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leached by surface waters, which take into solution various metals and 

 passing downward meet with and are decomposed by the sulphides of 

 iron present in the unaltered ore, resulting in the redeposition of new 

 sulphides of the metals. 



This is followed by a description of the mode of occurrence of sec- 

 ondary sulphide ores of copper, silver, and. zinc, as studied by the 

 author and others. The enrichment in many cases proceeds along 

 barren fractures producing bonanzas. In others it forms films, pay- 

 streaks, or ore shoots in the body of leaner original ore. In still other 

 cases the alterations are produced by deep-seated uprising waters acting 

 upon the vein. As a consequence of these processes veins do not 

 increase in richness in depths below the zone of enrichment. 



J- P. I. 



Origin and Classification of Ore Deposits. By Charles R. Keyes. 

 Trans. American Inst. Mining Eng., Vol. XXX ; 34 pp., 

 1900. 



The various attempts that have been made in the past to formulate 

 a rational and at the same time a useful classification of ore deposits 

 have met with only indifferent success. The fundamental factor in 

 the proposed scheme by Dr. Keyes is geological in nature. It is based 

 upon the principle that local deposition and specific form of the ore 

 body is dependent upon geological structures, and these largely govern 

 also the exploitation of the ores. This is believed to be as nearly as it 

 is possible to approach a purely genetic arrangement. The great geo- 

 logical processes are made the governing principles. 



Although the present memoir discusses only the classificatory 

 aspects of the ore deposits, it is an application of the modern principles 

 of petrology, and especially those dealing with the processes involved 

 in general rock metamorphism, and it opens the field for fuller expla- 

 nations and applications of these principles. 



Three propositions are emphasized : First, ore bodies with few 

 exceptions are regarded as essentially surface deposits — that is, they 

 are considered as confined to a very thin zone near the earth's surface, 

 or more precisely in the outer belt of the zone of fracture of the litho- 

 sphere ; second, most of the worked deposits of the globe are thought 

 to be of very late geological formation, probably few dating back before 

 the Tertiary ; third, ore bodies are believed to be concentrated chiefly 



