268 Willis and Hayes — Appalachian Faulting, etc. 



ing have not been considered in presenting the mechanics of 

 the problems ; although it is too large a subject for full discus- 

 sion here it cannot be altogether omitted. It has long been 

 assumed that the force in the Appalachian province came from 

 the southeast, because the folds are overthrown toward the 

 northwest. Heim disposed of this argument in pointing out 

 that the overturn must always be toward the deeper syncline ; 

 and it is here shown that, in accordance with the conditions of 

 deposition, the deeper syncline in original folds must be off the 

 land. Therefore the direction of the. force cannot be inferred 

 from the direction of the overturn. 



Force and resistance must be equal or movement ensues. 

 Acting against flat strata, a sufficient pressure must be opposed 

 by a sufficiently resistant buttress or the strata must move for- 

 ward. A continental mass of crystalline rocks might consti- 

 tute such a buttress and flat strata pressed against it might 

 remain in static equilibrium so long as pressure and resistance 

 were not strong enough to crush them. If the pressure were 

 a growing one and the resisting buttress were firmer than the 

 strata, then they must eventually yield either by crushing or 

 bending. Observation shows that the latter is the common 

 result and the lines of flexure are determined by preexisting 

 deflections from a horizontal attitude. When anticlines begin 

 to develop as competent structures the force is exerted against 

 the load upon the rising arch and the resistance is correspond- 

 ingly diminished. Hence it follows that strata between a 

 growing force and a resistant mass are subjected uniformly if 

 at all to compression which is equal in opposite directions. 

 Therefore, in the Appalachians the compressing force was 

 directed both northwestward and southeastward. But from 

 the time folding begins there will be movement from the force 

 toward the resistance. This movement we conceive to have 

 been in the Appalachians a superficial flow of a broad zone 

 from northwest to southeast, from the sea toward the land. In 

 the development of thrust-faults the relative movement at the 

 surface was determined by the direction of fault-dips which 

 depended in turn upon the direction of overturn in the original 

 and consequent stepfolds; these are generally overturned to 

 the northwest, and the apparent direction of the pressure is 

 thus the opposite of the real direction. If we consider the 

 mass above a thrust-fault as stationary, the faults are all under- 

 thrusts / or if we consider the motion with reference to the 

 mass below the fault plane, they are all overthrusts. 



