190 
GEOLOGY OF THE FOURTH DISTRICT. 
The lower divisions of the group are seen in Allen’s creek, south of Le Roy ; and the 
higher portions, with the Moscow shale, at Pavilion and Bethany. A little distance to the east 
and south of Batavia, the lower part of the group is well exposed, and in Alexandria the higher 
divisions of the same. 
In the town of Darien, a little west of the Centre, there is a good exposure of the upper 
part of the Ludlowville division and the Encrinal limestone; the latter is a compact mass 
about three feet thick, and its upper part strongly stained from decomposing iron pyrites. 
The shale abounds with Atrypci ciffinis, A. concentrica, Delthyris mucronatus, and other spe¬ 
cies of these genera. Cyatliophylli, Strombodes, Favosites, and other corals are almost as 
abundant as at York in Livingston county. The Moscow shale is tolerably well exposed at 
this place, though its decomposition upon the surface has obscured its characters, and it pre¬ 
sents the appearance of an ordinary clay bank. 
On the road westward from the last named place, the rocks of the group are exposed in 
several places. On a small stream crossing the road near Alden, the shales are well exposed ; 
and a little south of this, the encrinal limestone is quarried. It is here highly fossiliferous ; 
containing, in addition to the usual abundance of crinoidal columns, Delthyris, Fleurotomaria, 
and Calymene bufo. At this and many other localities, this thin calcareous mass appears to 
be a common depository of many or all the species of the shale below, with several others 
which are peculiar to itself. It was of course produced during a cessation of the mud deposit; 
and the forms living on the bed of the latter would consequently be inclosed in this deposition, 
with others produced on the calcareous bottom, some of which are different. 
Along the Cayuga creek, on the Indian reservation, these shales can be traced from near 
their base, throughout, to their connexion with the higher rocks. And here 1 should not omit 
to state what has appeared, only more indistinctly, at other localities. The division noticed 
as the olive shale, and the band of blue calcareous shale, with the sandy part of the Ludlow¬ 
ville division, have, almost or entirely, disappeared, apparently from a gradual diminution in 
the quantity of matter. The gradual thinning becomes very apparent at all the localities 
west of the Genesee river. At the same time all those portions which are persistent, except 
the encrinal limestone, have diminished in the same direction ; and although few localities offer 
good exposures, it is, nevertheless, very evident that such is the fact. 
This becomes more palpable when we arrive at the shore of Lake Erie, which offers 
the best continuous exposure west of Seneca lake. It is here quite evident that the rocks 
have diminished to less than one half the thickness which they have on Seneca lake. At the 
same time, however, many of the fossils have maintained their position, and some species 
seem even to be more abundant than eastward. The Cypricardia, Avicula, Nucula, Belle- 
rophon, and a few others appear to have diminished in numbers according to the diminution 
of the sandy calcareous shale in which they so abound at the east. 
To show, however, how fixed are their habits and place of residence, there is a stratum of 
this kind on Lake Erie, which is indicated by the letter a, in the section Plate 5, in which 
Cypricardia, Turbo, Bellerophon and Orthoceras occur to the almost entire exclusion of every 
thing else. This stratum, too, holds the place which the thick mass of similar character does 
