42 NEW YORK STATE MUSEUM 
proves that the Potsdam sea occupied all these regions. Along the 
southwestern border of the Adirondacks the Potsdam is absent and ~ 
there is not the slightest evidence that it ever was present, so that 
region must have been dry land in Potsdam time. In the south- 
eastern Adirondacks the Potsdam sea certainly extended in as far 
as Wells (southern Hamilton county) and North River (north- 
western Warren county), because small outlying masses of Pots- 
dam sandstone occur at those places. These outlying masses were 
formerly connected with the larger areas but have become com- 
pletely separated from them by extensive (Postpaleozoic) erosion 
and downfaulting. There is no evidence whatever that the sea 
covered the heart of the Adirondacks. To summarize for northern 
New York, we may say that the Potsdam (Upper Cambric) sea 
covered the whole region except the central and southwestern 
Adirondacks which stood out as a great island in the midst of that 
ocean. Southeastern New York certainly, and the middle eastern 
border of the State probably, were covered by the Upper Cambric - 
sea, but whether that sea extended over the rest of the State has 
not been determined because all early Paleozoic strata, if present, 
are now deeply buried. 
What do we know about the character of the topography of the 
land over which that ancient Potsdam sea spread? As a result of 
the very long erosion during late Precambric and early Paleozoic, 
thousands of feet of material had been removed so that rocks which 
had been so deeply buried were exposed at the surface, and the 
whole country must have been well worn down. Was the region 
worn down to the condition of a peneplain? Recent detailed studies 
on all sides of the Adirondacks furnish a very satisfactory answer 
to this question. In many places the Potsdam has been seen in 
actual contact with the Precambric rock whose surface oftentimes 
clearly proves that the whole region had reached a peneplain con- 
dition. Along the northeastern Adirondacks this peneplain was 
considerably rougher than along the northwestern and southwestern 
portions. This is explained by the fact that the northeastern area 
subsided first and consequently was not subject to wear quite so 
long as the latter named areas. 
The accompanying figure (no. 15) affords an interesting example 
of the kind of peneplain topography here considered. It demon- 
strates that occasional low knobs of more resistant rock (for ex- 
ample, Grenville quartzite) protruded above the otherwise nearly 
featureless plain, because when the Potsdam sea overspread the 
