The bentonite deposits described in this report, which are roughly and tentatively estimated to include a minable reserve of 110 million short tons of montmorillonitic clay, are shown on a geologic map covering approximately 1,280 square miles, mostly in the Crow Indian Reservation, Big Horn County, Mont., but extending a short distance southward into Sheridan County, Wyo. Bentonite has been mined in recent years from two small workings in Sheridan County, one of which is north of Gay Creek and the other near West Pass Oreek; the clay has been processed within the district in a plant at Aberdeen Siding, Mont., a shilJping point on the Chicago, Burlington and Quincy Railroad.
The bentonite beds of minable thickness are interspersed among strata belonging to Cretaceous formations ranging from the Thermopolis shale to the Bearpaw shale. Two beds in the Cody shale and two other beds in the Bearpaw shale are extremely thick locally; these, and a number of other beds, are favorably situated for mining along some parts of their outcrops.
Primarily, the beds are described with reference to their location, stratigraphic positions, thicknesses, geologic structural relations, accessibility and content of material that might be suitable for industrial use. Also outlined are the laboratory procedures employed in testing samples of the bentonite as molding-sand bonding materials for use by foundries and as ingredients of rotary oil well drilling muds.
