Page:Popular Science Monthly Volume 85.djvu/148

144 decomposition of ancient guano deposits. In defending this theory Penrose has assumed, in the same way as those who favor a marine origin for the nitrates, that the pampa region was once a part of the ocean bottom, but as the region gradually rose it became a more or less enclosed basin. At this time guano beds were supposed to have been deposited along the borders of these waters, just as they are now deposited in the neighboring shores of the Pacific. It is considered quite possible that marine plants might also have collected in the basin at the same time, and that these constituted the source of the iodine, although it is pointed out that this element might also have originated in minerals, or mineral springs occurring in the region. The formation, as suggested, of inland basins of sea-water, which would ultimately evaporate, would furnish also a source for the common salt associated with the nitrates, as well as for the soda of the nitrates.

The guano theory, however, has been objected to on the ground that no accumulation of phosphate has ever been found in the nitrate country, and such must necessarily occur in amount corresponding to the nitrates if the latter have been derived from guano. It is argued, on the other hand, that such phosphates may actually exist, but that they have not yet been discovered, and it is further explained that the absence of the remains of birds and of sea shells may be accounted for on the ground that sufficient time has elapsed since the beds were deposited to admit of the decay of all such materials.

There is still, however, a further objection which applies to both the sea-weed and guano theories. Thus, if the region was at one time a sea-beach it must have taken ages, as Newton has pointed out, for the nitrate pampa to be elevated to its present level. During these ages the region must have passed through varying climatic conditions, including most probably rains. It has, therefore, been argued that the nitrate deposits are, geologically speaking, of very recent origin.

In suggesting another organic source from which the nitrates may have been derived, Kuntze has called attention to the fact that vicunas, and llamas, which are at home in this portion of the Andes, have the peculiar habit of always depositing their manure in one and the same place. Immense herds of these animals are supposed to have roamed over the region from time immemorial, each herd having a definite dunging place at some convenient point. As the manure accumulated its nitrification would progress rapidly under the prevailing arid conditions. The common salt would be derived from the urine and excrements, while the decomposition of rocks throughout the region is considered sufficient to account for all other salts occurring in the crude niter.

Newton is of the opinion that the source of the nitrates is the