HAMILTON GROUP. 
191 
further east; and directly above it we find bluish calcareous shales, with Delthyris mucronata, 
Atrypa concentrica and A. affmis, being the characteristics of the upper part of the Ludlow- 
ville division of this group. 
This fact is sufficient to show that there were distinct periods of formation, and that the 
thick mass of sandy shale so abounding in the conchiferous mollusca in the eastern part of 
the State, and which in the central part is still in great force, extended westward entirely to 
lake Erie. It is there recognized by the same lithological characters, the same or a similar 
association of fossils, and holds the identical place in the strata. 
The section of this group on Lake Erie (see Plate 5) commences with the shale immedi¬ 
ately above the Marcellus, and from thence the continuation is nearly unbroken throughout. 
The characters of the lower and higher divisions are more nearly alike than farther east, the 
whole being a soft bluish shale, readily crumbling on exposure to the atmosphere, and decom¬ 
posing to a tenacious mud. The sandy portion is scarcely noticed, and thin beds of black 
slaty shale, so common on Seneca lake and other localities in the central part of the State, 
have entirely ceased. The peculiar fossils of these portions are likewise wanting, showing 
the association of certain forms with products of similar nature. The fossils here alluded to 
are a species of Orbicula, the Atrypa congregata of Conrad, and some others. 
Notwithstanding that fossils are exceedingly numerous ifi this exposure along Lake Erie, 
it must be acknowledged that there is a manifest decrease in the number of species. Scarcely 
any new ones are noticed, and many which have been common at the east are rare or wanting 
There are several species which abound in immense numbers, and which in this respect fully 
compensate in number of individuals for the absence of a larger number of species. This 
fact of the gradual disappearance of fossils is equally obvious with the decreasing thickness 
of rocks which is observed as we progress westward. Whether this was caused by increased 
depth of water, distance from shore, or by the thinning of the deposit alone, it may be inter 
resting to inquire; probably, however, all these causes operated in some degree to produce 
the result. 
The lower divisions of this group on Lake Erie, or that part below the stratum marked a 
in the section, are nearly destitute of fossils, and the forms which seem peculiar to this part 
on Seneca and Cayuga lakes are not found here. The three upper divisions are here the 
only ones which are interesting from their organic contents. 
Throughout the whole period of this deposition, there appears no evidence of any distur¬ 
bances. The character of the materials, the condition of the fossils, all show the absence of 
violent currents or of powerful waves. The deposit was probably made at a depth below 
water, beyond the reach of the waves which may have agitated the surface ; and there ap¬ 
pear to have been no oscillations by which one portion of its extent was elevated, while 
another was depressed. It was emphatically a long period of repose. 
