UPPER CRETACEOUS. 665 



Owing partly to a certain amount of erosion preceding the deposition of the succeeding 

 Eocene formation, though mostly to the general thinning out of the measures toward the north, 

 the thickness of the Laramie varies considerably. Thus, along the southwestern border of the 

 district and in the Raton Mountains near the southern border it is not less than 2,000 feet, 

 and doubtless exceeds this amount near the crest of the high ridge immediately south of the 

 Une; wliilc in the central and northern portions it does not exceed 1,700 feet, and is even less 

 than tliis in the northeastern area near the north boundary, where it is about 1,500 feet. 



The sections of the Laramie in di.Terent parts of the district vary but httle in their general 

 features. There is always an alternation of massive or thick-bedded gray sandstone beds with 

 thick shaly beds. The latter predominate toward the base of the group, the former toward the 

 top. The shaly beds of the upper half of the group are shale or clay shale, but in the lower 

 half, noticeably in the lower portion of it, they consist of sand shale — that is, thin layers of 

 greenish-gray fine-grained sandstone with partings of shale of the same color. The lower shaly 

 beds are occasionally interrupted by bands of sandstone a few feet in thickness and by beds 

 of clay shale associated with seams of coal, wliich may occur toward the center of the shale bed 

 or entirely above it and may equal the latter in thickness. Some of the upper sandstone beds 

 are disposed to weather into cavernous forms, and some of the alternating beds associated with 

 them consist of fine-grained greenish-gray fissile sandstone instead of shale. But aside from 

 the general features the sections possess little in common. It is only the lower, main sandstone 

 bed and the shaly beds above and below it that are really persistent throughout the 

 district. * * * 



The group is characterized by a rich semitropical flora, very similar to what is found in the 

 Gulf States to-day, and leaf imprints of certain species of oak, fan palm, fig, poplar, willow, and 

 tulip tree are of common occurrence in the thin-bedded sandstone and lower shaly beds. 



Lee's recent work in southern Colorado and northern New Mexico ^^ throws 

 doubt on the Laramie age of the coal-bearing rocks of this region, or at least of the 

 upper part of them. 



The Cretaceous of the Denver district was described in great detail by Eld- 

 ridge^^^ in the Denver monograph. The original description should be consulted. 

 Fenneman^^^^ has pubhshed a report on the Boulder quadrangle, which lies partly 

 in the Denver Basin, and his descriptions are here abstracted as follows : 



Above the Jurassic [Morrison] comes the prominent Dakota formation, easily recognized 

 and well known in the great hogback which parallels the Front Range for hundreds of miles. 

 It is a firm sandstone that is often quartzite, is generally thick bedded, and is characterized by 

 frequent cross-bedding and ripple marks. * * * At its base, though not everywhere present, 

 is a pebble conglomerate. The constituent pebbles include "abundant limestones, quartzites, 

 clays, flints, jaspers, and rocks of granitic composition, together with the separate mineral con- 

 stituents of the last." The whole is so firmly cemented that, where unweathered, the rock 

 fractures in broad planes which pass through the pebbles. Frequently a series of thin pebble 

 beds in a mass of sandstone takes the place of the continuous conglomerate. This alternation 

 of beds makes the thickness of the basal zone indefinite, but there are few pebble beds above the 

 first 20 or 30 feet. Considerable variation may be discerned in the sandstone, but with the 

 exception of the basal conglomerate no one bed has a constant position in the column. Gen- 

 erally the sandstone is composed of quartz grains, with a sihceous cement, and is gray or yellow- 

 ish gray. With increase of iron oxide the sandstone exhibits striking features of differential 

 coloring. * * * At intervals between the stronger ridge-making ledges occur layers of thinly 

 laminated shaly sands, aggregating in thickness only a few feet. * * * The fine between the 

 Dakota and the overlying Benton shales, while very distinct in a large way, is not so easily 

 located in detail. Alternations of sandstone and shale occur over a zone of 50 feet or more. 

 The thickness of the Dakota at Bear Canyon is about 320 feet, and at Fourmile Canyon a little 

 greater. 



