28 W. B. Dwight — Recent Explorations in the 



A few days later, a very important outcrop of Potsdam, rich 

 in fossils, was found on the Spackenkill Creek road, and in 

 the same belt of limestone. This road runs nearly east and 

 west, consequently crossing the belt; it is one mile and a half 

 south of the fossiliferous locality at the Driving Park (Smiley's 

 farm). The fossiliferous Potsdam ledge is at the junction of 

 the Spackenkill road with a short road leading to Varick's 

 farmhouse, and is about one mile east of the western edge of 

 the belt, that is, of the line of fault between the Potsdam and 

 Hudson Eiver group previously described. It is thus situated 

 near the eastern edge of this limestone belt. 



The distribution of these fossiliferous localities, as now 

 known, suggests the probability that almost the entire breadth 

 of this limestone belt (in most parts more than a mile), from 

 its western margin nearly to its eastern margin, is Primordial 

 rock ; and this continuously southwest to the Hudson Eiver, 

 where it terminates with a river-front, oblique to its line of 

 strike, of about one mile and a half, between Mallory's Dock 

 and the mouth of Casper Creek. The lithological features 

 favor, rather than oppose this supposition. It also seems prob- 

 able that the eastern edge of this primordial is a line of fault, 

 parallel with the strike, against Trenton strata in some places, 

 and perhaps the Hudson Eiver shales in others ; but more re- 

 search will be needed to determine this. At one spot in this 

 Spackenkill locality a thin layer was found crowded with 

 well-defined fossils. These consist of excellent specimens of 

 Lingulepis pinniformis and the kindred species as found at the 

 Smiley farm, commingled with masses of glabellas, free cheeks 

 and pygidia of two species of trilobites which I have not yet 

 identified at the Smiley locality. These trilobites are species 

 previously found by Mr. C. D. Walcott in Saratoga county, 

 N. Y., and described by him (as Calciferous fossils,) in the 

 Thirty-Second Annual Eeport of the New York State Museum 

 of Natural History, under the names Conocephalites calcifera 

 Walcott, and Bathyurus ornatus Billings. They are now how- 

 ever, referred by Mr. Walcott to the Potsdam strata, and on 

 page 21 of Bulletin 30, of the United States Geological Survey 

 they are listed by him in the Saratoga Potsdam fauna under the 

 names Ptychoparia calcifera., and P. Saratogensis ; closely re- 

 lated respectively to P. Wisconsensis and P. Oweni. The fact 

 of the correlation of these Poughkeepsie strata with the Sara- 

 toga County Potsdam is thus established. There is another 

 locality of Potsdam rock along the summit of the most eastern 

 one of these three belts of limestone in this vicinity. It was 

 mentioned as a probable but not a certain fact, in a note to my 

 preceding paper ; but I feel now quite certain of the fact. It 

 would appear as if the Potsdam here presented a "strike-fault" 



