Dake — Episodes in Rocky Mountain Orogeny. 251 



identity with the type Fort Union. It is also by no means 

 finally proved that the beds are not Wasatch, though the 

 writer believes that their interpretation as Fort Union ( ?) 

 is the more reasonable. 



In addition to the data above presented, attention 

 should also be called to the following facts. Hewett,^^ 

 in his discussion of the Fort Union, notes that the lowest 

 conglomerates of the group carry no igneous material. 

 It is only near the middle of the formation that granite 

 pebbles begin to be noted, and in the upper part is abun- 

 dant red clay. The presence of red clay beds and granite 

 pebbles in the pebbly beds studied by the writer would 

 tend to suggest that they represent the upper, rather 

 than the lower part of the Fort Union. The fact that the 

 granite pebbles occur only in the middle and upper part 

 of the sandstone probably indicates that the granite had 

 not yet been uncovered by erosion, in early Fort Union 

 time, and may quite probably be taken to mean that uplift 

 was still progressing, and hence accelerating erosion, 

 while the lower Fort Union was still in process of being 

 deposited. 



It seems clearly established, then, by the above facts, 

 that there were in northwestern Wyoming two distinct 

 episodes of Rocky Mountain deformation, both of which 

 were post-Lance, as the Lance formation is differentiated 

 in the latest reports on that region. The first is post- 

 Lance, and pre-Fort Union(f), the second post-Fort 

 Union(?), and clearly pre-Wasatch, since Hewett and 

 Lupton^^ indicate widespread unconformity at the base of 

 the Wasatch, ''commonly recognizable b}^ discordance in 

 dip.'' 



A late epoch of deformation includes the thrusting of 

 great blocks of Mississippian limestone many miles out 

 over the Fort Union(?) pebbly sandstones. According 

 to recent work by Hewett^^ this thrusting is post-Bridger, 

 since on McCulloch Peaks he finds Madison (Mississip- 

 pian) limestone resting on beds of undoubted Bridger age. 

 The major thrust plane has been notably warped ; but this 

 warping may have occurred as the faulting was going on. 

 After the faulting, the fault blocks were deeply trenched 

 hj erosion, before the epoch of vulcanicity that buried the 



'^Hewett, D. F.; op. cit., pp. 104-106. 



2=^ Hewett, D. F.. and Lupton, C. T.; op. cit., pp. 29-30. 



^^ Hewett, D. F. ; in a recent paper before the Geol. Soc. of Washington. 



