26 GEOLOGY OF THE DENVER BASIN. 



have been derived from the degradation of Paleozoic limestones resting on 

 the flanks of the range immediately above the Dakota beach, for these had 

 already been overlapped and buried beneath the Triassic sediments. They 

 must be assumed, therefore, to have been brought into this position by 

 strong long-shore currents from some Paleozoic exposures to the southward, 

 either at Perry Park or still farther south; possibly from remnants left upon 

 the flanks of the Rampart Range. 



COLORADO FORMATION. 



The cycle of deposition inaugurated in Dakota time was continued in 

 a progressively deepening sea through the Colorado Cretaceous. During 

 the Benton division of Colorado time there were deposited up to 600 feet of 

 beds, mainly argillaceous shales, characterized by their dark, almost black, 

 leaden hue, and containing thin fossiliferous and often bituminous lime- 

 stone beds of widely varying- thicknesses and frequent concretionary clay- 

 ironstones. The shales carry considerable disseminated pyrites, together 

 with gypsum and sulphur. The succeeding Niobrara division of the Colo- 

 rado is characterized by a persistent limestone at the base, reaching 50 feet 

 in thickness, of light color, and here somewhat dolomitic in composition, 

 though at other points of remarkable purity, succeeded by about 100 feet 

 of gray and txp to 250 feet of buff shales, with some thin limestone bands 

 and iron concretions. All these clays contain considerable amounts of 

 alkaline salts. 



Remains of a remarkable series of vertebrate animals have been found 

 in chalky beds that apparently correspond to the Niobrara limestone of this 

 horizon, along the Solomon, Saline, Smoky Hill, and other rivers in Kan- 

 sas. These animals include marine swimming reptiles, birds with teeth, 

 and pterodactyls, and from the latter they have been called by Marsh the 

 "Pteranodon beds." 



MID-CRETACEOUS MOVEMENT. 



Until within a comparatively few years it has been assumed that the 

 cycle of deposition of the Cretaceous deposits of the Rocky Mountains was 

 entirely uninterrupted. The character of the sediments of the middle part 

 of the series, which is prevailingly shales with unimportant and unpersistent 



