Part 12

The Piedmont plain, where it borders upon the Catoctin Belt, is composed in the main of the previously described Newark strata, red sandstone, and limestone conglomerate. East of the Newark areas lies a broad belt of old crystalline rocks, whose relations to the Catoctin Belt are unknown.

The rocks, in a transverse line, beginning a little to the east of Dranesville, in Fairfax County, and extending to the Catoctin Mountain, near Leesburg, occur in the following order, viz: Red sandstone, red shale, greenstone, trap, reddish slate, and conglomerate limestone.

Heavy dykes of trap rock extend across the lower end of the County, from near the mouth of Goose Creek to the Prince William line. "These, being intrusive rocks, have in some places displaced the shale and risen above it, while in other places a thin coat of shale remains above the trappean matter, but much altered and changed in character." A large mass of trap rock presents itself boldly above the shale at the eastern abutment of the Broad Run bridge, on the Leesburg and Alexandria turnpike. Not far to the east the shale is changed to a black or blackish brown color, while at the foot of the next hill still farther eastward the red shale appears unchanged. The summits of many of these dykes are "covered with a whitish or yellowish compact shale, highly indurated and changed into a rock very difficult to decompose."

Lafayette Formation.

A great class of variations due to rock character are those of surface form. The rocks have been exposed to the action of erosion during many epochs, and have yielded differently according to their natures. Different stages in the process of erosion can be distinguished and to some extent correlated with the time scale of the rocks in other regions. One such stage is particularly manifest in the Catoctin Belt and furnishes the datum by which to place other stages. It is also best adapted for study, because it is connected directly with the usual time scale by its associated deposits. This stage is the Tertiary baselevel, and the deposit is the Lafayette formation, a deposit of coarse gravel and sand lying horizontally upon the edges of the hard rocks. Over the Coastal plain and the eastern part of the Piedmont plain it is conspicuously developed, and composes a large proportion of their surfaces. As the formation is followed westward it is more and more dissected by erosion and finally removed. Near the area of the Catoctin Belt it occurs in several places, all of them being small in area. One is three miles northeast of Aldie. Here, a Newark sandstone hill is capped with gravel. This gravel is much disturbed by recent erosion and consists rather of scattered fragments than of a bedded deposit.

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.

The Weverton sandstone has suffered less from metamorphism than any of the sediments. In the Blue Ridge it has undergone no greater change than a slight elongation of its particles and development of a little mica. Along Catoctin Mountain, from the Potomac River south, however, increased alteration appears together with the diminution in thickness. What little feldspar there was is reduced to quartz and mica, and the quartz pebbles are drawn out into lenses. Deposition of secondary quartz becomes prominent, amounting in the latitude of Goose Creek to almost entire recrystallization of the mass. A marked schistosity accompanies this alteration, and most of the schistose planes are coated with silvery muscovite. Almost without exception these planes are parallel to the dip of the formation.

Metamorphism of the Loudoun formation is quite general. It commonly appears in the production of phyllites from the argillaceous members of the formation, but all of the fragmental varieties show some elongation and production of secondary mica. The limestone beds are often metamorphosed to marble, but only in the eastern belt. The recrystallization is not very extensive, and none of the marbles are coarse grained.

The metamorphism of the igneous rocks is regional in nature and has the same increase from west to east as the sediments.

In the granite it consists of various stages of change in form, attended by some chemical rearrangement. The process consisted of progressive fracture and reduction of the crystals of quartz and feldspar, and was facilitated by the frequent cleavage cracks of the large feldspars. It produced effects varying from granite with a rude gneissoid appearance, through a banded fine gneiss, into a fine quartz schist or slate. These slaty and gneissoid planes are seen to be parallel to the direction and attitude of the sediments, wherever they are near enough for comparison.

Dynamic alteration of the Catoctin diabase is pronounced and wide-spread. Macroscopically it is evident in the strong schistosity, which is parallel to the structural planes of the sediments when the two are in contact. In most areas this alteration is mainly chemical and has not affected the original proportions of the rock to a marked extent. Its prevalence is due to the unstable composition of the original minerals of the rock, such as olivine, hypersthene, and pyroxene. Along Catoctin Mountain, however, both chemical and mechanical deformation have taken place, so that the original rock structure is completely merged into pronounced schistosity. This was materially assisted by the weak lath shapes of the feldspar and the mobility of the micas.

The average dip of the schistose planes is about 60 deg.; from this they vary up to 90 deg. and down to 20 deg.. In all cases they are closely parallel to the planes on which the sediments moved in adjustment to folding, namely, the bedding planes. In regions where no sediments occur, the relation of the schistose planes to the folds can not be discovered.

Parallel with the micas that cause the schistosity, the growth of the quartz and epidote lenses took place. These, too, have been deformed by crushing and stretching along Bull Run Mountain and the south part of Catoctin Mountain. From this fact, taken in connection with the folding of the schistose planes at Point of Rocks, it would appear that the deformation was not a single continuous effort.

The ratios of schistose deformation in the igneous rocks are as follows: diabase, with unstable mineral composition and small mechanical strength, has yielded to an extreme degree; granite, with stable composition and moderate mechanical strength, has yielded to the more pronounced compression.

MINERAL AND KINDRED DEPOSITS.

In point of mineral wealth Loudoun ranks with the foremost counties of the State. Iron, copper, silver, soapstone, asbestos, hydraulic limestone, barytes, and marble are some of the deposits that have been developed and worked with a greater or lesser degree of success.

A large bed of compact red oxide of iron lies at the eastern base of the Catoctin Mountain, on the margin of the Potomac River. Long before the Civil War a furnace was erected here by Samuel Clapham, Sr., for the reduction of this ore, and considerable quantities of it were formerly transported moderate distances to supply other furnaces. The Clapham furnace continued in operation until all the fuel at hand was consumed and then went out of blast. Water power was supplied by the Catoctin Creek, which flows into the river immediately above the mountain. To obtain this a tunnel was cut through a spur of the mountain projecting into a bend of the creek. This tunnel, about five hundred feet long and sixty feet beneath the summit of the hill, was cut through almost a solid wall of rock, and, at that day, was considered a great work.

Magnetic iron ore has been found in certain places, and this or a similar substance has a disturbing effect upon the needle of the surveyor's compass, rendering surveying extremely difficult where great accuracy is required. In some instances the needle has been drawn as much as seven degrees from its true course. This effect is more or less observable nearly throughout the Catoctin Mountain, and has been noted elsewhere in the County.

Chromate of iron was long ago discovered along Broad Run, and, about the same time, a bed of micaceous iron ore on Goose Creek below the Leesburg turnpike. Copper ore is associated with the last-named mineral.

In 1860, the output of pig iron in Loudoun was 2,250 tons, and its value $58,000. Rockbridge was the only Virginia County to exceed these figures.

In several localities small angular lumps of a yellowish substance, supposed to contain sulphur, have been found, embedded in rocks. When subjected to an intense heat, it gives forth a pungent sulphurous odor.

Small quantities of silver ore are discovered from time to time; but the leads have never been extensively worked and many of the richest veins are still untouched.

Deposits of copper in the schists have long excited interest and led to mining operations. The amount of ore, however, appears not to have justified any considerable work.

Near the base of the Catoctin Mountain, where it is first approached by Goose Creek, marble of an excellent quality is found but has been little worked. Among the varieties at the quarry are included pure white, white and pink, blue and white, white and green, serpentinized and chloritic serpentinized marble. These marbles are of great beauty and susceptible of a good polish. The calcareous bed here is about fifty feet thick and reaches southward for three miles with increasing thickness. At its southern end it is not entirely metamorphosed into marble, but retains its original character of fine blue limestone. Northward along this range the thickness of the marble constantly diminishes and rarely exceeds ten feet. Sometimes there are two beds, sometimes only one. At Taylorstown, just south of the Potomac, the bed is about three feet thick; on the north side of the Potomac about four or five feet. Here, as elsewhere, the beds of marble are inclosed in a bluish green micaceous schist, which has been thoroughly transformed by mechanical pressure.

In the vicinity of Leesburg and north of that town, and between the Catoctin Mountain and the Potomac River, the conglomerate limestone or brecciated marble is found in abundance, associated with red shale. It is a calcareous rock, apparently formed in part of pebbles cemented together and, when burned, produces an inferior lime. It is commonly known as Potomac marble. Of this variegated marble were formed the beautiful columns in the old Representatives' chamber of the Capitol at Washington. The soil in which this rock occurs is extremely productive and valuable.

The exhibition at the World's Fair, at New Orleans, of the following specimens of Loudoun minerals claimed much interest from visiting mineraloguists: