58 GEOLOGY OF THE BLACK HILLS. 



the fei'riferous strata, however, is highly siliciferous and entirely useless as 

 an iron ore, consisting of thin strata an inch or less in thickness of specular 

 hematite alternating with silicious slate or with pure white quartz in seams 

 or irregular masses, the whole presenting a remarkable resemblance to the 

 silicious banded hematite of the Huronian of the Lake Superior region. In 

 other localities on the same creek hematites were also found in the silicious 

 slates, but nowhere of any practical value because of their highly silicious 

 character. The slates associated with these iron deposits are commonly 

 highly argillaceous as well as silicious, as is indicated by their color, tex- 

 ture, and strong clayey odor. Similar ferrugineous slates occur also on 

 the headwaters of Rapid Creek a short distance north of the Elkhorn 

 Prairie. 



The quartzites vary in thickness from seams only a few inches wide to 

 masses 400 or 500 feet in width, though the more moderate thickness of 75 

 to 100 feet is more common. Several of these ledges have been traced 

 long distances, maintaining their regular relations with the adjoining strata 

 and holding to their own dimensions and character, running across the 

 country like dikes, bristling suddenly in peaks, or outcropping in the canons 

 of the creeks in sharp, well-defined walls. A prominent quartzite ledge on 

 Spring Creek, known as the "Mammoth Lode," has been traced continously 

 five or six miles in each direction, and others are distinguishable still greater 

 distances. They seem to be limited in their extent, in thickness as well as 

 in length, only by changes in the composition of the original deposits from 

 which they have been formed by metamorphism. 



The color of the quartzites is varied, being sometimes an almost pure 

 white but more often a light or dark gray or an impure blue or pink or, 

 when much iron is contained, a dark reddish brown. In texture they are 

 very compact and homogeneous, and on a fresh fracture, which is conchoi- 

 dal or fragmental, they have a vitreous or glassy luster. They are almost 

 pure quartz. 



Though they are found throughout the entire series of the slates, they 

 are of greater thickness and more numerous in certain parts of the district. 

 As has already been mentioned, there extends on the eastern border of the 

 Archsean area a long ridge or succession of prominent peaks and bluffs, 



