762 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Cross ^'^ recently contributed a paper to the discussion of the age of the Laramie 

 formation in which he proposed to distinguish those strata which unconformably 

 succeed the latest conformable Cretaceous as the Shoshone group. After describ- 

 ing in much detail the various local formations which may be referred to the 

 Shoshone group, he says : '^^^ 



In the preceding discussion I have avoided the question as to the age of the Shoshone 

 beds, whether Cretaceous or Eocene. I desire now to urge their reference to the Eocene. The 

 Denver beds were originally referred by me to the Eocene, but the great weight attached to 

 the-Mesozoic affinities of the vertebrate fauna by paleontologists led to a tentative acquiescence 

 in the assignment of the Arapahoe and Denver formations to the Cretaceous, in the Denver 

 monograph. In that volume I reviewed various aspects of the question and can add but little 

 to what was there said. The main point seems to be that the Laramie and Shoshone beds 

 belong to a transition series between the Cretaceous and Eocene and that whatever break 

 occurs between any two formations is possibly bridged over by deposits of some other locality. 

 The Laramie is related to the Judith River and other brackish-water formations of the Montana 

 Cretaceous, the Shoshone to the great succession of Tertiary local deposits. The retreat of 

 marine waters and the decided uplift of a large continental area marked the appropriate boundary 

 between Cretaceous and Eocene from the stratigraphic side. 



The name Lance formation has recently been adopted by the United States 

 Geological Survey for the difiosaur-bearing strata variously known as the " Ceratops 

 beds," "Lance Creek beds," "Hell Creek beds," and "Somber beds," which have 

 by some geologists been assigned to the Cretaceous and referred to or correlated 

 with the Laramie and by others included in the Fort Union and assigned to the 

 Eocene. The typical area is in Converse County, Wyo., where the formation has 

 yielded an abundant and varied vertebrate fauna, including Triceratops and several 

 other genera of dinosaurs and a number of small primitive mammals. Recent 

 areal work in connection with the examination of coal lands by the United States 

 Geological Survey has shown that the Lance formation is widely distributed in 

 Wyoming, Montana, North Dakota, and South Dakota. It is mapped together 

 with the Denver, Arapahoe, etc., as "Earliest Tertiary or latest Cretaceous" (5a). 



The Fort Union formation was named by Meek and Hayden,^*®^ who stated that 

 it consists of "beds of clay and sand, with round ferruginous concretions, and 

 numerous beds, seams, and local deposits of lignite" and "occupies the whole 

 country around Fort Union [at the mouth of Yellowstone River], extending north 

 into the British possessions to unknown distances; also southward to Fort Clark." 

 The formation as developed in this original area is now generally regarded as 

 Eocene, and it is so mapped with the other formations of the " Continental Eocene" 

 (5b 1), but the underlying strata which Knowlton (see pp. 763-764) called thfe lower 

 member of the Fort Union are excluded from it and assigned to the Lance formation. 



The argument for placing the Lance formation in the Cretaceous has been 

 presented by Stanton,^*^ who does not share with Cross the view that there is a 

 widespread unconformity of great significance between the Laramie and the Sho- 

 shone, although he, like all other geologists, clearly recognizes that there are very 

 pronounced local unconformities. Stanton's argument is based on the actual 

 stratigraphic sequences and upon the relations of the vertebrate and invertebrate 



