250 THE BASIN OF EASTERN PERSIA AND SISTAN. 
ment. On the fine gravel, however, they had no difficult)'. The junction between 
the gravel and the silt is ven' 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 ven,' 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 ver)- 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 ba.sin deposit 
come to an end and the creep from the mountain slopes begins. 
The basin of Selabad, 60 miles southeast of Birjand, is of much tlie same 
character. The center of this basin is not occupied by fields, but b)- 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 
w-a.ste 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 e\en- 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 fonn 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 instru- 
ments 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 moun- 
tain 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 ever)- writer on Persia speaks of the astonishing aljundance of gravel. 
The largest gravel plain which I saw was on the northwestern border of tlie lake of 
Sistan. From Beudun to Bering a smooth plain extends toward the southeast 
with a uniform slope so gentle that in 30 miles it amounts to but little more than 
800 feet. From mountains to lake the plain is composed of pebbles of dark lime- 
stone 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 
