WILLIAM B. DWIGHT. 133 
above mentioned in the ridges of the western margin, 
- suggesting a later date of deposition for the latter. In 
many places these Potsdam rocks exhibit very distinctly 
oblique lamination. 
The strike and the dip are quite variable. The most 
general strike of these Primordial strata appears to be 
about N. 21° E. (true), and the most general dip about 
55° easterly. Some further statements as to local 
variations will be given subsequently. 
_ In order to determine the true stratigraphic relations 
of these Potsdam and related strata, I have made a 
special detailed examination of a district covering about 
a mile and a half square in the vicinity of this locality, 
and embracing about a mile and a quarter of longitudi- 
nal extension in the direction of the strike. This dis- 
trict is mapped in plate LI. 
This Potsdam rock is one of the component members 
of the most western, and by far the broadest of three 
parallel belts of the Wappinger valley limestones, 
which, in this vicinity, rise between the Hudson river 
shales. In general, all these limestones and shales lie 
in a series of abraded folds, having usually a conforma- 
ble strike of from N. 20° E. to N. 30° E. These folds 
are closely compressed, and pushed over to the west, so 
that the earlier limestones usually overlie by inversion 
the later shales lying to the west of them. 
The particular belt of limestone which stretches south- 
west from the driving park is about three thousand five 
hundrea feet in width at its northern extremity, and 
about six thousand feet wide at the southern portion of 
the limited district now under consideration. This 
southern transverse line of six thousand feet passes 
through the middle of the estate of W. S. Johnston on 
the Albany post road. 
. This belt terminates toward the north, quite abruptly, 
along an almost straight line which cuts it apparently 
ab 7 
