252 Dake — Episodes in Rocky Mountain Orogeny. 



whole deeply under the Early Basic Breccia, believed by 

 Hague^* to be Neocene in age. The writer has fonnd sev- 

 eral clear cases of the lava and breccia overlapping the 

 deeply eroded fault plane, from Mississippian above onto 

 Cretaceous below. The fault, then, is post-Bridger and 

 pre-Early Basic Breccia. 



It is interesting in this connection to note that the Ban- 

 nock overthrust^^ is known to pass beneath the Almy con- 

 glomerate said to be of Wasatch ( ?) age. Veatch, quoting 

 Knowlton, however, says of the Almy,^^ *^If correctly 

 determined, the age should be Fort Union or near it.'^ 

 This would make the Bannock fault pre-Fort Union, 

 whereas the Hart Mountain thrust is post-Bridger. 



This, then, would appear to add to the post-Lance and 

 pre-Fort Union folding, and to the post-Fort Union and 

 pre-Wasatch deformation, a third episode, the post- 

 Bridger and pre-Early Basic Breccia overthrusting. 



Since in the region of the Big Horn Basin, these epi- 

 sodes are repeatedly shown in a single section, or at least 

 in a very limited locality, this distinct nature does not 

 depend for proof upon distant correlation. Wlien one 

 tries, however, to correlate these episodes with those of 

 other localtities, the problem becomes vastly more compli- 

 cated. 



The first episode noted by Chamberlin^^ in Colorado 

 is post-Montana and pre-Arapahoe. He does not state 

 definitely that this disturbance involved more than uplift 

 and erosion, but by inference he carries the impression 

 that it is to be correlated mth ]3rof ound folding and fault- 

 ing in Wyoming. If, as he states, the pre-Arapahoe in- 

 terval involves the removal of 14,000 feet of sediments, it 

 seems certain that folding, of at least moderate intensity, 

 must be assumed, since broad uplift of the general region 

 to such an extent is hardly to be considered. 



Now it is a well-known fact that the Arapahoe beds of 

 the Denver Basin are rather generally correlated with the 

 Lance of the type area. Further, the three episodes of 

 diastrophism noted in Wyoming are post-Lance, as that 

 term is used by Lupton and by Hewett in the Big Horn 



2* Hague, Arnold, Absaroka Folio, U. S. Geol. Survey, Folio 52. 



2^ Eichards, R. W., and Mansfield, G. E. ; The Bannock Overthrust, Jour. 

 Geol., 20, pp. 681-709, 1912. 



^^ Geology and Geography of part of Southwestern Wyoming, TJ. S. GeoL 

 Survey, Prof. Paper 56, p. 90. 



2^ Chamberlain, E. T. ; op. cit., p. 153. 



