its must be extremely old, as they are covered up with sand and
other _debris_, and are of considerable depth. Especially is this the
case with deposits occurring on the mainland, such as those at Pabellon
de Pica, where the layer of sand or conglomerate covering up the deposit
varies in depth from a few feet to over a hundred. The effect of this
superficial covering has been to protect the guano, to a certain extent,
from loss of nitrogen.

Although guano of the best class has been derived from the neighbourhood
of Peru, deposits have also been found in many other parts of the
world--viz., in North America, West Indies, Australia, Asia, Africa, and
among the islands of the Pacific.[186]

_Variation in the Composition of different Guanos._

The guano found in these different deposits varies very considerably in
composition. This is due to the difference in the nature of the
prevailing climate of the places where these deposits occur. Where the
climate is dry and warm, as is the case in Chili and Peru, the
excrements dry quickly and remain very little changed, as one very
important condition of fermentation--viz., moisture--is absent.[187] In
a damp climate, on the other hand, speedy fermentation ensues, resulting
in the loss of nearly all the organic matter, including nitrogen, in
such volatile forms as carbonate of ammonia, carbonic acid gas, water,
&c. The soluble alkalies, the most important of which is potash, as well
as the soluble phosphates, are also, under such conditions, lost to the
guano by being washed out by the rain. We have thus a wide difference in
the quality of the different deposits, depending on the extent to which
decomposition has taken place. Guano thus ranges from the rich
nitrogenous Peruvian kind, which has undergone little or no change from
the time of its deposit, to the purely phosphatic kind (such as those of
Malden and Baker islands), in which everything of manurial value has
been lost except the insoluble phosphate of lime. Even among the
nitrogenous guanos we find a considerable difference in quality, some
deposits being partially impoverished by the action of the atmospheric
moisture, dew, spray or sea-water, but still containing a considerable
proportion of their nitrogen. Other deposits, again, are largely admixed
with sand, which has been blown in upon them to such an extent as to
make them unsaleable. We can divide guano, therefore, into two great
classes--viz., _nitrogenous_ an