360 Campbell — Rogers's Geology of the Virginias. 



border is the Carolina line to its western limit. The "back- 

 bone" of the Blue Eidge marks out, with a few limited varia- 

 tions, its N.W. border from the Tennessee line to Harper's 

 Ferry on the Potomac, a distance of about 340 miles. The 

 narrowest part of the area thus defined borders on the Potomac 

 between Alexandria and Harper's Ferry — about 50 miles by a 

 right line. About one-third of this distance, however, is occu- 

 pied by overlying beds of Mesozoic age, to be noticed hereafter 

 in connection with a number of other large patches of the same 

 age similarly situated. 



In his State Eeports, Prof. R. makes no subdivisions of his 

 "Primary" formation, except of a very general lithological 

 character ; but in his notes for Macfarlane (note 6, p. 722), he 

 indicates four subdivisions, and singularly puts their notation 

 in the order, A, D, B, C. His D, which he calls, " Norian or 

 Upper Laurentian," seems to appear on the railroads, according 

 to his notes, at only two stations, one at the junction of the 

 Chesapeake & Ohio with the Virginia Midland Railway, about 

 one mile southwest of Charlottesville; the other seven miles 

 farther in the same direction, at Ivy station on the Chesapeake 

 & Ohio Railway. It is true that there are "four rather distinct 

 groups of Archaean rocks found in Virginia, viewed lithologi- 

 cally," but we regard the question, as to whether or not they 

 belong to so many distinct horizons, not sufficientiy settled to 

 justify our recognizing more than the two general divisions, 

 Laurentian and Huronian, la, and lb. 



One of the mistakes alluded to above, and which the reader 

 will encounter early in his perusal of the volume, is found in 

 two places in the Report on the " Geological Reconnoissance " 

 of the State. On page 27 he says : " Observations render it 

 doubtful whether in the Blue Ridge any truly primary rocks 

 occur." Again on pp. 83, 84, in discussing "the region west 

 of the limestones (below Lynchburg), as far as the western 

 flank of the Blue Ridge" — that along his profile section No. I, 

 which crosses the southern portion of Albemarle and northern 

 corner of Nelson county — he comes to the "conclusion, that by 

 far the largest portion of its surface is occupied by rocks which 

 do not belong to the primary system, while they have, at the 

 same time, served to display the modifying effects of igneous 

 agents, as manifested in the changed structure of many of these 

 rocks, on a scale of wonderful variety and extent. Early in 

 the present report, allusion was made to the prevailing errors 

 on the subject of the true geological character of this region, 

 inclusive of the Blue Ridge ; and enough has already been 

 stated, in regard to the structure of the region, to satisfy the 

 enlightened geologist of the entire impropriety of the designa- 

 tion, primary, which it has heretofore uniformly received." 



