s the appearance presented by these pampas as resembling
"a country after snow, before the last dirty patches are thawed." The
_caliche_, or raw nitrate of soda, is not equally distributed over the
pampas. The most abundant deposits are situated on the slopes of the
hills which probably formed the shores of the old lagoons. An expert can
tell from the external appearance of the ground where the richest
deposits are likely to be found. The _caliche_ itself is not found on
the surface of the plain, but is covered up by two layers. The
uppermost, known technically as _chuca_, is of a friable nature, and
consists of sand and gypsum; while the lower, the _costra_, is a rocky
conglomerate of clay, gravel, and fragments of felspar. The _caliche_
varies in thickness from a few inches to 10 or 12 feet, and rests on a
soft stratum of earth called _cova_.

_The Method of mining the Nitrate._

The mode in which the _caliche_ is excavated is as follows: A hole is
bored through the _chuca_, _costra_, and _caliche_ layers till the
_cova_ or soft earth is reached below. It is then enlarged until it is
wide enough to admit of a small boy being let down, who scrapes away the
earth below the _caliche_ so as to form a little hollow cup. Into this a
charge of gunpowder is introduced, and subsequently exploded. The
_caliche_ is then separated by means of picks from the overlying
_costra_ and carried to the refinery.

_Composition of Caliche._

Both in appearance and composition it varies very much. In colour it may
be snow-white, sulphur, lemon, orange, violet, blue, and sometimes brown
like raw sugar.

The _caliche_ found in the Pampa de Tamarugal contains generally about
30 to 50 per cent pure nitrate of soda; that in the province of Atacama
contains from 25 to 40 per cent. The subsequent refining processes,
which consist in crushing it by means of rollers and then dissolving it,
need not here be described. It may be sufficient to mention that the
process used is that known as systematic lixiviation, and is analogous
to the method introduced by Shanks in the manufacture of soda. The chief
impurity in the raw material is common salt: gypsum, sulphates of
potassium, sodium, and magnesium, along with insoluble matters, are the
other impurities. The manufacture of iodine, which, as has been already
noticed, is found in the nitrate-beds, is also carried on at these
_oficinas_.

_Extent of the Nitrate Deposits._

The question of t