Within the confines of the empire are four large chains, some of the peaks in their course rising to stupendous elevations, but the ridges generally falling below the snow line. The first is the Tien shan or Celestial Mountains, called Tengkiri by the Mongols, and sometimes erroneously Alak Mountains. This chain begins at the northern extremity of the Belur-tag in lat. 40° N., or more properly comes in from the west, and extends from west to east between longitudes 76° and 90° E., and generally along the 22° of north latitude, dividing Ílí into the Northern and Southern Circuits. Its western portion is called Muz-tag; the Muz-daban, about long. 79° E., between Kuldja and Aksu, is where the road from north to south runs across, leading over a high glacier above the snow line. East of this occurs a mass of peaks among the highest in Central Asia, called Bogdo-ula; and at the eastern end, near Urumtsi, as it declines to the desert, are traces of volcanic action seen in solfataras and spaces covered with ashes, but no active volcanoes are now known. The doubtful volcano of Pí shan, between the glacier and the Bogdo-ula, is the only one reported in continental China. The Tien shan end abruptly at their eastern point, where the ridge meets the desert, not far from the meridian of Barkul in Kansuh, though Humboldt considers the hills in Mongolia a continuation of the range eastward, as far as the Nui Hing-an. The space between the Altai and Tien shan is very much broken up by mountainous spurs, which may be considered as connecting links of them both, though no regular chain exists. The western prolongation of the Tien shan, under the name of the Muz-tag, extends from the high pass only as far as the junction of the Belur-tag, beyond which, and out of the Chinese Empire, it continues nearly west, south of the river Sihon toward Kodjend, under the names of Ak-tag and Asferah-tag; this part is covered with perpetual snow.