CLARK: TACONIC REVOLUTION. 157 
The se^ction is self-explanatory, and is direct proof of tilting at 
the end of the Ordovician in this region, though it does not in- 
dicate folding. 
The Taconics and the Green Mountains. 
That the bulk of the crystalline rocks of the Taconics and of 
the Green Mountains is of Cambrian and Ordovician age no 
one now seriously doubts. That these ranges contain rocks of 
later date no one has yet been able to prove. With the details 
of the structure of these mountains we are not here concerned, 
for we are interested chiefly in the relations between Ordovician 
rocks and rocks of later age. On the face of it, the only conclusion 
which is strictly dependent upon the facts is that these moun- 
tains were folded after the deposition of the youngest contained 
sediments (Trenton— Utica). Nowhere are these Ordovician 
crystalline rocks overlain, conformably or unconformably, by 
Silurian sediments, and only near Bernardston, Massachusetts, 
and in northeastern Vermont are they associated with Devonian 
strata. The structure at these two localities has been discussed 
above. 
It has been said that the late or post-Ordovician age of the 
Taconic revolution has been determined partly by the occur- 
rence of Silurian strata on the upturned edges of the Ordovician. 
In the mountainous region east of the Hudson th(>re an^ no known 
Silurian rocks overlying the Ordovician crystallines. The Ren- 
sellaer grit was once supposed to be of Silurian age but as it 
contains no fossils its place in the stratigraphical column cannot 
be determined (see above p. 156). 
The fact that no Silurian or higher strata are known to be 
folded in with the Ordovician of this region has been urged as a 
reason for placing the folding at the close of the Ordovician. 
This is unsafe reasoning; all the more so because geologists agree 
that eastern North America must have been uplifted (in an 
epeirogenic, if not orogenic sense) at the close of the Ordovician. 
The early Silurian (Medinan) sediments are not such as we 
should expect from the destruction of a mountain-chain of the 
first order of magnitude. Rather do we get sands in no way dif- 
fering from normal beach sands, only local conglomerates, and 
