Page:Popular Science Monthly Volume 85.djvu/145

Rh nitrates, they are not considered of much commercial importance, and the same may be said of all other deposits so far discovered in this country.

Small nitrate deposits are also to he found in various other parts of the world, as in the Sahara, in Russian Turkestan, and in Egypt, where nitrate earths occur which contain about 15 per cent, of calcium and sodium nitrates. The earth has long been used locally as a fertilizer, and its use is supposed to be increasing. The source of the nitrates in this region is not known.

All known deposits, however, which occur, like the ones just referred to, in various desert regions throughout the world are insignificant compared with the well-known deposits in the deserts of Atacama and Tarapaca in the north of Chile. These deposits command a great deal of interest, not only on account of their commercial importance, but also for the many attempts which have been made to explain why the quantity of nitrates in this particular region should be so large compared with any other known deposit.

The first shipment of nitrates to Europe from Chile was made in 1825. Since then the annual exportation has continuously increased until in 1912 the total quantity exported amounted to 2,485,860 tons of which 1,925,590 tons went to Europe, 469,100 tons to the United States, and 91,170 tons to other lands.

The arid region in which the nitrates are found extends for about 430 miles between 13° and 25° south latitude and lies between the Andes in the east and the Coast Range on the west. This area lying between the two mountain ranges does not form a continuous valley, but is broken up by transverse ranges into a series of elevated basins or plateaus. These plateaus are generally flat or undulating, and have an elevation from less than 2,500 feet to more than 5,000 feet. They have a general slope from the foot of the Andes towards the Coast Range, and as a result the lowest part of this plateau region, or pampa as it is called in Chile, lies along its western border where it joins the foothills of the Coast Range. It is along this zone that the nitrate deposits occur. The surface of the surrounding region is dry and sandy and vegetation is totally absent.

The nitrate beds as they occur in different parts of this region vary in thickness up to about six feet. They are usually found at or near the surface, but may in some cases be covered with an overburden to a depth as great as thirty feet. The nitrate deposits are never found pure, but are always mixed with sodium chloride and other salts, and are impregnated with insoluble earthy material. Crude nitrate may sometimes run as high as 60 to 70 per cent. of sodium nitrate, but a deposit running 50 per cent. is considered high-grade material. Material containing less than 16 per cent. is too low grade to be mined at a profit at present.