IN THE CAMBEIAN ROCKS. 385 



geological inquiry. That ratio once established, other calculations 

 might be made. The phosphates in these as in other sedimentary 

 rocks must be regarded as a sort of residuum or ash, the result of 

 the decomposition of nitrogenous organic matter which was left 

 after the more volatile carbon, hydrogen, nitrogen, and oxygen had, 

 under various combinations, more or less completely disappeared. 

 Calcium phosphate is known to be the chief constituent of the ash 

 of most albuminous principles : analyses of these were quoted. The 

 question of the loss of phosphoric acid in the rock adjoining the trap 

 was one of much difficulty ; but as an instance somewhat similar, 

 Bischof was quoted to the effect that large quantities of calcium 

 phosphates occur in basalt in the vicinity of a guano deposit at the 

 Isle of Ascension. 



2d2 



