230 ROLLIN T. CHAMBERLIN 



frequently considerable areas over which suitable rock-exposures were 

 wanting. In all, nearly four hundred dip-angles were recorded 

 between the nearly horizontally bedded uplands west of Tyrone and 

 the outskirts of Harrisburg. 



In plotting the dip-angles to scale on co-ordinate paper, it was 

 found most convenient to represent the distance between two telegraph 

 poles, or one thirty-eighth of a mile, by two millimeters, which was 

 the smallest unit available on the style of paper used. Therefore 

 each mile in nature is represented by seventy-six millimeters on paper. 

 From these plotted dip-angles and the available information upon 

 the location of the contacts of the different formations and their vary- 

 ing thickness, partly obtained in the field and partly from the reports 

 of the Pennsylvania State Survey, the writer has attempted to restore 

 the complete folded section as it is supposed to have been before the 

 ridges were truncated by erosion. Necessarily the uncertainties in 

 the projection of folds are so considerable that this can be regarded 

 only as a rough approximation to the original conditions following 

 the period of folding. It is on the basis of this restored section that 

 the present study has been made.' 



THE SHORTENING OF THE CRUST 



Several estimates of the amount of crustal shortening involved in 

 the folding of these mountains have already been made. Lesley 

 placed the lateral movement of the Appalachian thrusting at forty 

 miles. ^ Claypole^ divided the folded tract into two parts; the first 

 from the approximately horizontal formations on the northwest, 

 across the eleven principal ranges of mountains to Blue Mountain on 

 the southeast, a total of forty-nine miles; the second, sixteen miles 

 in length, crosses only the Cumberland Valley. By deducting twenty 

 miles of the first section for the flatfish tops of the anticlinal crests and 



1 For an alternative profile of the Appalachian flexures, a series of dip-readings 

 was made along the Susquehanna River between Harrisburg and the vicinity of Wil- 

 liamsport. But this proved to be a less representative section, and since, in addition, 

 good 'outcrops were less numerous, only the Tyrone-Harrisburg section will be treated 

 in this article. 



2 J. P. Lesley, cited by Chamberlin and Salisbury, Geology, Vol. II, p. 125. 



3 E. W. Claypole, "Pennsylvania before and after the Elevation of the Appa- 

 lachian Mountains," Am. Nat., Vol. XIX (1885), pp. 257-68. 



