300 S. F. EMMONS UINTA MOUNTAINS 



could not coincide with the faults north of the crest, Avhich probably are 

 arranged en echelon. Along the southern flanks the faulting is apt to 

 take place near the sudden steepening of the dip, which is observable in 

 most every cross-section, and beyond which the beds resume their former 

 low angle and extend southward under the Tertiaries in a broad, shallow, 

 syncline. Thus the uppermost of the Mesozoic beds involved in the orig- 

 inal Uinta arch, the Laramie formation, first reaches the surface again 

 100 miles to the south in the Book cliffs, just north of the Eio Grande 

 Western railway. The sudden steepening of the dip on a given line 

 along the flanks of a mountain uplift is a common phenomenon, and 

 notably well developed along the east face of the Front range of Colorado, 

 and can best be explained as the result of a tangential shove, as illus- 

 trated by some of Bailey Willis's experiments in mountain building. On 

 this hypothesis the faulting would be an expression of the relief of strain 

 along lines of extreme folding tension. 



Correlation 



Having had an opportunity in recent years of personally examining 

 the Grand Canyon section, on which Powell based his Green Kiver Pale- 

 ozoic section in the Uintas, I am better able to correlate the latter with 

 that exposed in the Duchesne region as shown above. My interpretation 

 would be as follows : The remnants of red beds on the Coconino plateau 

 correspond to the Permo-Carboniferous beds on the Duchesne ; the Upper 

 Aubrey limestDne to the cherty limestone immediately above the Weber 

 quartzite; the Lower Aubrey to the Weber quartzite itself, and the. Red 

 Wall to the Wasatch limestone. In either region there is uncertainty, 

 through want of fossil evidence, as to the Devonian and Silurian. In the 

 Grand Canyon region a Devonian fauna has been discovered in a thin 

 series of rocks separated from the Eed Wall limestone by a slight uncon- 

 formity, but the Silurian is apparently wanting. In the Uintas no fossils 

 have yet been found below the Wasatch limestone. The Ogden quartzite, 

 whose name has been temporarily retained for the Duchesne section, has 

 in the Wasatch recently been determined to lie below beds of Ordovician 

 age. It is probable, therefore, that both Devonian and Silurian are 

 wanting in the Uintas, and that the Tonto of the Grand canyon is repre- 

 sented by the Ogden quartzite or the Shale series, or by both together. 



Powell's Lodore series is supposed to represent the Tonto series of the 

 Grand canyon, though in his Uinta report he said Carboniferous fossils 

 had been found in it, which was evidently an error, probably caused by a 

 displacement of labels during transportation. 



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