WILLIAM B. DWIGHT. ssil 
and Trenton groups. Incarrying out the work, in which 
I have for several years been engaged, of preparing a 
detailed stratigraphical chart, on April 25th of the 
present year, to my great surprise, I struck a ledge of 
rock in the Wappinger valley limestone belt, which 
_ proved rich in Potsdam fossils. 
This remarkable locality is on an estate owned by Mr. 
Albert K. Smiley, propriétor of the Lake Mohonk house. 
It is in the outskirts of the city of Poughkeepsie, to the 
southeast. It is about one mile southwest of Vassar 
college, eight hundred fifty feet south of the southwest 
corner of the driving-park on Hooker avenue, and about 
two thousand two hundred feet west of the road which 
passes south along the east side of the same. | 
The Potsdam rocks are found in a series of low hills or 
ridges trending northeasterly and southwesterly in par- 
allel lines. The most interesting paleontological features 
appear to be concentrated in a ridge lying at the north- 
east corner of the group (hill A, plate VI). This is sit- 
uated immediately to the southwest of Smiley’s detached 
barn, which is itself south of the southwest corner of 
the driving-park. This hill is about three hundred feet 
wide and one thousand four hundred feet long; it is 
mostly covered with soil, the rock cropping out on each 
side, but chiefly on the east, in a few narrow, quite in- 
conspicuous ledges; it would never have attracted my 
attention but for its occurring within the range of a sys- 
tematic survey. 
Lithologically this Potsdam rock is exceedingly varia- 
ble, and all the varieties described occur within the 
small compass of a few acres of ground. It is every- 
where (so far as already examined) calcareous, all its 
varieties effervescing with acids. It is also everywhere 
more or less arenaceous; often conspicuously such. 
Large portions of it area tough, compact limestone, often 
quite dark, and frequently filled with a conspicuous 
15 
