HOW DEFORMING FORCE WAS TRANSMITTED 339" 



between the layers through the original sedimentary partings. Free 

 movement of one layer on another is clear in any section of closely folded 

 rocks, and indeed there conld have been no other result unless the layers 

 were stretched apart or piled together. Theoretical considerations, ex- 

 perimental results, and observation establish this principle beyond ques- 

 tion. In fact, no one with a natural section of closely folded rocks before 

 him would raise the question. 



When this principle is applied to the larger groupings of facts whicli: 

 cannot be taken in witli a sweep of the eye, one large feature of distribu- 

 tion is highly important. With respect to rigidity the rocks have sub- 

 stantially the following order : shale, thin limestone, massive limestone, 

 dolomite, sandstone, quartzite, gneiss, granite and other plutonic rocks. 

 In applying this order of strength to the various areas in the Appa- 

 lachians, it is seen that only in certain areas are large thicknesses of 

 one kind of rock combined in one unit. Instances of such units are the 

 Middle Cambrian shales of East Tennessee and northwest Georgia, a 

 thousand feet thick ; the Knox dolomite of East Tennessee and southwest 

 Virginia, over 3,000 feet thick; the Kittatinny limestone of ^ew Jersey 

 and Pennsylvania, 4,000 feet thick; the Lower Cambrian quartzite group^ 

 of East Tennessee and ^N'orth Carolina, 2,000 feet thick; the Carbonif- 

 erous Lee conglomerate of East Tennessee, Kentucky, and Virginia, over 

 1,000 feet thick; the Lower Cambrian Great Smoky conglomerate of 

 North Carolina and Tennessee, 10,000 feet thick, and the Precambrian 

 granites, diorites, and gneisses of the Appalachian Moimtains and the 

 Piedmont. Groups of mixed rocks of alternating hardness, such as shales 

 and sandstones, are seen in the Devonian of jSTew York, Pennsylvania, 

 and Ohio and the Pennsylvanian rocks of all the coal measure basins. 



Viewed in a broad way, the stratigraphic column of the Paleozoic beds 

 is strongest at the base in Vermont and Canada and also in East Ten- 

 nessee and Georgia. It is also almost equally strong at the top of the 

 section of Tennessee and southwest Virginia. Elsewhere in the Appa- 

 lachians the rigidity of the column is greatest near the base, although 

 less so than in Tennessee and Vermont, while the upper part of the 

 column is markedly weaker. By far the most rigid mass of rocks wdiich. 

 is visible, however, is the Precambrian. The resistance of granite to 

 compression is well known both in scientific tests and in natural phe- 

 nomena. The strength of diorite and other plutonic rocks approaches 

 that of granite, and the Precambrian gneisses also have a relatively high, 

 degree of resistance to deformation, their minerals being the same as 

 those of granite and the individual grains being interlocked with each 

 other. The thin seams of mica in the gneisses, and to a greater degree- 



