of the Peruvian guano sold in commerce, over 10,000,000 tons
having been exported from them alone. Some of this guano contained 14
per cent of nitrogen (equal to 17 per cent ammonia); and although part
of the guano shipped from these islands was not quite so rich, yet it
was all of a high-class order. The deposits on these islands were in
many cases 100 to 200 feet in depth, and rested on rocks of granite. The
lower layers were consequently found to be poorer in quality, and mixed
with pieces of granite. The Chincha island deposits have been long
exhausted,[189] and the chief deposits of Peruvian guano since worked
have been those on Guanape and Macabi islands--a considerably inferior
guano, containing only 9 to 11 per cent of nitrogen (equal to 11 to 13
per cent of ammonia)--which in their turn have become exhausted; from
Ballestas, almost as rich as the Chincha island guano, also now
exhausted; and from Pabellon de Pica, Punta de Lobos, Huanillos,
Independence Bay, and Lobos de Afuera. Quite recently a deposit of very
high-class guano was discovered in Corcovado, and a good many cargoes
have already been shipped to this country. It is found to contain
nitrogen equal to from 10 to 13 per cent ammonia, 30 to 35 per cent
phosphates, and some potash, being thus a most valuable guano.

_Appearance, Colour, and Nature._

In colour it varies from a very light to a very dark brown, the richer
samples being generally lighter. Samples taken from even the same
deposit have been found to differ very considerably in appearance, those
taken from the lower and older layers being usually darker than those
taken from the more recent upper layers. It was soon found also to vary
very much in composition. After a deposit had been worked for some time,
the quality of guano it yielded was found to be inferior and coarser,
and in many cases mixed with pebbles or pieces of granite, porphyry, &c.
This led to the custom of screening it on arrival in this country,
before it was used as a manure. In the richer qualities--_e.g._, in the
Chincha guano--little round concretionary nodules, varying in colour
from pure white to dark brown, were occasionally found. Analysis showed
these nodules[190] to be composed chiefly of potash salts. Sometimes,
also, little crystals of almost pure ammonia salts were found. It soon
became customary, therefore, to prepare guano for the market by
separating the stones and reducing the whole to a fine uniform powde