188 
GEOLOGY OF THE FOURTH DISTRICT. 
With this mass terminates both the peculiar shales of which the group is composed, and 
also the greater part of those fossils typical of the same, so far as regards the Fourth District. 
The divisions here enumerated may be found of service to the careful observer, and they 
afford some interesting facts regarding changes in fossils; still they cannot all be described 
as distinct, neither is it of practical importance that they should be. 
From the fact that the position of the last is well marked, being embraced between the 
Encrinal limestone below and the Tully limestone above, and also containing a peculiar asso¬ 
ciation of fossils, it is everywhere recognizable, and as a subdivision is more important than 
some of the others. 
The subdivisons here enumerated can all be seen on the eastern shore of Cayuga lake, be¬ 
tween Springport and Ludlowville ; the latter place also presents the two next higher rocks of 
the system.* The eastern shore of Seneca lake exhibits the same between its outlet and 
Lodi, in Seneca county. The same rocks appear, but not so well exposed, on the western 
shores of both lakes. This fact is of interest, as showing the influence of the prevalent 
westerly winds, which, from the constant action of the water driving against the eastern 
shores, has undermined the mass, and a perpendicular cliff is thus constantly kept exposed. 
The operation of the same cause has produced a fine exhibition of the rocks of this group 
upon the shores of Lake Erie. 
This group of rocks occupies a belt of country from five to eight miles wide, extending 
through the counties of Seneca, Ontario, Livingston, Genesee and Erie. 
In the county of Seneca it occupies the southern part of Fayette, the whole of Yarick, 
nearly the whole of Romulus, and that portion of Ovid bordering the shores of both lakes. 
The superior rocks have been greatly denuded, and the surface of the group is greater than 
in any part of the country westward. Its southern limit, which is just north of the village of 
Ovid, is several miles farther south than any other point before coming to Lake Erie. The 
two lakes, Seneca and Cayuga, seem to have been the great outlet of northern waters, and 
along their channels the great excavating and transporting force seems to have operated with 
peculiar energy. 
On the eastern side of Seneca lake the group is well exposed in the outlet of Crooked lake, 
which in its passage to Seneca lake has excavated a channel from rocks of the Portage group, 
through the Genesee slate, the Tully limestone and Moscow shale, exposing the two latter to 
great advantage, and presenting localities where great numbers of the finest fossils can be 
obtained. The illustration at the head of this group presents a conical hill of the Moscow 
shale, succeeded near the top by the Tully limestone, and above this a thin band of Genesee 
slate, upon which is a covering of alluvium. The sloping hills on either side are indented 
by numerous ravines which expose the rocks of the same group. It also occupies the eastern 
margins of the towns of Benton and Milo in Yates county. 
In Ontario the group occupies nearly the whole of the town of Seneca, the southwestern 
part of Phelps, the northern part of Gorham, nearly all of Hopewell, the whole of Canandai¬ 
gua, East and West-Bloomfield and Lima. 
* See Section, Plate VII. 
