328 F. H. KNOWLTON CRETACEOUS-TERTIARY BOUNDARY 



interval is cumulative, since it occurs at the same horizon throughout 

 four great States. Of much greater significance, however, is the fact that 

 this pre-Lance interval marks the boundary between the last of the under- 

 lying series of chiefly marine Cretaceous beds and the succeeding exclu- 

 sively continental deposits which prevailed thereafter in the Eocky Moun- 

 tain area. In other words, with a single exception this boundary marks 

 the final retreat of the marine waters from the Eocky Mountain province. 



The most complete measure of this pre-Lance unconformity is to be 

 found in the vicinity of the mouth of the Medicine Bow Elver, in Carbon 

 County, Wyoming. This unconformity was first detected and studied by 

 A. C. Veatch in the vicinity of the town of Carbon, which is about 25 

 miles south of the Medicine Bow Eiver. Veatch holds that this time 

 interval represents the removal of more than 20,000 feet of strata. The 

 horizon below this unconformity was called "Lower Laramie" by Veatch ; 

 but it is now regarded by the writer as the true Laramie, while the hori- 

 zon above was called "Upper Laramie," now, in the writer's opinion, 

 proved to be the dinosaur-bearing Lance formation, since it contains the 

 remains of Triceratops. The line then recognized as the boundary be- 

 tween these two formations was structurally correlated northwest to the 

 vicinity of the mouth of the Medicine Bow Eiver, and thence up that 

 stream for a distance of some 25 miles above its junction with the Xorth 

 Platte. According to Ball, at a point about 20 miles above the junction 

 the "Upper Laramie" and 'T/ower Laramie" were found in contact with 

 marked angular discordance. 



During the present field season [1913] C. F. Bowen, of the United 

 States Geological Survey, found remains of dinosaurs 1,000 feet or more 

 below the horizon at which Veatch, in his reconnaissance work, had 

 drawn the line between his "Lower Laramie" and "Upper Laramie." On 

 account of the lithologic similarity of the rocks of this region, careful 

 'dreal work may be necessary- before the line can be drawn exactly. The 

 line taken by Doctor Peale and the writer as that of the major uncon- 

 formity may prove to be only one of the minor breaks known in the 

 Lance, in which event Bowen's discover}- merely reduces the supposed 

 thickness of "Lower Laramie" rocks from 6,000 feet to 4,000 or 5,000 

 feet. On the other hand, indeed, it may be possible that dinosaurs act- 

 ually occur in the "Lower Laramie," ^ since they undoubtedly existed 

 somewhere at this time, but none properly identified have as yet been 

 found in undoubted Laramie. 



2 Since the above was written Bowen has informed me that the only dinosaur found 

 in the lower part of the so-called "Lower Laramie" is about 700 feet above the top of 

 the Lewis shale and 100 feet or more heloiv a horizon containing Halymenites, This 

 shows that probably it is not in the "Lower Laramie" at all, but in the Fox Hills unit, 

 which in that area is mapped with the "Lower Laramie" ! 



