THE APPALACHIAN FOLDS OF CENTRAL 

 PENNSYLVANIA 



ROLLIN T. CHAMBERLIN 



The observational basis of this study of the Appalachian folding 

 in central Pennsylvania was laid during a special trip on foot from 

 Tyrone to Harrisburg during the summer of 1905. The chief purpose 

 was to measure the dip-angles of the strata at as many stations as 

 possible, that they might be subsequently plotted to scale as a ground- 

 work for restoring the folded structure. Nearly 400 dips were 

 measured, but on plotting them and attempting to restore the structure 

 it was found that they only scantily covered several portions of the 

 section where critical data were especially desirable. This proved 

 to be particularly true of the neighborhood of Harrisburg, where the 

 anticlinal arches are overturned and the shales and slates are so 

 crumpled that, with the time available, it was not possible to trace 

 out many of the minor, but none the less important, complications 

 of structure. As a result it was felt that the material at hand was 

 scarcely adequate for a serious study, and in the hope that a later 

 opportunity might arise to make a further search for the desired data, 

 the work was laid aside. But up to the present no opportunity to 

 again visit this region has presented itself and it has seemed, on 

 reflection, best to proceed with the original purpose, since this was not 

 so much to gain a truer view of this particular case of folding, as to 

 put to working trial certain recent suggestions as to the deductions 

 that may be drawn from data of this sort. In Chamberlin and 

 Salisbury's Geology, Vol. II, pp. 125-126, a method is given for dedu- 

 cing the thickness of the shell involved in folding. The present study 

 is a preliminary attempt to make a special application of this method 

 and to see what collateral suggestions might spring from it in practice. 

 For this purpose it is not so material, though it is desirable, that the 

 actual data be complete. In the very nature of the case, most studies 

 of this class must, for the present, deal with incomplete data, since 



228 



