526 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



a commingling of the muds derived from the erosion of the purer dolo- 

 mites with the mud obtained from older siliceous sediments. Eastward 

 the chemical character of these deposits changes in the reduction of the 

 amount of magnesium and silicate of aluminum. Here the source was 

 most likely the purer Trenton limestone, and the deposits were formed 

 near the open sea. The Trenton limestone beds, which covered much, 

 if not all, of the Adirondack region at that time, and which were exposed 

 by the erosion of the covering Hudson strata during Salina time, form 

 an admirable source for the water-limes and the greater part of the 

 Manlius limestone of eastern Xew York. These late Siluric strata merge 

 southward into open sea limestones of organic origin (Lewistown lime- 

 stones of Pennsylvania), which, during periods of marked transgression 

 (Wilbur, Cobleskill), extended into the region where the terrigenous lime 

 muds were being deposited. 



Of special interest in this connection is the fact that the genera and 

 species of the Eurypterids are peculiarly long lived, and as such do not 

 form exact horizon markers. Thus the same species seem to have existed 

 in the same region from Xormanskill to late Trenton time, and from the 

 beginning of Xiagaran time (Medina-Clinton) to the beginning of Salina 

 (Shawangunk), or perhaps better, end of Guelph time. 123 This is not 

 unlike the persistence of characters in modern river crayfish. Thus the 

 Cambarus primmvus Packard of the Green Eiver beds (Eocenic) of 

 Wyoming is a close relative of the modern C. affinis of the same region. 

 This persistence of species in the same watercourses is in strong con- 

 trast with the modifications which they undergo as they enter different 

 regions, where, presumably, they are cut off from constant intercrossing 

 with the parent stock. With river organisms, geographic dispersion 

 seems to be conducive to specific modifications, but persistence in a given 

 region during a long, period of time seems to bring with it a persistence 

 of type. 



Conclusions regarding the origin of the Shawangimk and Longwood 

 deposits. — Having now shown that the salt deposits of the Salina are 

 best explained on the hypothesis of desert conditions without connection 

 with the sea, and that Eurypterids are river rather than marine animals, 

 we have no longer any excuse for withholding our recognition of the 

 subaerial or deltaic origin of these two deposits. The Shawangunk was 

 formed as a delta by streams coming from the highland of Appalachia, 



»" The Pittsford shales, since they contain the last of the Guelph sediments, might 

 just as well be united with the Xiagaran as a closing deposit. The deposition of the 

 Shawangunk in the east probably commenced before Guelph deposition in the interior 

 bad come to an end. 



