32 TACONIC PHYSIOGRAPHY. 
longitudinal troughs occurred in the axis are taken for comparison 
It is also evident that the general descent of the original surface west 
ward as it emerged from the Ordovician sea was much more markec 
than the present surface indicates. 
At this point it is well to examine afresh the peneplain theon 
as applied to this region/' This supposes that during the period; 
between the end of the Ordovician and the end of the Cretaceous 
the entire region was, with the exception of a few residuals (monad 
nocks), reduced to a peneplain or base-level, remnants of which cai 
now be seen on the Green Mountain Plateau. But, commencing witl 
the beginning of the history, the supposition is that in pre-Cambriai 
time a mountain system existed in New England and another in the 
Adirondacks, and that after the Cambrian-Ordovician transgressioi 
the denudation of these mountains furnished materials for thi 
mechanical sediments which now occupy the Hudson-Champlain val 
ley and the Ta conic region. However much uncertainty may attenc 
the reconstruction of Green Mountain Cambrian and Ordoviciar 
geography, we know the approximate minimum thickness of Cambro 
Ordovician mechanical sedimentation, and thus have at hand sonic 
measure of the denudation of the pre-Cambrian land masses during 
Cambrian and Ordovician time. It may be seen from the map (PI 
I ) that after deducting the Green Mountain and Adirondack border 
areas, together with those Cambrian areas which may have been above 
water during Ordovician time, there are yet about 2,400 square mile^ 
over which mechanical sediments measuring at least 3,500 feet, and 
quite possibly 4,500 feet, in all probability extended. This amount ol 
detritus, if added to those portions of the Adirondack and Green 
Mountain islands whose drainage entered this arm of the Atlantic, 
would suffice for a small mountain system. Whether the Cambro- 
Ordovician period of erosion alone was sufficiently long to reduce this 
mountain system to sea level can hardly be determined. 
The Ordovician movement folded the entire region, sea bottom and 
sea deposits, as well as portions of the land masses, and thus arose 
another mountain system. If another transgression took place in 
Silurian time over a portion of the Taconic region, as seems probable 
from a study of the Rensselaer Plateau and Bird Mountain,'' the 
second great period of erosion must have been interrupted in Silurian 
time and to have had a duration bearing the same proportion to the 
earlier cycle as Devonian, Carboniferous, Jurassic, Triassic, and 
Cretaceous do to Cambrian and Ordovician, and it is during this 
time that the Taconic and Green mountains are supposed to have 
been base-leveled. Have we any satisfactory evidence of this? As 
has been shown, the surface of portions of the Green Mountain Range 
a See the quotations from Davis, Emerson, and Tarr, on pages 13, 15, 1G. 
h See Bull. U. S. Geol. Survey No. 242, 1904, p. 53. 
