AMOUNT OF FOLDING 335 



strata the problem was comparative!}^ simple, but for the Precambriaii is 

 far more difficult. Folds in the sedimentary gneisses and schists can be 

 determined fairly well, but in the granites and other plutonic or massive 

 rocks dislocation took place only slightly by folding and mainly by a 

 multitude of shear planes on which slipping was possible. The amount 

 of motion on such planes can be determined only here and there where 

 the alteration of original forms, such as pebbles and phenocrysts, could 

 be observed. Wherever these measures were possible the deformation 

 w^as found to be uniformly great, and original pebbles, etcetera, were 

 drawn out to lengths from two to ten times the original. Furthermore, 

 the general mashing and sheeting of plutonic rocks is abundant evidence 

 that they have suffered enormous compression and shortening. In the 

 sedimentary rocks a shortening of 50 per cent is common in folds of all 

 sizes from those of microscopic dimensions up to those a mile across. 

 In fact, the general order of magnitude of the shortening wherever de- 

 terminable seems to be in the vicinity of 50 per cent. The shortening 

 diminishes northwestward and is greater in the north and south portions 

 of the system where faulting is more prominent. Taken as a whole, 

 therefore, an estimate of 40 per cent as the ^ average shortening of the 

 Appalachians seems conservative. 



The visible width of the strongly folded belt is now 270 miles in the 

 Tennessee-Carolina cross-section and in the Maine-New Brunswick cross- 

 section. In the Carolina section Paleozoic beds are now found in a belt 

 125 miles wide; in Virginia the belt is of the same width. In the Massa- 

 chusetts-New York section it is a little wider, while in the Maine section 

 the Paleozoics are found in the whole 270-mile width of the system. 

 This figure does not take into account the southeastward extensions of 

 the Paleozoic into Nova Scotia. It is likely that further detailed infor- 

 mation in Georgia and the Carolinas will extend the width of the known 

 Paleozoic belt still farther southeastward. 



If, therefore, the Appalachians as a whole were shortened 40 per cent 

 in a northwest-southeast direction, it is evident that the present visible 

 -width of 270 miles, plus 30 miles for probable extension under the Cre- 

 taceous, represents an original width of 500 miles and that the amount 

 >of shortening of the crust is thus 200 miles. This amount is more than 

 three times as great as any previously stated, but it is believed by the 

 author to be a conservative measure. Previous estimates of 40 to 60 

 miles of shortening for the Appalachians, of 40 miles for the Eocky 

 Mountains, and of 75 miles for the Alps indicate clearly the magnitude 

 •of the movements in the Appalachians. It is obvious that this great 

 .amount of shortening makes heavy demands on certain theories of 



