XVI 
DECOMPOSING SHELLS 
393 
mounds, called Huacas, are really stupendous ; although in 
some places they appear to be natural hills encased and 
modelled. 
There is also another and very different class of ruins 
which possesses some interest, namely, those of old Callao, 
overwhelmed by the great earthquake of 1746, and its 
accompanying wave. The destruction must have been more 
complete even than at Talcahuano. Quantities of shingle 
almost conceal the foundations of the walls, and vast masses of 
brickwork appear to have been whirled about like pebbles by 
the retiring waves. It has been stated that the land subsided 
during this memorable shock : I could not discover any proof 
of this ; yet it seems far from improbable, for the form of the 
coast must certainly have undergone some change since the 
foundation of the old town ; as no people in their senses would 
willingly have chosen for their building place the narrow spit 
of shingle on which the ruins now stand. 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 
