and the mines at Caceres have been worked on a large scale for
seventeen years, and about half a million tons have been raised. In 1882
the imports into this country amounted to over 56,000 tons; but latterly
they have only been about a fourth of this amount. Dr Dauberry visited
the deposits in 1843, and wrote a most interesting account of them. They
do not seem, however, to have been imported for purposes of
superphosphate manufacture till a number of years afterwards. Of
Estremadura phosphate there are three classes, containing respectively
50, 60, and 70 per cent of phosphate of lime, the lowest quality being
the commonest.[223]

_Norwegian Apatite._

This apatite has ceased to be imported of late years, owing to a duty on
exportation.

_Charleston or South Carolina Phosphate._

For a number of years these deposits have formed the chief source of
phosphate of lime used in the manufacture of mineral superphosphates in
this country (in fact they have furnished two-thirds of our phosphate
supply during recent years). Discovered twenty-five years ago, some four
to five million tons have already been shipped. About half a million
tons were raised in 1886 from these mines, which are the most abundant
in the world. There are two kinds--the so-called "land" and "river"
phosphates. The former contains more oxide of iron and alumina, and is
therefore less pure than the latter, in which the iron and alumina do
not exceed 2 per cent. The river phosphate is dredged from the Bull,
Coosaw, and Beaufort rivers. Of phosphate of lime it contains from 50 to
60 per cent. It is generally sold in three grades--50 to 52 per cent, 55
to 56 per cent, and 58 to 60 per cent of phosphate of lime. It will thus
be seen to be incapable of producing very high-class superphosphates
--_i.e._, containing more than 30 per cent "soluble" phosphate.
This point will be more intelligible when we describe the manufacture of
superphosphate. The demand for these phosphates in the United States has
increased enormously in recent years, owing to the increase in the
quantity of manure used.

_Belgian Phosphate._

Another very important source of mineral phosphates are deposits
discovered some years ago in Belgium near Mons. These phosphates are of
different qualities, and are found, some in layers near the surface in
pockets forming the richest class, and containing from 45 to 65 per cent
of phosphate, and some in the form of a friable phosphatic ro