Philadelphia. - Area, 130 square miles; population in 1880, 847,170.
A special map of the Philadelphia belt of the older and younger gneiss
formations, first appearing at the surface at Morrisville, opposite
to Trenton, N.J., and widening westward so as to cover most of Delaware
county, will be found in this Hand Atlas, in the place of a map of Philadelphia
alone. (See the description of Delaware and Montgomery counties already
given.) The special feature of the geology of the city is its brick-clay
and gravel beds, deposited at various levels; the oldest (Bryn
Mawr gravel) at 400` A.T. a patch of which remains about Chestnut Hill;
the less ancient sands and brick-clay of Nicetown and the terrace
west of the Schuylkill on which the railroad to Media and West Chester
is built, at 200` A.T.; the more recent gravel, sand, and brick-clay
encountered in laying the foundations of the city houses, from 80 to
100` A.T. to below the river mud of the Neck. The abundance and excellence
of the Delaware valley clay has conduced to the celebrity of the Philadelphia
house-brick, and its almost universal use in the construction of its
houses, a comparatively small number of public and private edifices
being built of marble, New Red brownstone, or Delaware county serpentine.
The red color of the brick, due to a constant particular percentage
of iron in the clay, contrasts strongly with the yellow bricks manufactured
from the Drift clay in other parts of America. As a curiosity it may
deserve mention that the Assayers of the U.S. Mint found by calculation
that there was enough disseminated gold in the bricks of the
houses of the city to pay off the National debt; and they calculated
that it would cost $10 to extract one dollar’s worth of the metal from
the clay at the brick-yard.
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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