442 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 



dipping Vespertine grits and shales — represented in Great Britain by the lowest 

 Carboniferous strata, — forming the other wall of the fault. 



General Parallelism of the Faults to the Axis Planes. 



It is a very general feature of the great longitudinal faults, whether these 

 coincide with the anticlinal and synclinal axis planes, or occur, as they more fre- 

 quently do, on the steep sides of the flexures, to dip in the same direction with 

 the axis planes. In the Appalachian chain their inclination, therefore, is almost 

 invariabty towards the south-east. A consideration of the nature of the forces 

 which have folded and ruptured the strata, shows that such a direction of 

 their dip is an almost inevitable consequence of the undulatory movement. It 

 is only in districts of low symmetrical crust undulations, or those where the strata 

 are absolutely flat, that the great fractures descend perpendicularly. 



In corrugated zones, like those of the Appalachians, the Alps, and the 

 Ardennes, the magnitude of these main longitudinal fractures, both as respects 

 length and vertical displacement of the dislocated strata, is in proportion to the 

 sharp bending or close folding of the waves to which they belong. Thus they 

 invariably possess their grandest dimensions, in the south-eastern or most pli- 

 cated belt of the Appalachians, or on that side of the zone where the crust move- 

 ments have been most energetic. 



l & v 



Uninverted side of Wave usually shoved over the Inverted. 



This obliquity or dip towards the quarter whence the movement has proceed- 

 ed, is evidently the cause of that overlapping of the newer, less-lifted side of the 

 wave in which the fault lies, upon the steeper, more perpendicular, or inverted 

 flank ; for the forward or horizontal thrust which accompanied the propagated 

 wave-movement resulting in the fracture, has, when this once occurred, found 

 an inclined plane, up which the uninverted slope of the wave slid over the 

 edges of the strata composing the inverted side. In many instances, as the 

 Appalachian sections will prove, one flank of the wave has been shoved forward 

 and upward, unconformably, upon the crushed and buried flank to an enormous 

 distance. Subsequent erosion having cut down the higher strata of theupdriven, 

 gently-sloping side of the wave, its lower beds are exposed to view, in immediate 

 contact with the unremoved upper strata of the other side. Where the lower 

 formations, cut into by the water on both sides of the fault, have been equally 

 easy of excavation, especially when they are all of identical composition, as in the 

 case of the great lower Appalachian limestones, the denuding waters have so ef- 

 fectually planed down the great inequalities of surface at first caused by the dis- 

 location, as sometimes to have left in the landscape almost no external traces of 

 the gigantic rupture which lies beneath the soil. It is, then, only by a recogni- 

 tion of the ages of the respective strata, thus abruptly placed in contact, and 



