The Loudoun formation, of course, followed a period of erosion of the Catoctin Belt, since it is the first subaqueous deposit.  It is especially developed with respect to thickness and coarseness to the west of Catoctin Mountain.  Elsewhere the outcrops are almost entirely black slate.  This is true along the Blue Ridge, through almost its entire length, and also through the entire length of the Catoctin Mountain.  On the latter range it is doubtful if this formation exceeds 200 feet in thickness at any point.  Along the Blue Ridge it may, and probably does, in places, reach 500 feet in thickness.

The distribution of the coarse varieties coincides closely with the areas of greatest thickness and also with the synclines in which no Weverton sandstone appears.  The conglomerates of the Loudoun formation are composed of epidotic schist, andesite, quartz, granite, epidote, and jasper pebbles embedded in a matrix of black slate and are very limited in extent.

Weverton Sandstone.

The formation next succeeding the Loudoun formation is the Weverton sandstone.  It is so named on account of its prominent outcrops in South Mountain, near Weverton, Maryland, and consists entirely of siliceous fragments, mainly quartz and feldspar.  Its texture varies from a very fine, pure sandstone to a moderately coarse conglomerate, but, in general, it is a sandstone.  As a whole, its color is white and varies but little; the coarse beds have a grayish color in most places.  Frequent bands and streaks of bluish black and black are added to the white sandstones, especially along the southern portion of the Blue Ridge.  The appearance of the rock is not modified by the amount of feldspar which it contains.

From the distribution of these various fragments, inconspicuous as they are, considerable can be deduced in regard to the environment of the Weverton sandstone.

The submergence of the Catoctin Belt was practically complete, because the Weverton sandstone nowhere touches the crystalline rocks.  Perhaps it were better stated that submergence was complete in the basins in which Weverton sandstone now appears.  Beyond these basins, however, it is questionable if the submergence was complete, because in the Weverton sandstone itself are numerous fragments which could have been derived only from the granite masses.  These fragments consist of blue quartz, white quartz, and feldspar.  The blue quartz fragments are confined almost exclusively to the outcrops of the Weverton sandstone in the Blue Ridge south of the Potomac, and are rarely found on Catoctin.

The general grouping of the Loudoun formation into two classes of deposit (1), the fine slates associated with the Weverton sandstone, and (2), the course sandstones occurring in deep synclines with no Weverton, raises the question of the unity of that formation.  The evidence on this point is manifold and apparently conclusive.  The general composition of the two is the same—­i. e., beds of feldspathic, siliceous material derived from crystalline rocks.  They are similarly metamorphosed in different localities.  The upper parts of the thicker series are slates identical in appearance with the slates under the Weverton, which presumably represent the upper Loudoun.