FLORA OF TIBET OR HIGH ASTA. 129 
travellers, he is careful to state) Mongolia and North Tibet 
as elevated plateaux, forming three terraces, separated from 
each other by chains of mountains running from west to east. 
Mongolia, the lowest of these terraces, is from 2000 to 4000 ft. 
above the level of the sea, followed by a second at 10,000 ft., and 
separated from Mongolia by the Nan Shan chain, which is called 
Tsaidam. The third is separated from the second by the Tan La 
chain, and rises to a height of 15,000 ft. This is North Tibet, 
properly so-called ; but Tsaidam is commonly included in Tibet, 
as it partially is in the present paper, though we have few 
plants from that part. The Tibetan plain at 14,000-15,000 ft. 
and upwards above sea-level may be divided into two unequal 
parts by an imaginary diagonal line from the desert of Odontala 
in the north to Tengri Lake in the south. The rivers and 
rivulets to the west of this line drain into numerous, often large 
salt-lakes. To the east of this line, as well as the country 
south of Lhasa, the drainage is to the sea, chiefly to the east by 
the feeders of the Hoangho and the Yangtze and the upper 
Brahmaputra, while a relatively small south-western portion is 
drained by the Sutlej and Indus. The western plain or plateau 
consists of vast wide valleys between lofty parallel mountain- 
chains running from the west to the east, and rising to 20,000 ft. 
and upwards. With few exceptions the mountain-chains are 
naked and arid in the extreme; but some of the higher peaks 
are permanently covered with snow. The valleys and inequalities 
of the plain are largely filled or covered with a deposit called 
loess, the product of erosion, wind-borne to its present position. 
In places the loess deposit is of great thickness, the result of 
centuries of almost incessant dust-storms, and intersected by 
streams and rivers, more especially in North China; and where 
there is sufficient moisture it is exceedingly fertile. The com- 
position of loess varies, but it is more or less calcareous and 
argillaceous and of a friable nature. In some districts it is mixed 
With sand; in others gravel predominates. We sometimes find 
the word loess translated by mud, but it would be mostly dry 
mud within our limits, and loam is perhaps a more intelligible 
rendering of the word. 
CLIMATE *. 
The data concerning the climate of Tibet are very incomplete, 
but sufficient to give an idea of the general characteristics. in 
* Temperatures have throughout been converted to Fahrenheit's thermometer, 
and the + and — refer to the zero, and not to the freezing-point. 
