60 GEOLOGY OF THE BLACK HILLS. 



clearly defined structure. The silicious materials composing it display 

 great variety in color and character. Among them are a deep red jasper, a 

 grayish quartzite and a pure white milky or translucent quartz which in 

 places has a bright red crystalline iron oxide running through it like the 

 dendrites in moss-agate. 



On Box Elder Creek, south of this deposit and on the summit of a 

 high elevation, there is found running parallel with the stratification a ledge 

 of pure milky quartz which from its position attracts the attention of the 

 passer by. It is some feet in thickness and is traceable for nearly a mile. 

 Samples were taken from it and tested for gold, but yielded no trace by fire 

 assay. The quartzites also frequently carry seams of quartz which in 

 many cases seem to be true veins produced after their formation and con- 

 solidation. The banded structure already noticed is found also in the seams 

 in the quartzites. Bands of iron ore, ferruginous strata of the inclosing rock, 

 and seams of ferruginous quartz are found in the quartzites, and though the 

 separation between the ordinary quartzite and the ferruginous portions is 

 generally ill-defined, the latter are sometimes so well marked as to run like 

 true veins within the body of the quartzite itself. There is, however, no 

 slickensiding nor other direct evidence of the existence of fissure veins In 

 many instances the ferruginous quartzite ledges are probably due to the 

 original dissemination in the quartzite of iron oxides, which in the process 

 of weathering are oxidized and hydrated to brown hematite. Sometimes, 

 however, the ferruginous portions arise from the decomposition of iron 

 pyrites which existed in seams or irregular masses of later date than the 

 quartzite. The Mammoth Lode, for example, which is highly ferruginous, 

 owes its iron largely to the decomposition of pyrites, some pieces of which, 

 unchanged and slightly auriferous, were found in place beneath the level of 

 the creek. 



A fuller exposition of the occurrence and character of the veins is 

 given by Mr. Jenney in his chapter on the Mineral Resources. 



The dip of the strata of the eastern series is always high, 70° to 90°, 

 and though it varies in amount and direction, it is usually toward the west. 

 The general strike is about north 30° west, but it swings now in one direc- 



