316 A. KEITH OUTLINES OF APPALACHIAN STRUCTURE 



have been well metamorphosed to phyllite or schist, while the sandstone 

 was only silicified. Xot only were some, like pure limstone, readily 

 metamorphosed to marble, bnt in that process their strength was over- 

 come and they flowed freely into the cracks and openings of more rigid 

 and massive beds like quartzite or dolomite. This result is strikingly 

 shown in some of the older marbles of western Xew England. In many 

 of the quarries it is clear that a given marble has been in the zone of 

 flowage for calcite marble^, while the included dolomite layers were still 

 in their zone of fracture. These features of rupture and flowage are so 

 beautifully shown in many of the Vermont quarries that the region will 

 become classic. 



EFFECT OF LOAD 



Another generalization relates to the effect of load in determining the 

 kind of folding. Thin-bedded rocks like shale, as has been stated, fold 

 readily — so freely, in fact, that the mere removal of a score or two of 

 feet of overlying rock in a ledge Avill cause the underlying beds to buckle 

 and lift. This fact shows that the rocks of the earth^s crust, even close 

 to the surface, are under a compression which is sufficient to condense 

 them materially. The well known expansion of blocks when loosened in 

 granite and marble quarries shows the same thing. It is, therefore, 

 obvious that a load of thousands of feet of overlying rocks will condense 

 and restrain the lower layers; their freedom of movement upward into 

 folds when laterally comjDressed grows less as the overlying rocks are 

 thicker. The pressure due to the weight of the upper rocks is added to 

 the pressure of the momitain-building forces, and together they may be 

 sufficient to overcome the strength of the rocks (as in the Vermont mar- 

 bles) where neither force alone would be sufficient. Thus, a rigid bed 

 when held down by a great thickness of shale above may be forced into 

 broad folds, while the same rigid bed without much load above it may 

 be so free as to be broken by faults rather than to be folded. The Ivnox 

 dolomite of Tennessee exhibits both of these conditions and results. The 

 maximum thickness of sediments is about 30,000 feet, and all, from top 

 to bottom, are folded. 



BEDDIXG ,SLIP 



The importance of bedding planes in rocks is twofold. As was stated 

 above, thin-bedded rocks with numerous planes are less rigid and conse- 

 quently fold freely. In addition the bedding planes form natural sur- 

 faces of parting, and along these the layers slip in a-djustment to folding. 

 Inspection of any set of folded beds shows clearly a large amount of 

 sliding of one layer over another. This .is theoretically necessary in 



