to
a marked degree, for the three main synclinal lines are lines of great
elevation, and the anticlines are invariably valleys.

In order of solubility the rocks of the Catoctin Belt, within the
limits of Loudoun County, to which section all subsequent geologic
data will be confined, stand as follows:

1. Newark limestone conglomerate; calcareous.

2. Newark sandstone and shale; calcareous and feldspathic.

3. Newark diabase; feldspathic.

4. Granite; feldspathic.

5. Loudoun formation; feldspathic.

6. Granite and schist; feldspathic.

7. Catoctin schist; epidotic and feldspathic.

8. Weverton sandstone; siliceous.

All of these formations are in places reduced to baselevel. The first
three invariably are, unless protected by a harder rock; the next
three usually are; the Catoctin schist only in small parts of its
area; the Weverton only along a small part of Catoctin Mountain.

The Catoctin Belt itself may be described as a broad area of igneous
rocks bordered by two lines of Lower Cambrian sandstones and slates.
Over the surface of the igneous rocks are scattered occasional
outliers of the Lower Cambrian slate; but far the greater part of the
surface of the belt is covered by the igneous rocks. The belt as a
whole may be regarded as an anticline, the igneous rocks constituting
the core, the Lower Cambrian the flanks, and the Silurian and Newark
the adjoining zones. The outcrops of the Lower Cambrian rocks are in
synclines, as a rule, and are complicated by many faults. The igneous
rocks have also been much folded and crumpled, but on account of their
lack of distinctive beds the details of folds can not well be traced
among them.

They are the oldest rocks in the Catoctin Belt and occupy most of its
area. They are also prominent from their unusual character and rarity.

An important class of rocks occurring in the Catoctin Belt is the
sedimentary series. It is all included in the Cambrian period and
consists of limestone, shale, sandstone and conglomerate. The two
border zones of the Catoctin Belt, however, contain also rocks of the
Silurian and Juratrias periods. In general, the sediments are sandy
and calcareous in the Juratrias area, and sandy in the Catoctin Belt.
They have been the theme of considerable literature, owing to their
great extent and prominence in the topography.

_Granite._

The granite in the southern portion of the County is very important in
point of extent, almost as much