chap. x. Nitrate of Soda. 305 



by Mr. Belford Wilson, the Consul-General at Lima) 

 of 420 miles. In a well near the works, thirty-six 

 yards in depth, sand, earth, and a little gravel were 

 found : in another well, near Almonte, fifty yards deep, 

 the whole consisted, according to Mr. Blake, 1 of clay, 

 including a layer of sand two feet thick,' which rested 

 on fine gravel, and this on coarse gravel, with large 

 rounded fragments of rock. In many parts of this now 

 utterly desert plain, rushes and large prostrate trees in 

 a hardened state, apparently Mimosas, are found buried, 

 at a depth from three to six feet; according to Mr. 

 Blake, they have all fallen to the south-west. The bed 

 of nitrate of soda is said to extend for forty or fifty 

 leagues along the western margin of the plain, but is 

 not found in its central parts : it is from two to three 

 feet in thickness, and is so hard that it is generally 

 blasted with gunpowder ; it slopes gently upwards from 

 the edge of the plain to between ten and thirty feet 

 above its level. It rests on sand in which, it is said, 

 vegetable remains and broken shells have been found ; 

 shells have also been found, according to Mr. Blake, 

 both on and in the nitrate of soda. It is covered by 

 a superficial mass of sand, containing nodules of common 

 salt, and, as I was assured by a miner, much soft gyp- 

 seous matter, precisely like that in the superficial crust 

 already described : certainly this crust, with its charac- 

 teristic concretions of anhydrite, comes close down to 

 the edge of the plain. 



The nitrate of soda varies in purity in different 

 parts, and often contains nodules of common salt. 

 According to Mr. Blake, the proportion of nitrate of 

 silver varies from twenty to seventy-five per cent. An 



1 See an admirable paper, 'Geolog. and Miscell. Notices of 

 Tarapaca,' in Silliman's 'American Journal,' vol. xliv. p. 1. 



