Page:The American Cyclopædia (1879) Volume I.djvu/626

 590 APPALACHIAN MOUNTAINS and naturalists in 1842, by the Profs. Rogers, who were at the head of the geological surveys of Pennsylvania and Virginia, and who had extended their observations into the continua- tions of the chain N. and S. from these states. This paper is still the most complete treatise upon this subject. Prof. Guyot has also given much attention to the physical structure of these mountains, and made careful barometrical measurements of several of their highest sum- mits, both near their northern and southern extremities. The general course of the Alle- ghanies is that of the coast line opposite to them. The sea makes its nearest approach to them at the mouth of the Hudson river, which is only 50 m. from the passage of this river through the Highlands. Thence as far S. as Cape Hatteras, the width of the Atlantic slope gradually increases, till the space between the coast and the Blue Ridge is about 200 m. ; and so it continues to the southern extremity of the mountains. This space is a hilly district, gradually becoming of higher elevation as it extends back from the coast. In New England its average height at the base of the mountains is about 500 ft. above the sea ; in Pennsylvania, 300; in Virginia, 500; and further S. 1,200. From the mountains to the lowest falls of the streams over the edge of the granitic platform, this is for the most part a region of the low- est stratified, metainorphic, and granitic rocks. These lowest falls mark the head of navigation of the streams, and the descent to the lower and more level platform of the upper second- ary and tertiary formations, which in the south- ern states stretch along the coast in a belt sometimes reaching 100 m. in width. The eastern ridges of the chain, rising from their elevated base, do not present the appearance of the height above the sea which they actu- ally reach ; and on their western slope; which stretches far away toward the Mississippi, their height is still more completely lost in the elevated and wide-spread plateau. Between Lake Champluin and Lake Ontario, this west- ern table land is 1,500 ft. above the sea. and from it as a base rise the high summits of the Adirondack mountains. In Virginia and Ten- nessee, as observed by Prof. Guyot, the bot- tom of the valley W. of the Alleghanies is from 1,000 to 2,000 ft. above the sea, and be- yond it for 100 m. W. extends a plateau of 1,500 to 2,000 ft. elevation, traversed by lon- gitudinal ridges. All the cross sections from the eastern edge of the granite present first the slightly undulating profile of the Atlan- tic slope, which is succeeded by the sudden rise to the highest elevation, and this by the wave-like descent and ascent across the valleys and the ridges, and finally terminate in the gradual descent on the western table land. As first pointed out by Prof. Rogers, the same law is found to obtain in this chain and in the Jura mountains, of steepest general slopes toward the east ; but of individual ridges the gentler slopes are toward the east, and the steepest inclina- tions toward the west. In the mid-region of the chain in New Jersey, Pennsylvania, and Mary- land where the breadth is greatest, the height appears to be correspondingly diminished. The summits, valleys, and table land all reach here their least elevation. The highest summits are but little over 2,000 ft. above the sea. Still the barrier between the eastern and western waters is complete ; and no clean cut through the range is anywhere found, excepting that of the Mohawk river in New York, the highest elevation of which is only 400 ft. above the sea. Toward the north and the south from this central portion, the plateau becomes more elevated, as well as the summits that rise up from it. In North Carolina, near the borders of Tennessee, and in the northern part of Bun- combe county, the base of the Black moun- tains, which have been an especial subject of examination by Prof. Guyot, is found to extend from 100 to 150 m. in length, with an eleva- tion of 2,000 ft. Above this many summits are found reaching more than 4,500 ft. high- er, as the Black Dome, the height of which above the sea is 6,760 ft. ; the Balsam- Cone, 6,668 ; the Black Brother, 6,671 ; Cat-tail peak, 6,595 ; Hairy Bear, 6,597, &c. The great ele- vation of this group makes it the culminating point of the system. Mt. Washington in New Hampshire, though found by the measurement of Prof. Guyot to be but 6,288 ft. above the sea, which measurement differs only three feet from that made by the officers of the coast survey, appears much more elevated than the summits of the Black mountains, from its ris- ing from a plateau of not half the height of the base of this group. In the southern part of Pennsylvania other parallel ridges succeed to the Alleghany mountains : Negro mountain, Laurel hill, and Chestnut ridge, each a repeti- tion of the other, at distances about 10 m. apart, and each occupying nearly as great a breadth as the valleys which separate them. The capping of their summits is the con- glomerate rocks, which underlie the coal meas- ures. These strata arch over the crests of the ridges, projecting in bold cliff's, and on each slope dipping beneath the coal measures, which in the valley hills attain their greatest thickness. Thus the same strata appear upon the summits, and in undulating lines pass be- neath the valleys to reappear upon the crest of the next ridge, and so on till, dipping down the western slope of Chestnut ridge, the coal measures spread in nearly horizontal strata over the western portion of Pennsylvania, Maryland, and Virginia. Their lowermost lay- ers reappear as they rise to the surface upon the other margin of the great coal basin, as far into Ohio as Zanesville, and thence along a line extending to the mouth of the Scioto. In the gentleness of the dips of the strata, this west- ern slope presents a striking contrast to the highly disturbed stratification of the Atlantic slope. There the rock formations, nearer the disturbing causes which have elevated the