The area to which this report relates is shown in Pl. II. It comprises the greater portions of South Dakota, Nebraska, and Kansas, and the eastern portions of Colorado and of Wyoming, an area of about one-half million square miles. It is the result of my investigations during the past eight years, but includes also all available data from many sources. I have been aided by various field assistants, those deserving special mention being Messrs. C. A. Fisher, C. C. O'Harra, J.E. Todd, the late C. M. Hall, G. B. Eichardson, W. S. Tangier Smith, E. H. Barbour, and the late J. E. Macfarland.On account of its size, the region presents relatively diverse geologic conditions, but comprises comparatively few formations, most of which are widespread. The question of water supply, both overground and underground, is one of great interest to the people in this district, and although considerable progress has...