FOOTNOTES:
[183] Bones, it is true, were in use long before guano; but popular as they deservedly were, they had not been used, at the time of the importation of guano, to any very considerable extent.
[184] The total annual imports at present may be taken at under 30,000 tons, whereas in 1855 they amounted to over 200,000 tons. For statistics on this point the reader is referred to the Appendix, Note I., p. 327.
[185] With regard to the origin of certain guano deposits, which are of very recent date—e.g., Angamos and Ichaboe—there can be no doubt whatever, because we can witness the process of formation still taking place. It is not so, however, with regard to older deposits, for which some have been inclined to claim mineral origin. The best proof that such deposits owe their origin mainly to bird excrements is the comparatively large quantity of uric acid they contain. On the other hand, the evidence in support of the belief that they are also formed from the remains of the birds themselves and other animals, is to be found in the large proportion of phosphates they contain, and the presence in the deposits of feathers and the fossilised skeletons of the animals above mentioned.
[186] A complete list of the various deposits will be found in the Appendix, Note II., p. 327. It may be noticed that nearly all the deposits lie within 10° to 20° north and south of the Equator.
[187] See Chapter on Farmyard Manure, p. 257.
[188] According to Nesbit, some of the cargoes of this guano contained hard saline lumps of very little manurial value—over 50 per cent being common salt.
[189] The salt exports were made in 1868.
[190] For analyses of these nodules and crystals, see Appendix, Note III., p. 328.
[191] See Heiden, vol. ii. p. 356.
[192] See Appendix, Note IV., p. 329.
[193] The Ichaboe guano at present exported is a fresh deposit, and is annually collected for shipment.
[194] Further chemical changes have occurred in certain cases between the guano and the limestone rock beneath, resulting in the formation of what is called a "crust" guano. Such guanos form a soft phosphatic rock, and are extremely rich in phosphates. As examples of these "crust" guanos may be mentioned Sombrero, Curaçao, Aruba, Mexico, and Navassa phosphates.
[195] The presence in the old Peruvian guano of concretionary nodules has already been referred to.
[196] According to Vogel the nitrogen as urates is converted by the sulphuric acid into ammonia salts.
[197] See Appendix, Note VI. p. 330.
[198] It must be remembered, however, that even genuine guano contains a certain quantity of carbonate of lime, and will give a slight amount of effervescence when so treated.
[199] The annual imports may be stated at from 3000 to 4000 tons.
[200] Agricultural Chemistry, vol. i. p. 367.
[201] See Appendix, Note VII., p. 331.