466 A TREATISE ON METAMORPHISM. 



places in the valley of the Orinoco the amount of nitrates in the soil 

 amounts to 30 per cent of the mass. The vast amount of these nitrates 

 is doubtless explained by the very great abundance and activity of the 

 bacteria; for it is well known that the high temperature of the Tropics, 

 combined with the high humidity of these regions, is veiy favorable to the 

 action of bacteria.' 1 In this connection Schloessing and Miintz state that 

 the maximum activity of the bacteria is at 30° C, or approximately that of 

 the Tropics. 6 



Much of the combined nitrogen is lost to the belt of weathering in the 

 following ways. In so far as nitrogen is set free by the action of the bac- 

 teria ana by the passage of the ammonia into the air, it is lost. There are 

 further great losses in the nitrogen compounds by the transportation of the 

 nitrates to the sea by the streams. The quantity of nitrates thus lost has 

 been greatly increased in recent years by the introduction into the streams 

 of sewage containing much combined nitrates. Further, as pointed out by 

 Conn, in so far as the combined nitrogen of the soil is manufactured into 

 powders which are exploded, the combined nitrogen is freed and passes into 

 the atmosphere. The ammonia which passes into the -atmosphere may be 

 brought back in part to the soil by the rain. The other losses must, how- 

 ever, be compensated by the synthesis of nitrogen compounds from the 

 nitrogen of the air by bacteria and leguminous plants combined. Until 

 very recently the latter process has preponderated, and the crust of the 

 earth has gained in combined nitrogen. Man in recent times has 

 undoubtedly increased the loss in combined nitrogen, and it is possible, 

 perhaps probable, that the balance is now on the other side, but by care in 

 cultivation, and possibly by manufacture, it will doubtless be possible to 

 continue the process of adding combined nitrogen to the soil faster than it 

 is lost from it. 



OXIDATION OF INORGANIC COMPOUNDS. 



The most important of the inorganic compounds oxidized are iron and 

 carbon. Other substances subordinate in cpiautity are also oxidized, but 

 these have small importance from a purely geological point of view. How- 

 ever, from the point of view of ore deposits, and therefore in reference to 



a Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1897, p. 372. 

 *Aikman, cit., p. 52. 

 «Conn, cit., p. 106. 



