954 A. W. GRABAU THE TULLY LIMESTONE 



of fishes, probably of river types, in the limestone layers show that the 

 rivers were still bringing in sediments and organic remains from the 

 south. 



This advance of the sea apparently flooded the region in Allegany 

 County, Maryland, and converted it into the estuary in w^hich were de- 

 posited the black Portage muds, which are continued on into ISTew York 

 as the West River and Middlesex shales. These shales thin westward, as 

 does the Genesee, but they extend farther than the latter as the result of 

 the transgression of the sea over the former land-masses. The origin of 

 the West River, and especially the black Middlesex shale, is apparently 

 to be sought in the same river systems which furnished the mud for the 

 Genesee shale. The higher black shales of the Portage, however, thicken 

 toward the west — a reasonable indication that the source of supply of this 

 material is to be sought for in that direction. Indeed, we now know that 

 the upper black Portage shales are the eastward continuations of the Ohio 

 shale series, which likewise had its origin in the south, but in another 

 river system situated farther to the west. These higher black shales and 

 their significance have been fully discussed in the report on the Devonic 

 submitted to the Geological Survey of Michigan. 



It must be borne in mind that throughout the Genesee period, as well 

 as during the period of deposition of the West River and Middlesex shales, 

 the basin in which, these deposits were forming was limited on the east 

 by the Sherburne bar, now represented by that paii: of the Sherburne 

 sandstone which carries only plant remains and the distribution of which 

 is approximately outlined by the area inclosed between the Chenango 

 and the Unadilla rivers of i^ew York. East of this bar the Hamilton 

 fauna continued through the lower 200 feet of Portage beds. Still far- 

 ther east the continental sediments of the Oneonta sandstone were accu- 

 mulating on the sea margin as part of a long-continued delta deposit. 

 These rest upon shales and sands 500 or 600 feet in thickness, which rep- 

 resent the non-marine terminal phase of the Hamilton. These shales 

 and flags contain no fossils other than plant remains, and they pass down- 

 ward into the normal fossiliferous marine Hamilton, as can be seen in the 

 cut of the Ulster and Delaware Railroad east of West Hurley Station. I 

 propose to name the lower fossiliferous Hamilton beds of the Ulster 

 County region the Mt. Marion beds, and the higher, non-marine series, the 

 AshoJcan beds. It is this latter series which Prosser erroneously called 

 Sherburne in his monograph on the Hamilton and Portage beds of eastern 

 New York.^ The Ashokan series forms the principal bluestone formation 



8 New York State Museum. Seventeenth Annual Report of the State Geologist, 1890. 



