11.] FORMATION OF NITRATE OF SODA 4? 



involves, in the first place, an extravagant amount of 

 seaweed, and our knowledge of the nitrification process 

 is quite opposed to the idea that it would take place in 

 a rapidly concentrating medium containing common 

 salt. Nor have we any reason to suppose that salt 

 would supply a base for nitrification ; even if its hydro- 

 chloric acid could be turned out the liberated acid 

 would at once suspend the process. And again, if the 

 iodates are to be taken as indicating seaweed, why 

 are not bromates also present in the caliche, since 

 both bromine and iodine are associated in seaweed. 



A much more probable theory is that the deposit 

 represents the saline residues of fresh-water streams 

 flowing off the Corderillas, containing nitrates and 

 other salts derived from old rich soils or rocks on the 

 heights. The evaporation of such waters for a long 

 period of progressive desiccation would result in the 

 accumulation of the dissolved salts in the dry region 

 over which the waters formerly spread when the rainfall 

 was greater. The occurrence of iodine cannot be 

 explained until more is known as to the amount of 

 this element present in the waters and soils of the 

 Corderillas. 



The only other deposits of nitrate of soda v\^hich 

 assume any economic importance are those which 

 occur in Upper Egypt, where certain shale beds of 

 Eocene age, out-cropping on both sides of the Nile 

 between Qena and Assouan, contain enough sodium 

 nitrate to make the clay worth carriage as a manure 

 known locally as " tafia." Analyses of a series of these 

 shales by F. Hughes show an average of ^-y per 

 cent, of nitrate of soda associated with lo-i per cent, of 

 sodium chloride, and 5-4 per cent, of sodium sulphate. 

 The material is disseminated throughout the whole 

 bulk of the clay ; and as this is not permeabk to any 



