366 SHELL TERRACES. [chap. xvi. 



is not opposed to the belief, of a small sinking of the ground 

 having subsequently taken place. The side ot 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 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. I have 

 no doubt that this upper layer originally existed as a bed of 

 shells, like that on the eighty-five-feet ledge ; but it does 

 not now contain even a trace of organic structure. The 

 powder has been analysed for me by Mr. T. Reeks ; it 

 consists of sulphates and muriates both of lime and soda, 

 with very little carbonate of lime. It is known that common 

 salt and carbonate of lime left in a mass for some time 

 together, partly decompose each other ; though this does 

 not happen with small quantities in solution. As the half- 

 decomposed shells In the lower parts are associated with 

 much common salt, together with some of the saline 

 substances composing the upper saline layer, and as these 

 shells are corroded and decayed In a remarkable manner, 

 I strongly suspect that this double decomposition has here 

 taken place. The resultant salts, however, ought to be 

 carbonate of soda and muriate of lime ; the latter is present, 

 but not the carbonate of soda. Hence I am led to imagine 

 that by some unexplained means, the carbonate of soda 

 becomes changed Into the sulphate. It Is obvious that the 

 saline layer could not have been preserved In any country In 

 which abundant rain occasionally fell ; on the other hand, 

 this very circumstance, which at first sight appears so 

 highly favourable to the long preservation of exposed shells, 

 has probably been the Indirect means, through the common 



