Andkew. — On the Clarendon Phosphate-deposits. 465 



thin layers of clay, while thin bands of phosphatic clay are abun- 

 dant among the hard nodules. The weight of these nodules 

 varies from 1 lb. or 2 lb. up to several tons. 



The physical and chemical properties of the rock-phosphate 

 vary greatly : its colour varies from white to dark-yellow ; its 

 hardness from 1 to 8 ; its specific gravity from 2-068 to 2 - 988 ; 

 its percentage of contained tribasic calcium - phosphate from 

 20 (and even lower) to 80 per cent. A detailed description of 

 several varieties will render this more evident : — 



a. (H = 6-5, SG = 2-915). — To the eye this variety suggests 

 the appearance of marble ; it is white, hard, of dull lustre, 

 and contains grains of quartz and glauconite ; it is a rich 

 variety, containing from 65 to 80 per cent, calcium-phosphate — 

 Ca 3 (P'0 4 ) 2 . 



b. (H = 8, SG = 2-988).— This is yellow, finely grained, and 

 flint -like ; it breaks with a conchoidal fracture, has an enamel- 

 like lustre, and often a banded character ; it is high-grade, 

 containing from 70 to 75 per cent. Ca 3 (P0 4 ) 2 . 



c. (H = 5-5, SG = 2-910).— A yellow variety like b, but 

 coarser-grained, not flint-like, and never with a banded appear- 

 ance ; it also is of good quality. 



d. (H =6, SG = 2-816).— This is the very siliceous " grit 

 phosphate," which is in fact a sandstone cemented together by 

 lime - phosphate ; it often contains enclosed nodules of the b 

 variety, and in places is found incrusted with hyalite, a form 

 of opal ; it is poor in phosphate, containing only from 20 to 

 30 per cent. Ca 3 (P0 4 ) 2 . 



e. (H = 1, SG = 2 - 446). — This is a pure-white, chalky, unc- 

 tuous variety, rich in phosphate, of which it contains about 

 75 per cent. 



/. (H=l, SG= 2-068). —This variety can scarcely be termed 

 rock-phosphate, as it is only a finely laminated phosphatic clay 

 which has hardened to a slight extent, and now occurs in 

 irregular seams among the masses of hard rock-phosphate. 



The first four are the most common ; when they are " burnt " 

 the iron which they contain is converted to ferric oxide, which 

 gives them a pink or reddish colour. Under the microscope 

 the yellow varieties are seen to have no trace of crystalline 

 structure ; they are almost opaque in thin sections, a dull-brown 

 colour being transmitted only in the very thinnest places ; 

 quartz and glauconite grains occur throughout. 



Analyses. — The following table of analyses will show the 

 chemical composition of these phosphates : — 



