o 



20 A. KEITH OUTLIXES OF APPALACHIAN STRUCTURE 



posed to the east of the fans, they are seen to lie between the batholiths. 

 The detailed structure in the cases now known is that of a narrow syn- 

 clinorium with marginal faults on which the anticlinal masses were 

 thrust over the synclinorium from both sides. Compression and motion 

 from both sides are clear, and the association of this fact with the neigh- 

 boring batholiths strongly supports the theory that batholiths have caused 

 mountain structures. 



PRECAMBRIAX DIVERGEXCE 



A feature of importance in Appalachian folding is the divergence of 

 structural trend exhibited in the folds involving Precambrian rocks. 

 These folds, and the allied faults and metamorphism, locally contain 

 included bodies of Paleozoic rocks. They also characterize the Precam- 

 brian in areas worn down far beneath the Paleozoic beds. Some of the 

 structures plainly antedate intrusive masses of Precambrian age and are 

 thus themselves Precambrian. It is probable, therefore, that where 

 Paleozoic rocks have structures parallel to these the post-Paleozoie 

 structures are posthumous, and that the Paleozoic deformation was con- 

 trolled by the Precambrian structure lines. The trend of the Precam- 

 brian structures departs materially from those of the Paleozoic in most 

 of the Appalachian range. There is an approach to parallelism along the 

 border of the general Paleozoic mass, but southeastward in the interior 

 of the Precambrian area differences of 10 or 20 degrees in trend are 

 common. The Precambrian trend is more nearly north and south than 

 the Paleozoic trend and in western New England, where the latter is 

 nearly north and south, the Precambrian trend is distincth' west of north. 

 In western Massachusetts the serrate outline of the Paleozoic and the 

 northwest trend of the Precambrian units are strongly brought out. 

 Similar relations hold in southern Pennsylvania, Maryland and Virginia^ 

 and the Carolinas, and wherever Paleozoic rocks are involved the control 

 of the structures of the Precambrian over the Paleozoic structures is 

 obvious. 



TRIASSIC TREXn LIXEl^ 



Although the structures of the Triassic are not classed with those of 

 the Appalachians, they have features in common, namely, the trend of 

 the structures and the control of the structures by the Precambrian. 

 The very preservation of the Triassic beds is due to the faults and the 

 tiJting forced on them by the Precambrian rocks. This is well brought 

 out in Massachusetts, New York, and Pennsylvania, and is especially 

 clear in Maryland just north of the Potomac. The trend of tlie Triassic 



