134 IOWA ACADEMY OP SCIENCES. 



deal of silt as well. This would be deposited near the 

 mouths of the streams for the most part. It is conceiva- 

 ble that many logs would drift beyond the zone of heavy 

 silt deposit, and that a woody deposit highly mixed with 

 silt near the stream delta, and growing purer with dis- 

 tance or other conditions that diminished current action, 

 might arise. Nevertheless, it is plain that if conditions 

 which would hasten the accumulation of woody matter 

 may be assumed, the problem will be simplified. 



The Rocky mountain uplift is generally credited to the 

 Laramie, for Laramie strata are found well upon the moun- 

 tain slopes. No evidence is at hand, however, to show 

 that the late Laramie ever wholly covered the Rocky moun- 

 tain area. If the uplift occurred all through the Laramie, 

 the explanation that has been offered for the lignites 

 receives material aid. The uplift, it may be conceived, 

 began with the region that is now the heart of the Rock- 

 ies, and continued till the region as far east as western 

 Dakota was slightly affected. The effect of the earliest 

 movement would be to quicken erosion in the region of 

 uplift and increase deposition at its edge, in the central 

 and eastern Montana country. Here the disturbance in 

 the west would manifest itself in abnormal drainage condi- 

 tions. Lakes would arise, fed by the streams coming from 

 the west. Streams thus rapidly quickened in a forest 

 country would carry much drift timber, for during the 

 former period of relative inactivity forest conditions would 

 have crept down close to the stream banks. Undercut- 

 ting, with landslides, would throw into the valleys the 

 giant redwoods, which the next flood would carry to the 

 lakes. As the uplift continued and its axis widened, the 

 region of deposition would be carried farther and farther 

 east, and there would be a gradual shifting of the lake 

 country in that direction. The Laramie strata to the west 

 would be tilted and faulted as they are to-day, and those 

 farther from the center of upward movement would lie 

 practically horizontal. 



Such an hypothesis seems to fit the nature of the Lar- 

 amie clays and sandstones, as well as the peculiarities of 



