Crosby.] 516 [March 7, 



quartzite, as well as the salient cliffs of the Carboniferous limestone 

 afford particularly striking examples of the jutting crags, pinnacles, 

 and knife-edges of rock formed, under favorable conditions, by the 

 quiet and discriminating erosion of rain and frost. The narrow, 

 craggy, vertical dikes or walls of quartzite and granite are from 

 50 to 500 feet high, and often transverse to the valleys or the nec- 

 essary direction of glacial movement. It is simply impossible that 

 they could have survived the glaciation of the hills, and highly im- 

 probable that erosion could have developed these features since the 

 close of the glacial epoch. 



The foot-hills, that is, the Cretaceous ridges or hog-backs out- 

 side the Red Valley, and the undulating or terraced surface of the 

 plains for a distance of many miles from the Hills, are covered by 

 a thin but very persistent layer of coarse gravel with numerous 

 small bowlders from six to twelve inches in diameter, the largest 

 observed being two feet. The pebbles and bowlders consist chiefly 

 of the streaked Archaean quartzites of the Hills, but the coarse 

 granite and diorite, the finer schists, and Potsdam and Carbon- 

 iferous strata, as well as the volcanic masses are all represented. 



This deposit, which rests alike upon the hardest and softest of 

 the Cretaceous strata, and is found on the highest hills as well as 

 in the valleys, could have been formed neither by an ice sheet nor 

 by currents of water strong enough to transport such coarse ma- 

 terial. Newton conceives that during or at the close of the glacial 

 epoch the region about the Black Hills was submerged ; and the 

 detritus in question was carried outward from the Hills in all di- 

 rections by icebergs. This explanation is, however, seriously 

 weakened by the complete absence of all other evidences of lacus- 

 trine conditions at this time. 



M}^ own observations have satisfied me that the gravel and 

 bowlders are a residual deposit ; and attention is invited to the 

 following general considerations which tend to establish this view: 

 (1) The Miocene beds must once have covered all the area where 

 the gravel and bowlders are now found. (2) The base of the Mi- 

 ocene is, as already noted, a bed of very coarse and loose con- 

 glomerate or gravel, which is essentially identical with the surface 

 gravel and bowlders, if we make a reasonable allowance for the 

 Miocene gravel having been coarser near the Hills than where it is 

 now exposed in situ in the Bad Lands, forty miles from the Hills. 

 (3) It is highly probable that the erosion of the Tertiary strata 



