260 SOILS AND MANUEES 



in considerable quantity in that part of the world, 

 and accumulations of seaweed. The first of these two 

 hypotheses seems to be untenable. Guano deposits are 

 often found to have undergone change, but it is always 

 the nitrogenous matter which disappears and the phos- 

 phates that are left behind. It is difficult to conceive any 

 kind of change by which the phosphates would be re- 

 moved and the nitrogenous matter left. If the deposits 

 had been formed by the nitrification of guano, phosphates 

 would almost certainly be found mixed with the nitrates 

 or near them, which is not the case. If the original 

 organic matter consisted of seaweed, it is clear that the 

 land must have undergone great elevation since it accu- 

 mulated. The geological evidence indicates that such ele- 

 vation actually has taken place even within comparatively 

 recent times, and though there is no certainty about the 

 matter, the seaweed theory is generally regarded as much 

 the more probable. It is, in fact, the most probable ex- 

 planation hitherto suggested. 



Extraction. The crude product, which is locally known 

 as caliche, is, of course, very impure. It is not only 

 mixed with sand and earthy matters, but contains also 

 a considerable proportion of the chlorides and sulphates 

 of sodium, magnesium and calcium, and smaller quanti- 

 ties of iodine, boracic acid, etc., all of which are, to a 

 large extent, eliminated in the process of refining, which 

 is carried out on the spot. The proportion of nitrate of 

 soda varies from about 25 to 50 per cent., the caliche 

 being much richer in some districts than in others, but 

 the average is about 35 per cent. 



The caliche is of varying thickness from a few inches 

 to several feet and lies at some depth below the sur- 

 face. The covering crust consists of two distincf layers, 

 the upper, called costra, being a mixture of sand and 

 gypsum, and the lower, called congelo, a conglomerate 



