REVIEW

What pre-eminently characterises the Duab mountain landscape is the regularity of the phenomena of erosion and the preservation of all the forms of young deposits. Nobody can fail to observe the huge terraces of the Zarafshan, nor the deep canyons cut through them by lateral streams. The lack of atmospheric moisture and the resulting absence of an evenly spread covering of grass or forest is responsible for this state of things. During summer and autumn the great rivers consist of almost nothing but glacier water, a fact which, by the way, sufficiently betrays the great extent of glaciation. The amount contributed by rain or springs during the drier half of the year is very small indeed. Therefore these river systems are made up of comparatively few lines running through barren matter. The symbol is a tree with long, gaunt ramifications, the ends of the stronger branches being glaciers, the leaves representing snow patches. With this let us contrast the northern slope of Alpine Europe, in which case the river tree must be compared to a succulent growth of innumerable capillaries filling up and draining the spaces between the trunk and branches, and between all the branches. The main channels appear embedded in a network of veins permeating the substance through which the rivers are traced. The Duabic river, on the other hand, sends out bare and lanky shoots interlaced only towards the top, like the thorny plants of the steppe. A formula by which we can express these different conditions is one giving the permanent points of confluence on a given surface.