EARLY CRETACEOUS MOVEMENT. 25 



variation, as Mr. Eldridge shows, is in part from the bottom upward, hence 

 caused liy nondeposition on more elevated portions of the sea-bottom, but 

 mainly from the top downward, hence to be ascribed to erosion previous 

 to the deposition of the succeeding Dakota beds. 



The Golden arch is assumed to have been further elevated l>v this 

 movement about 1,000 feet, for the reason that this thickness of the Creta- 

 ceous sediments (including the Dakota and the lower part of the Benton 

 Cretaceous) apparently did not cross it. A further effect of the movement 

 is seen in the apparent overlapping of Dakota sediments across the under- 

 lying Morrison beds onto the Archean floor at Coal Creek. 



DAKOTA FORMATION. 



The first beds deposited in this epoch were characteristic conglomerates, 

 consisting mainly of small, very well rounded grains of chert, jasper, 

 quartzite, and often of other of the more resisting rock varieties, together 

 with some limestone pebbles. The whole formation, which averages 250 

 to .">.">0 feet in thickness in this field, is essentially a sandstone, which 

 changes to the quartzitic condition with remarkable facility, and, thus 

 offering greater resistance to erosion than the other rocks, generally con- 

 stitutes the hogback ridge wherever the Cretaceous strata are upturned 

 against the Hanks of the mountains. From an economic point of view, this 

 formation is important for the remarkably pure beds of fire clay which 

 occur in bodies of varying thickness within the sandstones. 



In organic remains it is remarkable for the variety and great number 

 of fossil plants found within its beds. Dicotyledonous plants make their 

 first appearance in the Colorado mountain region at the Dakota horizon. 

 Marine mollusks, though not yet found in the Denver Basin, occur plenti- 

 fully at this horizon in the plains region of Kansas and Texas. 



Vertebrate remains are not known to the writer to have been found in 

 these beds in the vicinity of the Denver Basin. 



Significant from a structural point of view is the fact that among the 

 pebbles of the basal conglomerate of the Dakota formation in the Denver 

 Basin Mr. Eldridge has detected some Silurian fossils ami ill-shaped coral- 

 line forms. It is not conceivable under existing: conditions that these should 



