260 ASHLEY — GEOLOGY OF AEKAXSAS. [May 13, 



the suggestion thus given may often be strengthened by the presence 

 of shales, or of the thin bed of novaculite and silicious shale, all of 

 which are believed to be low in the columnar sections and there- 

 fore, if exposed at all, would be near the axis. In a few cases the 

 presence of the igneous rock or of parallel outcrops of the thin 

 bedded novaculite and silicious shales was considered as a possible 

 evidence of an anticlinal axis. In only a few cases could the actual 

 continuity of the beds be traced. 



When erosion has cut down to the level GH, it is next to im- 

 possible to recognize the axis unless a good exposure of rocks low 

 in. the series occurs directly in the axis. 



When all these fail there has been tried successfully, in cases 

 where the overturn is suspected, the plan of tracing an anticline 

 into the disputed locality. Thus in Fig. 15 a layer of sandstone is 



Figs. 15, 16. — Diagrams illustrating the changes of dip and strike in the 

 development of an overturned anticline. 



represented as rising from a low anticline and merging into a high 

 overturn. Were the block planed off, the outcrop would have some 

 such shape as that shown in Fig. 16, on one side the dip remaining 

 constant in direction, and probably not varying much in amount ; 

 on the side toward the observer not only is the outcrop curved 

 somewhat, but the low dip at the further side gradually rises, finally 

 becomes perpendicular, and then is reversed by the complete over- 

 turn of the fold. 



In no case were continuous outcrops of that character found, but 

 the plan used was to select some unusually hard layer or layers in- 

 volved in the overturn, and to trace them partly by means of out- 

 crops and partly by means of the scattered blocks on the surface, 



