278 3. F. EMMONS — OROGRAPHIC MOVEMENTS. 



region seeraa probable from their apparent absence in certain sections and 

 from actual proof of local movement and erosion discovered by Mr. Kldridge 

 at Golden, Colorado ; but it cannot yet be said that there was a general dy- 

 namic movement preceding Dakota time corresponding to that which Mr. 

 Hill assumes to have affected the northern portion of Texas before the depo- 

 sition of the upper Cretaceous there. 



The character of the sediments and of the contained organic remains of 

 the Dakota Cretaceous throughout the whole interior region, however, shows 

 that they were deposited in a slowly advancing ocean during a progressive 

 subsidence of the whole region. This subsidence continued to the middle 

 of the later Cretaceous time, and was followed by an equally gradual ele- 

 vation, which culminated in the shallow water conditions of Laramie time, 

 when the oceanic waters finally retreated from the interior region even more 

 slowly than they had advanced, never to penetrate it again. 



The same general succession or cycle in the character of sediments depos- 

 ited during later Cretaceous time may be observed throughout the interior 

 region, though a variation is found in the thickness and in the prevalence 

 of coarser or liner materials of the series as a whole, according as they were 

 deposited near elevated land-masses and in narrow bays, or in broader seas 

 at a distance from any considerable land-masses. While the sedimentation 

 during this cycle was essentially conformable and undisturbed in character, 

 a lew unconformities by erosion have been observed, which indicate at least 

 local movements about the middle of the period whose extent will probably 

 be increased by future investigations. These are, an unconformity by ero- 

 sion at the close of the Niobrara Cretaceous observed by G. Eldridge* at 

 Golden, Colorado ; one noted by F. 15. Meekf at the same horizon on the 

 Missouri; and a third at Austin, Texas, described by K. T. HillJ. 



The occurrence of lacustrine life in the Belly River ami Dunvegan beds 

 in Manitoba may likewise be found to be some way connected with these 

 movements. 



Correlations. — On the Atlantic border there is direct evidence of an oro- 

 graphic movement which seems to cqrres] I pretty closely in geological 



time with that jus! described. The Triassic series of the eastern slopes, which 

 include in places bed- that arc considered by some to be of Jurassic age. were 



uplifted, folded, and extensively eroded before the deposition of the succeed- 

 ing Cretaceous beds. The earliesl of the latter -cries, the Potomac forma- 

 tion, is essentially a shore-line deposit, and though its age is uol fullyagreed 

 upon, some regarding it as late Jurassic and others as early Cretaceous, it 

 may probably be considered to be the stratigraphical equivalent of the beds 

 first deposited after the Jurassic movement in the Rocky Mountain region. 



• Bull. Philosophical Boo. ••! Washlngl Vol \ i. [881 proi 



+ f 3urv. of the Territories, Vol. IX : invertebrate Palaeontology. Washington, 1 



XXXIII. 

 J Atncr. Jom Vol. XX X IV. I--T 



