APPALACHIAN FOLDS OF CENTRAL PENNSYLVANIA 251 



studies, viz.: (i) the present horizontal distance across the folded 

 section, (2) the original length of the same block before folding, and 

 (3) the estimated average height to which the folded beds were 

 thrust. Obviously the nature of the result obtained by this method 

 of inspection is dependent upon the relation between the amount of 

 crustal shortening and the height to which the beds have been raised 

 in consequence of this shortening. Clearly, the greater the horizontal 

 shortening of the folded block in proportion to the resulting vertical 

 bulge, the thinner that block must be; and likewise, the less the 

 horizontal shortening in proportion to the average height of up warped 

 beds, the thicker the deformed shell must be. In the Pennsylvania 

 section considered, the less closely folded strata between the Blue 

 Ridge and the western andclinorium stood considerably higher in 

 proportion to the amount of lateral shortening suffered than did the 

 intensely folded beds to the east. Because of this, the calculations 

 indicate a deformed shell increasing in depth to the maximum point 

 beneath the slightly deformed region in Section 2 and thinning again 

 beneath the anticline at the west end. These calculations, of course, . 

 assume that the folds have derived their height solely from the 

 upthrusts of the crumpling process, and that the height of each 

 particular area has been determined by the extent of the plication 

 directly beneath it. If these assumptions are at variance with the 

 facts, the conclusions are correspondingly at fault. The results, how- 

 ever, seem to imply that the assumptions are not seriously at fault. 



For the sake of simplicity, it has been assumed that in these 

 shortened blocks the amount of shortening deduced from the present 

 surface beds has continued undiminished throughout the whole 

 thickness of each block. But such uniformity is not to be expected 

 since the thrusts of the upper and lower parts of the shell are probably 

 not always the same and one mode of deformation doubtless grades 

 into another. This complicating factor must lessen somewhat the 

 significance of the numerical figures obtained, without however 

 detracting seriously from the general nature of the results. 



So far as a single test may go in justifying a method of inquiry, 

 this trial of the suggested mode of determining the thickness of the 

 shell involved in mountain-folding may be regarded as not only 

 sustaining the value of the method, but as indicating forms of appli- 

 cation whose values were not anticipated. 



