POSITION OF THE BLUE RIDGE SANDSTONE. 157 



author. Other members of the Survey, especially Messrs. McGee and Dar- 

 ton, have at various times made widely separated sections. 



The Problem of the Area. 



Vieivs of the Authors. — The general conclusion that the Blue ridge sand- 

 stones were not Potsdam, but later, was stated by the senior author and dis- 

 cussed in the Survey, but it was considered only tentative on account of its 

 radical departure from accepted views. The work of the junior author has 

 been to verify and elaborate the stratigraphy deduced by the senior and to 

 prepare the present paper. 



Former Views. — The mutual relations of the Shenandoah limestone and 

 the shale and sandstone of the ridges were considered by the Rogers brothers 

 and Lesley, who studied the formations over wide areas, to be limestone on top, 

 shale below that, with sandstone at the bottom. The limestone w^as consid- 

 ered Cambro-Silurian (Chazy-Calciferous), the shale and sandstone Potsdam. 

 This opinion has been accepted by subsequent geologists and emphasized in 

 various publications. The senior author, however, was unable to verify the 

 accepted ideas and concluded that the series stood in reality sandstone on 

 top, shale below that, with limestone at the bottom. To these rocks the 

 present discussion is limited. 



General Relations of the Beds. 



On a general view of the formations in question, it is obvious that the 

 Shenandoah limestone dips eastward under the shale and sandstone of the 

 ridges. This is in the great majority of cases true and is commented on by 

 W. B. Rogers in various parts of his reports. To explain it, earlier geolo- 

 gists have beer obliged to consider the series overturned, an actual easterly 

 dip thus representing a theoretical westerly dip. In some sections across a 

 single sequence of the formations this could not be gainsaid, but a repetition 

 of the beds brings out their actual relations and they are obviously not over- 

 turned, but normal. 



The limestone of the Shenandoah valley forms a wide series of open and 

 closed folds, disappearing under the mountain with easterly dips. The 

 mountain sandstone and shale form synclines throughout, w^ith the single 

 exception of Catoctin mountain, where the syncline is in places bisected by 

 a fault. These synclinal axes are prolonged from the mountain ends into 

 synclines of the Shenandoah limestone and the parallel bands of rock swing 

 around the mountain ends. 



On structural evidence, therefore, the sandstone is higher than the lime- 

 stone. To prove anything else requires the evidence of fossils, but none 



