218 GEOLOGY OF THE BLACK HILLS. 



the same position as the original water-shed ; and marks, therefore, the posi- 

 tion of the summit of the uplift at the time of its inception. An inspection 

 of the geological map, or of Figure 27, shows that the main water-shed has 

 a north and south trend and runs near the eastern edge of the Carboniferous 

 plateau. The summit of the irplift, as shown by the contours of Figure 26, 

 corresponds very nearly with the central part of the Archaean area. Its 

 trend is parallel with that of the watershed, but it lies fifteen or twenty miles 

 farther east. Thus it would appear that the portion of the uplift which 

 took the lead at the start was afterward overtaken and exceeded by another 

 portion, so that the present summit or axis of upheaval is not the original 

 axis of upheaval. They both fall within the tabular top of the main uplift, 

 and the present difference between them is only a few hundred feet. 



Notwithstanding the eccentricity of the main water- shed, the creeks of 

 the Hills afford a rare example of consequent drainage. It does not often 

 happen upon the slopes of the most symmetrical uplifts that so large a 

 number of water-ways assume and adhere to centrifugal and independent 

 courses. So perfect a drainage system could hardly have been established 

 except under the most favorable circumstances, and this seems to preclude 

 the idea that the uprising began upon a land surface. If it took place in a 

 region subject to subaerial conditions, there must have been a pre-existent 

 drainage system, and that system could not but have an influence upon the 

 new system inaugurated by the uplift. It seems more probable that the 

 movement began under water, imparting its curves to a surface smoothed 

 by the spreading of sediments, so that when it was subjected to meteoro- 

 logic influences its surface was an exact representation of the displacement 

 and guided the new drainage in perfect accordance with the dip of the 

 rocks. 



The evidence derived from the drainage system appears to show then, 

 first, that the Hills began to rise before the desiccation which eventually 

 succeeded the Cretaceous; second, that the form of the uplift was changed 

 during its progressive increase, the maximum line shifting from west to 

 east; and, third, that the flanks were so far buried by Miocene sediments 

 as to be given a new river system when the lake water was finally with- 

 drawn. 



