guelph dolomites being deposited in this more circumscribed sea. In some sections the change in deposition is inaugurated by the argillaceous beds of the Eramosa formation, and some of the late Niagaran beds are somewhat argillaceous. Beyond the farthest line of expansion of the Niagaran sea, the crystallines continued to form the rocky surface of the land. The contraction of the sea continued, until by the beginning of Salina time it had shrunk to such an extent that only a small epi-continental sea remained. It makes little difference whether we assume that this sea dried up entirely during the period when the salt formed in central and western New York and in Michigan, or whether we believe that the contracted remnant of the Niagaran sea persisted, the greater part of the North American continent is known to have become dry land during Salina time. Many writers have pointed out the evidences of arid conditions in the Salina, and I need not here repeat them. The entire country was exposed to drying winds, rain fell but seldom, and then it came as cloudbursts, filling river channels quickly and creating torrential streams of short duration. Whatever vegetation there may have been upon that ancient land was destroyed by the heat, and we may picture the country as a great desert where desiccation was in progress and where the winds and the rivers of flood seasons were the chief agents of transportation for the mechanically broken up rocks. The Salina was by no means a period of short duration; the thickness of the salt deposits alone shows that a long time was required for their formation. Throughout this whole period, disintegration of the Niagaran and earlier limestones was in progress, until there must have been piled up great limestone and dolomite dunes with fine beds of impure clayey material wherever shales were exposed to the clastation processes of the semi-arid climate. The crystallines likewise suffered the same destruction, and they added their quota to