256 GEOLOGY OF THE BLACK HILLS. 



action of waters holding silica in solution, or by the preponderance of clay 

 or sand in the original sediments, these rocks inperc(;ptibly merge into 

 each other, and the strata of qnartzite often cannot be distinguished from 

 jasper or pure quartz veins except by position and structure. There is a 

 most remarkable uniformity in the hardness and composition of these rocks, 

 not only for long distances along the outcrops of the same strata, but 

 throughout the whole extent of the geological formation of this belt rocks 

 from widely different localities most closely resembling each other. The 

 metamorphic action seems to have been the product of the mechanical 

 force producing the folding and upturning of the strata in Archaean time 

 The quartz veins are not igneous injections, but chemical precipitations 

 from waters holding silica in solution, partly, at least, derived from the 

 wall-rocks of the veins, and undoubtedly formed during the folding of the 

 strata, at which time the gold must also have been dejDOsited. The lower 

 layers of the Potsdam prove this, as they contain in places bowlders from 

 botli the veins and the wall-rock identical in hardness and appearance with 

 the rocks in the immediate vicinity. This chemical deposition of silica 

 would seem to be similar to the silicification of wood, the solution replacing 

 the original rock atom by atom, and samples of quartz were seen where 

 the original rock blended so imperceptibly with the white quartz that it is 

 difficult to explain the formation of the quartz in any other way. Quartz 

 occurs in the slates either as interlaminated fissure-veins, coinciding with 

 the stratification and often quite extensively developed, or as short and thin 

 veins, following for a few yards the jointing planes of the slates. It is in 

 the quartzites, however, that the greatest development of quartz is to be 

 found in this district. Strata of sedimentary rock lia-\'e, by the action of 

 water containing silica and iron in solution, been transformed into hard 

 quartzite intermixed with ferruginous quartz in all proportions throughout 

 the entire thickness and extent of the bed, until it resembles, on its outcrop, 

 an immense ledge of quartz and vein matter, traversing the slates for long 

 distances across the Hills. On examination, however, these peculiar forma- 

 tions are found to be altered sediments. There is no fiuccan or casing to 

 the ledges, and in the most favorable positions for the existence of slicken- 

 sides they are seen to be wanting. 



