ment. On the fine gravel, however, they had no difficulty. The junction between the gravel and the silt is very indefinite, and the two formations appear to merge into one another in many places. Elsewhere, however, the gravel lies over the silt, and we noted here and at Sistan that certain streams were engaged in the process of bringing small gravel and spreading it out in a smooth and very thin sheet upon the silt. Outside the band of finer gravel, the borders of the plain of Tabas are formed of coarser gravel, which increases in size and in the angle of slope of the surface as the mountains are approached. On the very edge the gravel becomes a mere mass of rough, angular fragments of all sizes up to a foot or more in diameter, and it is hard to say where the coalescing fans of the basin deposit come to an end and the creep from the mountain slopes begins.
The basin of Selabad, 6o miles southeast of Birjand, is of much the saine character. The center of this basin is not occupied by fields, but by a salt playa 6 or 8 miles in diameter. On the edges of this are broad deposits of silt, some of which are cultivated. Outside of these are the usual gravel slopes.
The surfaces of these tiresome, gently sloping expanses of gravel do not lie in one plane, as appears at first sight. Although they owe their origin to a sheet of waste which descends evenly from all parts of the mountains, this waste must first be gathered into valleys. Thus the immediate origin of the gravels which skirt the mountains is the innumerable fans which head in every valley, large or small, and expand outward until they coalesce with their neighbors on either side and merge into the plain of silt at the lower end. Each fan, no matter how flat it may seem, is really part of a cone ; hence the union of many fans must form a series of low swells and faint hollows. In only one way-can this rolling quality be brought to the notice of the eye, at least in the larger basins, without the aid of instruments of precision. This is well illustrated in the northern portion of the Desert of Despair. In looking across a piedmont slope at right angles to the mountains, it often happens that the lower half of some far-away mountain is hidden from view by the seemingly level plain as by a hill. For a mile, or even five, the mountain may be approached without apparent change in its appearance, but at last, slowly and almost imperceptibly, the whole of the distant blue mass is exposed to view, and one realizes that he has reached the arch of a huge flat cone of detritus. Beyond the arch the mountain again half disappears, and then reappears on the next cone, and so on indefinitely. The traveler feels that he is traversing a smooth plain, although his reason tells him that he is crossing a series of broad swells made by the coalescing of great fan-shaped cones of detritus. His aneroid may show that the arch of a fan is one or two hundred feet higher than the hollows on either side.
Almost every writer on Persia speaks of the astonishing abundance of gravel.
The largest gravel plain which I saw was on the northwestern border of the lake of Sistan. From Bendun to Bering a smooth plain extends toward the southeast with a uniform slope so gentle that in 3o miles it amounts to but little more than 800 feet. From mountains to lake the plain is composed of pebbles of dark limestone and slate, coarse and angular near the mountains, well-rounded and small near the lake. It is hard to understand how gravel, even though fine-grained, can