_Origin of Nitrate Deposits._—The probable origin of these niter deposits has given rise to a great deal of discussion, and a wide difference of opinion exists as to the source from which the nitrogen may reasonably be supposed to have been derived. According to the present state of our knowledge, it must be presumed that its sources have been organic, and that the niter has been produced by the activity of the same bacteria which now produce nitrates in our soils, rendering the nitrogen of humus available to plants. But it is by no means clear what that organic material could have been; for at the present time the plateau of Tarapacà is almost wholly destitute of vegetation, if not of animal life. The latest and apparently most reasonable suggestion is that of _Kuntze_, who calls attention to the fact that the vicuñas and llamas which are at home in this portion of the Andes, and are known to have roamed over that region in countless herds, have the curious habit of always depositing their manure in one and the same place whenever at liberty. Each herd of these animals has its definite dunging place at some convenient point. That such herds have existed in the region from time immemorial is obvious from historical as well as collateral evidence; and as their manure accumulated, its nitrification would progress rapidly under the prevailing arid conditions. The common salt would naturally be derived from the urine and excrements, and the alkaline salts which exist throughout this region as the products of soil decomposition, would be quite sufficient to account for the alkaline bases in the caliche. On the other hand, the presence of iodine points to seaweeds as the organic source.