el valleys,
impart to them and the enclosing mountains moisture borne from the far
away Gulf of Mexico.

GEOLOGY.

The geology of more than half the area of Loudoun County has received
thorough and intelligent treatment at the hands of Arthur Keith in his
most excellent work entitled "_Geology of the Catoctin Belt_,"
authorized and published by the United States Geological Survey.[5]

[Footnote 5: Credit for many important disclosures and much of the
detail appearing in this department is unreservedly accorded Mr. Keith
and his assistants.]

Mr. Keith's analysis covered the whole of Bull Run Mountain, the
Catoctin in its course through Virginia and Maryland to its
termination in southern Pennsylvania, the Blue Ridge and South
Mountain for a corresponding distance, all intermediate ridges and
valleys and contiguous territory lying outside this zone and
paralleling the two flanking ranges.[6]

[Footnote 6: The name "Catoctin Belt" is applied to this region
because it is separated by Catoctin Mountain from the Piedmont plain
as a geographic unit more distinctly than in any other area, and
because its geological unity is completed by Catoctin more fully and
compactly than elsewhere.]

In this important work the Catoctin Belt is shown to be an epitome of
the leading events of geologic history in the Appalachian region. It
contains the earliest formations whose original character can be
certified; it contains almost the latest known formations; and the
record is unusually full, with the exception of the later Paleozoic
rocks. Its structures embrace nearly every known type of deformation.
It furnishes examples of every process of erosion, of topography
derived from rocks of nearly every variety of composition, and of
topography derived from all types of structure except the flat plateau
type. In the recurrence of its main geographic features from
pre-Cambrian time till the present day it furnishes a remarkable and
unique example of the permanence of continental form.

With certain qualifications, a summary of the leading events that have
left their impress on the region is as follows:

1. Surface eruption of diabase.

2. Injection of granite.

3. Erosion.

4. Surface eruption of quartz-porphyry, rhyolite, and andesite.

5. Surface eruption of diabase.

6. Erosion.

7. Submergence, deposition of Cambrian formations; slight oscillations
during their deposition; reduction of land to baselevel.

8. Eastward