NIAGARA GROUP. 
87 
abundantly scattered over the surface. This part likewise exhibits numerous irregular cavi¬ 
ties, with calcareous spar and fluate of lime, and blende is more frequently found here than 
below. Nodules or accretions of light-colored brittle hornstone occur every where; and the 
fossils are often silicified, and stand out in relief upon the surface. The Catenipora escha- 
roides marks this part of the rock, though it is often nearly destroyed or absorbed; still, a 
careful examination will enable one to detect it, and in more favorable situations fine speci¬ 
mens are obtained. It is this part of the rock which appears at the rapids on the Genesee 
above Rochester, and elsewhere both on the east and west sides of the river. There are 
usually some thin-bedded regular strata above this, which are mostly used for burning to lime ; 
the concretionary and irregularly stratified portions of the central division being too impure, 
and too difficult to be quarried for this purpose. The upper strata are not only readily quar¬ 
ried, but are easily broken into fragments of suitable sizes. It breaks with a dull sound, and 
presents a slightly uneven fracture. 
The lithological characters alone of the two upper divisions are every where sufficient to 
distinguish this part of the rock from all other limestones in the State ; these are, its brittle 
nature, the glistening surface of the minute crystalline laminae of which the mass is com 
posed, and its harsh or apparently siliceous character. 
The characters, as described at Rochester, are the prevailing ones, for a great distance 
westward. The dark-colored mass, however, above the beds of passage or hydraulic lime¬ 
stone, thins out, and its place is occupied by a light grey crinoidal limestone, sometimes 
composed of extremely comminuted particles, at others exhibiting portions of the crinoidal 
columns and other fossils. 
The following arrangement will enable the reader to bear in mind the different parts of this 
division, which are persistent over a large extent of country, and seem worthy of being no¬ 
ticed, as the mass is so variable in its different parts that it cannot well be described as one. 
5. Thin-bedded dark grey or brownish limestone. Few cavities. Highly bituminous. Sometimes 
contains nodules of hornstone. 
4. Thick-bedded dark or bluish grey limestone with irregular cavities, and often siliceous accretions, 
or hornstone. Surface very ragged from weathering. Highly bituminous. 
3. A lighter colored subcrystalline mass, very irregularly stratified, contorted and concretionary. 
2. A bluish grey subcrystalline mass, mostly thin bedded, and separated by seams of dark shale. 
1. Grey or bluish grey siliceous limestone; hydraulic limestone, or beds of passage from the shale 
below. 
Few points west of Rochester offer so good an opportunity of examining all the parts of 
this limestone in the order of succession. It is exposed and quarried at numerous places, 
which will be noticed under the description of the rocks of Monroe county. 
In the town of Ogden, the lower part of the limestone, or that following the hydraulic layers, 
is charged with fossils, (principally Atrypa and fragments of the Trimerus .) The rock con¬ 
taining them is extremely thin bedded, though a pure limestone. The upper part of the rock 
in the same town is extensively developed, and appears in very heavy strata, abounding in 
coralline fossils. 
