Stevenson.] -L ' ^ [May 20, 



Ridge, for those rocks are just missed on the northerly side of the Draper 

 Mountain fault, at twelve miles from the Blue Ridge. Even now the 

 Coal Measures must be caught on some of the Washington county hills 

 beyond the Saltville fault. The Lower Potsdam is brought up under the 

 Lick Mountain anticlinal at not more than four miles from the line of the 

 Lower Carboniferous in the Draper Mountain area. The whole of the 

 Palseozoic column, then, or not less than 22,000 feet, has been removed 

 from the westerly side of the Blue Ridge in South-western Virginia; while 

 in the "Valley" and on the upthrow side of the Aiults the thickness ot 

 removed rocks is Irom say 18,000 feet in Lick mountain to 12,000 feet or 

 more along the faults. Additional proofs of this enormous erosion are 

 found in the occurrence of Lower Silurian valleys separated by ridges 

 carrying small areas of the Coal Measures. 



This enormous erosion occurs only on the upthrow side of the faults ; so 

 that on the downthrow side one may find even the Coal Measures, while the 

 lower beds of the Calciferous may be on the other. So, 15,000 feet or more 

 may have been removed from one side, while on the other, the whole loss 

 may not exceed 3000 or 4000 feet. This great contrast between the two 

 sides seems to suggest that the lateral thrust was enough in every case to 

 push tlic tqnhroicn beds far over on the downthrown, so that there could 

 be no erosion of the latter until after removal of the former. 



Of course, there are perplexing problems here; they usually abound. 

 One is suggested by the successive increase in height of the erosion planes 

 as one ascends New river. Thus the highest south from East River moun- 

 tain is at 1810 feet above tide ; that south from Big Walker mountain is at 

 about 1925 feel ; while that south from Draper and Lick mountains is at 

 about 2270 feet. These benches were all made during a long period when 

 the river in each area had practically ceased to deepen its cliannel -way. 

 It may be that the Medina sandstone of Draper mountain, the Chemung 

 conglomerate and Medina sandstone of the Walker mountains and the 

 Medina sandstone of the East River mountains may have proved sufficiently 

 hard to resist erosion for a long time. However that may be, corrasion 

 advanced regularly after it began, for the first bench below is reached at 

 140 feet south from East River mountain, at 150 feet south from the Wallier 

 mountains, and at nearly 200 feet in the space between Lick mountain and 

 the Blue Ridge.* 



A long halt in corrasion occurred during the formation of the second 

 bench ; but thenceforward no important obstacle seems to have been en- 

 countered and the interruptions were only long enough to admit of form- 

 ing narrow river terraces, which usually are found on but one side of the 

 river. 



University of the City of New York. 



* For the elevations along New river, I am indebted to Mr. W. W. Coe, Chief Engineer 

 of Norfolk and Western R. R. ; and for those on the Clinch and Holston rivers to Mr. 

 Oramel Barrett, Jr., of Abingdon, Virginia. 



