[MACKENZIE] HISTORICAL AND STRUCTURAL GEOLOGY 105 



to have been that of the Laramide revolution. The latest date, there- 

 fore, at which this revolution could have occurred is late Eocene, or 

 possibly early Oligocène. 



The evidence on the date of the Laramide revolution from the 

 Flathead valley is as follows: — ■ 



The front range of the Rocky mountains north of the boundary, 

 the Clarke range, is bounded on the west by the Flathead valley, the 

 east wall of which is marked by a profound normal fault (12, b), 

 45,d). This normal fault is later than the compressive stresses of 

 the Laramide revolution, for, as Willis has pointed out, the normal 

 fault has detached the Clarke range so that the strata could not in 

 their present position have received the pressure which overthrust 

 and flexed them (45,d). The fault probably took place during the 

 relaxation immediately following the compression of the Laramide 

 revolution (26,c). 



The age of the normal faulting is dependent on the age of the 

 Kishenena formation, which was laid down in the Flathead valley 

 in lake basins consequent on the faulting, and is, therefore, younger 

 than the beginning of the faulting (26,c). The evidence for the 

 age of the Kishenena formation is derived from fossils it contains, some 

 of which, collected by myself in 1914, were determined by Dr. W. H. 

 Dall, who reports finding (26,d) "two species of 'Planorbis,' crushed 

 flat. . .and the remains of a species of 'Physa.' The shells are 

 specifically indeterminable owing to their bad state of preservation, 

 but the larger one recalls ^Planorbis utahensis' White, and the smaller 

 multispiral one 'Planorbis cirratus' White, the former from the 

 Bridger group, and the latter from the Green River beds of Wyoming. 

 Only a guess is permissible, yet a probability of Eocene is existent so 

 far as I dare express an opinion." Fossils collected by Daly were 

 examined by Dr. T. W. Stanton, who reported the collection to 

 consist entirely of freshwater shells belonging to the genera Sphaerium, 

 Valvata (?), Physa, Planorbis, and Limnaea. Similar forms occur 

 as early as Fort Union, now regarded as earliest Eocene, but there is 

 nothing in the fossils themselves to prevent their reference to a much 

 later horizon in the Tertiary, because they all belong to modern types 

 that have persisted to the present day, though it should be stated 

 that their nearest known relatives among the western fossil species 

 are in the Eocene. Doctor Stanton lists the species as follows: — 



Sphaerium sp., related to Sphaerium subellipticum M. and H. 



Valvata (?) sp., resembles Valvata subumbilicata M. and H. 



Physa sp. 



Planorbis sp., related to Planorbis convolutus M. and H. 



Limnaea sp. 



