Page:The Habitat of the Eurypterida.djvu/106

 buried; but in eastern New York during the later stages of the Ordovicic the streams did not have a moderate gradient. The elevation of the land leading up to the tectonic movement known as the Taconic folding which displaced the rocks in some regions in the Hudson Valley as much as 90° was in progress at least throughout the Upper Ordovicic. One of the results of this movement was the steepening of the gradients of the streams which thus became torrential, not necessarily through increased rainfall, but through increased gradient. The streams consequently brought great quantities of clastic material to the margin of, and into the sea, where deposition probably went on in a sinking geosyncline.

. The physical characters of the Normanskill and Schenectady beds point to Appalachia as the source of the sediments. The mode of occurrence of the eurypterids, graptolites and plant remains is better explained on the hypothesis that the eurypterids and perhaps the plants also were fluviatile and not marine organisms. As yet the Ordovicic merostome faunas are too little known to say that the habitat can be proved to be one or the other; the most that we can say is that all the known facts are better accounted for by the fluviatile hypothesis, which is fully supported by the palaeontological and chorological data.

In the intercalated shales in the Shawangunk conglomerate at Otisville, Orange County, New York, and at the Delaware Water Gap and elsewhere a large eurypterid fauna has been discovered. The Shawangunk is distributed in the form of a semi-cone, having its greatest thickness, about 2000 feet, in the Delaware Water Gap region, and thinning away in all directions. That is, it has the form of a dry delta or alluvial fan rather than of a sea coast deposit, for if it were the latter, it would be of fairly uniform thickness and would not have the semi-cone shape. The pebbles in the conglomeratic portion of the Shawangunk are well rounded, but in some sections a certain amount of angularity is still retained as a rule. For riverworn pebbles to be perfectly rounded, they must be transported for a considerable distance. Again the complete destruction of all but the quartz argues for prolonged transportation and frequent reworking. As in the case of the other clastics, there was no source for the