on opposing dips, are nearly 1½ miles apart. The crest then remains nearly steady, preserving a strip of sandstone for about 5 miles in length, known as Shamokin mountain, but highest opposite New Berlin, where the Medina red sandstone shows on the crest.
At the west end of the ridge it again subsides, so that on both sides of Sweitzer run valley, the lower Clinton shales arch over its crest, until north of Centerville the axis again rises rapidly, raising the Medina sandstone a couple of hundred feet above Penns creek, which cuts diagonally through the mountain, eroding the south dip. The rise west is then more gradual, lifting the Oneida sandstone to daylight about 6 miles from Centerville, and finally the No. III Hudson river slate just before reaching the Mifflin county line.
Snyder county.
1. Jack’s mountain, of anticlinal structure, forming one half of the north county line has just been described; but in Spring township of Snyder county, west and south of Swift run, and about 3 miles west of Troxelville, the southern division of the mountain has elevated at the High Top an anticlinal spur of Medina sandstone IVc, between which and the northern division of the mountain occurs the synclinal trough of Swift run, composed of the lower Clinton measures and the Oneida sandstone along the Mifflin line. High Top mountain is soon parted by the increasing strength of the anticlinal going westward, admitting of an elevated plateau of the red Medina IVb, and in turn a second interior spur or knob of the Oneida IV at the Spring and West Beaver township line. This interior ridge is similarly parted in the next mile, and Hudson river slates No. III appear close to the county line, forming the head of the New Lancaster anticlinal valley of Mifflin county.
2. The Northumberland synclinal. The whole central tier of townships in Snyder county to the south of Jack’s mountain is occupied by one broad synclinal of Catskill and Devonian rocks, which may be conveniently called the “Northumberland Synclinal,” forming the center of the
