THE MINERALS IMPORTANT AS ORES, ETC. 27 



ore and gangue minerals has doubtless proceeded in nature with 

 great slowness, and from very dilute solutions. Both classes ex- 

 hibit a tendency to concentrate in cavities, even from a widely 

 dispersed condition through great masses of comparatively barren 

 rock. The formation may have proceeded when the walls were 

 far below their present position with regard to the surface, so that 

 to those inclined a wide latitude for speculation on origin is 

 afforded. It, is also possible that in the earlier history of the globe 

 circulations were more active than they are now a line of argu- 

 ment on which a conservative writer would hesitate to enlarge. 



