southeast rising to heights ranging generally between 2000 and 4000 feet above tide-level; and the North Mountain range on the northwest, almost equally high at many points. (2.) The axial line of the Blue Ridge (which consists chiefly of Archæan rocks) has but few gaps through which streams of water can pass. Not a single outlet of any considerable size is found for the waters of the valley through this ridge anywhere between Harper's Ferry on the Potomac and Balcony Falls on the James-a distance of one hundred and fifty miles. The only other water-gaps are the one through which the Roanoke (afterwards the Staunton) River passes towards the southeast, and the narrow, rugged ravines by which the waters of New River (Kanawha) and some of its tributaries run down from the Plateau formed by the bifurcation of the Blue Ridge towards its southwest extremity. But along the northwest side of the axial ridge, throughout the greater part of its extent, we find a large number of short broken ridges and irregular peaks, forming sometimes double, and often triple, lines nearly parallel with the main mountain, and indicating by their position and structure that they were once continuous ridges that have since been fractured and cut into deep gorges, through which small streams of water now run down into the limestone valley below. These broken ridges consist of Primordial rocks. The mountains on the northwest are far less regular and continuous than the main Blue Ridge, and are traversed by numerous water-gaps. Here the Upper Silurian (Medina) Sandstones constitute the material of which most of the ridges are constructed, and the heavy beds are frequently arched or folded, and cut through by ravines of considerable extent and grandeur, like that through which New River makes its way towards the Ohio, or the beautiful arch at Clifton Forge, or the grand "Goshen Pass" between the Chesapeake and Ohio Railroad and Lexington.

(3.) Those who have not visited this section of the State must not imagine that the "valley" is one vast continuous plain like some of the western prairies. It is a land of "hill and dale, of water-brooks and fountains of water." Its limestone and cherty ridges are frequently of such dimensions that in many parts of the world they would be called "mountains;" and where they are cut by the bold and rapid streams that abound here, they present many steep and naked cliffs, sometimes more than two hundred feet in height above the water. Such natural sections present features of great interest to the geologist; and afford important aid in ascertaining the real structure and relative position of the several sub-divisions of this, the most remote age of paleozoic history.

(4.) Any good map of Virginia will show that this valley is AM. JOUR. SCI.-THIRD SERIES, VOL. XVIII, No. 103.-JULY, 1879.

not single, whether viewed lengthwise or crosswise. From a few miles southwest of Winchester to a point nearly opposite Harrisonburg, it is divided into two subordinate valleys, by the Massanutton Mountains-a long belt of ridges of Upper Silurian and Devonian rocks that withstood the denuding agencies that uncovered so many hundreds of square miles of the Lower Silurian limestones. Less extensive ridges also interrupt the continuity on the northwest side; and some of them, like the House Mountain,* across which the accompanying section passes, present striking examples of mountains left in isolated positions by the sweeping away of the once adjacent rocky masses through the powerful denuding agencies of water and ice. That such agencies have operated in this region on an extensive scale will be considered more fully hereafter.

The cross divisions of the valley are marked by the watersheds that determine its drainage. Southwest of Wythe County we find the waters carried off by the Holston into the Tennessee River. Wythe, Pulaski and part of Montgomery, are drained by New River, which runs down from the Blue Ridge plateau, crosses towards the northwest and makes its way to the Ohio. Thus we have "New River Valley." A small portion of Montgomery and nearly all of Roanoke County, are drained by the Roanoke River-one of the three rivers that have cut water-gaps through the Blue Ridge in a southeasterly direction. Next to this "Roanoke Valley comes the upper "James River Valley," occupied by Botetourt and Rockbridge. Extending from the water-shed (crossing near the line between Rockbridge and Augusta) to the Potomac, we find the extensive "Shenandoah Valley."

(5.) Elevations.-At Harper's Ferry, where the Potomac leaves the Great Valley, the height above tide-level is only about two hundred and forty feet; but when we reach the head waters of the Shenandoah, we have arrived at a water-shed having an average height of nearly 1800 feet. Then, in passing on to the south corner of Rockbridge, we come to the "pass" of the James, at Balcony Falls, having an elevation of about 700 feet. The Roanoke Valley has about the same average elevation as that of the James Valley, 1200 feet; but on rising to the margin of New River Valley, near Christiansburg, in Montgomery County, we are about 2000 feet high; and on the southwest margin, at Mount Airy-the summit of the A. M. & O. Railroad-2600 feet. Many points on the Blue Ridge are not higher than this highest part of the great limestone valley. At the Tennessee line the height is less than

1700 feet.

*This is often spoken of as if it were a single mountain-and so it appears to be as seen from Lexington-while, in reality, there are two short parallel ridges nearly a mile apart, cut off abruptly at both ends.

2c Sa

3b

2c

Sa 3b

36 2c 26

2c

26 26

2c 2b 2a 1g 1f le ld 1c 1b 1a SECTION OF SILURIAN FORMATION, ROCKBRIDGE CO., VA.-From S. E. to N. W. 20 Miles.

(b)

Arch. (a) E.

Here, then, we have a plateau, rather than a valley, with an average elevation above the sea of about 1200 or 1300 feet. This is much above the average elevation of the Mississippi Valley. It is in reality a part of the great belt of uplift that constitutes the Appalachian Range, but erosive agencies have stripped it of the greater part of its mountain-making masses. The Blue Ridge, which now forms its southeast border, was once the shore-line of the great primal ocean that covered the Mississippi Valley (including "Appalachia") during the remote ages of geological history.

At present the streams of water in the valley tend towards the southeast margin all the way from the Potomac to Salem, in Roanoke County. This is most strikingly the case in the basins drained by the Roanoke and the James Rivers, thus indicating less elevation on that side than on the other. I think we shall learn hereafter that this is most probably the result of difference in the amount of denudation on the two sides.

This brief summary of the most conspicuous physical features of the Great Valley and its surroundings is deemed sufficient to give the reader a tolerably distinct, though very general view of the present surface formed by the outcropping of the most extensive exposure of Lower Silurian rocks in Virginia. There are other less extensive exposures of the same rocks forming subordinate limestone valleys, but they must be left out of our present discussion.

Geology.-My purpose is to give in the first place a section extending from the Blue Ridge to the North Mountain, embracing some of the Archæan rocks at one extremity, and of the Devonian at the other. The discussion of this, with its divisions and sub-divisions, and some leading peculiarities of each, will, I think, illustrate the geology of this middle part of the State in a manner, and to an extent, not hitherto attempted by any one.

I am indebted to the partial survey of Virginia, made under the direction of the venerable and distinguished geologist, Professor W. B. Rogers, for guidance and aid in my own investigations and for many of the facts contained in this communication. The line of section here given has been carefully explored and re-explored throughout its whole extent, several times. It crosses a portion of the valley not heretofore represented in section, so far as I know; and while it may be regarded, to a certain extent, as typical of this region of the State for some miles on each side of its line, it presents some peculiarities worthy of special notice. These will be discussed in future. For the present a general description must suffice.

The southeast extremity is on the slope of the Blue Ridge beyond Robinson's Gap, and extends one mile past the line

between Rockbridge and Amherst Counties; while the northwest reaches about a mile beyond the crest of the North Mountain to the valley of the Rockbridge Alum Springs, where it cuts the Devonian shales from which the waters of those springs flow. A subordinate ridge of Medina sandstones, however, rises in the valley between the end of the section and the Springs.

The first general division includes the metamorphic and eruptive rocks of the main Blue Ridge. The other general divisions are those adopted by Professor Rogers in his survey of the State (1836-41). Only Nos. I to VII are included. The sub-divisions into which each of these is here divided are my own, and may be regarded as representative (with local modifications), not only of the limestones of the Great Valley, but also of the shales and sandstones of the bordering mountains and outlying ridges on both sides. They are marked, a, b, c, etc., in ascending order, and will be found to correspond with many of the subdivisions given by Professor Dana in his Manual of Geology (ed. 1875).

There is no natural section or gap through the metamorphic and eruptive rocks at this point on the Blue Ridge, but the outcrop is quite distinct, except that of a mass of syenite (E) protruded among the stratified rocks. The crest of the ridge is marked by a heavy bed of syenitic gneiss (or stratified syenite), (b) which might readily be taken for an igneous rock-so greatly has it been metamorphosed. This, with the thinner beds of like composition, and the interstratified slates (c) all dip steeply to the S. E.-or rather S.S.E. Beneath the mass of syenite we find first gneissoid rocks with considerable quantities of epidote and under these, slates and sandstones, all dipping conformably with those above. These are a of the metamorphic group on the section.

Against the upturned edges of these metamorphic strata we find the lowest of the Primordial beds, resting unconformably, and dipping in the opposite direction. Here begins No. I of Professor Rogers's divisions. It might be subdivided into very many alternations of sandstones and shales, but I have preferred to limit the number to seven, that are quite constant in their general features for many miles along the N.W. face of the range. At the grand natural section at Balcony Falls, where the James River passes through the mountain, about fifteen miles S. W. of my line, there is a very interesting exposure of all the divisions here given-similar in relative position, similar in lithological and fossil characters, and having the same general dip.

No. L-The group, No. I, a, as a general rule, has a layer of feldspathic and siliceous conglomerate near the bottom, then