158 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
no arkoses or other hetcrogonoous sedimentary materials. Ln 
other words, if all of the Ordovieian and earlier rocks were now 
inaccessible, one would not he likely to suppose, from an examin- 
ation of the Silurian sediments alone, that the Silurian period 
was immediately preceded by mountain-building of any inten- 
sity. At Green Pond Mountain, New Jersey, the initial Silurian 
deposits consist of sandstone and conglomerates from twelve to 
fifteen hundred feet thick; and at other localities such as Otisville, 
New York, Delaware Water (lap. New Jersey, Lehigh Gap, 
Pennsylvania, there are conglomeratic layers in the Medina 
sandstone.^ In all these cases the sandstones are clean and the 
pebbles are for the most part of vein quartz. So that even where 
the basal Silurian elastics are thickest there is no evidence of 
detritus from the wearing-down of neighboring high mountains. 
Vein-quartz pebbles in abundance are evidence of a nearby land 
mass which has been subject to long-continued erosion, and 
perhaps also to rejuvenation just prior to their deposition, but 
they never occur to the practical exclusion of other kinds of 
pebbles in the products of the wearing-down of young moun- 
tains. In support of the contention, we are thrown back upon 
the geological relations between the Ordovician and Silurian (or 
younger) rocks, and in this province the evidence fails us. 
I have stated above that all we can say about the age of the 
deformation of this region is that it must be post-Upper Ordovi- 
cian. We may, however, qualify this somewhat. Such meta- 
morphism as the rocks of the Taconics and the Green Mountains 
have suffered is not generally thought of as having been induced 
near the s\irface. Such metamorphism is a factor, among other 
things, of the depth to which the rocks are buried, and in the 
opinion of the writer," the metamorphism of the rocks of the 
Taconics and the Green Mountains could only have occurred 
under a cover of rock measured in thousands of feet. What this 
cover was we may never know but that it was not Ordovician is 
probable, for the Upper Ordovician is but scantily represented in 
eastern New York State. 
^Schuchert, C. Silurian formations of southeastern New York, New 
Jersej', and Pennsylvania. Bull. Geol. Soc. Amer., vol. 27, p. 543-547, 
1916. 
