EROSION. 33 
does not correspond to its structure, and it has been greatly denuded, 
but the.Taconic Range is far removed from the peneplain type. On 
the other hand, the Rensselaer Plateau is a typical peneplain. The 
preservation of its surface may be due to the massive character of 
its rocks, while the softer schists of the intervening ranges yielded 
j more readily to sculpture. 
If the lower western part of the peneplain represented by the 
Rensselaer Plateau be between the 1,500- and 2,000-foot levels and its 
eastern part fall between the 2,000- and 2,500-foot levels on the 
Green Mountain Range, then those portions of the intervening 
masses which belong to the plain will appear between the same levels 
as shown on the map. The few elevations above 2,500 feet on both 
ranges would then be residuals. But the theory requires an elevation 
of from 1,500 to 2,000 feet at the beginning of the Tertiary. The 
scantiness of the peneplain on the Taconic Range must, therefore, be 
ascribed to the denudation which has taken place since the uplift. 
As approximately one-half of the denudation of the Taconic topo- 
graphic belt has taken place below the 2,000-foot level, that part of 
it must have occurred during Tertiary and Quaternary time, and 
only an equal amount during the much longer Paleozoic and Mesozoic 
periods. 
In order to restore the relations as far as possible to their con- 
dition at the time of the uplift, a depression of the present surface 
some 1,500 or 2,000 feet, to its original " base-level " altitude, may be 
imagined. Such a depression would be sufficient to admit the 
Atlantic, whose waters now reach Albany, and are only 101 feet below 
Lake Champlain, through both the Hudson and the St. Lawrence, 
and transform the Taconic Range into an archipelago, whose con- 
figuration would be determined not by the present complex outline 
of the 1,500- or 2,000-foot levels, but largely by the contours of the 
original anticlines and synclines at those elevations. According to 
this idea the submerged areas of this archipelago were those upon 
which stream erosion began to operate in Cenozoic time, when the 
later uplift took place, while Paleozoic and Mesozoic erosion must 
have operated v upon whatever was above the 2,000-foot level. 
Again, it seems remarkable that if the Cretaceous sea invaded the 
Champlain Valley it should have left no vestige of itself in the form 
of either marine deposits or sea cliffs, although there seems to be a 
rock terrace on the southern side of the Greylock mass at the 3,000- 
foot level and indications of terracing at the 2,500- foot level. To 
explain this feature four alternative suppositions are available: (1) 
The valley was filled with Ordovician sediments to the 1,500- or 2,000- 
foot level, so as to shut out the Cretaceous sea and its sediments, 
in which case Cenozoic erosion must have removed all this Ordovician 
