240 ROLLIN T. CHAMBERLIN 



mately a horizon equivalent to the sea-level from which the strata 

 were up warped. Estimates of the height of the folded tract may 

 therefore be measured from the Kittatinny plain as a base. In this 

 way the Tertiary uplifts and any other disturbing factors since the 

 Cretaceous are eliminated. 



In the reconstructed section no beds younger than the Pottsville 

 conglomerate were included, for the reason that no younger forma- 

 tions now occur in the region studied. It seems highly probable, 

 however, that the Allegheny, Conemaugh, Monongahela, and possi- 

 bly the Dunkard formations once covered this portion of the Appala- 

 chians, and were incorporated in the folds but have since been removed 

 by erosion. These formations vary considerably in thickness in 

 different localities. In Westmoreland County the Lower Productive, 

 Lower Barren, Upper Productive, and Upper Barren Measures have 

 a total thickness of 1,477 feet.' Campbell gives 1,540 feet for the 

 Coal Measures above the Pottsville in southwestern Pennsylvania.^ 

 ■ These are on the southwest side of the area under consideration; in 

 the anthracite regions to the northeast, the Coal Measures appear to 

 be thicker — in the northern field 1,800 feet, in the middle 1,500 feet, 

 while they attain a total aggregate of 2,500 feet in fhe southern field, 

 and it is not certain that some layers may not have been removed by 

 erosion from each of these districts. ^ The thickness of these forma- 

 tions over the Tyrone-Harrisburg mountain-section was perhaps of 

 about the same order of magnitude, though this must always remain 

 a matter of conjecture. A belt representing the missing Coal Measures 

 therefore belongs above the Pottsville, but was omitted from the 

 cross-sections because of the uncertainty as to the thickness and 

 former extent of these beds. An average figure for the thickness of 

 these missing beds is to be added to the height of the folded section in 

 the following estimates. 



But the Kittatinny base-level maintains also somewhat fluctuating 

 elevations above the railroad tracks. Generally about 1,000 or 

 1,100 feet separate the two horizons, but at points in Huntingdon 



1 J. J. Stevenson, Second Geol. Surv. Pennsylvania, 1876, Fayette and Westmore- 

 land Districts. 



2 M. R. Campbell, Masonville-Unionville Folio, U.S. Geol. Surv. 



3 Penn. Geol. Surv.^ Summary Final Rept., 1895, Vol. Ill, Part i, "Carbonif- 

 erous," p. 1924. 



