The transition limestone is not, however, of frequent occurrence westward of the Alleghany ridge. It appears only in the vallies,[007] and is succeeded by clay-slate and the old sandstone lying almost horizontally. The coal, with the accompanying strata of argillaceous sandstone and shale, are, as far as we have seen, entirely horizontal. The country westward from the base of the Chesnut ridge has an undulating surface. The hills are broad, and terminated by a rounded outline, and the landscape, presenting a grateful variety of fields and forests, is often beautiful, particularly when, from some elevation, the view overlooks a great extent of country, and the blue summits of the distant mountains are added to the perspective. Pittsburgh has been so often described, the advantages and disadvantages of its situation, and the gloomy repulsiveness of its appearance, have been so often and so justly portrayed, that we should not think ourselves well employed in recounting our own observations. The Alleghany and the Monongahela at Pittsburgh, where they unite to form the Ohio, are nearly equal in magnitude; the former, however, on account of the rapidity of its current, and the transparency of its waters, is a far more beautiful river than the latter. Its sources are distributed along the margin of Lake Erie, and a portage, of only fifteen miles, connects its navigation with that of the St. Lawrence. {13} About the sources of the Alleghany are extensive forests of pine, whence are drawn great supplies of lumber for the country below as far as New Orleans. On French Creek, and other tributary streams, are large bodies of low and rather fertile lands, closely covered with forests, where the great Weymouth pine, and the hemlock spruce, are intermixed with beech, birch, and the sugar maple. The great white or Weymouth pine, is one of the most beautiful of the North American species. Its trunk often attains the diameter of five or six feet, rising smooth and straight from sixty to eighty feet, and terminated by a dense conical top. This tree, though not exclusively confined to the northern parts of our continent, attains there its greatest magnitude and perfection. It forms a striking feature in the forest scenery of Vermont, New Hampshire, and some parts of Canada, and New York; rising by nearly half its elevation above the summits of the other trees, and resembling, like the palms of the tropics, so beautifully described by M. De Saint Pierre, and M. De Humboldt, "a forest planted upon another forest."[008] The sighing of the wind in the tops of these trees, resembles the scarce audible murmurings of a distant waterfall, and adds greatly to the impression of solemnity produced by the gloom and silence of the pine forest. In the southern parts of the Alleghany mountains, pines are less frequent, and in the central portions of the valley of the Mississippi, they are extremely rare.