The peculiar features of this river, and its immediate banks, have led to most of the gross misrepresentations respecting the valley in general. The low water sur face of the Monongahela at Brownsville, has been found to amount to 850 feet above high tide in the Chesapeake bay. Estimating the fall in the Monongahela to be 20 feet from the borough of Brownsville to the city of Pius burgh, would produce 830 feet as the elevation of the latter place above the Atlantic Ocean. The apex of the hills in the vicinity of Pittsburg, are within a small frac tion of 460 feet above the low water level of the Monon gahela, Allegany, and Ohio rivers. By these elements we have 460 added to 830, or 1290 feet as the extreme elevation of the hills near Pittsburg above the Atlantic tides. We may, therefore, if the correctness of our pre vious theory is admitted, suppose, that the latter eleva tion was that of this part of the original plain from which the valley has been formed. This elevation evidently declined towards the now mouth of the Ohio, leaving a descent for the waters.
The actual elevation of the present surface of the country adjacent to the great central junction of the Mississippi, with its confluents, is not determined with the same precision with that of Pittsburg : but, from the length of the Mississippi below the mouth of Ohio, a near approximation may be obtained. From the mouth of Ohio to that of the Mississippi, the distance is within a small fraction of 1100 miles, which, at 31 inches to the mile yields 3850 inches, or 320.8 feet, as the entire de pression of the Mississippi river, from the mouth of Ohio to the Gulf of Mexico.
The fall in the Ohio river exceeds that of the Missis sippi, and has been estimated at about 5 inches to the mile, consequently, if we multiply 948 miles, the entire length of Ohio, by 5, we have 4740 inches, or 395 feet, as the amount of descent in that stream ; and, adding 395 to 320.8 feet, we find 715.8 feet as the declivity of the waters from the confluence of the Monongahela and Allegany rivers at the city of Pittsburg, to the Gulf of Mexico.
A very remarkable difference is perceptible between the different elevations of Pittsburg above the Chesa peake Bay and Gulf of Mexico, amounting to more than 125 feet, in favour of the former sheet of water. This apparent phenomenon is explicable on the simplest laws of hydrostatics. The Gulf stream flows from the Gulf of Mexico towards that part of the Atlantic Ocean adjacent to the Chesapeake Bay, demonstrating that a difference of let el must exist between the extremes, sufficiently great to admit a cut rent of considerable velo city. That velocity has never been very accurately de termined; but, if we allow only 13 inch per mile, would yield in 900 miles 112', feet. That the actual fall from Florida channel to the mouth of the Chesapeake Bay, considerably exceeds 100 feet, there can be no rational doubt, when the necessary geographical phenomena arc duly examined.
We may, from the data given, consider the valley of Ohio composed of an inclined plane, chequered by the deep channels of the rivers, hills, and extensive flats, the whole resting upon a base of secondary formation. In many parts of this interesting region, as in the state of Kentucky, the beds of the rivers are vast chasms, rather than valleys, in the true acceptation of the latter term. The two opposing slopes exhibit some very curious con trasts. Though considerably most extensive, the south eastern slope has no extensive remains of the ancient plane ; the north-western slope, on the contrary, con tains immense remains of the former plane. The rivers which enter the main recipient from the former, rising in or near the north-western ridge of the Appalachian Mountains, are precipitous from their sources, flowing, as already observed, in very deep channels: whilst those streams which enter from the north-western slope, rise out of a level continuous plain, in many places morass, as in part of the states of Ohio and Indiana. The latter
rivers are sluggish towards their sources, gaining velo city in their advance towards the Ohio river. .
The secondary rivers or the south-eastern slope, arc, the Monongahela, Little Kenhawa, Great Kenhawa, Sandy, Licking, Kentucky, Greene, Cumberland, and Tennessee. Those flowing from the north-western slope, are, the Allegany, Beaver, Muskingum, Hoek hocking, Sciota, Miami, and 1Vabash. Of these streams, the Allegany rises in Pennsylvania, flows into New-York, and winding from north-west to west, and south-west, again enters Pennsylvania, forming the north-western, or lather northern constituent of Ohio. No branch of the Allegany flows from the high table land ; the region it waters is in some parts mountainous, and every where hilly ; the sources of the Allegany flow, however, from the highest point of the valley of Ohio. Advancing westward from the region watered by the Allegany, the sources of the Beaver exhibit the commencement of the great central plain, which separates the two great basins of the Mississippi and St. Lawrence. This plain stretches westward, and, widening in extent through the states of Ohio, Indiana, and Illinois, reaches the Mississippi river. In its natural state, the valley of Ohio was, in the great est part of its extent, covered by a dense forest: but the central plain presented a very remarkable exception. As far east as the sources of 'Muskingum, commenced open savannahs, covered with grass, and devoid of tim ber. Similar to the plain itself, those savannahs expand ed to the westward, and on the waters of Illinois river opened into immense natural meadows, generally known under the denomination of prairies.• We have dwelt with more minuteness on this central plain than we would otherwise have done, from a con viction that it affords phenomena highly illustrative of the peculiar structure of that part of North Ame rica. It has been shown in this article, that the surface or Ohio, at the city of Pittsburg, was 830 feet above the nearest tide water of the Atlantic Ocean, and 715.8 feet above the surface of the gulf of Mexico. Lake Erie has been found, by actual and careful admeasurement, 565 feet above the tide water of the Hudson at Albany ; conse quently, the Ohio at Pittsburg is 265 feet above the sur face of Lake Erie ; the intermediate distance, in a direct line, 105 miles. Therefore, if a channel was open from the Ohio river at Pittsburg to Lake Erie, as deep as the bottom of the liver, and sloping towards the lake by gradual descent to its surface, the waters or Allegany and NIonongahcla would, in place of flowing down Ohio, rush into Lake Erie with the enormous velocity arising from a fall of 265 feet in 105 miles ; or upwards of 21 feet per mile. When these mathematically established facts are contrasted with the circumstance, that the ex treme north-western waters of Allegany, those of Lake Chataughque, rise within three miles from Lake Erie, the singular but real construction of the country becomes apparent. Nothing but actual measurement could render credible, that Pittsburg, situated at the confluence of two streams, each of which, following their meanders, have flown above 200 miles ; and one of which, (Allega ny,) having part of its source so near the very margin of Lake Eric ; and yet, that the city of Pittsburg should be elevated 265 feet above the surface of the lake.