Limits of Plants in North-West Yunnan. 335 
The country is watered by the south-west monsoon blowing 
over the plains of Assam from the Indian Ocean, and deluging this 
part of Asia during the summer months ; consequently we find a 
progressively diminishing rainfall from west to east, as I observed 
both from personal experience in the Salween, Mekong and Yangtze 
valleys respectively and by a comparison of the flora on the inter¬ 
vening ridges. Moreover a geologist would arrive at precisely the 
same conclusion by comparing the peaks of the two ranges, for 
while those on the Salween side shew the curved outline characteristic 
of water erosion (PI. 8, Fig. 2). the crest of the watershed on the 
Yangtze side is capped by extraordinary towers and pinnacles, 
plainly the work of dry denuding agents (PI. 7, Fig. 1); indeed the 
only two snow mountains I saw there were massive flat-topped 
buttresses not unlike the “ nunatacks” of the Arctic, whereas the 
peaks of the Mekong-Salween watershed are most commonly 
pyramids. 
Also, the rains begin earlier on the westernmost ridge than on 
those further east, since the first of the monsoon precipitates itself 
on the great bluff overlooking Assam before crossing the subsequent 
divides. 
The great altitude of the Salween-Mekong divide in this region, 
and the consequent terrific rainfall it receives (for its peaks rise 
higher than any of those further west) determine a comparatively 
low snow-line, which stands at about 16,000 feet. And then comes 
a change. 
The great bulk of the rain falls west of the Mekong, the ridge 
just referred to forming an effective rain-screen to the Mekong- 
Yangtze divide further east. The small precipitation it receives in 
consequence, and the magnificent autumn weather it enjoys, rapidly 
melting the snows of late summer, sufficiently accounts for the 
enormously elevated snow-line on the Mekong-Yangtze divide—I 
believe it to be hardly less than 19,000 feet. When we consider 
that there is eternal snow in New Guinea at 15,000 feet, the 
significance of this may be appreciated; it proves how little mere 
altitude affects the snow-line. 
Sketch-Map to Illustrate Part of the Country Traversed. 
■ ■ • 
Three high mountain ranges run north and south, and of those the western 
and the central act as rain-screens from the S.W. monsoon. The three main 
rivers, represented by double lines, run from south to north. The line of 
stars separates the arid from the rainy regions. The arrows show the 
directions of the winds and rivers. The writer’s route is indicated by a dotted 
line accompanying a continuous line. 
