EROSION. 29 
transverse hollows, neither material nor structure affecting the 
topography. 
The northwest-southeast limestone area in Whitecreek, N. Y ., 
corresponds to a peculiar change in the strike which recurs at the 
northeast edge of the Rensselaer Plateau. 
The easterly dipping cleavage so prevalent in the schist masses 
often determines the angle of the eastern slope of the hills, while 
strike and dip joints account for most of the north-south and east- 
west cliffs, and the rest are fault scarps. The sags in the crest lines 
are sometimes due to axial pitch, the larger saddles to limestone 
anticlines, and the lesser points to thick beds of quartz conglomerate 
or to large quartz veins in the schist. 
Of the two long incisions in the Green Mountain Range that of 
the Walloomsac is evidently more dependent upon structure than 
upon material, while that of the North Branch of the Hoosic is 
consequent upon structure as well as upon material, the limestone 
and schist extending up into the valley. The short incisions farther 
north are independent of structure and material. The gorge of 
Middlebury River has been cut for a length of 2| miles indifferently 
through gneiss, Schist, and quartzite and across their strike. But 
the north-south incision opposite Manchester coincides in its lower 
part with a limestone syncline, as shown in PL V, Z?, but in its 
upper part crosses an anticline of quartzite. 
The embayment in the Green Mountain Range may be attributed 
either, according to Dana, to an original irregularity in the pre- 
Cambrian surface or, according to Pumpelly, to a horizontal move- 
ment which altered the relations of the pre-Cambrian crystallines 
to the Cambrian and Ordovician sediments, or possibly merely to 
north-south compression along the axes of the folds, resulting in 
pitch (p. 27, and PL VI A). This last possibility would be more 
apparent in a section in which the Paleozoic sediments had been 
restored to the Green Mountain Range than it can be from the 
present surface relations of the' formations. Possibly all these factors 
had a share in producing the embayment. 
EROSION. 
GENERAL EFFECT. 
It is evident from what has been stated that neither rock material 
nor structure, singly or jointly, suffices to account for the land form. 
There remains to be considered the important factor of erosion. 
Before determining the character and amount of the denudation 
Avhich this region has suffered, the probable surface features as they 
emerged during the Ordovician movement should be restored, and 
