Mifflin. - Area, 380 square miles; population in 1880,
19,577. The western half of this long narrow county is a secluded Lower
Silurian limestone anticlinal valley, drained by Kishicoquillis creek
through Logan’s gap in Jack’s mountain, between which and Stone mountain
(on the west) the valley tapers to a point southward, and is split at
its northern end into three long, narrow, straight anticlinal vales,
separated by two picturesque synclinal spurs of the Buffalo mountains
coming from Snyder county. The limestone floor of the valley contains
deposits of brown hematite iron ore once extensively mined in open quarries.
Its sides consist of Loraine and Utica slate No. III, rising to a very
remarkable terrace of Oneida conglomerate (IV a,) broken at short regular
intervals by little ravines heading in the upper slope of red Medina
slates (IV b,) crowned by the mountain crest of white Medina sandstone
(IV c). The scenery is not only romantic in an artistic but in a geological
sense, and an end view of the northern spurs affords the finest illustration
of synclinal and anticlinal wave structure to be found in Pennsylvania.
The eastern county line (40 miles long) follows the crest of East Shade
mountain (IV,) crosses the synclinal vale of the Juniata "Long Narrows"
to Blue Ridge (IV) the crest of which it follows to the great bend of
the Juniata River. Between this eastern mountain line and Jack’s mountain
runs the Lewistown valley, 38 miles long and 6 miles (with great regularity)
wide; a trough (deeper at its two ends and shallower midway) of Upper
Silurian and Lower Devonian measures, crumpled into numerous sharp parallel
folds, producing at the present surface many zigzag outcrops of the
Lewistown limestone (Lower Helderberg No. VI) and Oriskany sandstone
No. VII, with the overlying Marcellus pyritous ferriferous black clay,
turned near the surface into a valuable brown hematite iron ore, extensively
mined west of Lewistown in the numerous low ridges bordering the north
bank of the Juniata River. The fossil ore beds of the Clinton No. V
are opened at many points along the slope of Jack’s mountain, and outcrop
also along the slopes of Shade Mountain and Blue Ridge. See in Report
F descriptions of the mines on both ore ranges, and on the Marcellus
ore bed, and also of the large broken down deposits of glass-sand mined
near McVeytown on the Oriskany outcrop. The instrumental measurements
of the formations at Lewistown, McVeytown, Mount Union and other points,
reduced to graphic cross sections and published in Report F, are the
most perfect made by the Survey in Middle Pennsylvania, and furnish
a useful handbook for field geologists. The Lewistown section, for example,
sums up its details as follows: Marcellus black slate 290`+; iron ore
bed; Marcellus* limestone, 40`; Scoharie? Dark shale, 53`; Cauda-galli?
clay, 40`; Oriskany sandstone, 110`; Oriskany shale, 205`; Lewistown
shale, 140`; Lewistown (L. Held.) limestone, 185`; Water-lime shale,
470`; Salina variegated shale, 358`; Niagara? limestone, 4`; Niagara?
shale, 70`; Clinton upper red shale, 305`; lower red shale, 260`; lower
lime shale and upper olive shale, 250`; fossil ore beds and ore-sandstone,
120`; middle olive shale, 128`; iron-sandstone, 7`; lower olive shale,
571`; Medina white sandstone, 820`; red sandstone and shale, 1280`;
Oneida red conglomerate, 309`; gray sandstone, 313`; (total of IV, 2722`);
Hudson river gray sandstone, 425`; gray shale, 190`; hard fine sandstone,
140`; dark ferruginous shale, 182`; Utica upper gray slate, 210`; middle
black shale, 302`; lower gray slate, 855` (total of III, 1367`); Trenton
limestone in Kishicoquillis valley (exposed) 320`.
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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