A year spent amongst the high mountain ranges of the Yunnan–Tibet frontier, where the Salween, Mekong, and Yangtze Rivers have been pinched together till they now flow parallel to one another for 200 miles in a belt of mountainous country averaging about 75 miles in width, enabled me, while prosecuting my botanical exploration, to ascertain a few facts of geological interest which form the subject of the present paper (see Plates V and VI and Figs. 1–4). The Tibetans of Kham, with more eye for the picturesque than one would have given them credit for, have, with amazing intuition, appreciated the subtle distinction between a land of high mountains and a land of deep valleys, and in their classical writings refer to Tibet under the name of Nam-grog-chi, which, according to Mr J. H. Edgar, may be translated “the land of deep corrosions”; and this it undoubtedly is.