Reviews 



Secondary Enrichment of Ore Deposits. By S. F. Emmons. Trans. 



American Inst. Min. Eng., Vol. XXX, 40 pp., 1900. 

 Enrichment of Gold and Silver Veins. By Walter Harvey Weed. 

 Trans. Amer. Inst. Min. Eng., Vol. XXX, 25 pp., 1900. 



In the exploitation of ore bodies it has been found that many 

 deposits decrease in richness as depth increases. The explanations 

 have been various. That the phenomenon is a far more general one 

 than was once supposed, has only recently been recognized in its full 

 significance. It may be expected to be very frequently met with, now 

 that its real character has been found out, by all students of ore deposi- 

 tion. 



It is now a well-known fact that as geological formations, ore- 

 bodies as a rule are to be regarded as deposits originating very near 

 the surface of the earth's crust ; or, to be more precise, in the thin 

 outer belt of the zone of fracture of the lithosphere. The unusual 

 richness of many ore deposits at very shallow depths has come to be 

 considered as due to local enrichment, often long after the first con- 

 centration has taken place. 



From the viewpoint of origin, diminution of richness with depth is 

 not, then, to be ascribed to actual depreciation in the original grade of 

 the ore. The real status of the case is that the deposition of ore has, 

 in the upper belt, undergone a greater or less augmentation in metallic 

 content since the body was first formed. 



Among those who have given the subject of ore genesis most atten- 

 tion, and especially among those who have approached the subject 

 from the geological side, the rival theories of ascending solutions, 

 descending solutions, and laterally moving solutions, no longer find 

 countenance as distinct processes. Ore deposition may take place 

 through all three means, which may have equal importance. After an 

 ore deposit has once formed under special geological conditions, the 

 secondary enrichment which it may undergo is believed to take place 

 largely under the influence of the descending solutions. 



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