Asia is the continent of plateaux which culminate in the Steppes of the Pamirs, appropriately called by their inhabitants "the Roof of the World." To the east of these, four great mountain ranges run, roughly, along the parallels of latitude--the Himalayas to the south, the Kuen-Iun, Thian Shan, and Altai to the north.

Between the Himalayas and the Kuen-lun is the great Plateau of Tibet, which runs into a sort of cul-de-sac at its western end in Kashmir.

Between the Kuen-lun and the Thian Shan we have the Gobi Steppe of Mongolia, running west of Kashgar and Yarkand; while between the Thian Shan and the Altai we have the great Kirghiz Steppe.

It is clear that only two routes are possible between Eastern and Western Asia: that between the Kuen-lun and the Thian Shan via Kashgar and Bokhara, and that south of the Altai, skirting the north of the great lakes Balkash, Aral, and Caspian, to the south of Russia.

The former would lead to Bassorah or Ormuz, and thence by sea, or overland, round Arabia to Alexandria; the latter and longer route would reach Europe via Constantinople.
Communication between Southern Asia and Europe would mainly be by sea, along the coast of the Indies, taking advantage of the monsoons from Ceylon to Aden, and then by the Red Sea.