532 



A. G. LEONARD 



east, since on the Fort Peck Indian Reservation, according to C. D. 

 Smith, "about 200 feet of somber-colored sands and clays, with 

 numerous carbonaceous layers and a few beds of impure lignite, 

 overlie the Fox Hills sandstone." 1 In that region there is no 

 apparent unconformity between the Fox Hills and the Lance 

 beds, the two so grading into each other that their contact is very 

 indefinite. Good exposures of the latter formation are found 

 on Cottonwood Creek and Poplar River in the Fort Peck 

 Indian Reservation, and in the badlands south of the Missouri 

 River. 



The Lance formation lies near the boundary line between the 

 Cretaceous and Tertiary, and for this reason it is difficult to 

 determine to which of these systems it should be referred. In 

 most places where the contact has been observed it is seen to rest 

 conformably on the Fox Hills sandstone, and everywhere passes 

 conformably into the Fort Union above. Deposition was thus 

 continuous from Cretaceous time on into the Tertiary, and there 

 is no break in the sedimentation which might form a line of separa- 

 tion. On stratigraphic grounds, therefore, the Lance formation 

 is as closely related to the Fox Hills sandstone below as to the Fort 

 Union above, and we are forced to depend on the fauna and flora 

 for the determination of the age. 



According to Dr. Knowlton, 193 forms of plants have been 

 found in these beds and of these, 84 species have been positively 

 identified. 2 Since the greater number of these plants (68 species) 

 are common to the Fort Union, he considers the Lance beds the lower 

 member of the Fort Union formation, and, therefore, of Tertiary 

 age. The writer formerly held the same opinion regarding the age 

 of the Lance formation, but on the basis of its vertebrate fauna, 

 including many dinosaurs, and its conformity with the underlying 

 Fox Hills, there is much ground for the belief held by Dr. Stanton 

 and others that the Lance beds should be regarded as of Creta- 

 ceous age. But to whichever system they are ultimately referred, 

 it is at least certain, as stated above, that these beds lie near the 

 border line between the Cretaceous and Tertiary. They have the 



1 Bull. U.S. Geol. Surv., No. 381 (1909), 39. 



2 Proc. Wash. Acad. Sci., XI, No. 3 (1909), 219. 



