APPALACHIAN FOLDS OF CENTRAL PENNSYLVANIA 237 



The variation in the amount of linear reduction due to mashing 

 appears to correspond to differences in the character of the material 

 used in the experiments. As a rule the softer the material in the layers 

 the greater the degree of mashing; when more plaster and less wax 

 and turpentine stiffened the layers, they were much less compacted. 

 An average figure for the shortening of the layers due to mashing in 

 these experiments by Willis would seem to lie in the neighborhood of 

 5 per cent. In the case of the Appalachians, however, the amount to 

 be allowed for shortening due to mashing of the strata in addition 

 to that resulting from the corrugation must be left largely to conjecture, 

 but as the rock-formations are relatively much stiffer than the wax- 

 and-plaster layers used in the experiments, it would seem likely that 

 the figure for the mountains should be considerably less than 5 per 

 cent. 



Tending to offset the shortening due to the mashing of the rocks is 

 the subsequent elongation of the strata arising from the opening of 

 fissures, jointing, cementation by infiltration, and the penetration of 

 igneous intrusions. Several former fissures near the junction of the 

 Juniata with the Susquehanna have been rendered conspicuous by 

 the intrusion of Mesozoic trappean magmas which have solidified 

 within them. No attempt at any quantitative estimate of their 

 importance in crustal shortening is made here. Whether the impor- 

 tance of these various secondary factors is material and whether, if 

 ignored, the balance of their sum-total tends toward an overestimate, 

 or an underestimate, of the true extent of the crustal shortening, is 

 here left to the individual judgment of each geologist guided by his 

 own experience and insight. The results reached later may have 

 some reflex bearings on these points. 



THE HEIGHT OF THE FOLDED TRACT 



For a study of the dynamics of crustal warping and the nature and 

 dimension of the mountain-building movements, one of the necessary 

 factors to be determined is the amount of vertical bulging. To 

 determine the extent of the upswelling connected with the folding it 

 is necessary to measure the height of the newly folded ranges above 

 the average height of the same region before the movements began. 

 The first requirement is a base plain to which may be referred the 



