The Appalachian Folds of Central Pennsylvania

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 groundwork 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. I25-126, a method is given for deducing 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