One is to the effect that it owes its origin to huge guano deposits, originally covering the shores of the large salt lakes which, by the subsequent overflowing of their shores, effected the mixture of the guano with the salts. In this way, by a slow process of decomposition, nitrate of soda would be ultimately formed.[209] This theory, apart from other considerations, seems at first sight extremely plausible, more especially when we remember that it is on this very coast that the greatest guano deposits have been found, and that the famous Chincha Islands, which alone have yielded over 10 million tons of this valuable fertiliser, are comparatively near the scene of the nitrate deposits. What seems further to support this theory, is the actual occurrence in the nitrate-fields themselves of small quantities of guano. But however plausible it may appear at first sight, it does not bear closer criticism. One very serious objection is the absence in these deposits of phosphate of lime, which is the largest constituent of guano. If they were really due to guano, how does it happen that the insoluble phosphate of lime should have disappeared, while the easily soluble nitrate of soda should alone be preserved? Again, assuming this theory to be correct, we should naturally expect to be still able to find evidence of the chemical changes which would under such circumstances have taken place, in the shape of portions of the guano in the transition stage. Such evidence, however, the most careful investigations have failed to detect. Apart, however, from the above objections, there seems to be little doubt, from evidence afforded by traces of birds' nests, &c., that the guano found in the nitrate-beds was deposited subsequent to the formation of the nitrate of soda. _Nitric Acid derived from Sea-weed._