Читать книгу History and Comprehensive Description of Loudoun County, Virginia - James William Head - Страница 6

[2] Bruce's Economic History of Virginia.

The patentees, some years afterward, sold the grant to the second Lord Culpeper, to whom it was confirmed by letters patent of King James II, in 1688. From Culpeper the rights and privileges conferred by the original grant descended through his daughter, Catherine, to her son, Lord Thomas Fairfax, Baron of Cameron—a princely heritage for a young man of 20 years.

BOUNDARIES.

The original boundaries of Loudoun County were changed by the following act of the General Assembly, passed January 3, 1798, and entitled "An Act for adding part of the county of Loudoun to the county of Fairfax, and altering the place of holding courts in Fairfax County."

1. Be it enacted by the General Assembly, That all that part of the county of Loudoun lying between the lower boundary thereof, and a line to be drawn from the mouth of Sugar Land run, to Carter's mill, on Bull run, shall be, and is hereby added to and made part of the county of Fairfax: Provided always, That it shall be lawful for the sheriff of the said county of Loudoun to collect and make distress for any public dues or officers fees, which shall remain unpaid by the inhabitants of that part of the said county hereby added to the county of Fairfax, and shall be accountable for the same in like manner as if this act had not been made.

2. And be it further enacted, That it shall be lawful for a majority of the acting justices of the peace for the said county of Fairfax, together with the justices of the county of Loudoun included within the part thus added to the said county of Fairfax, and they are hereby required at a court to be held in the month of April or May next, to fix on a place for holding courts therein at or as near the center thereof (having regard to that part of the county of Loudoun hereby added to the said county of Fairfax) as the situation and convenience will admit of; and thenceforth proceed to erect the necessary public buildings at such place, and until such buildings be completed, to appoint any place for holding courts as they shall think proper.

3. This act shall commence and be in force from and after the passing thereof.

As at present bounded, the old channel at the mouth of Sugar Land run, at Lowe's Island,[3] is "the commencement of the line that separates Loudoun from Fairfax County and runs directly across the country to a point on the Bull Run branch of Occoquan River, about three eighths of a mile above Sudley Springs, in Prince William County." The Bull Run then forms the boundary between Loudoun and Prince William to its highest spring head in the Bull Run mountain, just below the Cool Spring Gap. The line then extends to the summit of the mountain, where the counties of Fauquier and Prince William corner. From the summit of this mountain, a direct line to a point[4] on the Blue Ridge, at Ashby's Gap, marks the boundary between Loudoun and Fauquier counties. A devious line, which follows in part the crests of the Blue Ridge until reaching the Potomac below Harpers Ferry, separates Loudoun from Clarke County, Virginia, and Jefferson County, West Virginia, on her western border. The Potomac then becomes the dividing line between Loudoun County, and Frederick and Montgomery counties, Maryland; "and that State, claiming the whole of the river, exercises jurisdiction over the islands as well as the river."

[3] "What is called Lowe's Island, at the mouth of Sugarland Run, was formerly an island, and made so by that run separating and part of it passing into the river by the present channel, while a part of it entered the river by what is now called the old channel. This old channel is now partially filled up, and only receives the waters of Sugarland Run in times of freshets. Occasionally when there is high water in the river the waters pass up the present channel of the run to the old channel, and then follow that to the river again. This old channel enters the river immediately west of the primordial range of rocks, that impinge so closely upon the river from here to Georgetown, forming as they do that series of falls known as Seneca Falls, the Great, and the Little Falls, making altogether a fall of 188 feet in less than 20 miles."—Memoir of Loudoun.

[4] Designated in an old record as a "double-bodied poplar tree standing in or near the middle of the thoroughfare of Ashby's Gap on the top of the Blue Ridge." It succumbed to the ravages of time and fire while this work was in course of preparation.

This completes an outline of 109 miles, viz: 19 miles in company with Fairfax, 10 with Prince William, 17 with Fauquier, 26 with Clarke and Jefferson, and 37 miles along the Potomac.

TOPOGRAPHY.

Loudoun County is preeminently a diversified region; its surface bearing many marked peculiarities, many grand distinctive features. The broken ranges of hills and mountains, abounding in Piedmont Virginia, here present themselves in softly rounded outline, gradually sinking down into the plains, giving great diversity and picturesqueness to the landscape. They are remarkable for their parallelism, regularity, rectilineal direction and evenness of outline, and constitute what is by far the most conspicuous feature in the topography of Loudoun. Neither snow-capped nor barren, they are clothed with vegetation from base to summit and afford fine range and pasturage for sheep and cattle.

The main valleys are longitudinal and those running transversely few and comparatively unimportant.

The far-famed Loudoun valley, reposing peacefully between the Blue Ridge and Catoctin mountains, presents all the many varied topographic aspects peculiar to a territory abounding in foothills.

The Blue Ridge, the southeasternmost range of the Alleghanies or Appalachian System presents here that uniformity and general appearance which characterizes it throughout the State, having gaps or depressions every eight or ten miles, through which the public roads pass. The most important of these are the Potomac Gap at 500 feet and Snickers and Ashby's Gap, both at 1,100 feet. The altitude of this range in Loudoun varies from 1,000 to 1,600 feet above tide-water, and from 300 to 900 feet above the adjacent country. It falls from 1,100 to 1,000 feet in 4 miles south of the river, and then, rising sharply to 1,600 feet, continues at the higher series of elevations. The Blue Ridge borders the county on the west, its course being about south southwest, or nearly parallel with the Atlantic Coast-line, and divides Loudoun from Clarke County, Virginia, and Jefferson County, West Virginia, the line running along the summit.

Of nearly equal height and similar features are the Short Hills, another range commencing at the Potomac River about four miles below Harpers Ferry and extending parallel to the Blue Ridge, at a distance of nearly four miles from summit to summit, for about twelve miles into the County, where it is broken by a branch of Catoctin Creek. Beyond this stream it immediately rises again and extends about three miles further, at which point it abruptly terminates.

A third range, called "Catoctin Mountain," has its inception in Pennsylvania, traverses Maryland, is interrupted by the Potomac, reappears in Virginia at the river margin, opposite Point of Rocks, and extends through Loudoun County for a distance of twenty or more miles, when it is again interrupted.

Elevations on Catoctin Mountain progressively diminish southward from the Potomac River to Aldie, although the rocks remain the same, and the Tertiary drainage, which might be supposed to determine their elevations, becomes less effective in that direction.

Probably this mountain does not exceed an average of more than 300 feet above the surrounding country, though at some stages it may attain an altitude of 700 feet. Rising near the Potomac into one of its highest peaks, in the same range it becomes alternately depressed and elevated, until reaching the point of its divergence in the neighborhood of Waterford. There it assumes the appearance of an elevated and hilly region, deeply indented by the myriad streams that rise in its bosom.

On reaching the Leesburg and Snicker's Gap Turnpike road, a distance of twelve miles, it expands to three miles in width and continues much the same until broken by Goose Creek and its tributary, the North Fork, when it gradually loses itself in the hills of Goose Creek and Little River, before reaching the Ashby's Gap Turnpike.

The Catoctin range throughout Loudoun pursues a course parallel to the Blue Ridge, the two forming an intermediate valley or baselevel plain, ranging in width from 8 to 12 miles, and in altitude from 350 to 730 feet above sea level. Allusion to the physiography of this valley—so called only by reason of its relation to the mountains on either side—has been made elsewhere in this department.

Immediately south of Aldie, on Little River, near the point of interruption of Catoctin Mountain, another range commences and extends into Fauquier County. It is known as "Bull Run Mountain," but might rightly be considered an indirect continuation of the elevation of the Catoctin, its course and some of its features corresponding very nearly with that mountain save only that it is higher than any of the ranges of the latter, excepting the western.

East of the Catoctin the tumultuous continuity of mountains subsides into gentle undulations, an almost unbroken succession of sloping elevations and depressions presenting an as yet unimpaired variety and charm of landscape. However, on the extreme eastern edge of this section, level stretches of considerable extent are a conspicuous feature of the topography.

Three or four detached hills, rising to an elevation of 150 or 200 feet above the adjacent country, are the only ones of consequence met with in this section.

COMPARATIVE ALTITUDES.

The hilly character of Loudoun is clearly shown by the following exhibit of the elevation of points and places above tide-water. The variations of altitude noted in this schedule are based upon conflicting estimates and distinct measurements made at two or more points within a given circumference and slightly removed one from the other.

				Feet.
Sterling				415
Ashburn				320
Leesburg				321 to 337
Clarke's Gap				578 to 634
Hamilton				454 to 521
Purcellville				546 to 553
Round Hill				558
Bluemont				680 to 730
Snicker's Gap				1,085
Neersville				626
Hillsborough				550
Waterford				360
Mount Gilead				600
Oatlands				270
Little River, near Aldie				299
Middleburg				480
Potomac River, near Seneca Dam				188
Potomac River, at Point of Rocks				200
Potomac River, at Harper's Ferry				246

The whole of the county east of the Catoctin Mountain varies from 200 to 350 feet. The eastern base of the Blue Ridge has an elevation of about 730 feet, and the highest peak of that range in Loudoun rises 1,600 feet above tide-water.

The Short Hills have an approximate altitude of 1,000 feet, while that of the Catoctin Mountain varies from 300 to 700 feet. The valley between the Blue Ridge and Catoctin Mountains varies from 350 to 730 feet in elevation.

From many vantage points along the Blue Ridge may be obtained magnificent views of both the Loudoun and Shenandoah valleys. The eye travels entirely across the fertile expanse of the latter to where, in the far distance, the Alleghany and North Mountains rear their wooded crests. A few of the summits offer even more extensive prospects. From some nearly all of Loudoun, with a considerable area of Fairfax and Fauquier, is in full view. Other more distant areas within visionary range are portions of Prince William, Rappahannock, and Culpeper counties, in Virginia, Frederick and Montgomery counties, in Maryland, and even some of Prince George County, east of Washington City. Westward, the view embraces Shenandoah, Frederick, Clarke and Warren counties, in Virginia, Berkeley and Jefferson counties, in West Virginia, Washington County, in Maryland, and some of the mountain summits of Pennsylvania.

DRAINAGE.

The drainage of Loudoun can be divided into two provinces. One is the Potomac province, which is drained by a system of small tributaries of that stream. Its elevations are quite uniform and are referable to that master stream, whose grade is largely determined by its great basin beyond the "Catoctin belt." The second province is the region drained by smaller streams, chief of which is Goose Creek. In this province the drainage lines head entirely within the "Catoctin belt," and the elevations are variable according to the constitution of the rocks in the belt itself.

The tributaries by which the drainage of the two provinces is effected are Catoctin Creek, North Fork Catoctin Creek, South Fork Catoctin Creek, Little River, North Fork Goose Creek, Beaver-dam Creek, Piney Run, Jeffries Branch, Cromwells Run, Hungry Run, Bull Run, Sycoline Creek, Tuscarora Creek, Horse Pen Run, Broad Run, Sugarland Run, Elk Lick, Limestone Branch, and as many lesser streams.

The general slope of the county being to the northeast, the waters, for the most part, naturally follow the same course, as may be readily perceived by reference to maps of the section. The streams that rise in the Blue Ridge mostly flow to the eastward until they approach the Catoctin Mountain, where they are then deflected more toward either the north or south to pass that range by the Northwest Fork and Goose Creek, or by the Catoctin Creek which falls into the Potomac above Point of Rocks. East of Catoctin Mountain the streams pursue a more or less direct northern course.

Goose Creek, a right-hand branch of the Potomac River, is a considerable stream, pursuing a course of about fifty miles from its source in Fauquier County to its junction with the Potomac four miles northeast of Leesburg. It once bore the Indian name Gohongarestaw, meaning "River of Swans." Flowing northeastward across Loudoun, it receives many smaller streams until passing the first range of Catoctin Mountain, when it claims a larger tributary, the North Fork. Goose Creek represents subsequent drainage dependent on the syncline of the Blue Ridge and dating back at least as far as Cretaceous time. Its length in Loudoun is about thirty miles, and it has a fall of one hundred feet in the last twenty-two miles of its course. It drains nearly one-half the county and is about sixty yards wide at its mouth.

Catoctin Creek is very crooked; its basin does not exceed twelve miles as the crow flies, and includes the whole width of the valley between the mountains except a small portion in the northeastern angle of the County. Yet its entire course, measuring its meanders, would exceed thirty-five miles. It has a fall of one hundred and eighty feet in the last eighteen miles of its course, and is about twenty yards wide near its mouth.

The Northwest Fork rises in the Blue Ridge and flows southeastward, mingling its waters with the Beaver Dam, coming from the southwest, immediately above Catoctin Mountain, where their united waters pass through a narrow valley to Goose Creek.

Little River, a small affluent of Goose Creek, rises in Fauquier County west of Bull Run mountain and enters Loudon a few miles southwestward of Aldie. It pursues a northern and northeastern course until it has passed that town, turning then more to the northward and falling into Goose Creek. Before the Civil War it was rendered navigable from its mouth to Aldie by means of dams.

Broad Run, the next stream of consequence east of Goose Creek, rises in Prince William County and pursues a northern course, with some meanderings through Loudoun. It flows into the Potomac about four miles below the mouth of Goose Creek.

Sugarland Run, a still smaller stream, rises partly in Loudoun, though its course is chiefly through Fairfax County, and empties into the Potomac at the northeastern angle of the County.

In its southeastern angle several streams rise and pursue a southern and southeastern course, and constitute some of the upper branches of Occoquan River.

Perhaps no county in the State is better watered for all purposes, except manufacturing in times of drought. Many of the farms might be divided into fields of ten acres each and, in ordinary seasons, would have water in each of them.

There are several mineral springs in the county of the class called chalybeate, some of which contain valuable medicinal properties, and other springs and wells that are affected with lime. Indeed, in almost every part of the County, there is an exhaustless supply of the purest spring water. This is due, in great part, to the porosity of the soil which allows the water to pass freely into the earth, and the slaty character of the rocks which favors its descent into the bowels of the hills, from whence it finds its way to the surface, at their base, in numberless small springs. The purity of these waters is borrowed from the silicious quality of the soil.

The largest spring of any class in the county is Big Spring, a comparatively broad expanse of water of unsurpassed quality, bordering the Leesburg and Point of Rocks turnpike, about two miles north of Leesburg.

The springs, as has been stated, are generally small and very 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 8–½ 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]

[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]

[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 features from pre-Cambrian time till the present day it furnishes a remarkable and unique example of the permanence of continental form.

With certain qualifications, a summary of the leading events that have left their impress on the region is as follows:

1. Surface eruption of diabase.

2. Injection of granite.

3. Erosion.

4. Surface eruption of quartz-porphyry, rhyolite, and andesite.

5. Surface eruption of diabase.

6. Erosion.

7. Submergence, deposition of Cambrian formations; slight oscillations during their deposition; reduction of land to baselevel.

8. Eastward tilting and deposition of Martinsburg shale; oscillations during later Paleozoic time.

9. Uplift, post-Carboniferous deformation and erosion.

10. Depression and Newark deposition; diabase intrusion.

11. Uplift, Newark deformation; and erosion to Catoctin baselevel.

12. Depression and deposition of Potomac, Magothy, and Severn.

13. Uplift southwestward and erosion to baselevel.

14. Uplift, warping and degradation to Tertiary baselevel; deposition of Pamunkey and Chesapeake.

15. Depression and deposition of Lafayette.

16. Uplift and erosion to lower Tertiary baselevel.

17. Uplift, warping and erosion to Pleistocene baselevel; deposition of high-level Columbia.

18. Uplift and erosion to lower Pleistocene baselevel; deposition of low-level Columbia.

19. Uplift and present erosion.

Along the Coastal plain reduction to baselevel was followed by depression and deposition of Lafayette gravels; elevation followed and erosion of minor baselevels; second depression followed and deposition of Columbia gravels; again comes elevation and excavation of narrow valleys; then depression and deposition of low-level Columbia; last, elevation and channeling, which is proceeding at present. Along the Catoctin Belt denudation to baselevel was followed by depression and deposition of gravels; elevation followed and erosion of minor baselevels among the softer rocks; second depression followed, with possible gravel deposits; elevation came next with excavation of broad bottoms; last, elevation and channeling, at present in progress.

The general structure of the Catoctin Belt is anticlinal. On its core appear the oldest rocks; on its borders, those of medium age; and in adjacent provinces the younger rocks. In the location of its system of faulting, also, it faithfully follows the Appalachian law that faults lie upon the steep side of anticlines.

After the initial location of the folds along these lines, compression and deformation continued. Yielding took place in the different rocks according to their constitution.

Into this system of folds the drainage lines carved their way. On the anticlines were developed the chief streams, and the synclines were left till the last. The initial tendency to synclinal ridges was obviated in places by the weakness of the rocks situated in the synclines, but even then the tendency to retain elevation is apt to cause low ridges. The drainage of the belt as a whole is anticlinal 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 so as the diabase in the same section.

The areas of granite are, as a rule, long narrow belts, and vary greatly in width.

The mineralogical composition of the granite is quite constant over large areas. Six varieties can be distinguished, however, each with a considerable areal extent. The essential constituents are quartz, orthoclase and plagioclase, and by the addition to these of biotite, garnet, epidote, blue quartz, and hornblende, five types are formed. All these types are holocrystalline, and range in texture from coarse granite with augen an inch long down to a fine epidote granite with scarcely visible crystals.

Loudoun Formation.

Among the various Cambrian formations of the Catoctin Belt there are wide differences in uniformity and composition. In none is it more manifest than in the first or Loudoun formation. This was theoretically to be expected, for first deposits upon a crystalline foundation represent great changes and transition periods of adjustment among new currents and sources of supply. The Loudoun formation, indeed, runs the whole gamut of sedimentary possibilities, and that within very short geographical limits. Five miles northwest of Aldie the Loudoun formation comprises limestone, slate, sandy slate, sandstone, and conglomerate with pebbles as large as hickory nuts. These amount in thickness to fully 800 feet, while less than three miles to the east the entire formation is represented by eight or ten feet of black slate.

The name of the Loudoun formation is given on account of the frequent occurrence of all its variations in Loudoun County. Throughout the entire extent of the Catoctin Belt, and especially through its central portions, the Loudoun formation has frequent beds of sandstone, conglomerate, and limestone. The limestones occur as lenses along two lines; one immediately west of Catoctin Mountain, the other three or four miles east of the Blue Ridge. Along the western range the limestone lenses extend only to the Potomac. There they are shown on both sides of the river, and have been worked in either place for agricultural lime. Only the refuse of the limestone now remains, but the outcrops have been extant until recent years. Along the eastern line the limestone lenses extend across the Potomac and into Maryland for about one mile, and it is along this belt that they are the most persistent and valuable. As a rule they are altered from limestone into marble, and at one point they have been worked for commercial purposes. Nearly every outcrop has been opened, however, for agricultural lime. Where Goose Creek crosses this belt a quarry has been opened and good marble taken out, but want of transportation facilities has prevented any considerable development. The relation between marble and schist is very perfectly shown at an old quarry west of Leesburg. The marble occupies two beds in schist, and between the two rocks there is gradation of composition. In none of the western belts are the calcareous beds recrystallized into marbles, but all retain their original character of blue and dove-colored limestone. None of them, however, is of great thickness and none of great linear extent.

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.

A marked change in the thickness of the Weverton sandstone occurs along Catoctin Mountain, the formation diminishing from 1,000 to 200 feet in a few miles. This plainly indicates shore conditions, and the nature of the accompanying change of constituent material locates the direction of the shore. This change is a decrease of the feldspar amounting to elimination at the Potomac. As the feldspar, which is granular at the shore, is soon reduced to fine clay and washed away, the direction of its disappearance is the direction of deep water. Thus the constitution and thickness of the Weverton sandstone unite in showing the existence of land not far northeast of Catoctin Mountain during Weverton deposition.

Aside from this marked change in thickness, none of unusual extent appears in the Weverton sandstone over the remainder of the Catoctin Belt. While this is partly due to lack of complete sections, yet such as are complete show a substantial uniformity. The sections of the Blue Ridge outcrops range around 500 feet, and those of the Catoctin line are in the vicinity of 300. This permanent difference in thickness along the two lines can be attributed to an eastward thinning of the formation, thus, however, implying a shore to the west of the Blue Ridge line. It can also be attributed to the existence of a barrier between the two, and this agrees with the deductions from the constituent fragments.

Newark System.

An epoch of which a sedimentary record remains in the region of the Catoctin Belt is one of submergence and deposition, the Newark or Juratrias. The formation, though developed in the Piedmont plain, bears upon the history of the Catoctin Belt by throwing light on the periods of degradation, deposition, igneous injection, and deformation that have involved them both.

At the Potomac River it is about 4 miles in width, at the latitude of Leesburg about 10 miles in width, and thence it spreads towards the east until its maximum width is, perhaps, 15 miles. The area of the Newark formation is, of course, a feature of erosion, as far as its present form is concerned. In regard to its former extent little can be said, except what can be deduced from the materials of the formation itself. Three miles southeast of Aldie and the end of Bull Run Mountain a ridge of Newark sandstone rises to 500 feet. The same ridge at its northern end, near Goose Creek, attains 500 feet and carries a gravel cap. One mile south of the Potomac River a granite ridge rises from the soluble Newark rocks to the same elevation.

As a whole the formation is a large body of red calcareous and argillaceous sandstone and shale. Into this, along the northern portion of the Catoctin Belt, are intercalated considerable wedges or lenses of limestone conglomerate. At many places also gray feldspathic sandstones and basal conglomerates appear.

The limestone conglomerate is best developed from the Potomac to Leesburg, and from that region southward rapidly diminishes until it is barely represented at the south end of Catoctin Mountain.

The conglomerate is made up of pebbles of limestone of varying sizes, reaching in some cases a foot in diameter, but, as a rule, averaging about 2 or 3 inches. The pebbles are usually well rounded, but sometimes show considerable angles. The pebbles of limestone range in color from gray to blue and dark blue, and occasionally pebbles of a fine white marble are seen; with rare exceptions also pebbles of Catoctin schist and quartz occur. They are embedded in a red calcareous matrix, sometimes with a slight admixture of sand. As a rule the entire mass is calcareous.

The conglomerate occurs, as has been said, in lenses or wedges in the sandstone ranging from 1 foot to 500 feet in thickness, or possibly even greater. They disappear through complete replacement by sandstone at the same horizon. The wedge may thin out to a feather edge or may be bodily replaced upon its strike by sandstone; one method is perhaps as common as the other. The arrangement of the wedges is very instructive indeed. The general strike of the Newark rocks is a little to the west of north, while the strike of the Catoctin Belt is a little to the east of north. The two series, therefore, if extended, would cross each other at an angle of 20 to 30 degrees. The conglomerate wedges are collected along the west side of the Newark Belt and in contact usually with the Weverton sandstone. The thick ends of the wedges along the line of contact usually touch each other. Going south by east the proportion of the sandstone increases with rapid extermination of the conglomerate. The thin ends of the wedges, therefore, resemble a series of spines projecting outward from the Catoctin Belt.

The result of weathering upon the conglomerate is a very uneven and rugged series of outcrops projecting above the rolling surface of the soil.

The ledges show little definite stratification and very little dip. The topography of the conglomerate is inconspicuous and consists of a slightly rolling valley without particular features. It approaches nearer to the level of the present drainage than any other formation, and decay by solution has gone on to a very considerable extent. Where the draining streams have approached their baselevel, scarcely an outcrop of conglomerate is seen. Where the areas of conglomerate lie near faster falling streams, the irregular masses of unweathered rocks appear.

When but slightly weathered the conglomerate forms an effective decorative stone and has been extensively used as a marble with the name "Potomac marble," from the quarries on the Potomac east of Point of Rocks, Maryland. While it is in no sense a marble, yet the different reds and browns produced by unequal weathering of the limestone pebbles have a very beautiful effect.

The thickness of the Newark formation is most uncertain. The rocks dip at a light angle to the west with hardly an exception, and the sections all appear to be continuous. Even with liberal deductions for frequent faults, nothing less than 3,000 feet will account for the observed areas and dips.

Newark Diabase.

Description of the lithified deposits would be far from complete without reference to the later diabase which is associated with the Newark rocks.

These diabases, as they will be called generically, are usually composed of plagioclase feldspar, and diallage or augite; additional and rarer minerals are quartz, olivine, hypersthene, magnetite, ilmenite, and hornblende. Their structure is ophitic in the finer varieties, and to some extent in the coarser kinds as well. They are holocrystalline in form and true glassy bases are rare, rendering the term diabase more appropriate than basalt.

There is greater variety in texture, from fine aphanitic traps up to coarse grained dolerites with feldspars one-third of an inch long. The coarser varieties are easily quarried and are often used for building stone under the name of granite.

These forms are retained to the present day with no material change except that of immediate weathering, but to alterations of this kind they are an easy prey, and yield the most characteristic forms. The narrow dikes produce ridges between slight valleys of sandstone or shale, the wide bodies produce broad flat hills or uplands. The rock weathers into a fine gray and brown clay with numerous bowlders of unaltered rock of a marked concentric shape.

While the diabase dikes are most prominent in the Newark rocks, they are also found occasionally in the other terraces. In the Catoctin Belt they appear irregularly in the granite and schist. Rare cases also occur in the rocks of the Piedmont plain. The diabase of the Newark areas is almost exclusively confined to the red sandstone, and the dike at Leesburg cutting the limestone conglomerate is almost the only occurrence of that combination.

The diabase occurs only as an intrusive rock in the vicinity of the Catoctin Belt. Of this form of occurrence, however, there are two types, dikes and massive sheets or masses. The dikes are parallel to the strike of the inclosing sandstone as a rule, and appear to have their courses controlled by it on account of their small bulk. The large masses break at random across the sandstone in the most eccentric fashion. No dislocation can be detected in the sandstones, either in strike or dip, yet of course it must exist by at least the thickness of the intrusive mass. That this thickness is considerable is shown by the coarseness of the larger trap masses, which could occur only in bodies of considerable size, and also by the width of their outcrops in the westward dipping sandstones. The chief mass in point of size is three miles wide. This mass fast decreases in width as it goes north, without losing much of its coarseness, and ends in Leesburg in a hooked curve. The outline of the diabase is suggestive of the flexed trap sheets of more northern regions, but this appearance is deceptive, since the diabase breaks directly across both red sandstone and limestone conglomerate, which have a constant north and south strike. An eastern branch of this mass crosses the Potomac as a small dike and passes north into Pennsylvania. The diabase dikes in the Catoctin Belt are always narrow, and, while many outcrops occur along a given line, it is probable that they are not continuous.

At Leesburg the limestone conglomerate next the diabase is indurated, its iron oxide is driven off, and the limestone partly crystallized into marble.

Catoctin Schist.

The Catoctin schist is geographically the most important of the volcanic rocks of Loudoun.

Throughout its entire area the schist is singularly uniform in appearance, so that only two divisions can be made with any certainty at all. These are dependent upon a secondary characteristic, viz, the presence of epidote in large or small quantities. The epidote occurs in the form of lenses arranged parallel to the planes of schistosity, reaching as high as five feet in thickness and grading from that down to the size of minute grains. Accompanying this lenticular epidote is a large development of quartz in lenses, which, however, do not attain quite such a size as those of epidote. Both the quartz and epidote are practically insoluble and lie scattered over the surface in blocks of all sizes. In places they form an almost complete carpet and protect the surface from removal. The resulting soil, where not too heavily encumbered with the epidote blocks, is rich and well adapted to farming, on account of the potash and calcium contained in the epidote and feldspar.

Except along the narrow canyons in the Tertiary baselevel the rock is rarely seen unless badly weathered. The light bluish green color of the fresh rock changes on exposure to a dull gray or yellow, and the massive ledges and slabs split up into thin schistose layers. It is quite compact in appearance, and as a rule very few macroscopic crystals can be seen in it.

A general separation can be made into an epidotic division characterized by an abundance of macroscopic epidote and a non-epidotic division with microscopic epidote. These divisions are accented by the general finer texture of the epidotic schist.

The schists can be definitely called volcanic in many cases, from macroscopic characters, such as the component minerals and basaltic arrangement. In most cases, the services of the microscope are necessary to determine their nature. Many varieties have lost all of their original character in the secondary schistosity. None the less, its origin as diabase can definitely be asserted of the whole mass. In view of the fact, however, that most of the formation has a well defined schistosity destroying its diabasic characters, and now is not a diabase but a schist, it seems advisable to speak of it as a schist.

Sections of the finer schist in polarized light show many small areas of quartz and plagioclase and numerous crystals of epidote, magnetite, and chlorite, the whole having a marked parallel arrangement. Only in the coarser varieties is the real nature of the rock apparent. In these the ophitic arrangement of the coarse feldspars is well defined, and in spite of their subsequent alteration the fragments retain the crystal outlines and polarize together. Additional minerals found in the coarse schists are calcite, ilmenite, skeleton oblivine, biotite, and hematite.

Rocks of the Piedmont Plain.

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."[7] 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."[8]