The Appalachian table-lands fade gradually into the Mississippi Valley. Their distinct character continues to near the borders of that stream where it unites with the Ohio. As we come upon the table-land system of the Cordilleras, soon after we pass west of the Mississippi, this great valley may be considered as made up of the table-lands of two great mountain systems, with only a relatively small area of alluvial matter between the mouth of the Ohio and the Gulf. Unlike the Ganges, the Amazon, and most other great rivers of the first class, the Mississippi River has a small delta section: not over twenty to thirty thousand square miles has this character. By far the greater part of the basin is really table-land, and is thus free from the evil of low countries to a degree equalled by no other very great river basin. Its valley is characteristically a table-land valley, with a general surface of rolling plain, varying from three hundred to five thousand feet above the level of the sea. Outside of the Cordilleras and the Appalachians, this valley has few mountain folds within its ample space. The absence of included mountain systems is almost as noteworthy a feature as the small amount of delta. There are only two or three patches of mountains that lie far beyond the limits of the great mountain systems of the east and west; and only one of these, the Ozark Mountains of Missouri and Arkansas, is at any distance from the main ranges. This is an insignificant group of low hills having considerable geological but no geographical importance. On the western border of the Mississippi Valley rise the vast ridges of the Cordilleras. This great mountain region is, next after the mountainous area of Central Asia, the most extensive region of great altitude in the world. From Mexico northward this system of mountains widens, until, in the parallel of forty degrees, it has a width of about one thousand miles. This system is made up of many ridges lying upon an elevated table-land. The valleys of the lesser streams are generally over seven thousand feet above the sea; the main peaks, to the number of many hundred, rise over twelve thousand feet above the sea level; many of them attain to about fourteen thousand feet of altitude. Its table-land extends east to near the Mississippi River. The great height and width of this mountain system produce a very marked effect upon the climate of the vast area that it incloses, and upon the country which lies within a thousand miles to the east of its mountain walls. The winds from the Pacific are to a great extent drained of their moisture in the western or Sierra Nevada section of these mountains, and have little moisture to give to the central and eastern chains; and when these winds emerge on to the western plains, they are as dry as those that blow over the Sahara.