and
risen above it, while in other places a thin coat of shale remains
above the trappean matter, but much altered and changed in
character."[7] A large mass of trap rock presents itself boldly above
the shale at the eastern abutment of the Broad Run bridge, on the
Leesburg and Alexandria turnpike. Not far to the east the shale is
changed to a black or blackish brown color, while at the foot of the
next hill still farther eastward the red shale appears unchanged. The
summits of many of these dykes are "covered with a whitish or
yellowish compact shale, highly indurated and changed into a rock very
difficult to decompose."[8]

[Footnote 7: Taylor's _Memoir_.]

[Footnote 8: Ibid.]

_Lafayette Formation._

A great class of variations due to rock character are those of surface
form. The rocks have been exposed to the action of erosion during many
epochs, and have yielded differently according to their natures.
Different stages in the process of erosion can be distinguished and to
some extent correlated with the time scale of the rocks in other
regions. One such stage is particularly manifest in the Catoctin Belt
and furnishes the datum by which to place other stages. It is also
best adapted for study, because it is connected directly with the
usual time scale by its associated deposits. This stage is the
Tertiary baselevel, and the deposit is the Lafayette formation, a
deposit of coarse gravel and sand lying horizontally upon the edges of
the hard rocks. Over the Coastal plain and the eastern part of the
Piedmont plain it is conspicuously developed, and composes a large
proportion of their surfaces. As the formation is followed westward it
is more and more dissected by erosion and finally removed. Near the
area of the Catoctin Belt it occurs in several places, all of them
being small in area. One is three miles northeast of Aldie. Here, a
Newark sandstone hill is capped with gravel. This gravel is much
disturbed by recent erosion and consists rather of scattered fragments
than of a bedded deposit.

The materials of the Lafayette gravel are chiefly pebbles and grains
of quartz, with a considerable admixture of quartzite and sandstone.
The large quartz pebbles were probably derived from the large lenses
of quartz in the Catoctin schist, for no other formation above water
at the time contained quartz in large enough masses to furnish such
pebbles. On the hypothesis that they were of local origin and merely
worked ov