MARYLAND GEOLOGICAL SURVEY 211

SLATE. —One of the best known roofing slates in the United States
is the so-called Peach Bottom slate of Lancaster and York counties,
Pennsylvania, and Harford county, Maryland. The slate belt forms
a narrow zone which begins a short distance east of the Susquehanna
river in Lancaster county and passes in a southwest direction through
the southeastern corner of York county, terminating near Pylesville
on the Baltimore and Lehigh Railroad in Maryland. The age of this
slate has been determined on fossil evidence to be that of the Hudson
river shales of the lower Silurian. The slates of the Peach Bottom
region were worked as early as Revolutionary times, and show almost
no change after an exposure of a century. Several quarries are to-day
worked in Harford county, although the business is largely operated
by persons living in or near Delta, Pennsylvania. In 1896 the total
output of Maryland had a value of $90, 100.

THE SANDSTONE. —Sandstones are found at many horizons in Mary-
land. Many of these are well suited to furnish valuable building
stones, but as yet only one or two localities have furnished this
material for more than local use. These sandstones are found in the
Newark formation in the Frederick valley, in the quartzite belt of
Deer creek in Harford county, in the Weverton formation of Cam-
brian age in the Blue Ridge district, and in the Silurian and Devonian
formations of Allegany county.

The sandstones of Triassic age possess a recognized reputation in
the market and have been extensively developed throughout the
Triassic belt of the eastern United States, and large quarries have
been opened in this formation in Massachusetts, Connecticut and New
Jersey. A belt of this rock enters Maryland between Emmitsburg
and Union Bridge, rapidly narrowing southward through Frederick
county; while another area occupies the southwestern portion of Mont-
gomery county. The most extensive quarries of this sandstone in
Maryland are situated near the Potomac river, the largest at the
mouth of Seneca creek in Montgomery county, and a somewhat
smaller one near Washington Junction. Still smaller quarries for
local uses are found at more northern points. The Seneca sandstone
has been quarried in a more or less systematic way since 1774, when it