8o W. M. DAVIS 



elevation of the neighboring lands during the formation of the D'wyka 

 tillite. 



As to the second reason: The great unconformity between the Dwyka 

 tillite and the older formations cannot be interpreted in South Africa as 

 meaning a vigorous post-Carboniferous uplift. The underlying deformed 

 formations are all much older than the Dwyka, and were enormously eroded 

 after their deformation before the Dwyka tillite was deposited upon them; 

 hence the time of the pre-Dwyka deformation and uplift which initiated 

 the great erosion resulting in the unconformity must have been pre-Car- 

 boniferous, not post-Carboniferous. Indeed, as far as the surface of uncon- 

 formity has been traced, it everywhere shows forms of small relief; either 

 low, well-subdued mountains, or peneplains; hence a long time must have 

 elapsed between uplift and glaciation. Furthermore, the Waterberg sand- 

 stone of the Transvaal, supposed to correspond to the Table-mountain 

 sandstone farther south, and surely older than the Dwyka, because it is 

 unconformably covered by patches of Dwyka tillite, itself rests uncon- 

 formably upon the eroded surface of the strongly deformed older formations, 

 and here also the surface of unconformity is of small relief; and the Water- 

 berg sandstone is nearly horizontal or gently inclined, thus testifying to 

 the relative absence of vigorous uplifts in the Transvaal for a long period 

 before Dwyka time. 



Inasmuch as the Dwyka is associated with continental formations, 

 it may have been deposited in an inclosed continental basin; and if so, it 

 is evident that no safe inference as to the altitude of the pre-Dwyka land 

 surface above sea level can be drawn from the small relief to which the 

 surface had been worn down by pre-Dwyka erosion; for erosion in an 

 interior basin may produce a peneplain at an altitude unrelated to the general 

 baselevel of the oceans. Nevertheless, it is improbable that the interior 

 basin of South Africa stood at a great altitude in Dwyka time; for the 

 southern part of the Dwyka tillite follows conformably after a series of 

 presumably continental shales and sandstones, which in turn rest con- 

 formably upon marine Devonian (Bokkeveld) strata : and it is hardly prob- 

 able that the region which was low in Devonian time could have been 

 raised to a great altitude as an interior basin in Dwyka time without inter- 

 rupting the conformable sequence of stratified deposits that connect the 

 marine and the glacial formations. True, it is possible to imagine such a 

 case, but the imagined case includes conditions so improbable that they do 

 not form a satisfactory ground on which to build the explanation of the 

 Dwyka glacial climate; far less do they suifice to lead to a demonstrated 

 explanation. 



