Having thus discussed the origin of the nitrate-fields, we may now give a more detailed description of their appearance. The chief deposits at present being worked are those lying in the Pampa de Tamarugal, in the province of Tarapaca. They stretch to a distance of thirty or forty miles inland, from Pisagua southwards to somewhat beyond the town of Iquique. This huge desert, as has been already indicated, seems to be entirely destitute of all vegetation and animal life. Even in the immediately adjoining country the only kind of vegetation that seems to grow is a species of acacia. The few streams that are found in this neighbourhood are entirely fed by the melting snow from the Cordilleras. Darwin describes the appearance presented by these pampas as resembling "a country after snow, before the last dirty patches are thawed." The caliche, or raw nitrate of soda, is not equally distributed over the pampas. The most abundant deposits are situated on the slopes of the hills which probably formed the shores of the old lagoons. An expert can tell from the external appearance of the ground where the richest deposits are likely to be found. The caliche itself is not found on the surface of the plain, but is covered up by two layers. The uppermost, known technically as chuca, is of a friable nature, and consists of sand and gypsum; while the lower, the costra, is a rocky conglomerate of clay, gravel, and fragments of felspar. The caliche varies in thickness from a few inches to 10 or 12 feet, and rests on a soft stratum of earth called cova.
The Method of mining the Nitrate.
The mode in which the caliche is excavated is as follows: A hole is bored through the chuca, costra, and caliche layers till the cova or soft earth is reached below. It is then enlarged until it is wide enough to admit of a small boy being let down, who scrapes away the earth below the caliche so as to form a little hollow cup. Into this a charge of gunpowder is introduced, and subsequently exploded. The caliche is then separated by means of picks from the overlying costra and carried to the refinery.
Composition of Caliche.
Both in appearance and composition it varies very much. In colour it may be snow-white, sulphur, lemon, orange, violet, blue, and sometimes brown like raw sugar.
The caliche found in the Pampa de Tamarugal contains generally about 30 to 50 per cent pure nitrate of soda; that in the province of Atacama contains from 25 to 40 per cent. The subsequent refining processes, which consist in crushing it by means of rollers and then dissolving it, need not here be described. It may be sufficient to mention that the process used is that known as systematic lixiviation, and is analogous to the method introduced by Shanks in the manufacture of soda. The chief impurity in the raw material is common salt: gypsum, sulphates of potassium, sodium, and magnesium, along with insoluble matters, are the other impurities. The manufacture of iodine, which, as has been already noticed, is found in the nitrate-beds, is also carried on at these oficinas.
Extent of the Nitrate Deposits.
The question of the extent of the nitrate of soda deposits is naturally one of very great interest, especially from the agricultural point of view. M. Charles Legrange, a French writer, estimated a few years ago that they still contained about 100,000,000 tons of pure nitrate of soda. Opinions on this point differ very considerably, and it seems wellnigh impossible to arrive at any very accurate estimate.
The number of years they will last will depend, of course, on the amount of annual exportation. This, at present, falls little short of 1,000,000 tons. If this amount is maintained, they should last, according to experts, some twenty or thirty years at least. A consideration which has an important influence on this question, is the price obtained for the article. If this should be increased, it may be possible to treat the larger quantities of the inferior raw material (which at present prices are allowed to accumulate) at a profit. Undoubtedly this is what will ultimately take place, when the richer quality of the caliche has been exhausted.
Composition and Properties of Nitrate of Soda.