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.