Inside the Zagros and Elburz ranges, bounded on the north and west by those mountain-lines, on the east by the ranges of Suliman and Hala, and on the south by the coast-chain which runs from Persia Proper nearly to the Indus, lies a vast tableland, from 3000 to 5000 feet above the sea-level, known to modern geographers as the Great Plateau of Iran.  Its shape is an irregular rectangle, or trapezium, extending in its greatest length, which is from west to east, no less than twenty degrees, or above 1100 miles, while the breadth from north to south varies from seven degrees, or 480 miles (which is its measure along the line of Zagros), to ten degrees, or 690 miles, where it abuts upon the Indus valley.  The area of the tract is probably from 500,000 to 600,000 square miles.

It is calculated that two thirds of this elevated region are absolutely and entirely desert.  The rivers which flow from the mountains surrounding it are, with a single exception—­that of the Etymandrus or Helmend—­insignificant, and their waters almost always lose themselves, after a course proportioned to their volume, in the sands of the interior.  Only two, the Helmend and the river of Ghuzni, have even the strength to form lakes; the others are absorbed by irrigation, or sucked up by the desert.  Occasionally a river, rising within the mountains, forces its way through the barrier, and so contrives to reach the sea.  This is the case, especially, on the south, where the coast chain is pierced by a number of streams, some of which have their sources at a considerable distance inland.  On the north the Heri-rud, or River of Herat, makes its escape in a similar way from the plateau, but only to be absorbed, after passing through two mountain chains, in the sands of the Kharesm.  Thus by far the greater portion of this region is desert throughout the year, while, as the summer advances, large tracts, which in the spring were green, are burnt up—­the rivers shrink back towards their sources—­the whole plateau becomes dry and parched—­and the traveller wonders that any portion of it should be inhabited.

It must not be supposed that the entire plateau of which we have been speaking is to the eye a single level and unbroken plain.  In the western portion of the region the plains are constantly intersected by “brown, irregular, rocky ridges,” rising to no great height, but serving to condense the vapors held in the air, and furnishing thereby springs and wells of inestimable value to the inhabitants.  In the southern and eastern districts “immense” ranges of mountains are said to occur; and the south-eastern as well as the north-eastern corners of the plateau are little else than confused masses of giant elevations.  Vast flats, however, are found.  In the Great Salt Desert, which extends from Kashan and Koum to the Deriah or “Sea” in which the Helmend terminates, and in the sandy desert of Seistan, which lies east and south-east of that lake, reaching from near Furrah to the Mekran mountains, plains of above a hundred miles in extent appear to occur, sometimes formed of loose sand, which the wind raises into waves like those of the sea, sometimes hard and gravelly, or of baked and indurated clay.