ALLUVIAL DEPOSITS. 
21 
consists chiefly of peat, of the same character as 
that of Pevensey, containing leaves, nuts, branches 
of trees, &c., and the bones of ruminants * : and at 
Isfield, in sinking the well near the jiaper-mill, a 
bed of similar materials was passed through ; it is 
nearly twenty feet thick, and contains oak-leaves, 
nuts, branches of trees, &c. 
3 . Calcareous Tufa^ deposited hy Springs. 
The deposition of calcareous earth from water 
flowing through beds of limestone, is a fact so well 
known as to require but little comment. 
8 })rings of this kind occur in many parts of 
England, particularly in Derbyshire, where the in- 
crustations they form are generally considered as 
petrifactions, although certainly having no claim to 
that title. The chemical changes which give rise 
to the phenomena in question admit of an easy 
explanation. 
At the temperature of 60 °, lime is soluble in 
700 times its weight of water ; and if to this solu- 
tion a small portion of carbonic acid be added, a 
carbonate of lime is formed, and precipitated in an 
insoluble state, t If, however, the carbonic acid 
be in such quantity as to supersaturate the lime, it 
is again rendered soluble in water ; and it is thus 
that carbonate of lime, held in solution by an ex- 
cess of fixed air, not in actual combination with 
the lime, but contained in the water, and acting as 
a menstruum, is commonly found in all waters. 
* In 1817, Thomas Smith, Esq. F.R. S., discovered in this alluvial 
bed the bones of a species of Bos. 
f Organic Remains, vol. i. p. 373. 
