ALTEBNATION OF HAED AND SOFT. 211 



parts: first, the order of sequence and the thickness of the several strata, 

 and, second, the position in which they lie, or their altitudes and angles of 

 dip. The position of the strata of the Black Hills is given by the form of 

 the displacement. The thickness and character of the several beds have been 

 described in the preceding sections. 



Considering the beds of the conforming series from Potsdam to Cre- 

 taceous simply with reference to their ability to resist erosion, we have the 

 following classification : 



Feet. 



A. Soft; beds of the upper Cretaceous . . 600 



B. Hard ; Dakota sandstone . 300 



C. Soft ; beds of the Jura and Trias 540 



D. Hard ; beds of the upper Carboniferous 300 



E. Very hard ; Carboniferous Limestone 350 



F. Soft; beds of the lower Carboniferous and Potsdam 290 



Here are three zones of soft rock separated by and alternating with 

 two zones of hard rock. From the summit of the uplift all these rocks 

 have been removed by erosion. About its flanks each of them outcrops in 

 a ring. The degradation of the dome has been rudely equivalent to the 

 paring of a slice from its summit, so that the successive envelopes of rock 

 are laid bare in concentric rings, just as the leaves of an onion bulb are laid 

 bare when a slice is cut from the side. Here, however, the resemblance 

 ceases, for the resistance to atmospheric degradation is greater with hard 

 beds than with soft, and they have not been carried away so rapidly. The 

 result is that the outcrops of the zones of hard rock have a relatively great 

 altitude and constitute ridges, while the outcrops of the soft zones are rela- 

 tively depressed. Taking them in order from circumference to center, we 

 have the upper soft zone (A) composing the plain from which the Hills 

 rise ; then the first hard zone (B) represented by the ridge of foothills ; then 

 the second soft zone (C) marked by the Red Valley ; then the second hard 

 zone (DE) constituting the Carboniferous ridge on one side and the Car- 

 boniferous plateau on the other ; and finally the third soft zone (F) marked 

 by the central area. 



Within the circle of the Potsdam there is exposed an entirely distinct, 

 series of rocks with a structure of its own, which has no apparent relation 



