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port, about the same thickness. It forms the northern slope of the 
mountain ridge, from the Genesee river westward; and a line from the 
upper falls at Rochester to the falls at Wolcott, in Wayne county, would 
be its southern limit, east of the Genesee river. Its width is from one 
to two miles, according as the surface is level or inclined. It abounds in 
fossils throughout its whole extent, containing five species of Trilobites, 
several species of Producta and Orthis, Encrinites, &c. The decompo- 
sition of this shale affords sulphate of magnesia and muriate of soda: 
small nodules of gypsum are of frequent occurrence, and also pyrites, in 
disseminated particles, extending through a considerable thickness of the 
strata. Owing to the latter circumstance, as well as to its composition 
it is readily decomposed by the action of air and water. It is this shale 
which underlies the limestone of the Niagara falls, and by its decompo 
sition leaves the limestone unsupported, when it falls of its own weight. 
This formation retains the character of shale, or slaty argillaceous 
limestone, for a thickness of about eighty or one hundred feet, when, 
from the increase of siliceous and calcareous matter it loses the charac- 
ter of shale, and becomes a compact, silico-argillaceous limestone. — 
Some of the strata of this part of the formation have been used for hy- 
draulic cement, but there is no rock to which the name, hydraulic lime- 
stome is exclusively applicable.* This portion of the formation is irre- 
gularly stratified, the strata being from a few inches to three feet in thick- 
ness, often divided in different directions, by seams of greenish shale. 
This rock is of a greyish blue colour j coarsely granular, compact and 
sub-crystalline. In its upper portions it becomes a more pure lime- 
stone, and finally, graduates into the geodiferous limestone of Eaton. 
In the same manner, througjhout all the limestones and shales, above the 
sandstone, there is a gradual and almost imperceptible change from one 
to the other. 
The rock of which we are speaking is better developed at Rochester 
than at any other place in our district: it is here quarried in the bed of 
the river for laying the foundation of the aqueduct. When free from 
seams of shale, it is a valuable and durable building stone. The lower 
strata of this rock form the bed of the river at the upper falls, of which 
the water of the Genesee is precipitated upon the shales below. At 
this place it has a thickness of sixty or seventy feet, but diminishes 
westerly, so that at Lockport and on the Niagara river it is no more 
than eight feet thick. 
* The upper strata of the gypeeous formation are used far the same purpose, and also the si 
lico-argillaceous portions of many limestones. 
