162 GEIGER AND KEITH — STRUCTURE OP THE BLUE RIDGE. 



have embraced a view so at variance with these facts. It is due to them to 

 say that not all the facts are against them. In various portions of the Blue 

 ridge there are sandstones dipping northwestward or toward the valley lime- 

 stone, and the facts are susceptible of the interpretation that the sandstones 

 are beneath the limestone. The section at Balcony Falls appears to show 

 an underlying sandstone. One unequivocal case of underlying sandstone 

 has been found by Darton five miles east of Dublin, where a visible anticline 

 of sandstone lies under an extensive arch of valley limestone. Without 

 doubt other cases will be demonstrated by closer study, and doubtful cases 

 are already known. 



Existence of two Sandstones. — It seems from the foregoing facts that there 

 are two sandstones, one above and one below the valley limestone. There is 

 nothing unusual about such an arrangement ; among the fossiliferous series 

 it is very common. Three or four times in the vertical column the same 

 lithologic character recurs ; but this simply indicates a renewal of similar 

 conditions of sedimentation and has no bearing on age. There is no reason 

 to suppose that these conditions did not exist before the limestone was de- 

 posited. Among the fossiliferous rocks the fossils enforce a discrimination 

 of the sandstones ; in the Blue ridge they do not. In their absence nothing 

 but structural evidence can discriminate, and that, in the case of Rogers at 

 least, was forbidden by the amount of ground to be covered. The mistake 

 was made of correlating distinct and distant sections with insufficient con- 

 nection by areas. It was, apparently, enough that they contained a grc^up of 

 rocks similar in texture and lay in the same topographic belt. In other 

 words, instead of structure, lithology was made the basis of correlation, in 

 spite of its unreliability in adjacent areas. 



Certain cross-sections were taken as typical, and from them a stratigraphy 

 was deduced. Into this mold the other observations were poured with the 

 inevitable result that some of them, to put it mildly, lost their original char- 

 acter. Rogers' published sections of the Blue ridge at Harper's Ferry and 

 Ashby's gap are distinctly wrong. In the former the limestone does not 

 dip northwestward and the shale under it, as represented ; nor does any 

 sandstone bed reach water level until the main ridge is reached ; nor is the 

 sandstone-shale series a simple monoclinal sequence, but a highly contorted 

 synclinal depression. His section a little south of Harper's Ferry gives the 

 open syncline of Blue ridge as it is, but adds thereto a series of vertical sand- 

 stones that have no existence whatever. In his section at Ashby's gap, at 

 the southwestern corner of the region under discussion, a synclinal ridge-cap 

 is turned into a monoclinal bed ; the low southeastward dips on the main 

 ridge are shown, but the equally plain northwestward dips are not. 



Rogers' sections show that he appreciated the want of harmony of his differ- 

 ent observations and the difficulty of reconciling them. In view of the ob- 



