220 
GEOLOGY OF THE FOURTH DISTRICT. 
sions, are owing to deep valleys, as those of the lakes and the Genesee river. As it approaches 
Lake Erie, it takes a broad sweep to the southwest, and disappears beneath the surface of the 
lake in the town of Evans in Chautauque county.* 
The uniformity in lithological character of this mass throughout, and its finely levigated par¬ 
ticles, indicate a quiescent period, and the operation of an ocean moved only by gentle cur¬ 
rents. There is no great accumulation in one part over another, and the deposition appears 
only to have been affected by the common law of gravitation. At the time of its deposition it 
possessed the character of a soft, black, carbonaceous mud, with scarcely an admixture of 
siliceous matter in any part. 
Concretions. — The well defined concretions or septaria have been noticed, and these are 
all that occur throughout the mass to interrupt its sameness. These bodies are often without 
any apparent seams ; at other times they are crossed in various directions by seams of crys¬ 
talline carbonate of lime, etc., which apparently fill cracks previously formed by the desicca¬ 
tion of the mass. These concretions are interesting as showing the occurrence of calcareous 
matter at one or two positions in the rock, which, though small in quantity, spread over a large 
extent of surface. 
These bodies vary from the diameter of a few inches to two or three feet, and are usually 
pretty purely calcareous and nearly spherical. 
Localities. — A few points will suffice to give the observer a knowledge of this rock. In 
Seneca county it can be examined in nearly all the gorges which communicate with the lake, 
in the towns of Ovid, Lodi and Covert. Below the Lodi falls it abounds in fossils peculiar 
to it, though of few species, Orbicula and Lingula being the most abundant. About two 
miles south of Big-stream point, in Yates county, it is well exposed on the lake shore, and 
contains its usual fossils. Along the outlet of Crooked lake it is well developed, and also in 
the ravines on the south side of this place. There are several good points for examination 
on the banks of Canandaigua lake, and in the ravines leading to the same. 
The gorge of the Genesee river at Mount-Morris affords, probably, the best exposition of 
this rock, where it is laid open in the perpendicular cliffs on either side, for more than a mile 
in length. The principal fossil at this place is a species of Avicula with a very fragile shell, 
which will be seen figured in illustration No. 94. The shale is well exposed in a ravine, 
and at a fall on a small stream about two miles northwest of the village of Moscow, in Living¬ 
ston county. 
From this place to Lake Erie it may be examined in nearly every ravine which extends 
southward, and in the lateral ones joining the north and south valleys. Its constant fossil is 
the little Avicula just referred to, and in nearly every locality it abounds, frequently covering 
the surface of the slaty laminae, through several inches in thickness, and more sparingly- 
scattered through a greater extent. 
* It will be recollected that the surface of Lake Erie is about 120 feet higher than Seneca lake, and consequently this rock 
disappears beneath its surface four or five miles farther north than if it were at the same level. 
