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[Davis 
must have been united in the continuous surface of a broad 
plateau, at a height of about 1500 feet in the Highlands, and 
rising gently northward to 2000 or 2500 in the Catskills ; and 1 
believe that this ancient broad plateau was produced by the eleva- 
tion, with slight southward tilting, of a rolling lowland, a surface 
of deep denudation during Jurassic and Cretaceous time ; but the 
original mass on which the forces of denudation worked to wear 
out this ancient lowland is lost in the distant past. The ancient 
lowland once being uplifted, probably about the close of Cretace- 
ous or the opening of Tertiary time, the excavation of the present 
valley lowland began ; being cut deep in the north, where the 
old lowland was raised high, and shallower in the south, 
where the elevation was less ; and being worn wide open where 
the rocks are weak, as above Newburgh, but still remaining 
narrow where the rocks are resistant, as in the gorge of the 
Highlands. 1 If this view is correct, the present Hudson valley 
must be essentially of Tertiary excavation. When sitting on the 
front cliffs of the Catskill mountains, A, fig. 1, and looking across 
the beautiful lowland to the eastern remnants of the old plateau 
in the mountains of western Connecticut and Massachusetts, B, 
w E 
one must certainly at first doubt the correctness of assigning so 
recent a date for the beginning of so great a work ; and in this 
feeling of general incredulity, I have had my share; but general 
incredulity should not be allowed to guide the judgment against 
legitimate geological arguments, such as have been urged else- 
where, and which seem to lead fairly to the conclusion here stated 
regarding the age of the valley. Moreover, if our measure of 
Tertiary time is taken from the west, where this late division of 
the geological scale is well represented, and not from our Atlantic 
slope, where Tertiary deposits are comparatively insignificant, it 
becomes less difficult to admit that Tertiary time may have wit- 
nessed large changes in our Atlantic slope topography. 
1 See The geological dates of origin of certain topographic forms on the Atlantic 
slope of the United States. Bull. Geol. Soc. Amer., ii, 1891, 570. 
