This paper consists essentially of a revision of paleogeographic data published in 1933 and 1937. Since the so-called Ancestral Rockies had a profound effect on Late Paleozoic and Mesozoic sedimentation in an area extending considerably beyond the boundaries of Colorado, the areas discussed include much of the Rocky Mountain region.

The revisions in Permian and Pennsylvanian paleogeography are comparatively slight and consist mostly of the elimination of some islands in Utah, a union of the Zuni and Defiance positive elements in New Mexico, and some changes along the edges of the Front Range and San Luis positive areas. There are also small positive elements added in northern New Mexico.

Only slight revisions are made in the Triassic maps, but there are considerable changes in Jurassic maps. The Navajo-Nugget sandstone is extended much farther east across Wyoming and questionably into the Black Hills of South Dakota. The Carmel-Twin Creek map is also extended, recent work having established the presence of Middle Jurassic and early Upper Jurassic beds over a larger area than that shown on previous maps. The sandstone facies of the Entrada becomes more shaly, silty, and predominantly red toward the west and north. The Curtis marine beds and their equivalents are shown as having a marginal extension east and south, consisting of evaporites and shales. The Morrison fresh-water deposits are shown as extending farther north and east than in previous maps.

Former maps by the author, of three stages in the Cretaceous, are revised, using data from various sources, especially from Preliminary Map 10 of the Oil and Gas Investigations Series by John B. Reeside, Jr., of the United States Geological Survey. A fourth map is added. Eastern limits of some individual sandstone wedges of the Upper Cretaceous are also shown from maps by the writer made in 1937.