158 GEIGER AND KEITH — STRUCTURE OF THE BLUE RIDGE. 



have been found outside of the Shenandoah limestone. To this datum, 

 then, we must refer the sandstone and shale by structural relations alone. 



To determine the exact structural relations, the areas of the formations 

 have been carefully worked out and large numbers of observations have 

 been made as to the attitude of the beds. 



Areal Distribution and Structure of the Rocks. 



Symmetrical Distribution of Rocks. — The accompanying map (plate 4) 

 shows the distribution of the rock-masses in practically all the detail of 

 nature. Inspection of the areas of distribution, especially in the northern 

 part, discloses a symmetry among them ; belts of one rock eucircle others. 

 The symmetry indicates a definite relation between the beds involved, and 

 its precise nature is brought out by the accompanying sections. The sections 

 (plate 5) are at intervals of one to six miles, as shown on the map by broken 

 lines, and are so placed as to bring out all material changes of structure. 



Synclinal Structure of Ridges. — At a glance it is apparent that the syn- 

 clinal type prevails in the mountains. There are some sections of close folds 

 that are not in themselves decisive of structure, but a few miles away, along 

 the same outcropping line, all doubts vanish and the structure is simple. 

 This is notably the case at the Harper's Ferry gaps in the Blue ridge and 

 South mountain. 



The clean-cut Potomac section is so folded and complex as to be open to 

 discussion at least. But in three miles either northward or southward along 

 the Blue ridge, indecision is replaced by certainty when the tangle of the 

 gap section is transformed to open normal folds. Southward the folds con- 

 tinue open throughout ; northward they open and close alternately. 



South mountain presents the same synclinal features. Northward from 

 the Potomac, erosion cuts three times through the sandstone belt into the 

 underlying .slates. Eight miles southward from the Potomac, the closed 

 syncline of the river section is cut through by erosion. South of this gap a 

 patch of the sandstone is left in Short hill before the axis finally pitches 

 upward and South mountain disappears. 



On Catoctin mountain the synclinal sandstone is entirely cut through by 

 the Potomac, leaving the underlying slates along the surface. Both north- 

 ' ward and southward from the river the structure is the same. The syn- 

 clinal axis pitches down until in three miles its eastern half is cut off by the 

 post-Newark fault. Some distance north, in Maryland, Catoctin and South 

 mountains nearly if not quite unite. 



The lesser sandstone knobs before mentioned are mere remnants in a gen- 

 eral synclinal axis. Some parts of the axis are flat, some are closely folded. 

 In many cases no ledges occur to furnish dip observations, but the sand- 



