. The Spanish-American republics . long as water does not touch them, forwater dissolves the cementing elements, and then the costra crum-bles. Below this is the caliche (c), and in the lower part of the calichea layer of congelo (d\ resembling externally a very moist gravel whichhas been frozen. This layer contains common salt, chloride of mag-nesium, sulphates, and only a small percentage of nitrate of this is a pale reddish-brown loose clay, sometimes mealy, some-times sandy, with many glittering crystals of anhydrite. This layer(e) is called coda, contains no nitrate, and rests
. The Spanish-American republics . long as water does not touch them, forwater dissolves the cementing elements, and then the costra crum-bles. Below this is the caliche (c), and in the lower part of the calichea layer of congelo (d\ resembling externally a very moist gravel whichhas been frozen. This layer contains common salt, chloride of mag-nesium, sulphates, and only a small percentage of nitrate of this is a pale reddish-brown loose clay, sometimes mealy, some-times sandy, with many glittering crystals of anhydrite. This layer(e) is called coda, contains no nitrate, and rests upon the primitiverock or clay bed forming the basis of the geological formation of theregion. The layer of caliche varies in thickness from a few centimetres to2 and even 2\ metres. It varies also in quality, purity, and raw material contains from 17 to 50 per cent, of nitrate of soda,and even more in very favored spots, for instance, at Agua Santa. Itschemical composition is a mixture of nitrate of soda and chlorure of. THE NITRATE DESERT OF TARAPACA. 165 sodium in very variable proportions as principal elements, combinedwith clay, sand, stones, and other earthy matter insoluble in accessory salts found in it are chlorures of potassium and mag-nesium, nitrate of potassium, gypsum, iodates and iodure of is crystalline in structure, slightly salt in taste, and very solublein water. Its color varies from all shades of gray and brown to snow-white, lemon, sulphur, violet, blue, and green. The yellow tints indi-cate the presence of chrome or bromide of sodium, while oxides of iron,copper, and manganese account for the red, green, and black shades. The origin of these deposits has been and is still the subject ofingenious conjectures, no one of which is entirely satisfactory. Tomention these hypotheses at length would require much space. Letit suffice to say that the most probable seems to be that which at-tributes the formation of this substance
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