442 N. H. DARTON — STRATIGRAPHY OF THE BLACK HILLS, ETC. 



hills it is represented by from 300 to 400 feet of deposits, but these thin 

 gradually to the southward in the Laramie range and disappear in the 

 northern portion of Colorado. The thinning appears to be general at the 

 outset and the upper beds probably disappear first, but this point has not 

 been definitely determined, and it may be that the upper shales merge 

 into sandy beds and these thin out gradually together with the under- 

 lying sandstones. 



The formation is evidently of marine origin, as indicated by its numer- 

 ous molluscan remains, and its age is regarded as late Jurassic. It has 

 not been divided into subordinate members, but in its regular succession 

 it presents a succession of beds and faunas which are constant over a 

 wide area, especially the sandstone near the base and the green shales 

 above containing numerous Belemnites densus. It is probable that the 

 Sundance formation does not extend far east of the Black hills, nor to the 

 southeastward of the locality at which it disappears in surface outcrops 

 in northern Colorado, but there is no direct evidence on this question. 



The Unkpapa sandstone which succeeds the Sundance formation along 

 the eastern side of the Black hills is a relatively local feature of unknown 

 age. It appears to represent a local shore deposit in late Jurassic times, 

 prior to the deposition of the Morrison beds. The horizon may possi- 

 bly be represented in other regions by the almost general occurrence of 

 a yellowish sandy bed at the top of the Sundance formation. If not, 

 it is probable that in the area in which it is absent there is a small un- 

 conformity or hiatus at this horizon. There is a very abrupt change 

 from the Sundance to Morrison sediments, but no direct evidence of 

 unconformity has ever been found. 



MORRISON FORMA TION 



As the western portion of the Great Plains is explored, it has been 

 found that the Morrison formation is very extensive in its distribution. 

 It is seen nearly all the way around the Black Hills uplift, it extends 

 along both sides of the Bighorn mountains, appears extensively in the 

 Hartville uplift, and is traceable along the Laramie range, across Colo- 

 rado, and far southward into New Mexico. Mr Willis T. Lee has found 

 that it extends eastward in the canyons of southern Colorado and down 

 the Cimarron river, where, near the Oklahoma- New Mexico line, appar- 

 ently it merges into upper members of the Comanche series or both 

 occupy the same horizon relative to the adjoining beds. I have re- 

 cently found a similar relationship in the Two Butte uplift in the south- 

 ern corner of Colorado. 



The character of the formation is strikingly uniform throughout, con- 

 sisting mainly of a mixture of clay and fine sand having a massive 



