No. 200.] 
am 
Tufa J Marl J and Materials for Manures. 
The substance usually denominated marl, is a calcareous deposit from 
springs, but partially decomposed. Calcareous shales are properly marls, 
and will be so considered. 
Rain water has the property of dissolving the calcareous rocks over 
which it flows, but the water of many springs dissolve much more. 
This property is owing to the presence of carbonic acid gas m the wa- 
ter; when the acid is dissipated in the atmosphere, the calcareous matter 
before held in solution is thrown down. The loose, porous portions of 
this deposit, usually containing incrusted plants, leaves and other sub- 
stances, is called tufa; the more compact parts are called Travertin, 
Besides these, a portion of the deposit in some situations is of a fine 
pulverulent texture, often containing great numbers of the shells of Cy- 
claSj Lymnea^ Planorbisj &c. and sometimes a great proportion of the 
deposit is made up of fragments of these shells. This substance is usu- 
ally termed shell marl, though in many of the deposits the shells are al- 
most entirely absent. When the deposit is made in a swamp, or beneath 
water, it assumes the fine pulverulent texture; but when exposed, or 
when the deposition is disturbed, it forms tufa. Tufa is also formed in 
small quantities, in hundreds of localities, where shell marl is not found. 
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 rock, 
till^ from their increasing weight, they fall into the stream. 
Along the Genesee river, below Rochester, are considerable accumu- 
lations of this substance. 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 
f Assem. No. 200.J 39 
