But however valuable the eggs and the oil, the feathers and the flesh of the hyperborean bird-republics may be to man, they are far from equalling in importance the guano producing sea-fowl of the tropical seas. This inestimable manure, which has become so indispensable to the British agriculturist, is found scattered over numerous localities in the intertropical regions. It abounds on many of the rocky islets of the Red Sea, where the life-teeming waters afford sustenance to innumerable sea-gulls, cormorants, and pelicans; but its most widely celebrated stores cover the small Chincha Islands, not far from Pisco, about a hundred miles to the south of Callao, where they form enormous layers 50 or 60 feet deep.
The upper strata are of a greyish-brown colour, which lower down becomes darker; and in the inferior strata the colour is a rusty red, as if tinged by oxide of iron. The guano becomes progressively more and more compact from the surface downwards, a circumstance naturally accounted for by the gradual deposit of the strata and the increasing superincumbent weight. As is universally known, guano is formed of the excrements of different kinds of marine birds; but the species which Tschudi, the celebrated Peruvian traveller, more particularly enumerates are--_Larus modestus_ (Tschudi), _Rhynchops nigra_ (Linn.), _Plotus anhinga_ (Linn.), _Pelecanus thayus_ (Mol.), _Phalacrocorax Gaimardii_ and _albigula_ (Tsch.), and chiefly the _Sula variegata_ (Tsch.).
The immense flocks of these birds, as they fly along the coast, appear like aërial islands; and when their vast numbers, their extraordinary voracity, and the facility with which they procure their food are considered, we cannot be surprised at the magnitude of the beds of guano which have resulted from the uninterrupted accumulations of countless ages. During the first year of the deposit the strata are white, and the guano is then called _Guano blanco_. In the opinion of the Peruvian cultivators, this is the most efficacious kind. As soon as the dealers in guano begin to work one of the beds, the island on which it is formed is abandoned by the birds. It has also been remarked that, since the increase of trade and navigation, they have withdrawn from the islands in the neighbourhood of the ports. Under the empire of the Incas, the guano was regarded as an important branch of state economy. It was forbidden, on pain of death, to kill the young birds. Each island had its own inspector, and was assigned to a certain province. The whole distance between Arica and Chaucay, a length of two hundred nautical miles, was exclusively manured with guano. These wise provisions have been entirely forgotten by the Spaniards, but the Peruvians now begin to discover the error of their former masters, and look forward with anxiety to the period when the guano will no longer suffice for the wants of husbandry. At the present day they use it chiefly in the cultivation of maize and potatoes. A few weeks after the seeds begin to shoot, a little hole is made round each root and filled up with guano, which is afterwards covered with a layer of earth. After the lapse of twelve or fifteen hours, the whole field is laid under water, and left in that state for about half a day. Of the guano blanco a less quantity suffices, and the field must be more speedily and abundantly watered, otherwise the roots would be destroyed. The effect of this manure is incredibly rapid. In a few days the growth of the plant is doubled; if the manure is repeated a second time, but in smaller quantity, a rich harvest is certain;--at least the produce will be three times greater than that which would have been obtained from the unmanured soil. The uniformity of climate, along a coast where rain is _never_ known to fall, contributes essentially to the superior quality of the Chincha guano, as atmospherical precipitations naturally dissolve and wash away many of the most fertilising salts.