Geological and paleontological work on the Cretaceous deposits in California and Oregon was begun as early as 1854, but in great measure the foundations upon which all subsequent work has been based were laid by J. D. Whitney, W. H. Brewer, and W. M. Gabb in the period from 1860 to 1884. This work was followed by that of G. F. Becker and C. A. White, and from 1891 to 1905 by the more important work of J. S. Diller and T. W. Stanton, from whose results all later work has proceeded. The knowledge of the stratigraphic succession and the faunas of the Cretaceous in California and Oregon developed by these later workers was a great advance from the generalized results of the earlier explorers. But it soon became evident, however, that other demands upon their time had left this work unfinished, far from the stage to which their interest and energy would have led them to attain. As concerns the early Cretaceous deposits, their stratigraphic limitations, relationships, and distribution, further advance was hardly possible prior to 1907 and the three succeeding years. During these years some notable advance in Cretaceous paleontology was made by Pavlow (1907), Diller (1908), Smith (1909, p. 347–349), Knowlton (1910), and others. The work of these men led the way to a solution of a primary problem—namely, a proper discrimination of the Lower Cretaceous from the late Jurassic sequence in these western States. A brief review of . . .