[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.