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History and Comprehensive Description of Loudoun County, Virginia by James William Head
page 46 of 250 (18%)
nearer to the level of the present drainage than any other formation,
and decay by solution has gone on to a very considerable extent. Where
the draining streams have approached their baselevel, scarcely an
outcrop of conglomerate is seen. Where the areas of conglomerate lie
near faster falling streams, the irregular masses of unweathered rocks
appear.

When but slightly weathered the conglomerate forms an effective
decorative stone and has been extensively used as a marble with the
name "Potomac marble," from the quarries on the Potomac east of Point
of Rocks, Maryland. While it is in no sense a marble, yet the
different reds and browns produced by unequal weathering of the
limestone pebbles have a very beautiful effect.

The thickness of the Newark formation is most uncertain. The rocks dip
at a light angle to the west with hardly an exception, and the
sections all appear to be continuous. Even with liberal deductions for
frequent faults, nothing less than 3,000 feet will account for the
observed areas and dips.


_Newark Diabase._

Description of the lithified deposits would be far from complete
without reference to the later diabase which is associated with the
Newark rocks.

These diabases, as they will be called generically, are usually
composed of plagioclase feldspar, and diallage or augite; additional
and rarer minerals are quartz, olivine, hypersthene, magnetite,
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