248 THE BASIN OF EASTERN PERSIA AND SISTAN. 
diate valley-sides, is graded, and thus covered more or less deeply with soil. Eastern 
Persia, however, is so arid that the ordinary state of affairs is reversed. All the 
mountains, whether young or mature, are characterized by nakedness. In the 
mountains between Neh and Sistan, for instance, which are now passing from youth 
to maturity, the aridity is so great that the growth of plants is prevented, and free 
play is allowed to the activity of the wind. The result is that the hills are almost 
ab.solutcly free from soil and present a remarkal)lc degree of roughness. On one 
small mountain near Aliabad, for instance, the highly lilted shale of which it is 
composed stands entirely naked, and e\en the cracks between the fragments are 
free from bits of soil. In stnicture, although not in color or texture, the mountain 
suggests a great mass of small quartz crystals tied loosely into enonnous bundles, 
with the {xjints up. Another good example of the influence of aridity is found in 
the neighborhood of Birjand. Here the moimtains are composed of dark volcanic 
rocks, and their main outlines are not shar]i and .steep in outline like tho.se among 
the regions of stratified rock to the south, wliicli have just been described. They 
lie rather in long ridges with rolling crests, rising to a nearly e\en height, but yet 
with considerable variation. The sides rise with a steep but by no means abrupt 
slope, and the drainage is completely established and seems to have become subse- 
quent. In a well-watered country such mountains would be wholly covered with 
soil and vegetation, and their rounded outlines and graded slopes would leave no 
doubt that they were in the stage of maturity. In the arid region of Birjand, 
on the other hand, but few of the slopes are graded, vegetation is \ery rare, naked 
rock is as prominent as soil, and many of the small valleys have precipitous sides. 
Everjwhere the same phenomena appear. Most of the mountains are rocky and 
jagged, with numerous small, sharp peaks and little points ; and ven' often a small 
hill, which forms but a tiny island in a plain of gravel, still retains the mggedness and 
sharpness of outline of an Alpine peak. Graded slopes are not a feature of maturity 
in an arid climate, for the mountains may be nuich reduced in height, the drainage 
may become completely adjusted to the strata, and all the other characteristics of 
maturity may be de\-eloped before the graded condition makes its appearance. 
Mature basins. — In the maturity of an arid mountain region the basins are the 
most important and characteristic feature. In the Persian basins the rock floors are 
hidden far from sight ; the deposits of Tertiary- age which were first laid down 
in them are also invisible for the most part. Only the deposits which are now in 
process of fonnation contribute largely to the geographic appearance of the country- 
as it exists to-day. In Eastern Persia these modem deposits are in part aqueous and 
in part seolian. The latter vary little in texture, and consist largely of fine sand, cover- 
ing the drier plains and sometimes mantling the leeward side of the hills. Their 
most remarkable development is at Sistan (Plate 4 and fig. 169), where the violent 
winds move the sand with phenomenal celerity and heap it into dunes of great height, 
which are to-day fast encroaching on areas of gravel and silt. The aqueous deposits, 
on the other hand, var\' from the finest clays of lacu.strine deposits, through the silts 
and sands of plazas, to coarse gravel and bowlders in the huge piedmont fans and in 
the mountain valleys. The basin deposits seem to occur ahnost invariably in one 
