COMPARISON OF CINNABAR AND COKEDALE SECTIONS. 361 



as to resemble volcanic tuffs ; indeed there is a striking resemblance to the 

 Tertiary volcanic leaf-bearing beds found in the volcanic breccias of the 

 park. The more friable sandstones crumble into coarse brown sand on 

 weathering, while the harder beds break into fine angular fragments. The 

 conglomerates are usually thin, and local layers, composed of pebbles of vol- 

 canic andesites rarely over three inches in diameter, occur ; no other material 

 being found. The first noticeable change in the rock is a thin belt of lime- 

 stone (number 33) beneath this series and some 210 feet above the coal. 

 This is underlain by lighter-bedded gray sandstones, forming the top of the 

 coal series. 



The coal-measure sandstones (number 28) are some 600 feet thick, and very 

 much like the same beds in the Cinnabar field. Being in strong contrast to the 

 earthen-gray shales beneath and to the somber volcanic sandstones above, 

 this coal belt is readily traced. The sandstones are very light-colored, cross- 

 bedded, with massive outcrops, and are characterized at certain horizons by 

 dark brown-surfaced bowlder-like concretions from one to two feet in diam- 

 eter. The beds at the base of the series are fissile, and weather out on slopes 

 into lines of tombstone-like ledges. 



Beneath these coal-bearing sandstones there are several great belts of 

 shale, with beds of sandstones between. 



As is generally the case at the base of the Rocky Mountains, the beds of 

 the Colorado group are not typically developed, the shales being quite arena- 

 ceous and containing thin beds of sandstone. No attempt has been made to 

 separate the series into the groups established by Hayden, nor into the more 

 recent divisions, Colorado and Montana ;* and it will be possible to make the 

 separation in this section only after careful search for and collection of fossils. 

 The Colorado is, however, readily separated from the quartzite forming the 

 summit of the Dakota group — a dense pink quartzite, whose detritus covers 

 the hillside below the crest of the " hog-back " formed by its outcrop. The 

 red magnesian limestones beneath grade into a conglomerate of quartz peb- 

 bles, the typical Dakota conglomerate of this region, which forms the usual 

 " hog-back " ridges ; the sandstones and conglomerates resting upon the 

 Jurassic sandstones (number 2 of the section) and being separated from the 

 Paleozoic limestones by a sag cut in the soft Jurassic limestones (number 1 

 of the section). 



A comparison of the Cokedale section with that of Cinnabar shows a satis- 

 factory correspondence, and I have little doubt that the coal-bearing sand- 

 stones of the two fields are of the same age. The two sections show that the 

 lower seam of workable coal occurs at about 3,600 feet above the Jurassic 

 limestone. 



* "Some suggestions upon the Method of Grouping the Formations of the Middle Cretaceous, 

 and the Employment of an Additional Term in its Nomenclature ; " by George H. Eldridge : Am. 

 Jour. Sci., 3d ser., vol. XXXVIII, 1889, p. 313. 



