1915-lt^.] BOTANICAL SOCIETY OF EDINBURGH 81 



Broadly speaking, this part of Asia is built up chiefly 

 of granite and slate, with some limestone, occasionally 

 crystalline. Slates commonh' occur in the river beds, and 

 are generally on edge, but metamorphic rocks are also 

 found at 15,000 or 16,000 feet on some of the divides, and 

 perhaps higher still. Similarly granite is usuall}^ found 

 forming the bulk of the ranges {e.g. the Salween-Irrawaddy 

 divide, at least in the south, and parts of the Mekong- 

 Salween divide), but it also crops out both in the Yangtze 

 and Mekong valleys. However, the plain of Hkamti in 

 Northern Burma, between the eastern and western branches 

 of the Irrawaddy, and the mountains to the south 'and west, 

 are composed of sands, gravels, clays, and- conglomerates, 

 with leaf beds and shells ; neai- Myitkyina slates and 

 mica-schists appear, the former in the river bed, on edge 

 as usual, the latter with sands and clays, heaved up in 

 north-and-south-trending ridges from 3000 to 5000 feet 

 high. The dip of these rocks is usually south-east, and the 

 schists give evidence of considerable pressure. 



It is Cjuite evident that the whole of this tract, at least 

 from the Mali-hka westwards to the Assam Hills, M'as once 

 a big lake — it is too big for an estuary, the area under 

 water being about a hundred and fifty miles long by forty 

 or fifty broad ; and we now see how it is that plants have 

 not migrated due east across the Burmese hinterland from 

 the Assam side, but must have travelled to the north-east, 

 and then come down the parallel ranges. At this period 

 the continuity of the Himalayas with the China axis was 

 probably complete, and the parallel ranges probably had 

 no existence, or were only just beginning to appear. 



One of the most peculiar features of the country is its 

 apparent westward tilt, as though it was on an inclined 

 plane. Thus it is found that while the general level of the 

 Mali valley is less than a thousand feet above sea-level (the 

 plain of Hkamti is about 1200 feet), the 'Nmai flows at a 

 higher level, the Sal ween higher again, the Mekong about 

 1500 feet above the Sal ween, and the Yangtze about 

 1000 feet above the Mekong : yet the Yangtze is the 

 biggest river of all, and the Salween a good second, so that 

 the difference of level cannot be set down to erosion, the 

 Mali being the smallest as well as the most sluggish of all. 



