230 



PERUVIAN GUANO, ETC. 



[chap 



brought by the parent birds. In addition to the excreta 

 the deposit will thus also contain many carcases of 

 young birds dying from some cause or other, fragments 

 of fish, feathers, seaweed, and even sand and stones 

 originally swallowed by the parent birds. When the 

 birds leave the island the tropical sun and the intense 

 dryness of the atmosphere rapidly desiccate the 

 accumulated materials and prevent any change or loss 

 by fermentation. 



The excreta of the birds, which is the starting-point, 

 is highly nitrogenous, consisting very largely of uric 

 acid, together with a fair amount of phosphoric acid 

 derived from the fish, which is the exclusive diet of the 

 birds. An old analysis of a white Peruvian deposit, 

 consisting mainly of recently deposited excreta, showed 

 as much as 18-3 per cent, of nitrogen and only 9-2 per 

 cent, of phosphoric acid. 



Table LXXIII. — Analysis of Freshly-deposited Guano. 



Dry as is the climate a certain amount of change 

 still goes on ; the uric acid is fermented to urea and to 

 ammonium salts, some of which are volatilised, while 

 the occasional rains dissolve out both the ammonium 

 compounds and soluble phosphates and the alkalis. As 

 a result, the composition of guano deposits is extremely 

 variable, both in the different strata of one deposit 

 and still more in passing from island to island. The 

 older a deposit is, and the greater the washing it has 



