NIAGARA GROUP. 
85 
structure is not often well exposed, though it does appear in a few of the strata. Cavities 
or geodes, lined with calcareous spar, gypsum, etc., are occasionally found, and there is a 
constant increase of their number and size in a western direction. 
The lower part of this limestone, possessing a siliceous character and properties of the 
hydraulic cement, occurs in the towns of Rose and Williamson. 
In Monroe county, the northern margin or outcropping edge of this limestone extends through 
the towns of Penfield, Brighton, Gates, Ogden and Sweden; in each of these places, nu¬ 
merous localities are presented for its examination. The principal outcropping portion is the 
dark grey upper mass, which is highly bituminous, and when weathered has a harsh feel like 
a friable sandstone. On fresh fracture, it is often dark blue, or approaching to black. It 
appears to be composed of small crystalline grains, which present numerous shining laminae 
of a resinous lustre; and the rock, from its aspect, often resembles a sandstone rather than a 
limestone. It every where furnishes a good lime, but of a yellowish color. Large quantities 
of bituminous matter are expelled in the process of burning; and this substance frequently 
flows from the kiln, of the consistence of tar. The rock, on examination, proves to contain 
no appreciable amount of silex, but is every where magnesian ; and it is probably from the 
mixture of the two earths that the harsh or siliceous-like character is presented. Exposed 
fragments are often quite porous, from the solution and removal of a portion of the matter. 
Its northern limit, through the eastern part of Monroe county, is marked by an accumulation 
of fragments of the rock, which are rounded rather from weathering in place than from trans¬ 
portation. These always exhibit on their outer surfaces the peculiar porous or spongy texture. 
That their form and decomposition is due to weathering in the places we find them, is evident 
from the frequent presence of some silicified fossil which stands out upon the surface in strong 
relief. In many places this stone is exceedingly brittle, particularly on first exposure. 
At the Penfield mills on the Irondequoit, the lower part of the limestone appears in the bed 
of the stream, and underlies the surface at no great depth for some distance around. It is of 
a bluish color, very hard and tough, mostly thin-bedded, and separated by seams of shale. In 
consequence of its hardness, it is rejected as a building stone, and quarries are sought at a 
greater distance. 
At Rochester, the lower beds have the same siliceous character as farther east. They pass 
upward into a coarsely granular or compact subcrystalline limestone, very irregularly stratified, 
and the layers separated by greenish shale. The strata are often wedge-form, or irregular in 
thickness. This portion of the rock is partially composed of fragments of crinoidal stems and 
other fossils, but so comminuted that their forms are usually undistinguishable. 
Still above this, the rock which forms the central portion of the mass presents itself in very 
irregular concretionary or contorted strata. The whole exhibits an appearance as of folding 
among the laminae, by which it has partially assumed the form of spheroidal concretions, but 
still inseparable from the adjoining and surrounding rock, which may be only partially folded 
or contorted, and connecting this with another spheroidal mass. A very similar example, on 
a small scale, may often be seen in a very curled and gnarled plank or stick of timber. 
