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Rh drain the widest extent of country, is the Monass. Its basin occupies the eastern half of Bhutan. There are four principal Monass affluents: the Matichu, which flows past Tongsu; the Tongchu, Korichu, and Monass. SeYeral passes into Tibet are said to lead up the gorges of these rivers into the basin of the Lopra-cachu. The region of the Monass is unexplored, except by Pemberton, and nothing is known beyond his route. Some of the peaks of the Bhutan Himalaya, eastward of Chumalhari, have, however, been measured from the Assam plain by Mr. Lane. One at the head of the Matichu is 24,737 feet; and two twin peaks, at the head of branches of the Monass, are respectively 20,965 feet and 20,576 feet above the sea.

The above topographical sketch is intended to embrace the regions with which the narratives in the present volume have to do; and to describe cursorily the orography and hydrography of Great Tibet, Nepal, Sikkim, and Bhutan.

Travellers and systematic geographers have devoted a large share of attention to the structure of the great Himalayan mass, and, in my 'Memoir on the Indian Surveys,' I have given some account of the views of the physical character of the Himálaya formed by Herbert, Henry Strachey, Forbes Royle, Cunningham, Thomson, Brian Hodgson, and Hooker. Mr. Hodgson has stated his view of the question with remarkable clearness, and his explanation of the hydrography of Nepal is a masterpiece of lucid description. Mr. Trelawney Saunders has also treated of the whole subject in his 'Memoir of the Mountains and River Basins of India,' and has illustrated his view of Himálayan geography by means of a large diagram which has not been published. But the Himalayan portion of the beautiful and very clear maps of India which illustrate my 'Moral and Material Progress Reports for 1871-72 and 1872-73,' are based upon the large diagram, the first attempt, of which I am aware, to give clear expression to the whole Himálayan system by means of cartographic illustration. Mr. Saunders