PREFACE. 



The survey of the Eureka District was authorized by the Hon. Clarence 

 King, the first Director of the United States Geological Survey, and the 

 field work, for the most part, was done during his administration. The field 

 season was confined to the summer and autumn of 1880, and was limited to 

 five months, the work being brought abruptly to a close early in December 

 owing to the inclemency of the weather. Visits of a few days' duration 

 were made by different members of the party during the two following 

 years, but these were simply to verify previous observations or to correct 

 apparently conflicting statements. 



This monograph is purely geological in its scope and is mainly a 

 careful study and survey of a comparatively small block of mountains, 

 which may be designated the Eureka Mountains, but which should not be 

 confounded with the Eureka mining district, as several other well known but 

 less important mining districts also lie wholly within this mountain area. 



As it was unmapped and only occasionally visited by geologists, little 

 had been accomplished, except for the immediate purposes of mining, toward 

 investigating its structure or solving its many geological problems. The 

 Eureka region was known to occupy an exceptionally broad expanse of 

 mountains, affording fine geological sections if carefully worked out, and 

 of special interest for the purposes of comparative study in other regions of 

 the Cordillera. In this direction scarcely anything had been accomplished. 



The field work, as planned, could not have been completed in the 



