GEOLOGICAL RELATIONS OF THE POCONO 39 



northern outcrop the thickness decreases rapidl}^ westward from the 

 area east of the Allegheny mountains, apparently in part from loss of 

 the lower beds, until it becomes approximately 400 feet in Clinton and 

 the adjacent counties, whence to the northwest outcrop in Pennsylvania 

 the variation is insignificant. The character of the rock changes slowly 

 in that direction. It is mostly sandstone, often coarse, along the north- 

 easterly outcrop and easterly outcrop, though in the central east shales 

 occur, one of which, midway in the mass, was found by Dr I. C. White 

 to be richly fossiliferous. An impure limestone makes its appearance 

 in the same relative position near Ralston, in the second basin of Rogers. 

 Hodge describes it in Tioga county of the third basin, and speaks as 

 though it were widely distributed, while later observers have found 

 limestones in the adjacent counties of Elk, Warren, and McKean. Doctor 

 Chance has shown clearly how the Pocono, after reaching its minimum 

 Pennsylvania thickness of about 400 feet in the third basin, changes in 

 composition westwardly, becoming more and more shaly below the She- 

 nango sandstone, so that Doctor W^hite's section, giving for the Pocono 

 of Crawford and Erie a series of shales and sandstones with several thin 

 limestones, is what one might expect to find in northwest Pennsylvania. 

 The Pocono holds an impure limestone in the Chestnut Hill gaps of 

 southwest Pennsylvania and northern W^est Virginia, where the mass is 

 once more a sandstone. 



Along the westerly outcrop in Ohio one finds the same general condi- 

 tions as in northwest Pennsylvania, the sandstone on top with shales and 

 irregular limestones below. Southwardly the limestones disappear and 

 do not reach into northern Kentucky, but midway in that state calcare- 

 ous beds appear, as at the southeast in southern Virginia, increasing 

 southward until they become important in Tennessee, while in northwest 

 Alabama one finds only limestone and chert. The thickness diminishes 

 southward from nearl}^ 500 feet at the Ohio river to 175 feet in northern 

 Alabama, and the whole disappears finally before one reaches the middle 

 of that state. Apparently the thinning toward the south is due to the 

 gradual loss of the lower members — a continuation of the conditions 

 observed in the upper Devonian. 



The physical geography is discussed later on ; here must be consid- 

 ered onh^ the place of the Pocono in the geological column. 



The Pocono of Pennsylvania, Ohio, Kentucky, and Virginia has been 

 regarded by most geologists as Lower Carboniferous throughout. The 

 Pocono of the eastern outcrops in Pennsylvania has been accepted as the 

 equivalent of that in the western counties, as though the westward de- 

 crease were due merel}^ to lessened thickness in each of the subdivisions, 



VI— Bui.i.. Geoi.. Soc. Am., Vol. 14, 1902 



