166 Review of the New York Geological Reports. 
under the names of Sherburne flagstones and shale, but from its 
superior development on the banks of the Genesee river near the 
town of Portage, (formerly Nunda,) the name was changed to the 
one at the head of this paragraph. During the progress of the 
survey a diversity of opinion existed in the minds of the geolo- 
gists, as to the most appropriate classification and subdivision of 
the rocks comprised in the upper part of the Erie division of the 
New York system; viz., of those members lying above the Gen- 
esee slate. The difficulties of reconciling the different views 
arose from the variations and modifications in lithological char- 
acter, of the equivalent beds at distant localities, together with 
the scarcity of fossils in most of the rocks. For, though in cer- 
tain situations, well defined lines of separation can be established, 
by the difference of mineral composition, yet these geological 
horizons become more and more obscure as the distance from the 
starting point increases. The final grouping was established 
mainly on paleontological evidence. | 
On the Genesee river the Portage group admits of a threefold 
subdivision : soft, green, argillaceous shale (Cashaqua shale) 
neath, resting on the Genesee slate; green and black shale and 
sandy shale (Gardeau shale and flagstones) in the middle ; thick 
bedded sandstone (Portage sandstone) above. Shales predomi- 
nate therefore beneath, sandstones above, with but very few fos- 
sils. In going east the arenaceous strata increase in importance, 
and near Cayuga lake the whole series consists of shale and shaly 
sandstone passing almost imperceptibly into thick bedded sand- 
stone not very different from those of the Chemung group, so 
that lithological divisions become unsatisfactory, not only between 
the subdivisions, but even between the Portage and Chemung 
groups. Going west the shaly matter augments and the sand- 
stone constantly diminishes, so that along the shore of Lake 
Erie, there is a thick mass of black shale, succeeded by green ~ 
and black shales for several hundred feet, with hardly any flag- 
stone or sandstone, and this change is accompanied by an in- 
crease of the number of fossils. 
“From its general similarity, and from the difficulty also of 
separating it from the higher rocks on its southern limit, it is col- 
ored the same tint on the map, being the northern part of the 
light umber tint.” By consulting the geological map, it will be 
seen that nearly the whole of that part of the state south of the 
lakes is occupied by these two formations. 
The scenery of that region is remarkable and highly pictur- 
“a as may be gathered from the following extract from Hall’s 
eport :— 
_ “The higher sandstones of the group, and in many instances 
some of the intermediate ones, produce falls in the streams which 
pass over them, and some of the most beautiful cascades in the 
