UNCONFORMITY AT BASE OF ONONDAGA LIMESTONE 307 
stratum of the Onondaga limestone immediately above the Oriskany 
in this section contains a considerable amount of sand, as it does 
at Splitrock where the Oriskany is absent. The large and numerous 
Oriskany fossils cease abruptly at a definite plane a little below 
the top of the arenaceous beds and thus indicate precisely the top 
of the Oriskany. The sandy element of the basal beds of the 
Onondaga limestone disappears within from 4 to ro in. of the top 
of the Oriskany. The contact of the two formations and the large 
Oriskany fossils in the lower formation which project beyond the 
surface of the weathered sandstone are shown in the photograph 
(Fig. 2). Southwest of Jamesville one-half mile, at the cascade 
west of the reservoir outlet, the thickness of the Oriskany is about 
twice that seen in the lake cliffs northeast of town. The section 
exposed at this point shows: 
JAMESVILLE SECTION 
Ft. 
Light-gray limestone with corals and other fossils (Onondaga).......... 7 
Hard dark-colored sandstone cemented with lime and holding Oriskany 
SSIS (Oral ech Bern aa rd ae ieee re os a 6 
Thin and evenly bedded dark lead-gray limestone (Helderberg)......... 15 
The Onondaga limestone forms the top and the Helderberg the 
bottom of the fall. The characteristic chert lenses will be noted 
in the Onondaga limestone above the hammer in the photograph 
(Fig. 3). 
West of the Finger Lake region the disconformity at the base 
of the Onondaga eliminates from the section not only the Schoharie 
and Esopus but the Helderberg as well, letting the Onondaga 
rest in the western part of the state upon rocks of Silurian age. 
Western New York.—The disconformity which marks the break 
in sedimentation between the Silurian and Devonian systems in 
western New York is very plainly indicated by both the physical 
and the faunal evidence in the vicinity of Buffalo. The contact 
of the Cobleskill and Onondaga limestones is marked by an irregular 
line in the Falkirk cement quarries and in many other places in 
the same region. One of the localities where eroded depressions 
in the Cobleskill may be seen at the base of the Onondaga is in 
the rock cut of the Lake Erie & Western R.R. near its intersection 
