THE NITRATE DEPOSITS OF CHILE ii 



A thickness of from i to i^ feet is common, of 2 to 3 feet is less so, 

 but not unusual, and of 4 to 6 feet is very unusual. Where the 

 surface of a deposit is not too heavily covered with earthy materials, 

 it often has a moist appearance due to the deliquescent character 

 of the nitrate or perhaps to the presence of calcium chloride. 



The nitrate deposits are usually covered by a capping composed 

 of sand, clay, gravel, and rock fragments, from a few inches to many 

 feet in thickness. In a few places this capping is absent and the 

 nitrate is covered only by a thin coating of desert dust, but usually 

 the overlying material is from 2 to 20 feet in thickness and some- 

 times, though rarely, 30 to 40 feet. This capping is called costra 

 and is generally more or less indurated; the usual condition being 

 alternating layers or patches of harder and softer material, and an 

 extreme condition being that of a hard mass like a breccia or con- 

 glomerate, in which the cementing material is nitrate and other 

 saline and earthy substances. The rock fragments are angular 

 or partly rounded and vary from the size of grains of wheat to masses 

 of a foot or more in diameter, pieces from a half-inch to 3 inches 

 in diameter being the most common. The fragments consist of 

 limestone, shale, sandstone, igneous rocks, etc., the preponderance 

 of one or the other varying in different places. They seem to some 

 extent to vary in character according to the nature of the rocks in 

 situ in the neighborhood, and seem to have been derived largely 

 from the slopes of the Coast Range hills and from the small knobs 

 protruding up through the pampa. The costra is often overlaid by 

 from a few inches to a few feet of loose, wind-drifted material called 

 chuca. 



In some places there is a sharp line of demarkation between the 

 costra and the nitrate; in others they seem to blend into each other. 

 In fact, they often seem to represent one and the same deposit, rich 

 in nitrate at the base and poor in nitrate above. The smaller quantity 

 of nitrate in the costra than below may possibly be due to impover- 

 ishment by leaching during the rare periods of rainfall or flood. In 

 a few of the mines where the rich part has been exhausted, the costra 

 has been worked as a source of nitrate. Underlying the nitrate is 

 an earthy material of a brown or buff color, generally soft and powdery 

 though sometimes sandy, gravelly, or indurated, called coha. Below 



