252 THE BASIN OF EASTERN PERSIA AND SISTAN. 
must shift inward until the center of the basin is reached and the basin is destroyed. 
Tliis must happen in ever)- country, pro\ided the streams are strong enough to reach 
the sea. The divide, in its migration, will come into regions deeply buried in gravel 
and silt, but the streams nuist cut through this in time and reach bed rock. When 
this happens the topography will lose the characteristic forms due to deposition and 
assume fonns detennined by erosion along lines indicated by rock structure and rock 
texture. The whole country will doubtless be deeply shrouded in gravel, the residue 
of decomposition left after the wind has swept away the finer material, l)ut the main 
topographic forms will be determined by the character of the rocks, and will so con- 
tinue until all relief disappears. Therefore, if the center of a basin region is suffi- 
cientlv elevated above the sea, and if the rainfall is great enough so that some streams 
reach the sea, the basin character will eventually be destroyed, the major forms due 
to deposition will disappear, and in old age the arid countrj- will present a topography 
similar to that of a moist countr}-. The chief difference will be that the moist region 
will be reduced to a peneplain deeply shrouded in fine soil and densely covered with 
vegetation, while the arid countrj- will be reduced to a peneplain deeply shrouded 
in wind-swept gravel and almost void of vegetation. 
Under still drier conditions another state of affairs is possible in old age. Sup- 
pose that the countr)- is so arid that no stream is able to reach the sea. The divides 
will migrate until the streams on the two sides have the same grade, and then 
will sink steadily, though exceedingly slowly, in one position. While this is in 
progress, the rainfall will still further diminish becau.se of the lowering of the 
mountains, the streams will grow even shorter, and the heads of the gravel fans will 
rise nearl)- or quite to the divides. At last there will come a time when the land 
forms produced by deposition will dominate the topography of practically the whole 
country. Ckavel fans, e.xtinct playas, and deserts will everywhere prevail, and the 
little rain which falls will so soon be evaporated or sink into the ever-deepening 
gravel that running streams will be practically unknown. Whatever transportation 
of solid matter toward the sea takes place by means of water will be almost infinites- 
imally small, and the whole result will be an immeasurably .slow melting away of 
the country which will not materially affect the surface. If this were the end we 
might conclude that in an extremely arid countiy all the topographic forms of old 
age are due to deposition, with the single exception of the divides, which to a 
certain extent survive as the last remnant of forms due to erosion. 
There is still one factor, however, which we have disregarded. As the power 
of aqueous erosion decreases that of rcolian erosion increases. In a countr)- which 
had reached the stage of old age which has just been described, the wind would pla)- 
an exceedingly important part. It would comminute and strip off the gravels 
on the surface, and then would begin to erode the underlying rock. The forms 
produced would be ver)- different from those of aqueous erosion in detail, but they 
would follow the same guidance of rock-structure and rock-texture. Thus in 
extreme old age the driest country must be reduced to a peneplain, parts of which 
may lie below sea-le\-el where the strata are ver)- soft, and all of which will follow 
the lines of the rock structure. The surface of the peneplain will be strewn with 
fragments of waste which will increase in size in proportion to the aridity. 
