594 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



The Chico is also present in Alaska (Lower Yukon and Alaska penin- 

 sula), but as yet very little is known of the marine fossils. 262 



The close of the Cretacic. 263 — The Laramie is the last of the conform- 

 able Cretacic series of the Coloradoan sea. Its formations consist of alter- 

 nations of brackish water and continental deposits. Stanton 264 states : 



"The brackish-water species have survived from earlier formations in the- 

 same region by living in the marine waters or advancing with the sea margin 

 when the submergence came. The fresh-water types must have been preserved 

 in the streams of the adjacent lands when marine or even brackish waters 

 covered the larger part of their habitat. A considerable number of fresh-water 

 types were thus enabled to survive into the Tertiary. . . . With the- 

 brackish-water forms of the Laramie the case is different. ... In areas 

 of non-marine deposition where the line between Cretaceous and Eocene has 

 not been sharply drawn, because the erosion plane that is supposed to separate 

 them has not yet been located, the occurrence of an oyster bed, or a stratum 

 full of Corbula, is sufficient evidence that the rocks are still Cretaceous and 

 below the major unconformity that separates Cretaceous from Tertiary." 



During the past few years a discussion has been going on among- 

 stratigraphers, in which the opinions centering around "What is the- 

 Laramie?" and "The systemic age of the Ceratops beds" differ consider- 

 ably. In the field, geologists have pointed out erosional unconformi- 

 ties which they believe to be of wide extent and of the greatest impor- 

 tance as representing an interval of long duration. The strata above- 

 this unconformity are said to contain the Fort Union flora of Eocene age,, 

 and also the well known Ceratops fauna of dinosaurs associated with 

 archaic mammals. Geologists therefore maintain that these formations 

 are basal Eocene, and that the distinctive Cretacic land animals persisted 

 into Eocene time or the Lower Port Union, but were soon and almost sud- 

 denly extinguished in the Upper Fort Union in Colorado, Wyoming, and 

 Montana. 



Throughout a large area in Wyoming and Montana the latest marine 

 Cretacic strata of this region are overlain by a formation that is not 

 typically marine, composed of light colored sandstones and darker sandy 

 shales. This formation has been called "Ceratops beds of Converse 

 county," "Lance Creek beds," "Hell Creek beds," "Laramie," etcetera. 



263 Stanton and Martin : Bull, of the Geological Society of America, vol. 16, 1905, pp: 

 408-409. 



263 The literature on this subject is as follows : Cross : Proceedings of the Washington 

 Academy of Sciences, vol. 11, 1909, pp. 27-45. This paper gives references to all others; 

 hearing upon this discussion. Knowlton : Ibid., 1909, pp. 179-238. Stanton : Ibid., 1909, 

 pp. 239-293. Hatcher and Lull : Monograph 49, U. S. Geological Survey, 1907. Brown :: 

 Bull, of the American Museum of Natural History, vol. 23, 1907, pp. 823-845. 



- Si Stanton : Journal of Geology, vol. 17, 1909, p. 423. 



