134 S. W. Ford—Distribution of the Fossils in the 
Art. XVIL—Remarks on the Distribution of the Fossils in the 
Lower Potsdam Rocks at Troy, N. Y., with descriptions of a few 
new species; by S. W. Forp. 
The only rocks of the Lower Potsdam at Troy that have 
thus far proved fossiliferous are certain thin limestone masses, 
associated with the great body of coarse slates of which the 
formation at this place is principally composed. These masses 
are four in number, or at least occur in four distinct north and 
south ledges. They all occur within a tract not to exceed half 
a mile from west to east and considerably less than a quarter 
of a mile from north to south. Within this tract there are 
several other ledges, some of them holding thin deposits of lime- 
stone; but in these I have not as yet succeeded in finding any 
fossils. The following is a section across the Hudson River at 
Troy, not drawn to scale, but designed simply to show the gene 
ral arrangement of the rocks in this vicinity. 
_ At the point of the section indicated by the vertical dotted 
line a great dislocation most probably occurs, bringing up the 
more ancient strata c and causing them to overlap or stan 
above the newer b. This would, however, hardly be suspected 
from the physical structure of the region ; since, so far as I have 
been able to ascertain, the rocks throughout the entire length 
of the section have the same average (easterly) inclination. I 
have nowhere observed the precise line of contact between the 
rocks of 6 and ¢, unless it be in a high ridge about a mile and 
a half north of this city, where the hard dark-colored slates and 
thin gray sandstones of the Lower Potsdam appear to overlap 
conformably the newer shales. At Troy there is a considerable 
space between them covered with drift. 
* “ Notes on the Primordial Rocks in the vicinity of N.Y.” This Jour- 
nal for July, 1871. sila lata 
