Many of the streams which ultimately come to the Atlantic rise in the western ridges of the Alleghany chain, and traverse its longitudinal valleys before leaving the mountains to cross the Atlantic slope, which terminates in a precipitous ledge for 300 miles parallel to the range. By falling over this rocky barrier in long rapids and picturesque cascades they afford an enormous and extensive water-power; and as the rivers are navigable from the Atlantic quite across the maritime plains, these two circumstances have determined the location of most of the principal cities of the United States at the foot of this rocky ledge, which, though not more than 300 feet high, has had a greater influence on the political and commercial interests of the Union than the highest chains of mountains have had in other countries. The Hudson in the north is navigable to Albany; the Delaware and Susquehanna, ending in bays, are important rivers; and the Potomac, which falls into Chesapeake Bay, passes Washington, the capital of the United States, to which the largest ships can ascend. The watershed of the Rocky Mountains lies at a greater distance from the Pacific than that of the Alleghanies from the Atlantic; consequently the rivers are longer, but they are few, and little known; the largest are, the Oregon or Colombia, and the Rio Colorado. The former has its sources not far from those of the Missouri and of the Rio del Norte; and after an exceedingly tortuous source, in which it receives many tributaries, it falls into the Pacific at Astoria. The Colorado is a Mexican stream, which comes from the Sierra Verde and falls into the Gulf of California. The Sacramento with its tributaries, a Californian stream, lying between the two, and much inferior to either, has been brought into notice of late from the extensive and rich auriferous country through which it flows in its course to the Bay of San Francisco on the Pacific. On the table-land of Mexico there is a basin of continental streams, which, rising from springs on the eastern side of the Sierra Madre, and fed by the periodical rains, flow northward and terminate in lakes, which part with their superfluous water by evaporation. Of these, the Rio Grande, which, after a course of 300 miles, falls into the Parras, is the greatest.