146 ROLLIN T. CHAMBERLIN 



Its surficial features may be fairly well appreciated ; but its deeper 

 portions are as yet very imperfectly understood. Furthermore 

 the visible expression is material, while the formative process is 

 dynamic. The deeper and dynamic qualities are thus to be 

 unraveled only indirectly by special methods. 



In Chamberlin and Salisbury's Geology^ a method has been 

 given for deducing the thickness of the earth shell involved in a 

 given case of folding. This method utilizes the principle that if 

 the amount of crustal shortening be known and also the amount 

 of uplift resulting from the folding, the depth of the deformed 

 block may readily be calculated. It is first tentatively assumed 

 that the folding process has occasioned a change of shape, but has 

 only produced a negligible change of volume, the cubic contents 

 of the deformed block being essentially the same after folding as 

 before. If this assumption be allowed, the product of the original 

 length of the section before folding times the amount of uplift 

 resulting from the folding, will equal the amount of crustal shorten- 

 ing times the depth of the folded zone. Since the indications are 

 that the increase in density and reduction in volume of the deformed 

 mass in cases of ordinary open folding are relatively slight and can 

 if necessary be covered by a corrective factor, this method appears 

 to be trustworthy. 



An application of these principles by a trial of this method 

 upon the Appalachian Mountain system led to some very unex- 

 pected results.^ The mountainous belt of central Pennsylvania 

 having been selected for the trial, because folding is so typically 

 displayed there, a strip across the mountains from Tyrone to 

 Harrisburg was measured and its cross-section plotted to scale. 

 The plotted section was then divided into six subdivisions. Apply- 

 ing the method above outlined, the depth of the folded zone was 

 calculated separately for each subsection. The result was the 

 discovery that the two shallowest sections were on the two margins 

 of the folded region, while the deepest portion was in the middle 

 of the range. This would indicate that the shape of the deformed 



' II (1906), 125-26. 



2 Rollin T. Chamberlin. "The Appalachian Folds of Central Pennsylvania," 

 Jour. GeoL, XVIII (1910), 228-51. 



