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.("Observaciones sobre el Clima del Lima" par Dr.H.Unanue page 4.--Ulloa's "Voyage" volume 2 English Translation page 97.--For Mr.
Cruikshank's observations, see Mr.Lyell's "Principles of Geology" 1st edition volume 3 page 130.)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 peculiar 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, calcareous 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.(See "Volcanic Islands" etc.by the Author.) The other saline bodies may perhaps have been partially thus derived, but chiefly, as Iconclude from the following facts, through a different means.
On most parts of the second ledge or old sea-beach, at a height of 170feet, there is a layer of white powder of variable thickness, as much in some parts as two inches, lying on the angular, salt-cemented fragments of sandstone and under about four inches of earth, which powder, from its close resemblance in nature to the upper and most decayed parts of the shelly mass, I can hardly doubt originally existed as a bed of shells, now much collapsed and quite disintegrated.I could not discover with the microscope a trace of organic structure in it; but its chemical constituents, according to Mr.Reeks, are the same as in the powder extracted from amongst the decaying shells on the lower ledge, with the marked exception that the carbonate of lime is present in only very small quantity.On the third and highest ledge, I observed some of this powder in a similar position, and likewise occasionally in small patches at considerably greater heights near the summit of the island.At Iquique, where the whole face of the country is covered by a highly saliferous alluvium, and where the climate is extremely dry, we have seen that, according to Mr.Blake, the shells which are perfect near the beach become, in ascending, gradually less and less perfect, until scarcely a trace of their original structure can be discovered.It is known that carbonate of lime and common salt left in a mass together, and slightly moistened, partially decompose each other (I am informed by Dr.Kane, through Mr.
Reeks, that a manufactory was established on this principle in France, but failed from the small quantity of carbonate of soda produced.Sprengel "Gardeners' Chronicle" 1845 page 157, states, that salt and carbonate of lime are liable to mutual decomposition in the soil.Sir H.De la Beche informs me, that calcareous rocks washed by the spray of the sea, are often corroded in a peculiar manner; see also on this latter subject "Gardeners'
Chronicle" page 675 1844.): now we have at San Lorenzo and at Iquique, in the shells and salt packed together, and occasionally moistened by the so-called Peruvian dew, the proper elements for this action.We can thus understand the peculiar corroded appearance of the shells on San Lorenzo, and the great decrease of quantity in the carbonate of lime in the powder on the upper ledge.There is, however, a great difficulty on this view, for the resultant salts should be carbonate of soda and muriate of lime; the latter is present, but not the carbonate of soda.Hence I am led to the perhaps unauthorised conjecture (which I shall hereafter have to refer to)that the carbonate of soda, by some unexplained means, becomes converted into a sulphate.
If the above remarks be just, we are led to the very unexpected conclusion, that a dry climate, by leaving the salt from the sea-spray undissolved, is much less favourable to the preservation of upraised shells than a humid climate.However this may be, it is interesting to know the manner in which masses of shells, gradually upraised above the sea-level, decay and finally disappear.
SUMMARY ON THE RECENT ELEVATION OF THE WEST COAST OF SOUTH AMERICA.