me "coprolites" has
been given, on the assumption that they consisted of fossilised animal
excrements. These coprolites, or osteolites as they have also been
called, vary in the percentage of phosphate of lime they contain.
Sometimes this amounts to 80 per cent, but as a rule it is very much
less. They also in the past have formed an important source of manure,
and will be referred to subsequently.

_Guano._

We have, lastly, phosphoric acid occurring in large quantities in
guano-deposits, chiefly found on the west coast of South America. These
deposits, which have been of enormous importance as a source of
artificial manure, are of animal origin, and will be discussed at
considerable length in a chapter specially devoted to the subject; so
that we need do no more than mention them here.

Phosphoric acid is also found in the form of phosphate of lime in
certain rocks as "layers" and "pockets."

_Universal Occurrence in Common Rocks._

But while it is thus found in considerable quantities in various parts
of the world, and while no anxiety need thus be felt as to its abundance
for artificial manurial purposes, its occurrence in the common rocks,
which, as we have already pointed out, is practically universal, is in
many cases very minute.

Fownes first identified it in the felspathic rocks in 1844; and since
then its percentage in granite, lava, trachyte, basalt, porphyry,
dolomite, gneiss, syenite, dolerite, diorite, and a number of other
rocks, has been determined by numerous investigators. For analyses of
these rocks the reader is referred to the Appendix.[124]

_Occurrence in the Soil._

That no soil is actually without phosphoric acid is highly probable, but
in many soils it is present in the merest traces, and even in fertile
soils it is rarely present in quantities over two-tenths of a per cent;
while half that amount may be taken as an average for most fairly
fertile soils. This would be about 3500 lb. per acre, calculating the
soil to a depth of 9 inches. In exceptional cases it has been found to
the extent of .3 per cent; and in the famous Russian _black earth_ it
has been found to amount to .6 per cent.[125] Like nitrogen, it is found
in greatest amount in the surface portion of the soil, but its amount at
different depths does not vary to the same extent as we have found to be
the case with nitrogen.

_Condition in which Phosphoric Acid is present in the Soil._

Unlike nitrogen, phosphori