Excerpt from Notes on the Northern Black Hills of South Dakota The Cambrian is represented only by the Potsdam and rests unconformably on the Archean schists. NO Silurian or Devonian is found, and the. Potsdam and Carboniferous are conformable. The great deposits are of Archean age and yield mainly auriferous pyrite. There are sometimes lenticular and seemingly inde pendent masses of the slate and schist series sharing the folds and contortions and having a columnar cleavage coincident with the bedding. The porphyry flowed over the Potsdam east of Deadwood and under that in the Homestake to the west, lifting it like a laccolite. The gold was deposited by chemical action after having been segregated. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.