i South Dakota School of Mines 33 



but a light brown color when damp. Some portions of it are 

 pale green when dry, or olive when wet. It is a hydrosilicate 

 of alumina, with some admixture of sand and clay, being in 

 reality fullers' earth, and different from ordinary clay in being 

 much less plastic. In the lower beds of the group it merges 

 into sand on the one hand, and into clay on the^ other. It is 

 often associated with, or gives place to, coarse materials occupy- 

 ing channels or broad sheets. In the vicinity of Hermosa the 

 principal material is coarse sandstone and conglomerate, mainly 

 of dark brown color, which mantles extensive plateaus. On the 

 high level ridge, north of Spring creek, there are coarse con- 

 glomerates which extend entirely across the hogback range. 

 About Fairburn and to the westward, there are long channels 

 filled with conglomerate consisting of limestone pebbles and a 

 calcareous matrix. These extend up several of the depressions 

 through the hogback range, either displacing the fullers' earth 

 deposits, or being intercalated among them. The limestone 

 pebbles appear to have been derived from Tertiary limestones, 

 for they do not represe n t any of the Mesozoic or Paleozoic 

 rocks of the Hills. On the higher lands in the Red Valley, 

 between Hermosa and Rockerville, there is an extensive deposit 

 of nearly pure limestone, giving rise to a high plateau of consid- 

 erable extent. The total thickness of the beds is nearly thirty 

 feet at some places, the limestone being underlain by fullers' 

 earth. Limestones of various degrees of purity are abundantly 

 intercalated in the fullers' earth deposits in the region west and 

 southwest of Fairburn, lying in depressions on the older rocks. 

 These limestones usually contain frejsh-water fossils, mainly 

 gasteropods, often in great abundance. The most southerly 

 occurrence of the limestone is on the ridge a short distance 

 northwest of the western entrance of Fuson Canyon, and on the 

 high divide just, north of Lame Johnny creek, and a short dis- 

 tance west of Fremont, Elkhorn and Missouri Valley (now the 

 Chicago and Northwestern) railroad. There are extensive ex- 

 posures of coarse materials of White River age in the railroad 

 cuts through this divide south of Fairburn, where the materials 

 are mainly cross-bedded coarse sands with a large proportion 

 of gravel largely derived from crystalline rocks of the hills. The 

 thickness of the White River deposits on the flanks of the Black 

 Hills varies from a thin remnant to 200 feet or more. In the 

 divide just south of Lame Johnny creek in the Red Valley, at 

 a point ten miles southwest of Fairburn, over 200 feet were 

 measured, consisting mainly of pale flesh-colored sandy clay and 



