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University of California Publications in Geology [Vol. 13 



INTRODUCTION 



The mines of the Cobalt district of Ontario, Canada, have long 

 been known as large producers of silver. The precious metal occurs 

 associated with the arsenides of cobalt and nickel in veins of calcite, 

 varying in width from a fraction of an inch to eighteen inches. Usually 

 the silver is disseminated through the arsenide in the native condition 

 or in such minerals as dyscrasite, argentite, and proustite. Frequently, 

 however, it is found native in calcite, and locally it entirely displaces 

 the gangue, filling the vein as a sheet of solid silver. 



Silver was discovered in Cobalt district in 1903 during railroad 

 construction ; and in the same year Willet G. Miller, provincial geol- 

 ogist, made a preliminary study of the district. An official report 

 was published by the Ontario government in 1904. Three later editions 

 were issued, the fourth one appearing in 1913. 1 



Many geologists and engineers have visited the Cobalt District and 

 much has been written about it. Various hypotheses concerning the 

 origin of the ores have been suggested ; but the deductions of Miller 

 are probably the best known and most generally accepted. On account 

 of the necessary limitations of this paper, I will not attempt to dis- 

 cuss the various views of others ; but, as a background for my own 

 conclusions, I will give a brief resume of the current conceptions as I 

 found them upon beginning work. 



Grateful acknowledgment is due Professor A. C. Lawson and 

 Dr. P. R. Bichowsky for helpful discussions and suggestions relating 

 to the igneous and chemical problems dealt with, also to my assistants, 

 Mr. W. L. Whitehead and Mr. Maurice Albertson. 



REVIEW OF IDEAS CURRENT IN 1915 



Formations. — The oldest formation recognized consists of altered 

 Keewatin lavas, tuffs, and rocks resembling sediments, penetrated by 

 pre-Laurentian finegrained intrusives, the lavas and intrusives rang- 

 ing from the most acid to the most basic. All these formations were 

 folded and were intruded by gray Laurentian granite, which in time 

 became exposed by erosion and mantled by the Timiskaming Series of 



1 Ontario Bureau of Mines, Report, vol. 19, pt. 2. 



