extremely low and narrow, that it is improbable in the highest degree that
a town should have been founded on it in its present state; and I have
lately heard that M. Tschudi has come to the conclusion, from a comparison
of old with modern charts, that the coast both south and north of Callao
has subsided. (I am indebted for this fact to Dr. E. Dieffenbach. I may add
that there is a tradition, that the islands of San Lorenzo and Fronton were
once joined, and that the channel between San Lorenzo and the mainland, now
above two miles in width, was so narrow that cattle used to swim over.) I
have shown that the island of San Lorenzo has been upraised eighty-five
feet since the Peruvians inhabited this country; and whatever may have been
the amount of recent subsidence, by so much more must the elevation have
exceeded the eighty-five feet. In several places in this 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. ("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