IV.] COPROTITES AND ROCK PHOSPHATE 115 



declined owing to the opening up of so many cheaper 

 foreign deposits, and has of late years entirely ceased. 

 The coprolites formed hard dark-coloured nodules, which 

 were ground clown to a grey powder containing from 50 

 to 60 J er cent, of calcium phosphate, about 10 per cent, 

 of calcium carbonate, and 3 percent, of calcium fluoride. 

 Though occasionally applied directly to the land in a 

 ground form, they were almost wholly used in the 

 manufacture of superpho.<^phatc. 



Another phosphatic material which is practically 

 mineral and at one time entered into competition 

 with coprolites and bone phosphates as material for the 

 manufacture of superphosphate, consists of these guano 

 deposits in which the nitrogen has been wholly removed 

 or nearly so by the action of rain. Such deposits are 

 found in the West Indies (Aruba, Navassa, Sombrero, 

 Curasao), the Pacific (Baker, Abrolhos, Christmas, and 

 Ocean Islands), Bolivia (Mejillones), and one or two 

 other places "of less importance. The action of the 

 weather, particularly where the climate is not absol- 

 utely rainless, is always removing the nitrogenous 

 compounds from guano, so that the proportion of 

 phosphoric acid tends to increase, until even among 

 the Peruvian deposits a guano is found on Lobos 

 Island containing little more than 2 per cent, of 

 nitrogen and 60 per cent, of phosphate of lime. In 

 some of the other deposits that have been enumerated, 

 Christmas Island for example, the nitrogen has entirely 

 disappeared and a phosphate rock is left behind which 

 can only be termed a guano in virtue of its origin. 

 These purely phosphatic deposits, many of which are 

 now exhausted or no longer pay to work, have been so 

 much mineralised that they are not sold as guanos but 

 are employed for the manufacture of superphosphate. 

 However, the Lobos phosphatic guano is still exten- 



