324 ABSTRACTS 



peneplain. On this supposition the Blue Ridge plateau was once con- 

 tinuous with the Piedmont plain, but the latter remained at sea level 

 while the former was slowly elevated 2000 feet. There were halts in 

 this movement, during which partial peneplains were formed on the 

 western side of Blue Ridge. The final result of such a combination of 

 crustal movements and periods of baseleveling is that all of the pene- 

 plains on the western side of the Blue Ridge, collectively, represent 

 approximately the same time interval as that represented on the east- 

 ern side by the Piedmont plain. 



This theory also explains the present condition of the New River- 

 Roanoke divide, and why the former stream has not been captured by 

 the latter, — a crucial test for all theories relating to the origin of this 

 scarp. 



The same explanation is also suggested for the Black Hills of South 

 Dakota, — a dome-shaped uplift in which the outer rim of Dakota sand- 

 stone is in the zone of maximum erosion, and consequently is kept at 

 baselevel, while the inner belt of limestone owing to its more rapid 

 upward movement is unreduced. This is offered merely as a sugges- 

 tion to geologists in the hopes that it will be entertained as a working 

 hypothesis and its applicability tested by persons familiar with the 

 field. 



Dikes in Appalachian Virginia. By N. H. Darton. 



An account was given of further discoveries of diabase dikes and of 

 certain dikes of acidic eruptives among the Palaeozoic rocks near Mon- 

 terey in Highland county, Virginia. The diabase dikes are similar to 

 those described by Messrs. Diller and Darton some years ago, but the 

 acidic rocks are very great novelties for this region. The rock is por- 

 phyritic diorite containing quartz, mainly in rounded grains, with 

 plagioclase, biotite, and hornblende phenocrysts, in a white groundmass. 

 The dikes are of small extent and appear to be restricted to a limited 

 territory. The area is one in which the most prominent anticline of 

 central Appalachian Virginia reaches its culmination. 



The abstracts in this and the previous issue of this Journal were read before the 

 Washington Meeting of the Geological Society of America. 



