Walter P. Jenney
Published: 2016-11-06
Total Pages: 74
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From the INTRODUCTION. The Black Hills of Dakota are included between the Belle Fourche and the South Fork of the Cheyenne River, extending in a direction north 20� west for one hundred and twenty miles, with a breadth of from forty to sixty miles. They cover an area of nearly six thousand square miles, two-thirds of which is in Dakota, the remainder in Wyoming, the boundary-line between these Territories, the hundred and fourth meridian of longitude, passing through the western portion of the area. Surrounded on every side by level or rolling plains and separated from the main chain of the Rocky Mountains, the Black Hills have a geological system perfect and complete in itself, with the records beautifully preserved in the rocks, and each successive formation fully exposed by uplift and erosion to scientific investigation. Conceive a nucleus of upturned metamorphic rocks, mica-schists, slates, and quartzites of Arch�an time, surrounded by encircling-belts of the subsequent geological formations, extending continuously around the Hills, arranged in the order of their deposition, with a general dip from the center toward the level plains. The mineral wealth of the Black Hills is derived from these Arch�an rocks; distorted, set on edge, and metamorphosed, they contain the auriferous quartz-ledges, and these, by decomposition and erosion, have yielded the gold to the placer-gravels. Covering an area of about nine hundred square miles, the metamorphic rocks are discovered, on examination, to naturally separate themselves into two distinct divisions, the schists and the slates. The schists, usually micaceous, occupy the southwestern portion of this area, reaching from Castle Creek southeastwardly, through Custer's Park and the Harney's Peak range, to the southern end of the Hills. The clay-slates and quartzites extend in a parallel belt from the extreme northern part of the main range of the Black Hills, near Crow Peak, to a point nearly east of Harney's Peak, a few miles from the edge of the plains. The slates are probably more recent in age, and rest unconformably on the schists, though both formations have been subjected to simultaneous folding and metamorphic action until the contact between them is very indistinct and difficult to identify. Among the rocks of the first division immense masses of white feldspar-granite have been intruded between the strata of schists, especially in the southeastern portion of the hills, where the Harney's Peak range, eight miles in length and two to four miles in width, is wholly made up of this granite. Narrow dikes of granite traverse the schists conformably to the stratification. The sides of those dikes often expose black, polished surfaces or slickensides. In places the plastic granite in its intrusion has completely enclosed huge fragments broken from the adjacent schists.
