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GEOLOGY OF THE FOURTH DISTRICT. 
The rock used in this county for cement, is usually blue or greenish blue, when first 
exposed to the atmosphere, but becomes rusty grey and partially decomposed by atmospheric 
agents. In the locality last mentioned, the original color of the stone is grey, or of the color 
of pipe-clay. 
Fire Stone. 
A stratum of light grey limestone, of a porous texture, extends through the towns of Ogden 
and Sweden. This stone resists the action of ordinary fires for many years with little appa¬ 
rent loss or change. Its character depends on the presence of magnesian earth, and the 
porous structure of the stone, which prevents it from cracking when heated. It is much used 
in this neighborhood for the backs and jambs of fire-places, and also for door and window 
caps and sills. At the place where the stone is quarried, the stratum is not seen in place, 
but masses of the broken outcropping edge are used. 
Some of the strata of the gypseous rocks are sufficiently siliceous to be a good fire stone, 
and have been used for hearths of iron furnaces. 
Materials for Construction. 
Common building stone is obtained in many places along the course of the Niagara 
limestone formation. A very durable stone is quarried from the bed of the Genesee river at 
Rochester, and has been used for the foundation of the new aqueduct. Some of the stone is 
injured by the presence of thin seams of shale, admitting water, which, on freezing, will expand 
and split the stone. The portions free from seams of this kind, furnish one of the best and 
most durable materials in the county. A very good stone for ordinary purposes of construc¬ 
tion is obtained at the rapids near Rochester; but owing to its geodal structure, it cannot be 
used where a smooth and even surface is required. Dark, bituminous limestone is quarried 
in several places in Penfield. 
In West-Mendon the upper strata of the Onondaga salt group are extensively quarried for 
step-stones, door and window caps and sills, lintels, blocks for corners, pedestals, etc. The rock 
is a silico-argillaceous limestone, sufficiently hard and compact for ordinary purposes. When 
first quarried it is soft and easily worked, but becomes harder on drying. This stone is much 
used in Rochester and the adjoining towns, and the demand is constantly increasing. The 
common thickness of the layers is from four to eight inches, and they can be procured of any 
required dimensions. Except the more siliceous portions, these stones should not be used 
where they will be subjected to the action of freezing water, farther than to rains, which seem 
scarcely to affect them. 
The Medina sandstone and the indurated marl of the same formation have been used for 
building stone; but experience has proved what a knowledge of their composition would have 
foretold, viz. that in Monroe county they are almost entirely unfit for any useful purpose 
whatever. Many apparently compact blocks of this rock will, in the course of a few weeks, 
if exposed, crumble into a loose mass. Where used in buildings, it has, in some cases, been 
