central ridge, and a little high land near the bottom of the formation; and finally the Marcellus slate, following the line of the Oriskany sandstone ridges, and extending in long narrow belts between the folded synclinals in Derry township.
The greatest thickness of the entire series cannot be over 2500′, near the Snyder county line. The series has somewhat the same character as at the Susquehanna, with rather less sandstone and limestone however in the Hamilton and Marcellus groups.
The Marcellus iron ore, a dull colored ash-gray pyritous carbonate when freshly broken, is well exposed in Derry township along the north side of the valley, at the Townsend mines of the Logan I. and S. Co. The Marcellus here shows about 30′ of black slate between the Oriskany sandstone No. VII (making a ridge behind the outcrop) and the ore-bed. The limestone in some places underlying this ore is generally absent here or decomposed to a clay. The mine will be referred to in the chapter on Township Geology. The bed is from 6′ to 9′ thick in the present workings, capped with a smooth black slate hanging wall.
Perhaps 300′ of black slate overlies it throughout the narrow valley between the ridges, all belonging to the Marcellus. In the neighborhood of Lewistown only part of the Marcellus formation is held within the tightly compressed synclinals, the largest area and greatest thickness being found in the trough of the first synclinal north of Lewistown. Some few attempts have been made to find the Marcellus ore in different parts of this valley and in Granville township, but no ore is being mined at present. Near its horizon however, the Marcellus or Upper Helderberg limestone was seen at several points, at its greatest development showing 40′ of impure greenish limestone, weathering rapidly into unctuous green shales.
At MceVeytown the same rock is developed 40′ thick, darker in color and consisting of numerous thin layers separated by clay slate.
The McCoy, Ross, and the Dull & Bradley mines are located in three different Marcellus slate synclinals north
