1888.] 507 [Crosby. 



nessed great continental and orographic changes in what is now 

 the western part of the continent, the only effect in the vicinity of 

 the Black Hills was a shallowing of the sea, none of the Jurassic 

 sediments emerging sufficiently to suffer erosion. Prof. Carpenter, 

 however, pointed out to me, on the south side of the gap of Rapid 

 Creek, near Rapid City, distinct evidence that the Jurassic and 

 Cretaceous beds of the Black Hills are not perfectly conformable. 

 An artificial excavation at this point has exposed the contact of 

 the two formations, and it is perfectly plain that the older beds 

 were considerably eroded before the deposition of the newer. 



Age of the Black Hills Uplift. 



What is the age of the Black Hills uplift? and were all the for- 

 mations now terminating in concentric lines of outcrop, often in 

 encircling escarpments or ridges, from the Potsdam up to the Ter- 

 tiary, once continuous across the central or Archaean area of the 

 Hills? These are certainly among the most attractive problems 

 presented by the geology of this region ; but, although the student 

 feels from the first that there is a fair prospect of reaching a sat- 

 isfactory conclusion, a diversity of opinion has arisen. 



That the Hills were in existence in Archaean time, and formed 

 an island in the Potsdam sea, is certain. Newton says that "the 

 inequalities of the Archaean shore became gradually filled up, and 

 as the sea rose higher upon the land all that was not worn away 

 at last became entirely covered by the Potsdam sea and its sedi- 

 ments. That the Potsdam stretched completely across the present 

 area of the metamorphic rocks, we have undoubted evidence. 

 There is nowhere observed any thinning out of the formation as 

 we approach the centre of the Hills, but it everywhere maintains 

 within narrow limits the same thickness. " 



This conclusion appears to me extremely improbable. (1) It 

 cuts off, by covering up the Archaean ledges, the only known source 

 of sand for the formation of the upper Potsdam beds. (2) Where 

 the Potsdam now approaches nearest to the higher parts of the 

 Archaean area, such as the Harney Range, its inclination or dip is 

 manifestly insufficient to carry its base over their summits, espec- 

 ially if we consider that the Harney Range has probably been low- 

 ered several hundred feet by erosion, since the Paleozoic strata 

 were removed from its flanks. My attention was first called to 

 this point by Professor Carpenter. (3) Although Newton cor- 



