Delaware. Area, 190 square miles; population in
1880, 56,101. The oldest or fundamental hornblende gneiss is laid
bare in three isolated areas; the northern spreading through Radnor
and Newtown into Chester County; the middle spreading from southern
Newton, Edgmont, Thronbury, northern Middletown, and northern Concord;
the southern spreading across Aston, Bethel, and U. Chichester into
the State of Delaware. These areas are separated and surrounded by
the Chestnut Hill micaceous and garnetiferous schist country holding
the serpentine beds. An irregular line through Haverford and U. Darby
to Chester Creek (2 ½ miles up from its mouth) divides this country
from the triangular area of Manayunk and Philadelphia mica-schists,
which no doubt extends southward beneath New Jersey. The county has
a rolling surface averaging 450` A.T., but drops to a terrace of 200`
A.T., and then to the mud flats of the Delaware. Patches of old Bryn
Mawrgravel remain in various townships on the divides at 400` A.T.
Patches and streaks of brick-clay remain on the terrace, and are extensively
wrought. Brick clay (holding bowlders) passes also under the river
mud. Cobb’s Creek (along the eastern line,) Darby Creek, Crum Creek,
Ridley Creek, and Chester Creek cross the county from north-west to
south-east, flowing in rock-cut channels, or tortuous glens, presenting
a lovely variety of picturesque scenery, and affording a considerable
amount of valuable mill power. The geological exposures are numerous;
but the rocks are so metamorphosed, decayed, crumpled, cross-laminated,
and probably faulted, that in the absence of fossils, and of well-defined
mineral strata like limestone and iron ore, it is not easy to arrive
at any definite opinions respecting the order of their super-position,
or the classical system to which they belong. Under an appearance
of vertical stratification, they really lie almost horizontal, as
may be seen at Griswold’s "granite" quarry in Darby, Ward’s quarry
in Ridley, Deshong’s quarries in Nether Providence, and the Avondale
quarries in Nether Providence and Ridley townships, lithograph views
of which are published for the purpose of showing the true structure
in Report C5 on Delaware County. It is undoubtedly the
real structure throughout the county. But as the genera dip (as shown
along the Schuylkill River) is north or north-westward, carrying the
Philadelphia schists under the Manayunk schists, and these again under
the Chestnut Hill schists, it is hard to understand why all three
should not be regarded as descending beneath the isolated areas of
"older" hornblendic gneiss. A serpentine belt extending from
Chester Creek at Lenni (or Rockdale) past Media to Darby Creek in
Radnor Township (9miles) has been quarried for building stone. It
consists of separate and parallel outcrops; and at least 27 other
local exposures of serpentine in various townships are marked upon
the map, all of them in the Chestnut Hill schist area, and
apparently belonging to the upper part of that series. Castle rock,
in Edgmont Township, is a huge exposure of enstatite (anhydrous
serpentine) of picturesque aspect, and doubtful geological structure.
(See plates in Report C5 .) Extensive mines of kaolin
are worked at the West End of the county, and an outcrop of pure feldspar
rock in Concord Township is exploited for use of dentists. (See
numerous heliotype views of the Kaolin mines in Report C5.)
Mineralogical cabinets, public and private, have been amply enriched
with fine specimens of corundum, tremolite, actinolite, asbestus,
beryl, chrysolite, garnet, the micas, feldspars, and quartzes,
tourmaline, andalusite, fibrolite, cyanite, staurolite, stilbite,
sepiolite, marmolite, chrysotile, deweylite, damourite, jefferisite,
margarite, apatite, autunite, mirabilite, magnesite, bismuthite, menaconite,
magnetite, chromite, rutile, molybdite, &c. from numerous
exposures in different parts of the county. A small percentage of gold has been obtained by analysis from
the brick clays; a few small deposits of iron ore have been tried
and abandoned; no other ore seems to exist in the county. A few small
local exhibitions of trap have been noticed.
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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