16 



Chemical Composition and Commercial Value of 



appropriate name, for this material is not a guano deposit, but in 

 reality the rock itself, of which the islet of Sombrero consists 

 almost entirely. Although fragments of bones are not often 

 found imbedded in the rock which is imported into England, it 

 cannot be doubted that the latter is a true bone-breccia. I have 

 in my collection a specimen of Sombrero Rock, in which several 

 pieces of bone are distinctly visible. These pieces of bone are 

 perfectly white, and free from organic matter. The rock varies 

 considerably in colour, texture, and other external characters. 

 Whilst some is porous and friable, other specimens are of con- 

 siderable density. Most samples have a light yellowish-green 

 colour, which is, however, sometimes varied by a bright green 

 or bright yellow, a violet, bluish, or pinkish hue. On the 

 whole, this rock is reduced to powder with tolerable ease. 



I have carefully and minutely examined some average sam- 

 ples taken from a bulk of several tons, which gave the following 

 results : 



Composition of Sombrero Rock or Crust Guano. 



Sombrero Rock has been used in America for agricultural 

 purposes, it is said, with considerable success, when simply 

 reduced to powder. Such a practice cannot, however, be recom- 

 mended ; for this, like most mineral phosphates, requires to be 

 treated with sulphuric acid in order to become really efficacious 

 as a manure. It will be noticed that th'ere is not much carbonate 

 of lime, but a good deal of alumina, and in some samples also a 

 good deal of oxide of iron. The proportion of phosphate of lime 



