REPORT OF THE CHIEF ASTRONOMER 629 



SESSIONAL PAPER No. 25a 



tion. During the Eocene deformation of the Cretaceous peneplain in the 

 southern Appalachians the uplift was, on the average, considerably less than 

 2,000 feet in the broader belts baselevelled in Tertiary time. We must believe 

 that, in the Cordillera, the Laramide revolution developed very much higher 

 land which was partly reduced during the Eocene, Oligocene, and early Miocene, 

 yet was again increased by local uplifts at the close of the Miocene. Smith 

 offers proofs that the topography at the close of the Miocene was strongly 

 mountainous. It follows from the suggested hypothesis of two-cycle erosion in 

 the Pliocene that this great relief of the Cordillera was, in the earlier part oi 

 that period, reduced to such flatness that later upwarps and downwarps could 

 displace most of the rivers of the earlier cycle and force the development of a 

 whole system of new streams flowing down the axes of the downwarps or drain- 

 ing the side slopes of those downwarps. 



Hayes and Campbell point out a further special reason why the southern 

 Appalachians were locally peneplained during the Tertiary. 



' Although crystalline rocks are generally regarded as offering great 

 resistance to erosion, they are, under baseleveling conditions, subject to 

 very deep decay and probably at the close of the Cretaceous cycle were 

 softened to a far greater depth than at the present time. As the elevation 

 succeeding the Cretaceous period of baseleveling was not great, the streams 

 quickly swept away this mantle of residual material down to baselevel. 

 Under such conditions the Tertiary peneplain was very perfectly developed 

 throughout the whole of the piedmont plain. The subsequent erosion of 

 this peneplain has been comparatively slight and in many parts, especially 

 in the vicinity of the James and Potomac rivers, it is almost . perfectly 



preserved.'* 



• 



On account of the much greater amount of uplift in the Cascades we can- 

 not credit a similar explanation for Tertiary baselevelling in that region. 



Moreover, Hayes and Campbell have concluded that the principal upwarp- 

 ing of the Appalachian Cretaceous peneplain occurred at the beginning of the 

 Eocene and that the baselevelling, so far as it went, was completed at the dawn 

 of the Neocene. Similarly, Davis has dated the upwarp as ' early Tertiary.'f 

 That is to say, in the Appalachians, as already sufficiently emphasized, all Neo- 

 cene time has been engaged in the excavation of narrow valleys beneath the 

 Tertiary peneplain. All post-Eocene time has been very far from sufficient to 

 baselevel any large portion of the Appalachian uplift, initially low as it was 

 after the mid-Tertiary upwarp. 



In conclusion, therefore, we may hold that the Appalachian chain gives us 

 a measure of all Tertiary time in terms of erosion, and that, by this standard, 

 it seems impossible to accept the view of Smith and Willis as to Pliocene 

 peneplanation followed by the later Pliocene mature dissection of the Cascade 

 block. 



* C. W. Hayes and M. R. Campbell, National Geographic Magazine, Vol. 6, 1894. 

 p. 86. 



f W. M. Davis, Bull. Geol. Soc. America, Vol. 2, 18&1, p. 578. 

 25a — vol. iii — 41£ 



