98 ~ ECONOMIC GEOLOGY 
coast towns were provided with fresh-water by sea, and sold 
at the price of often 5 gallons for a penny. 
The nitrate may be recognized by its inflammability on 
burning wick. According to tradition the deposits were 
discovered owing to the ground catching fire; the alarmed 
Indians took some to a priest to expel the evil spirit, which 
he recognized as nitrate. In the eighteenth century it was 
used instead of saltpetre (KNOg) for gunpowder, but the 
modern trade dates from the discovery by a Scottish settler 
near Iquique, that the white soil made his garden extremely 
fertile. He sent some to Scotland where its nature was 
determined, and the first nitrate works were established at 
Iquique in 1826. In 1830, 8300 tons were exported; the 
amount increased to 2§ million tons in 1913, fell to 1 million 
tons in 1922, but had risen to 24 million in 1925. 
The nitrate deposits or ‘‘ saltreras are generally on the 
edge of salt pans or ‘ salares.” The sequence of deposits 
is usually as follows. At the surface are a few inches of 
decomposed *‘ porphyry; below is a conglomerate which 
is cemented by sulphates of lime, potash, and sodium, with 
a little sodium nitrate; next a layer of sand and clay con- 
taining salt and anhydrite. This layer and the conglomerate 
are together from 1-3 feet in thickness; and beneath them 
is the caliche, the main bed of sodium nitrate; it varies in 
thickness from a few inches to 6 feet; the average of nitrate 
in the material mined is between 20 and 30 per cent.; and 
less than 17 per cent. usually does not pay. The caliche 
rests upon sand and clay containing salt and gypsum, below 
which may be a second nitrate layer, the banco. 
TrEORIES OF ForMaTioN—The origin of sodium nitrate 
has given rise to an unusual variety of hypotheses. It was 
attributed to the decay of seaweeds and fish in an arm of 
the sea which had been raised above sea-level (Darwin, 
1846, Geol. Observ. S. Amer., Chap. 111; C. Noellner, ¥. prakt. 
Chem. cii, 1867, p. 461). The main argument for this theory 
was the presence of marine shells, and it was discredited 
when they were found to be derived from the underlying 
Cretaceous rocks. = According to a second theory the nitrate 
was derived from guano, either deposited by birds on the 
shores of lagoons (Penrose, 1910) or blown inland from the 
coast {Ochsenius, 1888, Z.d.g.G., x1, pp. 153-65). A third