182 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

and below it, and contain a mixed Clinton and Niagara fauna. The
thickness of the Rockwood formation probably varies from 750 to
1000 feet in Maryland.

THE LEWISTOWN FORMATION (Niagara, Salina and Lower Helder-
berg). —The Lewistown formation, so named from its typical occur-
rence at Lewistown, Pennsylvania, is found in Maryland in the
central portion of the Appalachian Region, occurring like the Rock-
wood formation in several belts confined to the eastern and western
portions of this district, and occupying very nearly the same area as
those above described for that formation. The deposits of the Lewis-
town formation consist chiefly of limestones with here and there inter-
bedded shales. Three distinct divisions may be observed in the Lew-
town formation which are well exposed to the east of Potomac Station
in southern Allegany county, viz:

tipper limestone series................. 640 feet thick

Cement series........................ 465

Lower limestone series................. 200

Total........................... 1305 feet

The upper and lower limestone series contain fossils, but they are
apparently entirely lacking in the intervening cement series.

THE DEVONIAN PERIOD.

The deposits of Devonian age enter, together with the Silurian
rocks, into the formation of the central division of the Appalachian
Region, and together with the Carboniferous deposits into the forma-
tion of the Alleghany Plateau. They consist of sedimentary mate-
rials that have been but little altered since they were deposited,
although in places subjected to considerable structural disturbances.
Four divisions are recognized in the sequence of Devonian deposits,
known as the Monterey, Romney, Jennings and Hampshire forma-
tions.

THE MONTEREY FORMATION (Oriskany sandstone). —The Mon-
terey formation, so called from its typical occurrence at Monterey,