Excerpt from The Geology of Hardin County: And the Adjoining Part of Pope County
Francis W. Shepardson, Chairman, and Members of the Board of Natural Resources and Conservation.
Gentlemen: I submit here with a report on the geology of Hardin County,
including the adjoining part of Pope County, and recommend its publication as Bulletin 41.
Fluorspar mining is the chief mineral industry of the region, and is of national importance, since the output annually is more than two-thirds of the country's total production. During the war period the Hardin County mines were under guard to prevent interruption of work; for the steel industry depends on fluorspar as one of the important fluxes. Naturally, this report has special reference to the fluorspar deposits - their distribution, mode of occurrence, and origin - and presents in the text, and by display on the accompanying map, suggestions for prospecting In search of additional veins of commercial importance.
The region deserves study, however, for phases of geology and geography other than those of economic importance.
Rock formations of Denovian, Mississipplan, and Pennsylvanian age, measuring between 3,500 and 4,000 feet, are well exposed along Ohio River or in the dissected upland area of the interior, and offer excellent opportunities for study. This report is a notable contribution to geological knowledge of the Upper Mississipplan or Chester group, serving to portray with accuracy the stratigraphic succession and the areal distribution of formations which have been established by more than ten years of painstaking study extending from St. Louis to the southeastern limits of Illinois.
The structural geology of Hardin County is also of special interest; the rocks have been bowed up into the form of an immense dome, then broken and readjusted by differential settling of the blocks, and intruded by igneous rocks in the form of dikes, sills, and plugs. The rich ores of fluorspar, lead, and zinc occur chiefly along the lines of old fracture planes.
On the whole, this report is presented with assurance that the subject matter and the geologic map will be of great benefit to the economic and cultural development of the region, and of exceptional interest and value to professional geologists and students.
I desire to acknowledge the public service rendered by the several authors, and to thank residents of the district for their constant cooperation. Special credit is due to the U. S. Geological Survey for active participation in the topographic and geologic mapping of the county.
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