308 V\ REPORT OE PROGRESS. E. W. CLAYPOLE. 



the time required for the deposition of the following strata 

 in its neighborhood : 



Hamilton lower shale, 



Marcellus black shale, 

 " iron ore, 

 " limestone, 

 " shale and ore, 



Oriskany sandstone, 



Lower Helderberg flint shale, 

 " " shaly limestone, 



u " limestone, 



in all about 1000 feet of rocks. We cannot yet turn this 

 date in years, but on the basis assumed above we can make 

 the duration of the dry land equal to that required to erode 

 and destroy the lower four hundred feet of strata (Onondaga 

 shale) deposited during a preceding period. 



It is not necessary for our purpose to maintain any spe- 

 cial cause for the elevation of the land at this place. The 

 continued but probably intermittent subsidence now uni- 

 versally admitted during the deposition of the pala?ozoic 

 rocks of Pennsylvania is sufficient. Dry land is as easily 

 produced by a subsidence of the sea as by an elevation of 

 the land. If the bottom of the sea in the district lying un- 

 der the present Appalachian region slowly subsided, its bor- 

 der would be left dry. We have but to place Rye town- 

 ship on the southeastern side of the subsiding area to ac- 

 count for its relative elevation without improbability or dif- 

 ficulty. 



It is of course impossible in the present imperfect state of 

 our knowledge of the geology of this part of Pennsylvania to 

 determine exactly where the boundary between sea and land 

 at that distant epoch should be drawn. We do not know 

 where the missing beds begin to appear, or where the drain- 

 age went during the time of elevation. But facts connected 

 with the overlying deposits in this district suggest some 

 farther possibilities which must now be considered. What- 

 ever doubt may exist regarding the limits of the Onondaga 

 shales there is none in regard to those of the other group. 

 As shown in the diagram and detailed in the pages of the 



