290 ALLUVIUM. 



circumstances, that any other than marine vegetation could thrive or even 

 exist there. 



« The adjacent country, inland, is a marsh from which the sea has been 

 expelled, and is now kept out with great difficulty, and at a vast expense, 

 and there is no woodland nearer than four miles, on the hill adjoining 

 these levels." 



The marsh called the Wish, near Eastbourne, cliiefly consists 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 

 weU near the paper-mill, a bed of similar materials was passed through ; 

 it is nearly 20 feet tliick, and contains oak-leaves, nuts, branches of trees, 

 &c. 



3. Calcareous tufa, deposited by SPRiNas flowing through lime- 

 stone STRATA, &C. 



The deposition of calcareous earth from water flowing through beds 

 of limestone, is a fact so well known as to require but little comment. 



Springs of this kind occur in many parts of England, particularly in 

 Derbyshire, where the incrustations 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 solution a smaU portion of carbonic acid be added, a 

 carbonate of hme is formed, and precipitated in an insoluble state f . 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 Hme, held in solution by an excess of fixed air, not in actual combina- 

 tion with the hme, but contained in the water, and acting as a men- 



* In 1817 Thomas Smith, Esq. F.R.S., discovered in this alluvial bed the bones of a 

 species otBos. 



+ Organic Remains, Vol. i. p. 373. 



