492 INDEX TO THE STRATIGEAPHY OF NOETH AMERICA. 



able discussion as to the correlation of formations of the Black HUls, Bighorn Mountains, 

 and the Front Range presents reasons for supposing that the Upper Wyoming, as developed 

 near Denver, may be wholly referable to the Permian series. This is based on stratigraphic 

 relations and the belief that a limestone occurring shortly above the "Creamy sandstone" is 

 the equivalent of the Minnekahta linlestone of the Black HiUs, where it contains Bakewellia 

 and Edmundia, forms apparently identical with those characteristic of the so-called Permian 

 of the Mississippi Valley. 



Poorly preserved shells of similar appearance were found in the limestone at Morrison. 

 Darton believes that the Opeche and Minnekahta (Permian) and the Spearfish (Triassic?) 

 formations, distinguished by him in the Black Hills, extend southward through Wyoming 

 into northern Colorado, but as their exposures are not continuous, and as lithologic detaUs 

 vary in the different areas of outcrop, he proposed Chugwater as a group term, which is thus 

 practically equivalent to Upper Wyoming. Darton states that the upper or Triassic (?) por- 

 tion of the Chugwater group thins out and disappears before the Denver area of Red Beds is 

 reached, leaving only the supposed Permian as present in the Morrison and Golden sections 

 especially described by Eldridge. 



From the brief review just given it appears that the Red Bed section of the Front Range 

 foothills contains no member to be correlated with the fossiliferous Triassic (Dolores) of the 

 western portion of Colorado. The views of Darton and the writer harmonize in referring the 

 entire Red Bed section of the Denver region and southward, at least to the Canyon City embay- 

 ment, to the Paleozoic. The Fountain beds seem to me to be synchronous in origin with the 

 Pennsylvanian formations of the San Juan region. An unconformity separates each from the 

 Mississippian Carboniferous (MiUsap or Ouray) and above each in apparent conformable rela- 

 tion is a nonfossiliferous complex of Red Beds. On the west slope these are embraced in the 

 Cutler formation, while at the base of the Front Range is the more variable section of the 

 Wyoming group, succeeding the Fountain. That the beds of the Wyoming group (excluding a 

 possible Triassic portion) are coextensive with the group consisting of the Molas, Hermosa, 

 Rico, and Cutler formations of the San Juan is of course not to be assumed. 



J-K 17. PENNSYLVANIA, WEST VIRGINIA, AND OHIO. 



The Dunkard group is the highest Carboniferous of the Appalachian basin 

 and is classed as Permian on the evidence of fossil plants. Stevenson ^^^'' thus 

 states its occurrence and character: 



The Dunkard area is much smaller than that of the Monongahela, embracing httle more 

 than 7,000 square miles. It is confined to Washington and Greene counties of Pennsylvania, 

 the western-central counties of West Virginia, and Belmont, Noble, Monroe, Washington, and 

 Meigs of Ohio. Small outlying areas occur in other counties, but they are insignificant. At 

 one time the beds of this formation were continuous eastward to beyond the AUeghenies, as 

 fragments remain in Maryland and west-central Pennsylvania. The extreme thickness, as- 

 found in the southwest corner of Pennsylvania at the West Virginia line and determined hy 

 oU-weU records, is a little less than 1,200 feet. The thiclaiess decreases greatly toward the 

 north, the bottom 475 feet becoming about 165 feet at the most northerly exposure and the 

 succeeding 240 feet is reduced to 150 feet at its northernmost exposure, nearly 30 miles south 

 from that of the lower interval. There is a similar decrease in a northwestward direction, and 

 toward the southwest one finds the bottom 700 feet of the thickest area reduced to barely 500 

 feet in Tyler of West Virgkiia, 35 miles away. Nothing can be determined respecting conditions 

 toward the east, as erosion due to great anticlines prevents comparison with the fragments 

 east from the AUeghenies in the deep basins of Broad Top and Maryland. 



In the original description of this formation as it is in Pennsylvania, Stevenson divided 

 it into the Washington County and the Greene County group, placing the plane on top of the- 

 Upper Washington limestone. Aside from the convenience of a division ia a column of such 

 length and complexity^ one must recognize in the physical conditions good reasons for this. 



