APPALACHIAN FOLDS OF CENTRAL PENNSYLVANIA 243 



plat is 1,139 millimeters, while the total length of the other four sec- 

 tions is 3,910 millimeters. 



^, , (^.4c;XiiS9) + (2. 88X3910) ., , 



Therefore "^ ^ ^^ ^ ^^ — ^=3.01 miles, the average 



1139 + 3910 

 height of the top of the restored Pottsville conglomerate over the area 

 from Tyrone to Harrisburg. 



This figure of 3 miles applies to the whole distance plotted. For 

 the purpose of study it was also desirable to know the average height 

 of the folded tract for the section from Tyrone only to the Catskill- 

 Chemung contact just southeast of Marysville. The photo print of 

 Section 5 was therefore carefully cut at this point and the northwestern 

 portion placed on the balance. This portion weighed 0.2100 grams, 

 while Section 5 originally weighed o . 5402 grams. This northwestern 

 portion of Section 5 measured 559 millimeters on the large plot com- 

 pared with 921 millimeters linear measurement for the whole of 

 Section 5. Calculated on this basis the average original height of 

 the Pottsville conglomerate from Tyrone to Marysville comes out 

 2 . 80 miles. 



THICKNESS OF THE FOLDED SHELL 



If one knows the average height to which the freshly folded tract 

 was elevated, together with the amount of lateral shortening which 

 has caused this elevation, it is a simple matter to calculate the thick- 

 ness of the shell which suffered folding, neglecting compression, etc. 

 By using the figures just obtained — 81 miles compressed into 66 miles 

 with a resulting mean elevation of 3 miles — one might make an esti- 

 mate of the average thickness of corrugated strata across the whole 

 tract. But as the thickness of the wrinkled shell is liable to be vari- 

 able, it is necessary, in order to ascertain the true significance of the 

 thickness and its variability, to consider separately the several dissimi- 

 lar parts which make up the section. The most elevated tracts were 

 on the flanks of the mountain-belt, at the two ends of the section under 

 consideration. In both of these the Trenton limestone comes to the 

 surface at the present time. Between these two greatest upthrusts-is 

 a long tract of lesser elevation and less acute folding. To bring out 

 the significance of these variations, the six sections into which the 

 whole cross-section was cut will each be considered separately. 



