Excerpt from Investigations of Coal Fields in Indiana, North Dakota, and Montana by the United States Geological Survey in 1908 The coal measures have a total thickness of approximately feet, of which about feet occurs in Indiana. Of this feet there are 600 feet of barren beds at the top, then a 500-foot interval which contains most of the workable coals, followed in descending order by 200 feet or more of rocks consisting mainly of sandstone. Recent work of David White indicates that the rocks of the upper 600 feet are of post-allegheny age, those of the next 450 feet of Allegheny age, and the lower rocks of Pottsville age. 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.