by the Niagaran limestones which would furnish pure clastic limestones and not impure siliceous muds. The only area from which the muds could be derived, was the land to the east. That the black mud was merely an extension of the muds forming at intervals on the Shawangunk delta must be obvious when it is seen that in that direction was the only source of the muds and that the Shawangunk muds contain the same eurypterid fauna. This will be more fully discussed in Chapter V.
The areal distribution of the Pittsford is limited. The shale is known from Monroe county and from Oneida county, New York. Both eastward and westward it dies out, the Vernon red shale resting directly upon the Niagaran. In a few localities black shales have been found which have been correlated with the Pittsford, but they contain no eurypterids. Such is the black shale at Buffalo, on Grand Island, and the dark shale in Herkimer county above the Lockport dolomite which contains no fauna except a few Lingulas. The outcrop in Oneida county is at Oriskany creek, where in a bluff occur some dark gray shales, about 21 feet below the base of the Vernon red shale, with intercalated waterlimes and dolomite beds. These dolomites contain fragments of one species, Eusarcus vaningeni, together with lingulas and orbiculoideas. Both the areal and vertical distribution, then, are limited, in much the same way as in the Bertie, and the source of this calcareous material may likewise be the same. (See beyond, p. 234.)
If the eurypterids of the Pittsford shale were brought in by the rivers coming from Appalachia, the waters in the region of deposition would become freshened by the inpouring of the river waters, and marine forms would thus be kept out. It is often assumed that the Pittsford shale marks a periodic increase in the salinity of the water, but in that case we are faced by a double problem: if the muds were not deposited by rivers, where did they come from, since they could not have originated in the sea? and then again, the question arises, where did the eurypterids suddenly come from? The muds might be æolian, but not the eurypterids. The only possible conclusion seems to be that the eurypterids and the black muds both were brought by rivers from the land, i.e., that the eurypterids were river-living organisms.
In this connection attention may be called to the fact that the species of eurypterids in the Pittsford and Shawangunk and to some extent the genera as well, are entirely different from those of the Bertie. This is not alone accounted for by difference in age, but is
