THE NITRATE DEPOSITS OF CHILE 19 



remained. The occurrence of the nitrate in the form of sodium 

 nitrate is probably due to the abundance of sodium salts in the region. 

 The occurrence of the nitrate and other saline deposits along the 

 western edge of the pampa, where the latter is lowest in altitude, may 

 be due to one or both of two causes: 



1. The pampa, sloping as it does from east to west, gradually 

 caused the last of the waters of the inland basin, as they evaporated, 

 to collect along its western edge. If the nitrate in solution had not 

 become sufl&ciently concentrated to cause deposition before that 

 time, the deposits of course would be formed only on the west side. 



2. If, however, deposition had begun while the waters still washed 

 the slopes of the Andes, the greater rainfall there than on the west 

 side of the pampa may have dissolved the nitrate from the east side 

 and allowed it to be carried down into the loose soil of the pampa, or 

 else over to the west side, where it was again deposited as the waters 

 evaporated. 



As against the hypothesis of the derivation of the nitrates of Chile 

 from guano, the objection has been made that guano beds, and the 

 remains of dead birds such as generally occur in them, are often 

 notably absent in some of the nitrate districts. Very little guano is 

 found with the Tarapaca nitrates, but in Antofagasta it does occur 

 in the same region as some of the nitrate deposits. The scarcity 

 of guano at present in some of the regions is easily explained, for 

 the birds which formed it were essentially sea birds, dependent upon 

 fish for food. As the waters of the inclosed basin gradually evapo- 

 rated they became too saturated with saline matter for fish to live 

 in them, so that the birds had to abandon their old haunts in the basin 

 and seek other regions for their sustenance. Hence, though immense 

 accumulations of guano had probably been formed, no new supply 

 was maintained, and ample time has elapsed for the old guano to 

 have been carried away, as already described, and for the bones, 

 etc., to have disintegrated. The fact that guano is still abundant 

 in parts of the nitrate regions of Antofagasta and not so in Tarapaca 

 probably indicates that the birds left the latter region at an earlier 

 date than the former, or that the conditions for its preservation were 

 better in Antofagasta than Tarapaca. 



The notable absence of seashells and remains of other marine 



