(1) The outer or foot hills, adjoining the plains of India.
(2) The central belt lying between these hills, and—
(3) The uplands immediately under the high snow ranges on the Tibetan frontier.
The first zone includes the whole of the outer ranges for a depth of twenty to thirty miles. They rise sharply and abruptly out of the plains and are cut into deep valleys or gorges by rivers liable to sudden floods, sometimes rising 40 feet to 50 feet. The annual rainfall is excessive, from 200 inches to 250 inches, and the hill-sides are densely clothed with vegetation, forests as well as undergrowth. In this zone the valleys are unhealthy and very feverish in the rainy season, hot and steamy, while the higher hills rising in places to an altitude of 12,000 feet are cold, wet and misty.
The second, or central zone, consists mainly of valleys of an elevation varying from 3500 feet to 10,000 feet which, with their dividing ridges, extend northward behind the first zone for about forty miles. These valleys are healthy, comparatively broad and flat, with a moderate rainfall and fairly well populated and cultivated. They have not yet been worn into the narrow gorges so noticeable in the outer hills, and still bear evidence of their glacial origin; the rise of the rivers is moderate, according to the marks on the banks not more than four feet to six feet, and the slopes of the mountains are much more gradual and to a certain extent, cultivated.
The third zone comprises the high valleys, of an elevation of 12,000 feet to 18,000 feet, running down from the great northern barrier of snow, with snow ranges between them, the peaks of which attain occasionally a height of 24,000 feet. These valleys are only used for grazing in the summer months, when the hardy Bhutan cattle are taken up to 12,000 and 14,000 feet and yaks and sheep even higher.
This division is particularly noticeable in the tract
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