132 W. B. Dwight — Potsdam strata near Poughkeepsie. 



The more abundant of these fossils are the L. minima, whose 

 fragments are thickly scattered through some portions of the 

 shalv limestone, and next the movable cheeks of trilobites. 



It will be observed, as has been suggested to me by Mr. R. 

 P. Whitfield, that the fauna of this locality forms a connecting 

 link between the known fossils of the Appalachian region and 

 those of the Western States. Thus with the Lingulepis minima 

 and L. acuminata of the New York Potsdam is found the Lin- 

 guhpis pinniformis of Wisconsin and other western localities. 



It may also be remarked that the fauna of this Poughkeepsie 

 locality, as far as at present developed, is quite distinct from 

 that of the Potsdam strata recently discovered by Mr. S. W. 

 Ford at Schodack Landing and other places in the town of 

 Stuyvesant, N. Y. From these localities, lying about fifty 

 miles north of Poughkeepsie, Mr. Ford reports (this Journal, 

 July, 1884) the following eleven species: Paleophycus incipiens, 

 Obolella crassa, Sienotheca rugoso , Byolithes Arnericauus, H.impar, 

 Hyolithellus micans, Microdiscus lobatus, M. speciosus, Conocoryphe 

 Lrilineata, Olenellus asaphoides, and Fordilla Troyensis. 



No one of the above list has been found at Poughkeepsie ; 

 the distinctive character of the two faunas is evident, the latter 

 apparently representing an earlier stage of life. 



The discovery of the presence of fossiliferous Primordial 

 rocks among the Wappinger Valley limestones, while it adds 

 an extremely interesting feature to the geology of the region, 

 contributes another complication to the difficult task of master- 

 ing the stratigraphy of Dutchess County. 

 "Vassar College. Poughkeepsie, jST. T., June 25, 1885. 



Note. — Since the above paper was written I have conducted these researches 

 further, and have traced the continuation southerly of the fault between the Pots- 

 dam and Hudson River shale to the bank of the Hudson River. This line, which 

 had been traced previously only into W. S. Johnston's farm, has been found to 

 continue, well marked as before by ponds and gullies, in a straight line till it 

 crosses the Albany post road at the corner of the New Hackensack road near 

 School House Xo. 2. On the west side of the post road it passe-. straight across 

 R. T. Gill's farm, then crossing the road leading to the Milton Ferry, and striking 

 between the two roads leading in a southwesterly direction to the river. It ter- 

 minates finally in a high bluff on the river near Mallory's Moulding Sand Dock 

 and about one mile north of Clinton Point post office. Throughout this course 

 the Primordial limestone and the shales show themselves frequently in close 

 proximity in outcrops. In Gill's farm both formations are mostly covered by hills 

 of drift, but on this farm, just north of the Milton Ferry road, both the Potsdam 

 limestone (here as calcareous shale) and the Hudson River shales crop out dis- 

 tinctly. The latter yield at this locality some very excellent and characteristic 

 fossds, as encrinal columns, Leptcena sericea. etc. The terminal bluff at the river 

 is composed of both formations, the fault running along its summit, but near the 

 northwestern edge. A s the extreme southwestern point of the bluff is reached, 

 the line of fault drops down its northwestern side, and the shales at last disappear, 

 leaving the point in possession of the limestone. After careful examination I am 

 satisfied that some, if not all, of the moulding sand is produced by the decomposi- 

 tion of the arenaceous Potsdam. Indeed, the process may now be seen going on 



