854 PEIiI". .[CHAP. xvi. 



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 appear- 

 ance 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 analyzed 

 for me by Mr. T. Eeeks ; it consists of siilphates 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 decompo- 

 sition 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 salt not having been washed away, of their decompo- 

 sition and early decay. 



I was much interested by finding on the terrace, at the height of 

 eighty-five feet, embedded amidst the shells and much sea-drifted 



