The age of the Morrison formation is necessarily bound up with the question of its origin and the physiographic conditions under which it was deposited. The present paper is a study in this direction. The writer was sent into the field in the summers of 1913 and 1914 by Prof. H. F. Osborn to study the Morrison formation in connection with his forthcoming monograph on the sauropod dinosaurs. Considerable time has been spent in the winter of 1913–1914 and in the past fall in assembling the results of this field study, together with a thorough study of the literature of the subject. Some of the conclusions from these studies are given in the present communication. The Morrison formation is one of those series of beds which have been the subject of considerable controversy. By the workers on the Hayden and other early surveys they were known as “variegated . . .