48 FERTILISERS CONTAINING NITROGEN [cHAP. 



extent by water, the nitrate can hardly be due to 

 infiltration, but must have been formed in situ — a 

 conclusion which is much strengthened by the fact 

 brought out by Hughes' analysis that small quantities 

 of nitrogenous organic matter, ammonia and nitrites 

 are also present in the extract from the clay. 



In all probability the nitrates in these shales repre- 

 sent the results of nitrification of a mass of organic 

 matter originally contained in the deposit, but until 

 further data have been accumulated as to the depth to 

 which the nitrates extend, and their replacement or 

 not by unoxidised organic nitrogen compounds at 

 depths beyond the access of atmospheric oxygen, it is 

 impossible to say whether we are dealing with recent 

 or with what might be termed fossil nitrification, or 

 again whether there has been any concentration of the 

 salts in the surface layer analysed. 



In any case, these Egyptian deposits give a clue to 

 the possible origin of the Chile beds by washing from 

 similar strata (and the Corderillas consist of rocks of 

 recent age) into a rainless area, where the salts are 

 accumulated by evaporation. The two deposits present 

 this common difficulty : that the deposit is nitrate of soda 

 instead of nitrate of lime — the usual product of nitrification 

 in soil ; again, both are associated with a preponderance 

 of sulphates over chlorides, a fact which seems to put 

 any marine origin out of the question. We are, how- 

 ever, dealing with typically arid conditions, and in all 

 parts of the world sodium salts are characteristically 

 abundant in the soils and rocks of areas of small rainfall ; 

 indeed, sodium carbonate is always found in such cases, 

 and this would form the base for nitrification. At the 

 same time, similar oxidising processes to those which 

 give rise to nitrates would convert the sulphur of the 

 organic matter to sulphates. But to settle the problem 



