220 TRANSACTIONS OF THE Rec 
Messrs. Logan and Hall published in 1866.* Walcott + accepts 
these sandstones of the Adirondacks as typical of the upper 
member of the Cambrian. 
The principal exposure is at Flat Rock, the next promontory 
south of Rowley Bay, in Willsboro’, and is a ledge of white 
Potsdam quartzite of several acres extent, exposed in hare ledges 
for a long distance around the shore of the cove and traceable 
back from the shore into the pine grove where it attains a height 
of forty feet. The rock is similar to that of Au Sable Chasm in 
Chesterfield, but seems more compact, is fine grained, with no 
ferruginous cementation, and without fossils. Especially on the 
weathered surfaces ripple marks are abundant, and slabs showing 
the same, undoubtedly transported from the locality, are plentiful 
along the beach near the boundary between Essex and Willsboro’. 
A Bostonite dike, four and one-half feet wide, cuts the quartzite 
in a cliff twenty feet high around Flat Rock Point. It has al- 
tered the dip of the quartzite noticeably, the bedding north of 
the dike being 10° northwest, while on the south it is 8° south- 
east, although the contact is scarcely metamorphosed. The 
dike is cream-colored, macroscopically resembling a metamor- 
phosed sandstone. It shows in section, however, a compact 
trachytic ground-mass without flow structure. A little pyrite is 
present. The Potsdam meets the Calciferous sandrock in a cliff 
twenty feet high, about 400 feet south of the dike, the contact 
being weathered out, beneath a large tree. The rock contains 
fucoidal remains, probably Buthotrephis gracilis, and has an 
interbedded layer of crushed and metamorphosed shale, at a 
height of six feet above the lake. The Calciferous here is less 
siliceous than elsewhere, more nearly approaching the Chazy, and 
has been quarried somewhat for local use at 164, on the map. 
The other exposures of both Potsdam and Calciferous follow 
the Boquet river along the flanks of the Archean hills. Under 
the dam at Whallonsburg there is a fine exposure of Potsdam 
sandstone, at least thirty-five feet in thickness being visible, as 
it reappears just in front of the Tyrrell House at the top of the 
hill above the bridge. A coarse-grained, gray, very siliceous 
rock occurs on the west bank of the river about 300 feet north 
of the mill, which is probably Calciferous, although it contains 
no fossils. After leaving this exposure the river flows over a 
channel of fine dark-colored silt, through alluvial meadows, 
without outcrop, until the eld Boquet mill at Essex is reached. 
Here broad ledges of Potsdam occur, just south of the bridge, 
* Geoglogical map of Canada and pa:tof the United States from Hudson’s 
Bay to Virginia, Montreal, 166. 
+C. D. Watcort: Correlation Papers: Cambrian, Bull. 8!, U. 8. G.S., p. 341, 
and p. 360, 
