270 The Land of Deep Corrosions 
13,000 feet in June and at 12,000 feet in November, so that 
there is good reason to think that the snow-line on this side 
does not exceed 16,000 feet. 
For these reasons I regard the Mekong-Salween water- 
shed as having been raised—at all events to its present very 
considerable elevation—subsequent to the Mekong- Yang-tze 
watershed, and this at once suggests that the lateral pressure 
which I assume to have squeezed the entire region into its 
remarkable form came from the west, not from the east, 
since the last ridge to be raised up, granting a certain 
degree of rigidity to have been attained further east, would 
naturally be on the side from which the pressure acted. 
It is not unlikely that a comprehensive survey of the 
distribution of plants in Western China, particularly of the 
truly alpine species, together with a careful comparison 
of the floras on the dividing ridges, would lead to useful 
results as regards the history of these rivers. It is an 
interesting point of view from which to look at the problem, 
but we need a much more extensive knowledge of the facts 
than we have at present, before drawing definite conclusions, 
though certain broad principles are so strikingly illustrated 
that they are worth referring to. 
The outstanding feature of the alpine flora is that it 
is essentially—indeed entirely—a North Temperate one, 
agreeing with Drude’s Northern Floral Region. 
Characteristic orders which are well represented are 
Ranunculaceae, Papaveraceae, Rosaceae, Saxifragaceae, 
Umbelliferae, Compositae, Primulaceae, Gentianaceae, 
Scrophulariaceae, and Liliaceae, while the following genera 
present a richness of species I have never seen equalled 
elsewhere; Rhododendron, Gentiana, Saxifraga, Meconopsis, 
Primula, Pedicularis, and Corydalis. The formations charac- 
teristic of the Northern Glacial Zone and the Northern 
Zone of cold winters (i.e. the Arctic, and large parts of 
Europe) are also typical of these mountains, tundra being 
represented by the high alpine flora with ‘cushion’ plants, 
forest by conifers and catkin-bearing trees, grass-land by 
alpine meadow and turf, heath by dwarf rhododendron. 
The vegetative season lasts about four months, from May 
to August on the Mekong-Salween divide, and from June 
to September on the Mekong-Yang-tze divide. 
