chap. ix. Decay of Upraised Sea-Shells. 273 



neighbourhood, marks of sea-action have been observed : 

 Ulloa gives a detailed account of such appearances at a 

 point five leagues northward of Callao : Mr. Cruikshank 

 found near Lima successive lines of sea-cliffs, with 

 rounded blocks at their bases, at a height of 700 feet 

 above the present level of the sea. 



On the Decay of upraised Sea-Shells. — I have 

 stated that many of the shells on the lower inclined 

 ledge or terrace of San Lorenzo are corroded in a pecu- 

 liar manner, and that they have a much more ancient 

 appearance than the same species at considerably greater 

 heights on the coast of Chile. I have, also, stated that 

 these shells in the upper part of the ledge, at the height 

 of eighty-five feet above the sea, are falling, and in 

 some parts are quite changed into a fine, soft, saline, cal- 

 careous powder. The finest part of this powder has been 

 analysed for me, at the request of Sir H. De la Beche, 

 by the kindness of Mr. Trenham Reeks of the Museum 

 of Economic Geology ; it consists of carbonate of lime 

 in abundance, of sulphate and muriate of lime, and of 

 muriate and sulphate of soda. The carbonate of lime 

 is obviously derived from the shells ; and common salt 

 is so abundant in parts of the bed, that, as before 

 remarked, the univalves are often filled with it. The 

 sulphate of lime may have been derived, as has probably 

 the common salt, from the evaporation of the sea-spray, 

 during the emergence of the land ; for sulphate of lime 

 is now copiously deposited from the spray on the shores 

 of Ascension. 1 The other saline bodies may perhaps 

 have been partially thus derived, but chiefly, as I con- 

 clude from the following facts, through a different 

 means. * 



p. 4. — Ulloa's ' Voyage,' vol. ii. « Eng. Trans.' p. 97.— For Mr. Cruik- 

 shank' s observations, see Mr. Lye ll's ' Principles of Geology' (1st 

 edit.), vol. iii. p. 1 30. 



1 See my discussion on a calcareous incrustation in Chapter III. 



