244 ROLLIN T. CHAMBERLIN 



Section i, comprising the great anticlinal east of Tyrone, was 

 shortened from 17.8 miles to 14.9 miles, while the top of the Potts- 

 ville conglomerate was raised to a mean height of 3.45 miles. To 

 produce this relation between shortening and elevation, a thickness of 

 crust amounting to 17.7 niiles must have been compressed, provided 

 there were no increase in the density of the rocks.' 



Section 2 was found to have been shortened from 16.3 miles to 

 15.2 miles. As this block was raised 2.37 miles on the average by 

 this folding, it must have had an original thickness of approximately 

 32.7 miles, on the assumption, of course, that the same degree of 

 shortening persisted throughout the whole block. 



Section 3 appears to have been reduced horizontally from 13.56 

 miles to 12. i miles, and to have been elevated 2.88 miles. The 

 same method of computation would assign to this block a thickness 

 of 23 . 8 miles. 



Section 4, which is now 12. i miles in length, seems to have 

 covered originally 14.44 miles. Having been upthrust to the extent 

 of 2 . 71 miles, it should have a depth of 14.0 miles. 



Section 5a, shortened from 9.6 into 7.37 miles and upthrust 2.36 

 miles, should constitute a block extending 7.8 miles below the 

 measuring base. 



Section 5&, the Cumberland County upswelling between the 

 Susquehanna bridge and Harrisburg, rose to the extent of about 

 5.75 miles. I have assumed a shortening of two into one, or 9.5 

 miles reduced to 4.75 miles of horizontal distance. On this basis 

 the thickness of the crust required would be only 5.75 miles, assuming 

 uniform shortening throughout this thickness. If a greater amount 

 of lateral compression be taken, the thickness of shell required becomes 

 correspondingly diminished. Claypole, it will be remembered, 

 assumed a shortening from 6 to i for the whole of the Cumberland 

 Valley. On this assumption, provided the height of the folds remained 

 the same, only about one mile of strata would need to be compressed 

 to give the results. 



I This method of deahng with the folded block takes no account, either of the 

 possible increase in the density of the crumpled rocks, or of the possibility that there 

 may have been some relief from the strains by down-folding as well as up-folding. 

 But any changes in density must be slight, and any considerable down-folding against 

 the great resistance of the underlying rocks seems improbable. 



