236 ROLLIN T. CHAMBERLIN 



The various elaborate factors which enter into the problem of 

 crystal movement, and which embarrass quantitative determinations 

 of the shortening, have been ably discussed by Van Hise.' These 

 embarrassing factors include the following: (i) The thickening and 

 thinning of the strata in the different parts of the sharper folds. The 

 thickening and thinning are uncertain variables, for which it is 

 difhcult to make allowance when only a very limited portion of the 

 whole fold can be observed, as is usually the case. (2) The variation 

 in the closeness of the folding in passing upward or downward from 

 the layer on which the observations were made. The farther upward 

 from the recorded data the folds must be projected, obviously the 

 greater the error likely to creep in. It was for this reason that, in the 

 case in hand, the measurements on the plotted section were made 

 along those strata which departed the least from the railroad horizon. 

 (3) Subsequent relaxation of the strata under the influence of gravity. 

 This may take the form of gliding on the limbs of the folds where a 

 new series of corrugations may be developed; it may also be mani- 

 fested in the opening of fissures and in the phenomena of normal 

 faulting. The effect of these secondary phenomena involving dilata- 

 tion of the surface shell subsequent to the folding period is to cause 

 an overestimate of the extent of the crustal shortening. But the 

 quantitative importance of gravity wrinkling in the Appalachian 

 region arising from relaxation and creep since the period of folding is, 

 for the present at least, impossible of determination, but it is probably 

 not seriously large. 



It is also probable, on the whole, that in addition to the folding 

 the lateral thrusts have caused a certain amount of mashing and 

 compacting of the material of the beds. A few measurements upon 

 the wax-and-plaster folds developed experimentally by Willis^ show 

 that the decrease in the length of the layers due to mashing alone, 

 varied from i up to 10 per cent, of the original length. The total 

 shortening of all kinds in the illustrations selected for measurement 

 varied from about 15 per cent, to somewhat more than 60 per cent. 



1 C. R. Van Hise, "Estimates and Causes of Crustal Shortening," Jour, of Geol., 

 Vol. VI (1898), pp. 10-64. 



2 Bailey Willis, "The Mechanics of Appalachian Structure," Thirteenth Ann. 

 Rept., U.S. Geol. Surv. (1891-92), Pt. II, pp. 211-82. 



