OCCURRENCE OF BLACK PHOSPHATE 33 



into two principal classes each of which has a number of varieties. 

 The two main groups are designated by their color, the black 

 phosphate, which represents the original deposition, and the white 

 phosphate, which is a secondary deposition or replacement. The 

 first group belongs to the Devonian age, and its members have 

 been changed from their original form only by the process of con- 

 solidation which affects all deeply buried sediments ; that is, they 

 have been changed from a condition of mud and sand into com- 

 pact rock exactly in the same manner as the non-phosphatic bodies 

 above and below them. The white phosphates, on the other hand, 

 probably do not occupy the position and form they had when first 

 deposited. The material composing them has been translated 

 from its original position and redeposited in an entirely different 

 form. 



The white phosphates are of comparatively recent origin, prob- 

 ably having been formed in the last geological period preceding 

 the present. 



36. Occurrence of Black Phosphate. The black phosphates oc- 

 cur first in a nodular condition associated with green sandy shale. 

 The nodules vary in size and shape from nearly spherical bodies 

 from one-half to one and a half inches in diameter, to irregular 

 flattened ellipsoids, sometimes two feet in length and one-third or 

 one-quarter as thick. Their surfaces are smooth and show no 

 external evidence of organic origin. In weathering they produce 

 almost a white powder, with fine and concentric banding of dif- 

 ferent shades of gray. Thin sections examined under the micro- 

 scope appear to be chiefly amorphous, with grains of pyrite a'nd 

 organic matter. In some of the nodules there is a concentric 

 arrangement of the material, which is easily separated. The 

 nodules in the lower layers are the largest. The number of 

 nodules varies largely within short distances. The nodules con- 

 tain from 60 to 70 per cent, of tricalcium phosphate. There is 

 a somewhat larger percentage of this substance in the weathered 

 than in the unweathered rock. The nodules are easily separated 

 from the materials in which they are imbedded, so that even when 

 they are not sufficiently numerous to form almost continuous 

 layers they can be mined with profit. 



