PHILOSOPHICAL SOCIETY OF WASHINGTON. 127 



down into these older beds to which their relation becomes an 

 inconsequent one. These streams may be called superimposed drain- 

 age. 



The creeks that drain the Black Hills constitute a consequent 

 drainage system of rare perfection. Rising near the center of the 

 uplift, they follow the direction of the dip on all sides, and hold in- 

 dependent courses until they have passed outside the area of dis- 

 turbance. The symmetry of their arrangement indicates that the 

 formation of the arch began under water, so that its original sur- 

 face was not furrowed by a pre-existent drainage, but presented to 

 meteoric waters, when it was finally laid bare, an even curvature 

 the slope of which everywhere represented the dip. The creeks 

 came into existence when the summit of the arch appeared above 

 the water, and its axial line became at the same time a watershed. 

 As its slopes gradually emerged the creeks extended their lower 

 courses, but their upper courses remained the same, and so did the 

 watershed. With the progressive degradation of the arch, the 

 creeks have descended vertically from their first position to their 

 present without essential modification of their horizontal relations, 

 and we have no reason to doubt that the watershed of to-day is 

 directly beneath the position of the original watershed. 



The watershed therefore marks the place of the original axis of 

 deformation, and as it does not correspond to the present axis of 

 deformation but lies fifteen miles further west, it is concluded that 

 the growth of the arch was not uniform, but was at first more rapid 

 on one side and afterward on the other. 



The discovery of this irregularity of movement does not make 

 the Black Hills displacement an exception, but rather helps to 

 ally it to the other displacements of the west, the history of which, 

 as far as known, is never simple. 



The rivers which embrace the hills and receive the waters of 

 the creeks are of more recent date, and are superimposed on the 

 uplift. After the arch was formed, and to some extent degraded 

 by erosion, a lake came into existence, which surrounded its base, 

 and remained long enough to partly bury its flanks with lacustrine 

 sediments, the sands and clays of the White River Miocene. When 

 the lake finally disappeared a new drainage system was created on 

 its bed, and to this system belong the branches of the Cheyenne. 

 Subsequent degradation has carried away the lake beds from the 

 immediate vicinity of the Black Hills, but the rivers inaugurated 



