EFFECT IN KIND OF YIELDING BORDER FAULTS 317 



folding and it is abundantly visibje in fact. In igneous rocks, however, 

 there are no such initial parting planes, and on this account igneous 

 rocks as a class are more rigid and less deformed than sedimentary rocks. 

 When the load and the lateral pressure are sufficient, however, the 

 strength of even these rocks is overcome. They develop countless shear 

 planes and motion ensues on them; the original crystals of the rocks are 

 crushed and drawn out ; new minerals are formed and schists and gneisses 

 are thus produced. 



EFFECT IN KIND OF YIELDING 



If the foregoing generalizations are summed up and tested by the gen- 

 eral character of Appalachian folding it is seen that there is complete 

 accord. In Alabama, Georgia, and the adjoining part of Tennessee the 

 chief component part of the stratigraphic column is the Knox dolomite. 

 It is there underlain by thick shales and overlain by comparatively thin 

 Ordovician, Silurian, Devonian, and Carboniferous strata. Consequently, 

 there is an enormous development of thrust-faulting and comparatively 

 little folding. Similar relations are found in northern Vermont and 

 Canada, with the same result of tremendous overthrusting. In Pennsyl- 

 vania, Maryland, Virginia, and northeastern Tennessee the general pro- 

 portions of the stratigraphic column are reversed ; the lower part of the 

 section consists of various heavy quartzites, doloinites, and limestones 

 overlain by great thicknesses of shale and sandstone. In this portion of 

 the Appalachians, therefore, deformation is almost wholly by folding, 

 and there are few faults except near the mountain border. 



BORDER FAULTS 



The narrow zone of faulting along the northwest border of the moun- 

 tains is one of the most constant features of the Appalachians. Out of 

 the entire extent of the range (1,500 miles on the mainland) only two 

 regions are knoAvn where there is little or no faulting in that situation. 

 These are in central Virginia and in southeastern 'New York. It is to be 

 noted that these correspond precisely with the two major recesses in the 

 Appalachian trend. A third place where the border fault may be absent 

 is in northwest Georgia, also coincident with a smaller recess. The fact 

 of minimum thrusting at these recesses contrasts most strongly with the 

 maximum thrusting which is the fact at the three intervening salients. 

 The conclusion can hardly be avoided 'that the forward advance on the 

 thrust planes is the direct cause of the salient and that the laa' where 

 thrust is absent is the cause of the recesses. 



