History and Comprehensive Description of Loudoun County, Virginia | Page 8

numerous,
and many of them are unfailing, though liable to be affected by drought.
In such cases, by absorption and evaporation, the small streams are
frequently exhausted before uniting and often render the larger ones too
light for manufacturing purposes. Nevertheless, water power is
abundant; the county's diversified elevation giving considerable fall to
its water courses, and many sites are occupied.
CLIMATE.
Because responsible statistical data is usually accorded unqualified
credence, it is without undue hesitation that the following bit of
astonishing information, gleaned from a reliable source, is here set
down as positive proof of the excellence of Loudoun's climate: "It
(Leesburg) is located in a section the healthiest in the world, as proven
by statistics which place the death rate at 8-1/2 per 1,000, the very
lowest in the table of mortality gathered from all parts of the habitable
globe."
The climate of Loudoun, like that of most other localities, is governed
mainly by the direction of the prevailing winds, and, to a limited extent,
is influenced by the county's diversified physical features.

Though the rainfall is abundant, amounting annually to forty or fifty
inches, ordinarily the air is dry and salubrious. This ample precipitation
is usually well distributed throughout the growing season and is rarely
insufficient or excessive. The summer rainfall comes largely in the
form of local showers, scarcely ever attended by hail. Loudoun streams
for the most part are pure and rapid, and there appears to be no local
cause to generate malaria.
In common with the rest of Virginia the climate of Loudoun
corresponds very nearly with that of Cashmere and the best parts of
China. The mean annual temperature is 50° to 55°.
Loudoun winters are not of long duration and are seldom marked by
protracted severity. Snow does not cover the ground for any
considerable period and the number of bright sunny days during these
seasons is unusually large. In their extremes of cold they are less
rigorous than the average winters of sections farther north or even of
western localities of the same latitude. Consequently the growing
season here is much more extended than in either of those sections. The
prevailing winds in winter are from the north and west, and from these
the mountains afford partial protection.
The seasons are somewhat earlier even than in the Shenandoah Valley,
just over the western border of Loudoun, and the farmers here plant and
harvest their crops from one week to ten days earlier than the farmers
of that region.
Loudoun summers, as a rule, are long and agreeably cool, while
occasional periods of extreme heat are not more oppressive than in
many portions of the North. The mountains of Loudoun have a
delightful summer climate coupled with inspiring scenery, and are well
known as the resort of hundreds seeking rest, recreation, or the
restoration of health. This region, owing to its low humidity, has little
dew at night, and accordingly has been found especially beneficial for
consumptives and those afflicted with pulmonary diseases. The genial
southwest trade winds, blowing through the long parallel 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