448 A NEW JOURNEY SOUTHWARDS.
This day too hard rock was remarkably rare, despite the fact that we had crossed over a not inconsiderable mountain-system. But then it is composed almost entirely of disintegrated material, the ruins of a former mountain-range, which, as time goes on, is more and more approaching the state of complete dissolution and destruction. Its flat summit is already everywhere exposed to the attacks of erosion; its slopes are scored in every direction by gullies, like new ulcers and gaping sores. It was for this reason that we found it culminating, not in a crest, but in an entire system of parallel ridges. And every one of them now falls so much more easily a prey to the transporting power of the water in that erosion attacks them from all sides and is eating its way in deeper and deeper. Precipitation, frost, wind, and flowing water, all cooperate to wear down the heights more and more, paring off one layer after another from the surface and subsequently depositing the loose material in the bottoms of the glens on both sides of the range. And in this way the powers of denudation and down-levelling have been operative for thousands upon thousands of years. At the present period the Tibetan plateau presents a picture of inconceivable flatness — a rolling highland region, with long, sweeping undulations. Upon it is accumulated the material which has been brought down from the ancient mountain-ranges, and it is upon this alone that the power of erosion works its will. With regard to the hard rock, we have seen that the work of destruction is for the most part already completed. All that now uplifts itself above the ruins is an occasional bare crest. In the peripheral regions of the plateau bare rock plays on the contrary the predominant rôle ; while farther south, in the central parts of the highlands, we shall frequently come into contact with backbones of hard rock. Hence it would be untrue to say, that the gigantic hollow which stretches between the Kwen-lun and the Himalaya is everywhere uniformly filled with disintegration products, for the statement is only correct of the northern half of Tibet, and more particularly of the broad zone south of . the Arka-tagh. The equally broad zone which lies along the northern foot of the Himalaya is in a high degree unlike the northern zone. For one thing, its altitude above sea-level is considerably less; then it drains to the ocean, and is richly clothed with grass and steppe vegetation. In vol. IV we shall find ourselves in the northern part of this latter zone, that is to say in Central Tibet.
During this stage the only rock that I observed was gypsum, cropping out of the eroded terraces in light-coloured slabs and flakes in a single small glen. In this same glen, which ends at the salt lake, there was dark-grey schist and sandstone dipping 73° towards the N. 3o° E.; all the gravel that we met with during the day consisted of the same formation, though this was the only place where it showed
as solid rock.
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