METAMORPHISM COMPETENCY OF BEDS 315 



METAM0RPHI8M 



In addition to the visible bending and fracturing of the Appalachian 

 strata, deformation has also taken the form of metamorphism in which 

 new minerals have partly replaced the old ones. This form of yielding 

 to pressure has one large feature which is common to the other forms of 

 yielding — that is, it grows more intense toward the southeast. There 

 are no traces of this result in the horizontal rocks of the plateau, but the 

 new minerals begin to appear along the southeastern portion of the Appa- 

 lachian Valley and increase rapidly until east of the Blue Eidge and in 

 similar situations at the north scarcely any of the minerals of the rocks 

 retain their original forms. By this process limestones were transformed 

 to marble ; shales to slates and schists ; sandstone to quartzites ; and 

 granites and igneous rocks to gneisses and schists. Sediments that con- 

 tained clay, feldspar, or calcite were more readily altered than other 

 strata, and basic igneous rocks formed schists more freely than rocks like 

 granite. Most of this metamorphism took place in direct connection 

 with the formation of shear planes during deformation, but an appre- 

 ciable amount was due to the heat and solutions from batholiths, and 

 still a third variety was the static result of pressure with no differential 

 motion. 



COMPETENCY OF BEDS 



The individual rocks of the Appalachian vary enormously in the man- 

 ner of their yielding to pressure. The geologist in the field soon perceives 

 that thin-bedded rocks, like shales, slates, and schists, yield readily in 

 small folds with dimensions from a few feet down to minute wrinkles. 

 Heavy beds of solid quartzite, sandstone, or dolomite are bent in broad 

 folds measured by hundreds of feet or by miles. The arch of Powell's 

 Valley in Tennessee, for instance, is 6 miles across and is maintained by 

 massive dolomite capped by massive limestone, conglomerate, and sand- 

 stone. Similar folds are found in the Cambrian quartzites of Iron 

 Mountain in Tennessee and the Silurian sandstones of eastern Pennsyl- 

 vania. The contrast in manner of yielding between shales and sand- 

 stones can frequently be seen in a single hand specimen. This relation 

 is summed up in the statement that the thick massive beds are competent 

 fo resist pressure, while the thin beds are not. 



Accordant with the foregoing principle are the variations in degree of 

 metamorphism shown by the different rocks. Eocks containing argilla- 

 ceous matter, whether in large deposits of shale or in thin seams between 

 other rocks, were readily deformed and also readily metamorphosed. 

 Thu§, in a series of interbedded sandstones and shales, the shale may 



