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 over during submergence, they might be connected with the quartz veins of the Piedmont plain.  That theory, however, with difficulty accounts for their well-rounded condition, which shows either beach action or long carriage.  The quartz sand may well have been derived from the granitic quartzes, but that is an uncertain matter.  The sandstones and quartzites are usually massive and pure white, of the variety found along Catoctin and Bull Run mountains.  Other varieties of sandstone—­the blue-banded type, for instance—­are derived from the Weverton sandstone on the Blue Ridge.  The white sandstone pebbles in the terraces along Bull Run Mountain can be traced from the ledges to the deposits.  In this region, therefore, an absolute shore can be seen.  In other areas along Catoctin Mountain a shore can be inferred, because the mountain projects above the baselevel plane and contains no gravel deposits.  In fact, only a few points at the stream gaps are cut down to the baselevel.

Metamorphism.

Dynamic metamorphism has produced great rearrangement of the minerals along the eastern side of the Catoctin Belt, and results at times in complete obliteration of the characters of the granite.  The first step in the change was the cracking of the quartz and feldspar crystals and development of muscovite and chlorite in the cracks.  This was accompanied by a growth of muscovite and quartz in the unbroken feldspar.  The aspect of the rock at this stage is that of a gneiss with rather indefinite banding.  Further action reduced the rock to a collection of angular and rounded fragments of granite, quartz, and feldspar in a matrix of quartz and mica, the mica lapping around the fragments and rudely parallel to their surfaces.  The last stage was complete pulverization of the fragments and elongation into lenses, the feldspathic material entirely recomposing into muscovite, chlorite, and quartz, and the whole mass receiving a strong schistosity, due to the arrangement of the mica plates parallel to the elongation.  This final stage is macroscopically nothing more than a siliceous slate or schist, and is barely distinguishable from the end products of similar metamorphism in the more feldspathic schists and the Loudoun sandy slates.  The different steps can readily be traced, however, both in the hand specimen and under the microscope.