202 ECONOMIC GEOLOGY 
larger, until this chain of death concentrates phosphorus 
in the bodies of fish, which are in turn the prey of gluttonous 
sea birds. Small islands off the coast of South America are 
the nesting places of vast flocks of a cormorant, Phalacro- 
corax bougainvillei, which weighs 6 Ib., but will eat 33 lb. of 
fish at a meal, and from 8 to 10 lb. in a day. They cannot 
assimilate all the phosphate and it is deposited in their 
droppings as guano—the Spanish word for dung. 
The fertilizing value of guano depends also on its nitro- 
genous constituents, mostly ammonia, which may be washed 
out by rain, leaving the residue as phosphoric or leached 
guano. Thick guano deposits can only accumulate on 
islands where birds can nest safe from mammals and snakes, 
and in climates with an insignificant rainfall. These con- 
ditions occur off the western coasts of South America, in 
“Guano Islands” off South-western Africa, and in the 
Abrolhos Islands off West Australia, where the prevalent 
westerly winds pile water against the coast and cause a cold 
northward flowing current. The Humboldt current along 
South America is 20° cooler than the adjacent water, and the 
Antarctic fauna—fur seals, petrels, and penguins—ranges 
northward into the tropics. The wind that blows across this 
cold water has its temperature raised on reaching land and 
its capacity for carrying moisture thereby increased ; hence 
the sea wind has a parching effect until the air is chilled by 
rising up the mountains, where its moisture falls as rain. 
The supplies of guano are to some extent renewed, but the 
birds have at times suddenly abandoned the islands, as in 
1911 when millions of the young were left to die. Dr, H. O. 
Forbes explained their disappearance as due to fright at an 
earthquake shock. They began to return three months 
later, and Forbes estimated that 5,600,000 of them were 
nesting on the Central Chincha Island in 1913; and that 
they ate 1000 tons of fish a day.! 
Rock PuospHATE—In a rainy climate the phosphoric 
acid is leached out of the bird droppings, and carried into 
the underlying material which it converts into phosphate. 
Manv of the bird-frequented islands are coral reefs and 
L Cf. R. C. Murphy, Bird Islands of Peru, 1925 H. O. Forbes, 
{bis., 1913 (10), i, pp. 709-12.