SUCCESSIVE DEPOSITS IN THE APPALACHIAN REGION 417 



of the material. In these beds I have observed a remarkably fine devel- 

 opment of cross-bedding, mostly of the torrential type. The diminished 

 thickness may be in part due to erosion, during the deposition elsewhere 

 of the lower Juniata beds, and in part to original difference in deposition. 

 About 25 miles west of Logan Gap, where the Juniata breaks through the 

 mountain at Jacks Narrows, only the upper 250 feet of the Bald Eagle 

 are brought up in the center of the anticline. They are greenish white, 

 hard sandstones, generally free from pebbles. That the beds do not ap- 

 pear farther eastward, where higher formations rest on disturbed Hudson 

 strata, is due to the erosion which followed the late Ordovicic or early 

 Siluric folding of the eastern strata and which occupied all of early 

 Siluric time. 



If we turn now to the easternmost ridges, we find that the Bald Eagle 

 is absent in the Blue Mountain Ridge on the Susquehanna above Harris- 

 burg, where red beds, followed by Tuscarora sandstone, rest on the Hud- 

 son River series, which apparently is slightly disturbed here (see be- 

 yond). These Hudson beds themselves are rather hard, though as a rule 

 thin-bedded sandstones alternating with shales, in which repeated search 

 has as yet failed to reveal fossils. That these sandstones are the strati- 

 graphic equivalents, in part at least, of the Bald Eagle seems most prob- 

 able, unless there is more of a hiatus here than we at present suspect. 

 They are more arenaceous in texture than the Utica shales which under- 

 lie the Bald Eagle and rest on the Trenton in the Bald Eagle Ridge. 



Southward, throughout eastern Perry and Franklin counties, Pennsyl- 

 vania, the Bald Eagle formation is absent. The Upper Ordovicic of these 

 sections in general begins with the Martinsburg shale series, together 

 with some of the underlying calcareous beds referred to the Trenton. 25 

 The shale here replaces very largely the Trenton limestone, including in 

 its lower portion fossils of this horizon, while its upper part corresponds, 

 according to the determinations of Ulrich, to the Utica and Lorraine of 

 the New York section. In the Mercersburg region, Franklin County, the 

 Martinsburg shale is 2,000 feet thick, the lower 800 feet consisting of a 

 black, fissile shale, closely resembling the Utica shale of New York and 

 carrying calcareous layers at the base, while the upper 1,200 feel is 

 "sandy and calcareous, with locally hard siliceous beds, and grades up- 

 ward into ferruginous sandstones referred to th(> Juniata formation." M 

 The upper beds are also coarser, massive, and arkosic. The upper part of 



25 Upper Chambersburg limestone; see G. W. Stoso, Mercersburg-Chambersburg quad- 

 rangle. U. S. Geological Survey Folio 170, p. 10. 



20 G. W. Stose and Cbarles K. Swartz : Pawpaw-Hancock Polio 170, p. :? ; see also 

 Mercersburg-Chambersburg quadrangle, No. 170, i>. 10, 



XXIX— Bull. Geol. Soc. Am., Vol. 24, 1012 



