GENESEE SLATE. 
221 
On the shore of Lake Erie it may be examined in numerous places, but the best locality 
is at Eighteen-mile creek and along the banks of the creek a mile from the lake. It is here, 
if possible, more strongly marked by the Aviculae, and they are so crowded together that 
often, for many feet in extent, it is nearly impossible to distinguish their forms. 
Thickness. — On the shores of Seneca lake and in Ontario county the thickness of this rock 
is about 150 feet, as estimated by careful measurements along the descent of streams and in 
perpendicular banks, though there is no single point where the whole thickness is shown at 
once. After passing the Genesee river in a westerly direction, it soon becomes evident that 
the rock has diminished in thickness, though there are no points where good measurements 
can be obtained. On the shore of Lake Erie, however, the whole rock is well exposed, in 
connection with the lower and higher masses, in the perpendicular cliff at the mouth of Eigh¬ 
teen-mile creek (See section, Plate 5.) At this place its thickness is but twenty-three feet 
seven inches, or less than one-sixth of its thickness at the eastern limit of the district. This 
thinning is due to the cause before mentioned, viz. the diminishing power of the currents 
which transported the materials, and the consequent precipitation of a greater portion of the 
matter near the origin of the same. It bears no evidence of denudation, and, from the even 
surface of the rock below, this great diminution is not local, but has evidently been gradual 
and uniform through the extent of one hundred and fifty miles. 
Minerals of the Genesee Slate. 
The only minerals, except iron pyrites, which occur in this rock are found in the cavities 
of the concretions. These are crystallized carbonate of lime, in the common rhombic forms, 
and in hexahedral prisms with trihedral summits ; sulphate of baryta ; quartz crystals, with 
more rarely galena, or sulphuret of lead. Fluid bitumen is of common occurrence, and with 
it a bright blue fluid and a substance like spermaceti, but softer. These are volatile, and it 
has been impossible to preserve any of them. The fluid bitumen and the blue fluid have 
likewise been noticed in septaria in the Marcellus slate. 
This shale is so intimately blended with the rocks that follow, that alone it has exercised 
no influence on the soil or springs which is not common to the rocks above it, and are only 
such as will be subsequently noticed under the next group. 
Organic Remains of the Genesee Slate. 
The only constant fossil of this rock is the Avicula fragilis, figured below. The greater 
part of the mass is destitute of any organic remains, and it is only in the higher portion that 
we find them in very considerable numbers. The forms figured in the woodcut (No. 95) are 
usually found in the upper strata, and I do not recollect having ever seen any of them more 
than twelve or fifteen feet below its termination. In several localties on Cayuga and Seneca 
lakes, these forms are all found associated together, and in great numbers ; but farther west 
