THE NUETH PAEK. 121 



rado Cretaceous. These latter beds, which are of considerable geological 

 importance, as indicating the age of rocks through which the volcanic out- 

 bursts have poured, extend for several miles along the foot-hills in regular 

 ridges, rarely inclined more than a few degrees, stretching down to the 

 Park Basin, where they are unconformably overlaid by Tertiary deposits. 



At Ada Spring, this Cretaceous ridge is cut at right angles by a narrow 

 ravine, exposing sandstone strata, and leading to the south side of the ridge, 

 which presents a precipitous face to the main divide of the Parks. Here we 

 have an exposure of some 400 or 500 feet, near the base of which crop out 

 characteristic blue limestones and argillaceous marls, which form so marked 

 a lithological feature of the junction between the Fort Benton and Niobrara 

 divisions of the Colorado Cretaceous group. From the lower bed of lime- 

 stone were collected a number of specimens of a small Inoceramus, together 

 with an Ostrea, which Prof. F. B. Meek identified as belonging to the Fort 

 Benton division, and. similar to species found along the east base of the 

 Colorado Range, and on the Laramie Plains, from the same horizon. The 

 bituminous odor emitted from these limestones in many localities is here 

 quite strong. These beds have a thickness of about 75 feet, and are over- 

 laid by argillaceous shales of a yellowish-gray color, but very sandy in 

 texture, which probably represent the Niobrara division, although no fossils 

 were found, and the marls not very characteristically developed. They 

 were estimated at 100 feet in thickness. Overlying the yellow-gray shales, 

 and reaching to the top of the ridge, are beds of coarse, crumbling sand- 

 stone, with interlaminated beds of arenaceous clays; the latter occurring 

 quite hard, and of a fine-grained texture. Several of the sandstone beds 

 would appear to be derived directly from the decomposition of old crystal- 

 line rocks, as they are made- up largely of fragments of feldspar, flakes of 

 mica, and grains of quartz, firmly compacted together. On the summit of 

 the ridge, the sandstones carry impressions and fragments of stems and 

 leaves of a deciduous growth. To the southward of this ridge, the Cre- 

 taceous strata extend up the slopes of the main divide, until broken up and 

 concealed by the volcanic rocks of the summit. 



West of Parkview Peak, the summit of the divide is somewhat lower, 

 and presents a plateau-like character, formed by a heavy mass of dark-gray 



