330 F. H. KXOWLTOX CRETACEOUS-TERTIARY BOUNDARY 



Dawson to the north, and to the south with the Eocene Wilcox formation 

 of the Gulf region. The Wilcox fonnation is underlain hy the marme 

 Midway formation, at the base of which is the Tertiary-Cretaceous line 

 of the Gulf coastal plain. 



This post-Cretaceous hiatus has now been traced over a wide areal ex- 

 tent from the Canadian border to Xew Mexico, and it has been definitely 

 tied in with a marine section in the Gulf region. It has been shown to 

 occupy the same relative position throughout, and we are now in position 

 to measure its magnitude. It appears to have involved at least the re- 

 moval of the full thickness of Fox Hills and Laramie, where the.maxir 

 mum thickness of Fox Hills is 1,000 feet, and the thickest known section 

 of Laramie is about 5,000 feet, or a total of 6,000 feet that may have 

 been removed. It is not now known whether in Montana, eastern Wyo- 

 ming, and the Dakotas the Laramie and the full Fox Hills section was 

 ever deposited; but it seems certain to me that the pre-Lance hiatus is 

 the time interval during which they were deposited in various areas and 

 subsequently removed in whole or in part. 



Suppose, for the sake of argument, we deny the validity of this uncon- 

 formity as a criterion for establishing the Cretaceous-Tertiary boundary. 

 Where, then, shall this line be placed? Take first the Lance formation. 

 It is now established beyond question that the sedimentation was con- 

 tinuous and uninterrupted^ from the beginning of the Lance to and 

 through the Fort Union. At the hundreds, even thousands, of localities 

 where the two occur in the same section it has been found absolutely im- 

 possible to draw any satisfactory line between them on structural or 

 lithologic evidence. The Lance is not mapable as distinct from the Fort 

 Union. In one of the latest publications of the United States Geological 

 Survey,"* which covers a ver}* large area in eastern Montana, the two are 

 mapped together, for, as W. E. Calvert, its author, says : "As a result of 

 these conditions [outlined above] no attempt is made on the index map, 

 or on the maps of the various areas treated in this report, to differentiate 

 the Lance formation from the overlying strata described in connection 

 with the Fort I^nion formation :*' and he continues. ^"'The lowest persist- 

 ent lignite bed was in the field arbitrarily considered to be the upper 

 limit of the Lance ;" . . . and he concludes, "It cannot be emphasized 

 too strongly that the upper limit adopted is merely suggestive, as the 

 finding of Triceratops bones higher in the section will necessitate the 

 upward extension of the formation." It would seem that a geological 

 system ought at least to be a mapable unit ! 



3 Except possibly in the CannonbaU region of Nortli Daliota,, whlcti will be considered 

 later, and within a single township in the Bighorn Basin of Wyoming. 

 *Bull. 471, r. S. Geological Survey. 1912, p. 25. 



