The obvious difficulty of accounting for the formation of the larger deposits by such a theory he meets by saying that it is only necessary to suppose that, even after the partial isolation of the lagoons by the elevations of the coast, they might still have maintained tidal or occasional communication with the sea by means of lateral openings in the chain of hills separating them from the ocean. In such cases there would be a gradual accumulation of salts, very much greater in amount than that due simply to the evaporation of the water originally contained in the lagoons. The above theory of the origin of the lower saline deposits may go to explain the mode of formation of the nitrate-fields; but in this case several difficulties present themselves. One is the much greater altitude of the latter, as well as their greater distance inland. This difficulty, however, may be met by assuming that they are of older origin than the lower deposits, and have been subjected to a correspondingly greater amount of volcanic upheaval. There is abundance of proof that this part of the continent has been the scene in the past of such volcanic upheaval. Forbes is of opinion that there is the fullest evidence to prove that, even since the arrival of the Spaniards, a very considerable elevation of the land has taken place over the greater part, if not the whole extent, of the line of coast; while Darwin states that he has convincing proof that this part of the continent has been elevated from 400 to 1200 feet since the epoch of existing shells. Furthermore, elevations of the coast-line, amounting in many cases to several feet, are known to have happened within recent times, while earthquakes and volcanic disturbances of a less striking nature are still of common occurrence. Successive lines, indicative of old sea-beaches, can be distinctly traced stretching inland, one behind the other; and patches of sea-sand and water-worn stone, found at a great distance from the coast, both in valleys and at altitudes much greater even than 4000 feet, point to the same conclusion.[208] The difficulty, therefore, of altitude and distance from the coast cannot be regarded as insuperable. _Source of Nitric Acid._ A difficulty, however, which is not so easily met, is afforded by the presence of the nitric acid which, in combination with the soda, forms the nitrate of soda. It is scarcely necessary to inform our readers that nitrogen--except, of course, in small quantities in the free state--is not a normal constituent of salt water. The question, therefore, of greatest interest in connection with the formation of these nitrate-beds is, Whence has the nitric acid been derived? Several theories have been put forward to account for it. _Guano Theory._