Both these rivers come from the south-west: the Argun, or Kerulen as it is called above Lake Kulun (Dalai-nor), through which it flows about half way between its source and Ust-Stryelka, rises in 49° N.
long.; the Shilka is formed by the union of the Onon and the Ingoda, both of which have their sources a little farther north-east than the Kerulen (Argun).
It then deflects south-east till it touches the Kerulen affluent of the Amur river at a point which is shown in unofficial maps as about 117° 30' E.
In the same way the Kentei (or Gentei) Mountains, as they are called, to the north of Urga, and the Yablonoi Mountains of Transbaikalia, separate the higher terrace of north-west Mongolia (drained by the tributaries of the Selenga) from the lower terrace of the Gobi, which is drained by the upper tributaries of the Onon and the Kerulen, both belonging to the basin of the Amur.
The trade is chiefly concentrated at Urga, Ulyasutai and Kobdo in north-west Mongolia; Kalgan, Kuku-khoto, Kuku-erghi, Dolon-nur and Biru-khoto in southern and south-eastern Mongolia; and at Kerulen in the north-east.