decreasing as a rule from north to south, so that the ninth latitudinal valley lies only 173 m. lower than the crest of Arka-tagh III and 225 m. lower than the crest of Arka-tagh IV. If now we pass beyond the Koko-schili system, we shall find that the differences of altitude are still less when we contrast the latitudinal valley between these two ranges with the two ranges themselves, for it lies only 131 m. lower than the northern range and 95 m. lower than the southern. On the other hand my latitudinal valley forms an exception, for it lies no less than 35o m. lower than Arka-tagh IV and 226 m. lower than Koko-schili I. In the table to which I have just referred, we see that the Tschimen valley and the Kum-köl valley are relatively very deeply sunk between their containing mountains; and the same thing
is true, though to a far less extent, in my latitudinal valley of 1896, and to an even less extent still in Wellby's latitudinal valley. These four latitudinal valleys may therefore be pronounced as of the first rank, and constitute striking and relatively deep trenches between the various mountain-systems of northern Tibet, whereas the other latitudinal valleys merely separate the individual ranges one from another. It is very interesting to observe, that these four valleys, taking them from north to south, grow increasingly shallower in relation to their containing ranges; that is, the difference of altitude betwen the bottom of the valley and the nearest range grows increasingly smaller. The only exception is the Kum-köl valley, which lies 131 m. lower in relation to Arka-tagh I than the Tschimen valley does in relation to the Tschimen-tagh. And if we take the mean of all the altitudes which I obtained in the Kum-köl basin, then the law just enunciated holds good also of that valley. That an association of this relative character must indeed exist is rather what might be expected. The reason why the two southern valleys are so slightly trenched is explicable by the advanced stage of the denudation on the adjacent mountains.
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