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.