200 GEOLOGY OF THE BLACK HILLS. 



black limonite and manganese oxide, which, though closely associated, seem 

 to remain distinct These oxides fill the spaces between the feldspar 

 crystals and irregularly impregnate the rock, giving it a brecciated appear- 

 ance. They often show a cubodial form, indicating that the}- are derived 

 from the decomposition of pyrites, and small pieces of that mineral were 

 found in the interior of several masses. Though gold is invisible, careful 

 assays prove its existence in the mass, and from this source is probably 

 derived a portion at least of the gold found in this region. 



The encircling zones of sedimentary rocks include the Potsdam below 

 and the Jura above. Their dip is quaquaversal and is usually quite regu- 

 lar, the angle varying from 20° to 30°. 



The Potsdam sandstone which immediately overlaps the base of the 

 igneous mass has been greatly metamorphosed. Where the rock was shaly 

 it has been changed into a hard fissile slate, scarcely recognizable as of 

 sedimentary origin ; and the pure sandstone strata have been converted 

 into comj)act quartzite often of a very white color. In several places the 

 igneous matter seems to have penetrated between the strata, which are 

 scarcely distinguishable from the injected rock, and in many cases the 

 metamorphosis appears to have been performed by the action of heated 

 waters, for the sandstone was found penetrated irregularly by well formed 

 crystals of feldspar. On the west side, near the middle of the formation, 

 were found layers of argillaceous sandstone covered by the large branching 

 fucoid peculiar to the Potsdam, and some of the upper layers of the sand- 

 stone are perfectly riddled with ScolitJms holes. The Potsdam, Carbonifer- 

 ous, and Red Beds are well exposed in many of the canons which radiate 

 from the central area carrying the drainage eastward to the Redwater or 

 westward to the Belle Fourche. On the south and east, facing Sun Dance 

 Hills, the Red Bed limestone forms the outer slope, dipping under the red 

 clay of the Redwater Valley. On the north and west, however, the Jura 

 is well exposed and capping this the Dakota sandstone. 



Though on a grander scale and exposing a larger area of igneous 

 rock than the other centers of igneous extrusion, the Warren Peaks show 

 the same character of pustular eruption/ 



The Bear Lodge (Mato Teepee). — This name appears on the earliest 



