McKay. — On Diatomaccous Earth at PaJcaralca. 379 



P.S. — I would here add that, as the surface-layers are 

 formed wholly of living forms, and all arc extinct at about 1ft. 

 from the surface, it seems reasonable to suppose that at, say, 

 6ft. from the surface other and quite distinct species may be 

 found. And, as Mr. Williams informed me he dug into the 

 deposit to a yet gi-eater depth without passing through it, 

 other and quite distinct species, it is probable, will be found 

 in the first-formed and lower part of the deposit. 



If samples were taken not more than Gin. apart in the sec- 

 tion of the deepest part of the deposit, an examination of these 

 would be likely to set at rest any doubts as to the true origin 

 and mode of accumulation of the deposit, since, if it is mainly a 

 derived and secondary dejDOsit, then from about 1ft. from the 

 surface to the greatest depth there should be little variation 

 of the specific forms, while, on the other hand, if the species 

 changecl more than once, that would go far to prove the cor- 

 rectness of my theory on the subject. 



