1 8 R. A. F. PENROSE, JR. 



nous materials consist mostly of ammonium salts, especially of 

 urate of ammonium, and other urates, together with guanine' and 

 variable quantities of nitrates and nitrites, as well as with various 

 other organic products resulting from decayed animal matter. It 

 is a well-known fact that, under suitable conditions, the nitrogen 

 of all these materials that are not already in the form of nitrates, 

 passes eventually into that form through the agency of certain micro- 

 scopic organisms (bacteria). These bacteria are of many different 

 kinds, and different ones act at different stages in the transition. 

 The action, when other conditions are favorable, goes on most 

 efi&ciently in the presence of alkalies, or of alkaline earths like calcium 

 carbonate, etc., which are abundant in the pampa region. 



It seems probable that the nitrates of Chile were mostly pro- 

 duced in this way from nitrogenous animal matter of old guano 

 beds which once lined the waters of the interior basin, and which 

 have long since disappeared under the influence of erosion. The 

 nitrates were probably carried down into the waters of the basin and, 

 became mixed with the other saline materials already there. The 

 waters probably began to diminish in volume shortly after their 

 separation from the sea, for though they received the drainage of 

 the surrounding land, this was not enough to compensate for the 

 loss by evaporation. The evidence tends to show that the rainfall 

 in those early days was more abundant in this region than at present, 

 but it gradually grew less and its constant diminution doubtless 

 hastened the process of desiccation. 



Thus the waters gradually sank until they fell below the level of 

 any outlet they may have had to the ocean, and then, the drainage 

 being cut off, the materials in solution became more and more con- 

 centrated as desiccation progressed. These materials consisted of 

 the original salts of the sea water, the nitrates and other salts from the 

 guano beds, and other materials constantly carried down from the 

 surrounding land. The concentration continued until the waters 

 became saturated with saline materials, and then deposition began 

 along the edges and on the bottom. Eventually the whole body 

 of water disappeared and the dry desert pampa with its deposits of 

 nitrates, common salt (sodium chloride), and other materials alone 



■ Guanine is an organic base containing nitrogen and having thie formula C5H5N5O. 



