250 GEOLOGY OF THE BLACK HILLS. 



Certificate of assay. 



School of Mines, 

 New York, January 6, 1876. 

 Sir : The sample of placer gold from Spring Greek, Black Hills, submitted to me 

 for examination, contains, by assay of tbe melted bar, made by sampling about one 

 ounce of the raw dust and melting to free it from oxide of iron — 



Gold 940. 4 fine. 



Silver 50. 4 fine. 



Base metal 3.2 



Total 1000. 00 



The value in gold coin of one ounce troy of melted dust is — 



Gold $19. 562 



Silver 0. 062 



Total 19. 624 



Note. — The value of pure gold per ounce troy is $20. 67 ; the value of pure sil- 

 ver per ounce troy is $1. 24. 

 Yery respectfully, 



P. De p. eioketts, e. m. 



Walter P. Jenney, E. M., 



Geologist Black Sills Expedition. 



About 100 feet above the original discovery an attempt was made to 

 test the creek-bed by building a wing-dam to keep out the water and 

 sinking a prospecting hole inside of it. But after a day of hard work we 

 were obliged to abandon the working at a depth of 4 feet, springs of water 

 bursting through the gravel and flooding the pit. From three to four colors 

 of gold were obtained from the loose gravel. 



In a broad gravel flat a quarter of a mile below this point a prospect- 

 ing hole was sunk to bed-rock, which was reached at a depth of 6 feet, and 

 found to be hard quartzite, covered with loose gravel, resting on a surface 

 worn as smooth as a floor by the water. Several pans of dirt were washed 

 from this pit, but not a color of gold was found. Another pit located in an 

 old channel had to be abandoned at the depth of 5 feet, quicksand being- 

 struck so fluid that it flowed into the working and caused the walls of the 

 pit to cave in. 



Two prospecting holes in the flat where the Mammoth ledge crossed 

 the gulch struck a cement of quartz-pebbles and oxide of iron so hard as to 

 be penetrated with great difficulty. The same cement, several feet in thick- 



