24 PROCEEDINGS OF THE NATIONAL Ml BEl 1/. vol 38. 



these figures. An average of five determinations for specific gravity 

 gives 3.26. The silica content, calculated from this value, is 53.5 

 per cent. 



Genesis. — The specimens were probably formed during the consoli- 

 dation of the Nubian Sandstone in Upper Cretaceous or later time by 

 the deposition from solution of barium sulphate in the interstices of 

 a loose sand. The linear force of crystallization pushed the sain I 

 grains slightly apart, and the less well-understood power, which makes 

 for an orderly molecular arrangement, developed an outer form in 

 keeping with an inner structure. 



