43i SOUTH AMEEICA— THE ANDES EEGIONS. 



the world is there such an extensive tract of coast so unfitted for the habitation of 

 man," * Rain may be said never to fall in the Atacama Desert, where miners have 

 passed long years without ever observing a single refreshing shower. So entirely 

 is the absolute dryness of the climate depended upon that the old ravines formerly 

 excavated by the running waters are now chosen as the most convenient tracks for 

 the construction of railways. Thus the line from the port of Chailaral to the 

 Salado mines follows the bed of the permanent or intermittent coast stream which 

 now runs out in the sands some 30 miles from the sea. 



In these regions the process of denudation and the weathering of the rocks 

 cropping out above the surface is caused, not by the action of rain and snow, but 

 mainly by the great oscillations of temperature between day and night. After 

 exposure during the day to the continuous action of the solar rays, the rocks are 

 rapidly cooled at night, when the glass falls from 70° to 90° Fahr. below the 

 midday heat. They are thus subjected to alternating movements of expansion and 

 contraction, which cause them to scale and crack in various ways according to 

 their geological structure. Certain formations are decomposed in thin films like 

 the leaves of a book ; others break into concentric layers, scaling off like the bark 

 of the plane-tree. Under the action of the air the felspar crystals are transformed 

 to kaolin, and all this debris accumulates in earthy masses at the foot of the hills. 

 The more compact nuclei, which offer a greater resistance to the atmospheric 

 influences, assume the form of towers or obelisks rising above the surrounding 

 plains, which are themselves thickly strewn with vast quantities of shingle, 

 innumerable fragments of quartz, chalcedony, and other crystals. 



The Chilian Nitrate-fields. 



Pissis has advanced the hypothesis that the nitrate-fields, so extensive in the 

 Atacama Desert, and farther north in the Pampa de Tamarugal, are also due to 

 the same climatic conditions. But various theories have been proposed by Darwin, 

 Forbes, Noller and others. At first it was supposed that the deposits originated in 

 a chemical transformation of guano ; but, if so, other substances should also be 

 present which are absent. Nor have the nitrates their origin in the decomposition 

 of the seaweed stranded on the beach, for in none of these beds have any marine 

 shells been found. On the contrary, the nitrate is almost everywhere interspersed 

 with small unrolled stones, not such shingle as we should expect to find in basins 

 of pelasgic origin, separated from the sea by upheaval and dried by the process of 

 evaporation. 



Moreover, the nitrates, so far from occupying the beds of such lagoons, are 

 usually disposed along the eastern slopes and in proximity to the crest of the coast 

 range, far from all limestone formations and from all stratified rocks such as would 

 have been deposited in marine waters. But it may be asked whether volcanic 

 exhalations may not have transformed the salts assumed to have been originally 

 precipitated in the old lacustrine depressions of the plateau. Pissis explains the 



•John Ball, op. cit., p. 123. 



