206 



GEOLOGY OF THE BLACK HILLS. 



involved in the displacement, and that we know the altitudes of all points 

 in the Hills. Then by examining- the rock at any point and determining to 

 what part of the stratigraphic series it belongs we can estimate what depth 



4 3 I 2 2 i 3 4 5 



Fig. 25. — Ideal cross-section of the Black Hills. 



The vertical scale is about six times the horizontal. The clotted lines indicate the portion of the 

 uplift removed by erosion. 



1. Archaean slates and schists. 



2. Granite. 



3. Potsdam, resting nnconformahly on 1 and 2. 



4. Carboniferous. 



5. Red Beds, with included limestone. 



6. Jura. 



7. Cretaceous. 



8. White River Tertiary, restiDg unconform- 



ably on 7. 



of rock has been removed, and by adding this to the known altitude of the 

 point deduce the local altitude of the restored uplift. Ity repeating this 

 process at a great number of points we can determine the corresponding 

 points of the ideal surface and attain to a definite conception of its form. 

 Or if we take into consideration the fact that the inclination or slope of the 

 ideal surface must be everywhere very nearly identical in direction and 

 degree with the dip of the visible rock beneath it, then it will suffice to 

 determine a few points only by the additive method, and fill in the inter- 

 vening space by simply copying the slopes observed in the rocks beneath. 

 I think this latter is the process of reconstruction which the mind employs 

 when it contemplates such vestiges of erosion, either in nature or in a view 

 like the one before us. 



In the present case we do not know the thickness of all the formations, 

 but our only conspicuous ignorance is with reference to the highest forma- 

 tion of all, the upper Cretaceous, and is therefore not serious ; for any error 

 in the estimate of that affects only our determination of absolute height, 

 and not our determination of form. Each of the formations, too, varies 

 in thickness at different points of its outcrop, and may be assumed to have 

 varied within the area from which it has been removed ; but these irregu- 



