Page:The Habitat of the Eurypterida.djvu/107

 conglomerate to the north, south or west, the only possible one being to the southeast where lay the old land of Appalachia. This is also indicated with great certainty by the shape of the cone of this formation which is thickest in the southeast, thinning out to the north, west and south. The material of this alluvial cone could only have been transported by rivers and its river-borne character and deposition upon land are well shown in the frequent occurrence of the torrential type of cross-bedding, and in the absence of any typical marine organisms, such as would be found in the subaqueous portion of a delta. Apparently there was no sea-border portion of this deposit, unless the Pittsford shale is considered as such. The Shawangunk conglomerates found in Pennsylvania and New York, thin away towards the region of Pittsford shale deposition, but actual connection has not been traced. There can, however, be little doubt that the Pittsford shale represents the finest material brought by the Shawangunk river from Appalachia. This mud was deposited very near the sea border, but there is no evidence that it was deposited in the sea, since the typical marine organisms are absent. It may be that the influx of fresh water was sufficient to keep these out. Interfingering with the shale, are the dolomite beds, deposited during short incursions of the sea (see section 4). If we could trace these Pittsford shales to the northern part of the state or into Canada, we would expect to find them grading into pure marine limestones, but no outcrops are accessible. The black shales intercalated between the conglomerate beds of the Shawangunk at Otisville and elsewhere represent the muds carried down by the river during times of flood. If the eurypterids were living in those rivers, their exoskeletons would have been floated down, while dead and even living individuals would have been swept down by the force of the torrential floods. The exoskeletons and floating bodies would settle down with the mud on the drying up of the water from the flooded areas. That such drying occurred is indicated by the presence of mud-cracks in these intercalated shales. When this mud dried up and became cracked, the eurypterid exoskeletons would be broken up and blown about by the wind, until the fragments should be covered by the next torrential deposit. This breaking up of the tests as the mud dried would account for the fact that the larger eurypterids are always found in a fragmentary condition, while the smaller ones are found whole. How to account for this fact on any other hypothesis is difficult to see.