154 C. W. HAYES — OYERTHRUST FAULTS OF THE APPALACHIANS. 



the post-Triassic tilting. It is, moreover, reasonable to look to a thrust from 

 the southeast as the cause of this later disturbance; and such a thrust may 

 be also accountable for the extraordinary dislocations described by Mr. 

 Hayes. 



It appears, further, that some clue to the original westward extension of 

 the overthrust mass in Georgia may be found by examining the arrangement 

 of the rivers of the region. The overthrust there produced a structural un- 

 conformity, and streams whose courses had been taken in the overlying mass 

 would be superimposed upon the lower mass as they cut their channels down 

 to it. It would therefore be interesting to learn if the streams of the area 

 next west of the present margin of the overthrust manifest a want of adjust- 

 ment to the structures that they traverse, such as characterizes superimposed 

 streams. 



Mr. Bailey Willis : We may explain by reference to the geologic map 

 of the United States the relation existing between faulting and the arrange- 

 ment of strata in the vertical column in the Appalachian region. Accepting 

 the scale of rigidity of strata, as indicated by Mr. Hayes, we may show that 

 faults are important where the load is small on the bed of maximum rigidity, 

 the Knox dolomite, and that faults are insignificant where this load is great. 

 Furthermore, regions of great faulting are regions of broad gentle flexures ; 

 regions of little faulting are regions of numerous small folds, closely appressed. 

 These relations show that the alternative of the formation of a fold or a fault 

 in a given bed is determined by the amount of load to which the bed is sub- 

 jected. 



