Petrifaction of Wood.
It will be proper to notice in this place, a petrifaction of wood which is found on the road from Washington to Fredericksburgh, 16 miles from the latter, and four miles north of the court-house in Stafford county. It is remarkable for its size, rather than for any singularity in the composition. It was found by digging away the earth on the side of the road, and appears to have been the trunk of a considerable tree. It is firmly fixed in the ground, and penetrates it obliquely; how far has not yet been ascertained. At the time I saw it about two feet had been exposed. The diameter is about eight inches. Its colour is white, sometimes resembling that of wood. The fibres are well preserved, and so is the general structure. It is much to be desired, that some one would clear it from its bed, and give it entire to one of our mineralogical cabinets.
Geological Features.
Next to the alluvial and secondary formations, as you pass to the west and northwest, are to be found ranges of granite and shistose, and other primitive rocks; interspersed with these may be seen sandstone, clay, slate, quartz, and limestone. Granite ranges were particularly seen in the neighbourhood of Fredericksburgh, crossing the Rappahannock; and in Orange and Albemarle counties, extending nearly to the Blue Ridge. Great quantities of quartz and quartz rock, sometimes covering with their fragments the sides of hills, are frequent. Another, and more interesting rock in the same connexion, is found in Albemarle county. For some time I doubted to what class to refer it. But from its resemblance to the rocks of the east and west mountains near New-Haven, I ventured to call it trap or whinstone. It becomes more abundant as you approach the Blue Ridge, and the granite disappears. On the sides and summit of the mountain, its appearance is more decidedly that of greenstone. In crossing the southwest mountain, the range to which Monticello belongs, and distant from the Blue Ridge about 25 miles, I observed the same rock. Whether this opinion is just, you will be able to decide from the specimens which have been forwarded.
Blue Ridge.
I have repeatedly named the Blue Ridge. It is the first of those long and parallel ranges of mountains, called the Alleghany; and constitutes one of the most prominent features in the geology of the United States. Its height I cannot determine with accuracy. Probably it would not average more than one thousand feet. Its base may extend in diameter from one to two miles; and yet such is the influence it has on the climate, that vegetation on the eastern, is usually two weeks earlier than on the western side. And what is remarkable, this difference obtains, on the former side at least, until you arrive within a few hundred yards of the summit. I crossed the mountain in two places, distant from each other one hundred miles, but observed nothing essentially different in their mineralogy. At one of them called the Rockfish-Gap, on the road from Charlotteville to Staunton, I spent a few hours, and brought away specimens of all the varieties of minerals which I could find. These have been submitted to your inspection. Among them, you will, I think, see greenstone, epidote, and slate more or less allied to the first. These are the most common rocks, and excepting the second, are usually stratified. The epidote is generally associated with quartz, and sometimes is imbedded in it. In some instances it has a porphyritic appearance, and is very beautiful. In others, it is coated with small filaments of a greenish asbestos. Other minerals were found, whose nature I could not so easily determine. I regret exceedingly, that I cannot furnish you with a more complete description of this interesting mountain. That its character is peculiar, or different from the country on either side of it, must be obvious to the most superficial observer. Its principal rock does indeed bear a resemblance to the trap or whinstone of Albemarle county, and yet I think you will say it is not the same. One fact of importance cannot be mistaken; this mountain constitutes the great dividing line between the granite and limestone countries. For you no sooner reach its western base, than the greenstone and epidote disappear; and limestone pervades the country for hundreds of miles in every direction. In all the distance from this mountain to New-Orleans, I did not find a single specimen of granite, or greenstone. This may appear singular, since Mr. Maclure and Professor Cleaveland have a granite range on their maps, immediately west of the Blue Ridge; and even that mountain is on those maps, in some parts of it, covered with the granitic tinge. This may be true. I can answer for only two points of it, and for that part of the country beyond, lying near the main road to Tennessee. In this route I descended almost the whole length of the great valley included between the Blue Ridge on the east, and the north mountain on the west. But in no instance did I meet with specimens of granite; nor west of the Blue Ridge with any prevailing rock but limestone. I know of no reason why the Blue Ridge should not be regarded as the first great dividing line between the granite and limestone countries. The change in the geological formation is so sudden and striking, that it would be difficult for the most careless traveller with his eyes open, not to observe it. The face of nature, he cannot but perceive, wears a different aspect; the air is more cool and lively; even the water which he drinks possesses new properties perceptible to his taste. The inhabitants no longer speak of their "sandstone water;" but every where he hears of "limestone water." Indeed for 800 miles in the direction which I travelled, he tastes no other water. Every spring and every rivulet, is strongly impregnated with carbonate of lime. The vessels in which it is prepared for culinary use, soon become lined with a white calcareous crust. Nor is its taste the only inconvenience experienced by the traveller unaccustomed to it. It often injures the health of a stranger, and covers the surface of the body with cutaneous eruptions.
Limestone country in inclined Strata.
The geological observer has now entered upon a very interesting field. Its great extent, and its wonderful uniformity, give new facilities to investigation. Two divisions of it seem to have been made in nature.
The first is that which includes the limestone lying in INCLINED STRATA. This division extends from the Blue Ridge, to the Cumberland mountain in East Tennessee, a distance in the direction of my route of 500 miles. Of course it includes all the ranges, five in number, of the Alleghany mountains. The strata lie in a course northeast and southwest, the same as the general course of the mountains. The angle which they make with the horizon is very variable, from 25° to 45°. The colour of the rock varies from blue, and pale blue, to gray, or grayish white, frequently it presents a dull earthy appearance. The fracture is more or less conchoidal. Sometimes the rock assumes a different character, and the fracture is uneven, and the texture firm. This last is distinguished from the former, not only by the fracture, but by the colour. It is usually spoken of by the inhabitants as the gray limestone, the colour of the other being usually of a bluish cast. It differs from that also by being less brittle, and possessing the quality denominated by stonecutters, "tough." In consequence of this, and its enduring heat better, it is more frequently used in building than the other. This variety of limestone is not uncommon. Its colour is not always gray, sometimes it is a reddish brown, and sometimes white. Immense quantities of it, possessing either a grayish or reddish brown colour, are found in the vicinity of Knoxville, East Tennessee. One range of it is crossed by every road, passing to the south and east of Knoxville. Its appearance is that of some variegated marbles; white veins penetrate it, and wind through it in every direction. Whether any part of it has a texture sufficiently fine and firm to be wrought to advantage, is yet to be determined. To the eye of a superficial observer, there are many indications that it has. A specimen of very fine white marble, resembling the Italian white, was shown me in Augusta county, Virginia, which was found 15 miles from Staunton, where there is said to be a considerable quantity of it.
Limestone country in Horizontal Strata.