428 
GEOLOGY OF THE FOURTH DISTRICT. 
sandstone. Whether the decomposition of this marl affords chloride of sodium, I am not pre¬ 
pared to decide ; but we know that this mineral does result from the decomposition of the 
shales above the sandstone. The shale along the Genesee below Rochester produces chloride 
of sodium and sulphate of magnesia. 
The situation of the salt springs in Monroe county, is unfavorable to large accumulations 
of water, and this may be the reason why they have not been more productive. Those which 
I have observed are along the margin of small streams, and the salt water as it comes to the 
surface is carried off by the fresh water, or becomes too much diluted. 
Tufa , Marl, and materials for manures. 
The substance usually denominated marl is a calcareous deposit from springs, but partially 
decomposed calcareous shales are properly marls. 
Rain water has the property of dissolving the calcareous rocks over which it flows, and 
the water of many springs holds large quantities in solution. This property is owing to the 
presence of carbonic acid gas in the water ; when the gas is dissipated in the atmosphere, the 
calcareous matter before held in solution is thrown down. The porous or more compact por¬ 
tions of this deposit, usually containing incrusted plants, leaves and other substances, is called 
travertine or tufa. Besides this, a portion of the deposit in some situations is of a fine pulve¬ 
rulent texture, often containing great numbers of fluviatile shells. 
Wherever the calcareous rocks are exposed, the action of rain and the percolation of water 
from the soil produce depositions of tufa. Along all the streams and rivers where calcareous 
rocks form the banks, we find deposits of tufa collecting, and remaining attached to the rocky 
cliff, till, from their increasing weight, they fall into the stream. 
Along the Genesee river, below Rochester, are considerable accumulations of this sub¬ 
stance. Vast quantities of calcareous matter are also carried down this river and smaller 
streams, to be deposited in Lake Ontario, probably in much more extensive beds than those 
found on land. 
Tufa is mostly used for burning into lime, of which it affords a fine quality. The more 
compact parts of it have been employed for building, and form a cheap and durable material. 
When first removed from the ground, and while moist, it is easily cut or sawed, and is thus 
shaped into blocks of any required dimensions. After exposure the moisture evaporates, and 
it becomes very hard. 
Deposits of shell marl are found in various places in Monroe county. The most important 
locality is in the southern part of the county, along the course of Allen’s creek in Wheatland. 
The eastern part of this deposit extends southward into Livingston county. Tufa forms the 
upper portion in many places to the depth of three or four feet, and below it is the shell marl 
three or four feet thick. Some portions of the formation are nearly pure carbonate of lime, 
containing abundance of the shells mentioned ; in other places it is impure from admixture of 
earthy matter. It extends in length about three miles, and in breadth from half a mile to one 
