306 
[Assembly 
The rocks forming the termination of this series consist of regular 
strata of a silico-argillaceous limestoncj with thin seams of siliceous mat- 
ter, often resembling chalcedony or agate. In some localities there are 
strata which are almost entirely siliceous, and are used for a fire-stone. 
The rocks of this part of the series are extensively used for building 
stone and hydraulic cement. These rocks, in most places, change gra-- 
dually into the carboniferous limestone above, but sometimes the line of 
separation is very distinct. At Mendon, in Monroe county, the two 
rocks are separated by a thin stratum of siliceous conglomerate, of a 
green colour. This conglomerate contains fragments of the rock below, 
showing that it was in some degree indurated before the deposition of the 
carboniferous limestone* 
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In an economical point of view, this formation is the most important 
of any in the Fourth District; its resources throughout a great portion 
of its extent are at this time undeveloped, and it is only when its limits 
and extent shall become known, that its immense value will be appre- 
ciated. In our district, this formation occupies a considerable portion 
of the southern part of Wayne, Monroe, and Niagara counties, and of 
the northern part of Erie, Ontario, and Seneca counties. Its width is 
from eight to ten miles, and its aggregate thickness four to five hundred 
feet. The gypsum is mostly confined to the upper rocks of the series. 
Fossils are of rare occurrence in this formation, so far as our observa- 
tion has extended. Specimens of Orthocera, Trilobite, univalve and 
bivalve shells, have been found in Wayne county. 
In some parts of this formation there appears to have been a tendency 
to crystallization, or some peculiar arrangement of the particles. In 
breaking the rocks, small masses sometimes separate, generally of a flat- 
tened form, but often nearly cubical or approaching to octahedral forms; 
having on one part the adjacent planes of a solid angle deeply striated. 
These forms present some analogy to the hopper form crystals peculiar 
to the muriate of soda; and Prof. Eaton has considered them as pseudo- 
morphous crystals of this salt. Their position, however, and their rela- 
tion to the surrounding rock, sufficiently proves the contrary. They 
are found in the solid rock as well as in the decomposing marl; and 
when removed leave a corresponding impression in the surrounding mass; 
they are also of precisely the same composition as the rock in which 
they occur. The rocks of this formation are also far above those which 
supply the salt springs, and can never have had any communication 
with them. 
