Most of the uranium deposits in the Uravan and Gateway mining districts are in the persistent upper sandstone stratum of the Salt Wash member of the Morrison formation. Areas in which this stratum is predominantly lenticular have been differentiated from areas in which the stratum is predominantly nonlenticular. The most fav.orable ground for uranium deposits is in areas of lenticular sandstone where the stratum is underlain by continuous altered greenish-gray mudstone. Ore is localized in

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... -and-flll sandstone beds within favorable areas of lenticular sandstone. Regional control of the movement of ore-bearing solutions in the principal ore-bearing sandstone zone is indicated by belts of discontinuously altered mudstone transitional in a northerly and southerly direction from an area of un« altered mudstone to areas of continuously altered mudstone; and an area of unaltered mudstone in which no ore deposits are found and an increase in size, number, and grade of ore deposits from areas of discontinuously altered to continuously altered mudstone. Discrete regional patterns of ore deposits and altered mudstone are associated with Tertiary structures; where these structures and favorable host rocks occur in juxtaposition, regional controls appear to have localized ore deposits. GEOLOGICAL SURVEY Hl'I.I.KTIN 1 (Mill PLATE 12 -' '"*£ A. VIEW LOOKING TOWARD DOLORES GROUP OF MINES Salt Wash member of Morrison formation (It) overlain by Brushy Basin member (a), underlain by Summerville formation (c) and Entrada sandstone (d). Caprock of mesa is Burro Canyon formation of Early Cretaceous age. 400 feet B. VIEW LOOKING TOWARD EAST SIDE OF CLUB MESA Lenticular upper sandstone stratum of the Salt Wash member composed of scour-aml-hll sandstone beils («) that interfinger with mudstone at (b).