568 OROGRAPHY OF CENTRAL TIBET.
4616 m. Consequently their mean altitude is 487o m., and that is also of course the mean value for the lowest depressions of the latitudinal valley. The cross-thresholds in this same valley lie, from west to east, at altitudes of 4920, 5026, 4990, 4992, 5116, 5059, 5099, 5085, and 4863 m. Hence their mean altitude, and coincidently therewith the mean altitude of the highest levels of the bottom of the valley, are thus 5017 m. The difference between these two means is therefore only 147 m.; whereas the difference in the absolute altitude between the highest point (5116 m.) and the lowest (4616 m.) amounts to exactly 500 m. Any way this valley is remarkably flat, and the rise from each depression to the nearest cross-threshold is almost always so slight as to be hardly noticeable. The only things that betray the flat undulations of the surface are the nearness or remoteness of the horizon, the presence of the lake depressions, and the existence of watercourses.
The altitudes which Wellby took in the big latitudinal valley which runs south of mine appear to be reliable and may readily be used for purposes of comparison. It is however matter for regret that he has seldom or never determined the altitude of the cross-thresholds, probably because they are in most cases so flat that their positions escaped his notice. If we confine our attention to that part of Wellby's journey which really appears to follow the identically same latitudinal valley, that is to say the stretch from his Camp No. 22 (about 80° E. long.) to Camp No. 8o, and take the mean of fifty-seven altitudes which he obtained, we get as the result 5066 m. The reason why this datum is so much higher than the corresponding part of my valley is that the western half of Wellby's valley lies considerably higher than its eastern half, so that it is to the latter that the comparison ought strictly to be confined. The altitudes of the lakes in that part of Wellby's route which runs exactly south of my latitudinal valley are 4803, 4932, 5087, 4928, 4829, 4904, 5090, and 4800 m. Hence their mean altitude is 4921 m., or not less than 51 m. more than the mean for the lakes in my valley. I am however unable to accept his data in this case, because they are not the measurements of the water surfaces in the more prominent depressions; but some of them are measurements of small sheets of water lying at considerable elevations above the bottom of the valley, and others are the altitudes of his camps, though these were formed, it is true, quite close to the larger lakes, but no doubt at ten or twenty meters, or more, above their surface. Consequently Wellby's measurements of altitude admit of comparison with mine only in the case of those parts which lie between the same longitudes, that is to say the whole of that part of the northern latitudinal valley which I travelled through and that part of Wellby's valley which lies due south of it. Further we have to take into account all the measurements that were made, whether they are those of passes, lakes, or points intervening between the two. The mean of all my fifty-one points is 4912 m., and of all Wellby's thirty-one points 4922 m. It must however be observed, that the comparison does not run exactly on all fours, because not only has Wellby fewer data than I have, but he has given no attention to the cross-thresholds. As this latter would of course have increased his mean somewhat, but as, on the other hand, the inclusion of the altitudes at the lake-levels would have decreased it, we may take the figure quoted as being not very far from
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