During the summers of 19^6 and 19*4-7 several areas in southwestern Montana were investigated to determine the area! extent and structure of the Phosphoria and adjacent formations as well as the quality and thickness of the phosphate "beds in the Phosphoria formation. This work is part of a study of the phosphate deposits of southwestern Montana which is one phase of a program "by the. Department of the Interior to investigate the natural resources of the Missouri River Basin. The geology of the Small Horn Canyon, Daly*s Spur, and Cedar Creek areas (pi. l), and of the Dell area (pi. 2) was mapped in as much detail as the aerial photographs permitted; the geology of some intervening areas was mapped by reconnaissance. In the area west of Daly's Spur and U. S. Highway No. 91, the geology was mapped "by plane table and aerial photographs, section corners being used as control points. Topography was not mapped. Previous geologic investigations of the Phosphoria formation of this area were undertaken "by C. F. Bowen, I/; D. D. Condit, 2/; and A
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