Since our voyage, M. Tschudi
has come to the conclusion, by the comparison of old and modern
maps, that the coast both north and south of Lima has certainly
subsided.
On the island of San Lorenzo there are very satisfactory proofs of
elevation within the recent period; this of course is not opposed
to the belief of a small sinking of the ground having subsequently
taken place. The side of this island fronting the Bay of Callao is
worn into three obscure terraces, the lower one of which is covered
by a bed a mile in length, almost wholly composed of shells of
eighteen species, now living in the adjoining sea. The height of
this bed is eighty-five feet. Many of the shells are deeply
corroded, and have a much older and more decayed appearance than
those at the height of 500 or 600 feet on the coast of Chile. These
shells are associated with much common salt, a little sulphate of
lime (both probably left by the evaporation of the spray, as the
land slowly rose), together with sulphate of soda and muriate of
lime. They rest on fragments of the underlying sandstone, and are
covered by a few inches thick of detritus. The shells higher up on
this terrace could be traced scaling off in flakes, and falling
into an impalpable powder; and on an upper terrace, at the height
of 170 feet, and likewise at some considerably higher points, I
found a layer of saline powder of exactly similar appearance, and
lying in the same relative position.
Pages:
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723