The layer or covering which contains the nitrate is encountered at a very slight depth, covered by a fold of the conglomeration indicated, and which in general is altogether sterile, though in some parts, principally in the North, it contains a regular vein of nitrate.
The vein of nitrate of soda in the layer which contains it is quite variable, the highest proportion being in the Province of Tarapacá, where in some points the medium quality amounts to 60 per cent. In the southern region this quality of caliche, or crude material, diminishes, and does not exceed an average of 30 per cent.
The name caliche is given to the raw material which contains the saltpetre that is found in the beds of deposits mixed with common salt, sulphate of soda, clay, and other foreign substances. The thickness of the layer is decidedly variable, and fluctuates between a few inches and three feet. Deposits of greater thickness exist, but these never have a great extension.
The height above sea-level at which these deposits are met with varies from 3,600 to 13,000 feet.
The layers composing a saltpetre deposit are:
1st. Chuca.—This is formed by clay mixed with earth very fine and evenly spread. The thickness of the chuca generally does not exceed an inch and a quarter.
2d. Costra.—This layer, which forms the immediate covering for the caliche, has a thickness fluctuating between four-fifths of an inch and several feet.
3d. Caliche.—This is the layer which contains the saltpetre, and its thickness varies greatly in different places.
4th. Conjelo and cova.—This is the last layer, which rests upon the rock. It is formed by a mixture of common salt, various sulphates, and other salts, but contains no saltpetre. Its thickness is also very variable.
The limpid caliche is taken to the finishing establishments, where it is submitted to a process of purification which is founded on the great solubility of nitrate of soda, superior to the other salts which are in combination with it, in water heated to the boiling-point. The solutions which result are carried by means of troughs to great vats, where the nitrate of soda crystallizes along with the potash, which exists in small quantity together with a little common salt and a small amount of sulphates and impurities.