CONFIGURATION AND STRUCTURE. 159 



stones are caps of hilltops surrounded by slopes of shale, so that they can be 

 nothing but synclines in structure. 



Topographic Forms express Bock Character. — The difference in continuity 

 between the portions of the major and the minor ridges is the expression of 

 a difference in texture of the retaining sandstones. The continuous lines are 

 made by a white and gray sandstone, often a silicious couglomerate, which 

 is of great durability ; the interrupted lines are formed by a fine white sand- 

 stone, which is less silicious and consequently more subject to erosion. The 

 effect of this textural difference on the topography is exaggerated by external 

 relations in most places. Adjacent to the fine sandstone is the easily eroded 

 limestone. Over this the Shenandoah river plays so that this sandstone is 

 readily within reach of erosion. The heavy sandstone, on the other hand, is 

 usually flanked by an area of epidotic schist, which is exceedingly hard and 

 tough, and is also defended on the other side by the lesser sandstones. The 

 subordinate position thus maintained by the outer sandstone has led geolo- 

 gists to ignore its importance ; but the value of careful mapping of both 

 types can hardly be overestimated. Each occurrence multiplies the chance 

 for observing the structural relations in a given area and hence strengthens 

 the conclusions based on structure. 



The distinct difference between the types would indeed suggest a difference 

 in age, and Rogers has represented them as different. But this distinction 

 of type, like most others, is accompanied by an intermediate gradation, and 

 one sandstone visibly changes to the other in one continuous line. This is 

 the case at the northern end of the Blue ridge proper, and is made manifest 

 at a distance by the gradual tapering of the ridge. 



Significance of Structural Details : Superposition of Shale and Sandstone. — 

 The sections shown in plate 5 are constructed from observed dips. From 

 these sections it is plain that the shale-sandstone series rests on the limestone, 

 apparently deposited there as sediment. If deposition gave them their pres- 

 ent position, i. e., if the present sequence is original, they are unquestionably 

 Silurian. Can they by any process have attained that position so as to sim- 

 ulate deposition ? If so, what are the attendant features elsewhere, and are 

 they present here ? 



Possible Faulting. — There is one structure able to produce similar effects, 

 i. e., faulting succeeded by later folding. In this case the plane of overthrust 

 would afterward be folded with the adjacent beds. Such a structure has 

 been discussed by Mr. Hayes at this meeting of the Society.* It is decidedly 

 an exceptional structure and as such requires the best of proof, either fossils 

 or unconformity of rock-masses. Fossils of early forms in the overlying 

 beds contrasted with late forms in the underlying beds are of course, as Mr. 

 Hayes states, final proof Without fossils, visible unconformities of texture 



* This volume, pp. 141-154, 



