ACTION OF FORCES PRODUCING THE STRUCTURE.' 315 



witness to successive periods of compression and disturbance to which the 

 western schists could never have been subjected. No action of a force from 

 a single direction can be made to account for the implicated structure of 

 the eastern rocks, as it can for those further west. These must have been 

 wrenched, folded and faulted at different times, and in this respect the two 

 portions of the Piedmont region in Maryland present one of their strongest 

 contrasts. 



Interpretation of the Structure of the Piedmont Region 



IN Maryland. 



To account for the structure of the Piedmont plateau in Maryland as out- 

 lined in the preceding section, three hypotheses have successively suggested 

 themselves. Two of these have, however, been already found to be more or 

 less at variance with facts observed ; and although it cannot of course be 

 asserted that, as the work of mapping in detail progresses, the third hypoth- 

 esis in its present form will be found to stand the final test, it may be pro- 

 visionally accepted as best in accord with our knowledge of the facts at this 

 time. 



These three hypotheses are : 



1. That the rocks of both the eastern and western areas are of the same 

 age, and that they have been bent into a broad synclinal whose flanks are so 

 sharply folded,' faulted and thrust as to simulate the fan-structure observed 

 in high mountain chains; and that the eastern flank of this synclinal or fan 

 was much more highly metamorphosed than the western both by more intense 

 dynamic action and by intrusion of a great amount of eruptive material. 



2. That the more highly crystalline eastern area is greatly older than the 

 western schists, and served as a rigid buttress against which these were thrust 

 and folded. 



3. That the eastern area is composed of rocks far more ancient than the 

 western, which extend out under these, forming the floor upon which they 

 were deposited ; and that although already much folded and metamorphosed, 

 this crystalline floor underwent at least one more folding after the schists 

 had been laid down, carrying these with it and involving them in a consid- 

 erable but not an extreme amount of disturbance and metamorphism. 



The first of these hypotheses, which was held by Tyson, is naturally sug- 

 gested by the close correspondence of sedimentary rock types in the western 

 and eastern areas, and also by the structure in making a section across the 

 region. A sufficient cause of the increased metamorphism and disturbance 

 on the east was sought in the vast amount of eruptive rocks, which are 

 absent from the western area. As conclusive, however, against the identity 



