THE NAKTSONG-TSO, ITS SOUND, AND ITS WESTERN HALF. 69
high plateaus of northern and central Tibet are either round or elliptical, shallow and uniform, with flat shores and almost always without islands. Here in the south however we have a lake. that is remarkably peculiar and fantastic in outline. And that it must indeed be so is clear when we compare the dissimilarities which mark the mountains that shut in the round and oblong lakes and those that encircle the Naktsong-tso. The former are very flat and rounded, and consist almost exclusively of soft powdery disintegration products; and in consequence of the low pitch of their slopes it is not possible for bays, headlands, and islands to originate in the former lakes, at all events none worth speaking about. The latter mountains are, on the contrary, sharply accentuated, with steep or almost vertical sides, and are wild and fantastic in outline, and built up almost entirely of hard rock. Consequently the shape of the lake, being directly determined by the setting, must obviously be capricious and picturesque also. The Selling-tso may be regarded as a form intermediate between these two types. In some districts, for instance beside the big northern peninsula and in the bay of the Jagju-rapga, the lake is capricious in outline, elsewhere however it appears to be in general fairly circular. To this problem we shall however return later on. For the present I will only say, that the differences of relief between the relief forms of the northern mountains and those of the southern mountains are of course caused by the more or less advanced denudation and atmospheric levelling; these processes have advanced farther on the great high plateaus than they have in the south of Tibet. In the latter quarter however glaciation has been more extensively developed than in the north, giving rise to more fantastic superficial forms and lingering longer, whereby the southern mountainous regions have been relatively longer protected against the attacks of the atmospheric agencies. And finally, I would remark, that the reason why glaciation covered a wider area in the south than in northern and central Tibet is that the rainfall there was incomparably greater. Accordingly the factor which has been principally instrumental in determining the shapes, not only of the mountains, but also of the lakes, is the rainfall.
On the 16th September, the last day of our boating excursion before rejoining the caravan on the northern shore, the weather was unfavourable, and this upset my plans. At 7 a. m. the temperature of the air dropped to + 4.1°, while that of the water was 9.6°. The air turned raw and cold, and the sky became covered with clouds to such an extent that it was as dark as twilight. At first we steered towards the N. 85° E., in order to get round a flat cape, with a pool at its base. After that we inclined more and more towards the north-east, making for a rather large, low-lying island. On our left we still had the same mountain-range as before, while on our right the big island, or rather peninsula, receded farther and farther into the background, and the waterway widened out into one of the largest of the open basins in the lake. The water was of a glorious blue-green tint and transparent to a considerable depth. The weather was typically Tibetan. There was a strong head-wind, and the heavy banks of cloud, which hung directly over our heads, kept up for fully two hours such a bombardment of hail that sometimes the storm actually moderated the violence of the waves. After a while the hail passed over into snow, which came down so thick and fast, that the mountains all round and the big
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