166 T. Sterry Hunt on the Chemical and Geological History 
It now remains to speak of the geological distribution of pe- 
troleum in the Palseozoic rocks of this country. Apart from 
the matters just described from the Quebec group, bitumen oc- 
curs at two distinct horizons in the New York series, For 
reasons which will be apparent farther on, we recall the princi- 
pal divisions in this series. At its base are the siliceous sand- 
stones of the Potsdam formation, to which succeeds the Calcife- 
rous sand-rock. This is, for the most part, a dolomite, occasion- 
group, in Guilderland, near Albany, New York. Both of these 
probably have their source in the underlying limestones, which 
are characterized by beds and nodules of chert, and by silicified 
fossils, not less than by the presence of petroleum. 
To these limestones succeed, in ascending order, the py? 
schists of the Utica formation, followed by the shales of the Hud- 
son River group. 1is terminates the Lower Silurian or Cam 
The siliceous strata at the base of the first series are repeated in 
the Oneida and Medina conglomerates and sandstones,.while the 
“™ The term Cambro-Silurian, first suggested by Prof. Phillips, is adopted by Jukes 
to designate the Lower Silurian series of rocks. (Zrans. Royal baak pot xxiii, 
p. 564, yor 
