Lebanon. - Area, 350 square miles; population in 1880,
38,476. This county shaped like the spanker sail of a ship is cut
out of Dauphin by a line running along the Sharp mountain 10 miles,
and another line 20 miles long drawn south across the Stony creek
red shale valley of XI, Second mountain X, the Pinegrove valley IX,
VIII, and V, the Blue mountain IV, the Great valley with its belts
of slate III, and limestone II, and the Mesozoic New Red belt, to
the Conewago creek and trap dyke along the Lancaster county line.
Its geology is therefore an unchanged continuation of that of Dauphin
county eastward (which see already described) except that the Sharp
mountain begins to split and open into a coal basin, between two crests,
half a mile apart at the Schuylkill county corner. Another difference
consists in the reappearance of traces of the Helderberg limestone
VI and Oriskany sandstone VII, two formations quite wanting in Dauphin
County. The overturned condition of the formations at the Susquehanna,
gradually changing to vertical, becomes a regular but very steep north
dip in Lebanon county; and in front of the Blue mountains runs a sharp
anticlinal, in the trough south of which has been preserved a strip
of No. IV, making Hole mountain, and well illustrating the plicated
structure of the Great Valley. Three longitudinal trap dykes run along
in the slate belt south of Jonestown; and other exhibitions of trap
are made at the edge and in the body of the New Red district, especially
along the county line south and west of Cornwall. A triangluar area
of slates is noticeable just east from Shafferstown, and the dips
in the limestone on its north-west and north-east sides prove that
it is a trough, or dimple in which the lower strata of Formation No.
III have been preserved, covered along the south side of the triangle
by New Red. In the south-east corner of the county Potsdam No. I surrounds
the west end of the Mulbaugh hill. The breccia of Mt. Ararat, a ridge
running 3 miles north of Lebanon, is a remarkable exhibition of limestone
strata crushed to fragments by pressure and re-cemented in place.
The pride of Lebanon county is its wonderful mine of magnetic iron
ore, containing sulphurets of iron and copper, and encased in decomposed
trap, against a south wall of white marble, overlaid by New Red shale,
at Cornwall. The ore has been stopped in three adjoining low hills
for many years to supply a number of large blast furnaces near the
mine and at Lebanon, and for sale to most of the other blast furnaces
of Eastern Pennsylvania.
From: A geological hand atlas of the sixty-seven
counties of Pennsylvania :embodying the results of the field work
of the survey, from 1874 to 1884. By J. P. Lesley. (Report of
progress (Geological Survey of Pennsylvania), v. X ) Harrisburg, PA
: Board of commissioners for the second geological survey, 1885.
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