PREFACE. 



The present work was undertaken at the instance of the Hon. Clarence 

 King-, first Director of the United States Geological Survey, in 1879. It 

 was his intention that it should form part of a series of monographs which 

 would in time include all the important mining districts of the country, and 

 thus furnish an accurate and permanent record of the manner of occurrence 

 and geological relations of the metallic deposits of the United States, as well 

 as of all substantial improvements in the methods of obtaining the metals 

 from their ores. 



In preparing such a monograph the general plan adopted was : first, to 

 obtain an accurate knowledge of the geological structure of the region and 

 of the various rocks of which it is made up ; next, to study thoroughly the 

 ore deposits in their varied relations to the inclosing rocks ; and, finally, to 

 investigate any methods of extraction or of reduction of the ores that pre- 

 sented new or unusual features, without M^asting time upon what was already 

 so well known as to require no further comment. Various circumstances 

 rendered such modifications of this plan necessary in the present case that 

 the various stages of the work could not always be carried on in their log- 

 ical sequence. The great altitude of the region and consequent inclemency 

 of its climate practically prevented surface work being carried on to ad- 

 vantage during eight months of the year. The organization of the Survey 

 was as yet incomplete, and assistants familiar with this class of work could 

 not immediately be obtained ; moreover, a year elapsed after the inception 

 of the work before laboratory facilities could be obtained which rendered 



