J. D. Dana—Age of the Green Mountains. 197 
IV. UnconrorMasinity oF Uprrer SInurIAN BEDS. 
The three ranges of facts bearing on this point are the following: 
(1) The absence of Upper Silurian strata from among the con- 
formable formations of the Green Mountains. 
(2) The existence, near the western border of the moun- 
tain range, of fossiliferous Lower Helderberg limestone (upper 
divison of the Upper Silurian) resting wxconformably on the 
upturned Hudson River slates. 
(3) The existence of fossiliferous Lower Helderberg beets 
order 
of the mountains occur in the Hudson River Valley, not far 
from the river, and are described and illustrated in Mather’s 
New York Report (1848). The following figures are from 
his plates; they represent two cases east of the Hudson River 
SSS. 
, Columbia Co. 
Becraft’s Mt., Columbia Co. 
and four just west of it. Fig. 1. Becraft’s Mountain, east of 
the city of Hudson (from Mather, plate 24), in which fossilifer- 
ous Lower Helderberg beds are nearly horizontal; fig. 2. Mt 
Bob (from plate 88) a few miles south of Becraft’s; fig. 3 
3. ~ 
Mount Bo 
Between Glasco and the Great Falls 
of the Esopus, Ulster Co. 
(from plate 7), in Ulster county, west of the river, between 
Glasco and ae Great Falls of Esopus Creek ; fig. 4 (from plate 
26), in Ulster county, on the right bank of the Rondout ; fig. 
6. 
— : miles west of Cocksackie 
Two miles —— ot Le tet Two or a wees : 
5 (from plate 8), in Greene county, two miles prone 
Cocksackie ; fig. 6 (from plate 8), the same, two or three mi 
