THE REGION ABOUT BOULDER. Ill 



elevation followed, succeeded in turn by thai of the Niobrara and Pierre 

 formations, the lower 10 to 15 feel of Niobrara possibly no! having 

 been laid down upon the crown of the arch owing to the height of the 

 pre-Benton hill. 



The third period of development embraces the time in which the strata 

 were brought from the position they held a1 the close of the second period 

 into approximately that which they hold al the present day. The dynamic 

 causes to which the geological development of this region is due were of 

 the same nature as those originating the development of the Golden geol- 

 ogy — that is, the positions which the strata have held during the several 



periods in the history of the area are attributable to the unequal compres- 

 sion locally manifesting itself, the result of forces which are secondary in 

 their relations to, and developed from, the general forces which broughl 

 about the uplift of the < lolorado Range. The elevations of the two regions 

 are indeed reduplications of each other, synchronism in their occurrence 

 alone being absent. The folds induced by the action of the earlier forces 

 of compression were, upon the readjustment of these forces in Tertiary 

 times, or during the period of the great Rocky Mountain uplift, brought 

 into the structural relations they now hold. The denudation which has 

 taken place since the uplift of the range has afforded their sections in plan 

 as delineated upon the surface of the prairies at the present time. 



INFERENCES FROM THE SERIES OF UNCONFORMITIES \T BOULDER AM) GOLDEN. 



Joint consideration of the Boulder and Golden series of unconformi- 

 ties leads to the following possilile conclusion: That along the prairie 

 region bordering the early elevations of the Rocky Mountains, and even 

 extending through the several geological ages during which the gradual 

 uplift took place, there occurred a constant succession of oscillatory move- 

 ments in one locality or another, either contemporaneous, as, for instance, 

 at the close of the Trias in both the Boulder and the Golden ana, or 

 alternating — the movement in one region followed at a little later period 

 by that in another — as when the specially developed elevation at the close 

 of the Jura in the Golden region was followed by that at the close of the 

 Dakota in the Boulder, the latter again by the movement which took place 

 in the Golden region at the close of the Niobrara. 



