PRELIMINARY REMARKS. 
15 
are then less porous in structure, and consequently less pervious 
to water. The cemented stratum is often called hard pan. 
When impervious, or only partially so, water is retained too 
long in the soil for profitable cultivation. Beds of gravel and 
pebbles are also cemented under similar conditions. These are 
called pudding stones; they are also subaerial formations, and 
should be distinguished from conglomerates, which are sub¬ 
aqueous. Limestone, permeated in the same way, yields to the 
action of water. A shelving ledge, or the roof of a cave below, 
is often hung with pendent masses, like icicles from the eaves 
of a house. These are- called stalactites. Their formation 
begins with a deposit of lime in the form of a ring, which is 
gradually prolonged by additions from the water. A part of 
the water drops to the ground or floor, and there forms another 
deposit of carbonate of lime. This is called stalagmite. 
Numerous instances, illustrating the agency of water in the 
mode I have stated, are found in all parts of the Union. Thus 
stalactites and stalagmites occur on a large scale in all the 
great caves of Kentuckey, Tennessee, and Virginia; also in the 
smaller caves of New York in Albany and Schoharie counties. 
Tufa is deposited from numerous springs in Onondaga, and 
other western counties in New York. The hydrous peroxides 
of iron and manganese are derived from mineral springs form¬ 
erly existing in the tertiary formations of the southern states, 
as well as in the more ancient rocks of the primary belts. 
Silica is often separated from its solution in hot water and from 
steam, and has furnished the siliceous sinter surrounding hot 
springs in almost all regions of the globe. Amethysts and 
coatings of chalcedony upon common quartz in Nova Scotia 
and the trap ranges of our country, and upon crystals of calc- 
spar in Edwards, N. Y., were the products of hot water holding 
this substance in solution. 
But water is more eminently a solvent for those bodies which 
have taste, as common salt, alum, copperas, &c. Extensive 
rocks are known, which consist of nearly pure salt or chloride 
of sodium. The elements of common salt exist in many of the 
