XVI DECOMPOSING SHELLS 393 



mounds, called Hiiacas, 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 1 746, 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 w^hirled about like pebbles by 

 the retiring waves. It has been stated that the land subsided 

 during this memorable sliock : I could not discover any proof 

 of this ; )-et 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 low^er 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 deepl)' 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 scaHng 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 



