CHAPTER IV. 



MINERAL RESOURCES. 

 By Walter P. Jenney. 



SECTION I. 

 INTRODUCTION. 



The Black Hills are pre-eminently a gold-producing region ; the meta- 

 morphic rocks constituting the gold-field cover an area of nine hundred 

 square miles, extending north and south through the central portions of 

 the Hills for a distance of seventy miles, with a breadth of from five to 

 twenty five miles. 



Gold occurs in the following formations: 



I. In veins of quartz more or less ferruginous traversing the Arcluran 

 schists and slates. 



II. In strata of slate mineralized and altered by the action of waters 

 depositing silica and iron pyrites. 



III. In the conglomerate forming the lower layer of the Potsdam 

 sandstone, resulting in this case from the denudation of the ledges in the 

 Archaean rocks. 



IV. In trachyte and porphyry intruded at the time of the elevation of 

 the Hills in the interval between the Cretaceous and Tertiary periods. 



V. In deposits in the slates and sedimentary rocks produced by the 

 intrusion of the trachyte and porpln-ry. 



VI. In placer gravels resulting from the decomposition and erosion of 

 the above formations in Tertiary and recent time. 



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15 B H 



