332 PHYSIOGRAPHY OF CENTRAL-ASIAN DESERTS AND OASES. 
a sufficient general increase of precipitation or even some change that would give 
rise to a larger proportion over the plains would make the desert into grass and 
thus break down the protective barrier. Also a large volume of water would 
penetrate farther out in the desert, a less volume less far, so that the delta with 
its oases, ever since there were any, must have varied in distance from the moun- 
tains, shifting out or back; responding to all greater cycles of climatic change. 
It is also evident that a river whose grade is for roo miles that of a vast silt-made 
plain must have been extremely sensitive to any warping or tilting of its channel. 
But even greater must have been the changes wrought by floods, the ever-shifting 
of distributary systems or even bursting of the whole river out to build elsewhere 
anew. If we could look back through foreshortened geologic time, the Murg-ab 
would appear in course and kind fast changing, a river living through a marvelous 
variety; we might first see it flowing to the ancient Aralo-Caspian, and as that sea 
is cloven into shrinking remnants, and rivers wandering free-ended join, we see 
the Murg-ab now with the Tedjend, now with the Oxus, then shrunken alone and 
ever shifting, with meanders made to break into new straightness; a silt-laden 
flow that coils to burst and glide in some new wandering way; a river which with 
its season’s flood may spread rare water in a wide sheet far out among the dunes 
and from that flood subside into new channels; for millenniums be led far to one 
side, leaving what was garden so by chance transformed to desert. 
Thus were the oases of Merv controlled by Nature’s ways and, though man 
could not prevent the effect of long-changed climate or much alter that of serious 
crustal movement, if it happened, the capricious behavior of delta distributaries 
used by him so stimulated his ingenuity that in time he got them under control. 
The Murg-ab with her silting distributaries proved a costly school, but graduated 
engineers whose works—canals, barrages, water-gates, and meters—were a marvel 
to antiquity. 
An oasis so bountifully favored, and whose civilization was so stimulated 
by trade relations and the natural education forced upon its type, as well as the 
protective isolation of that type, bore a populous and wealthy growth with varied 
culture; a growth that always recuperated rapidly after falling to the power of such 
organized attacks as those of Alexander, Genghis Khan, and Timur. 
The present outlines of bare clay upon the Murg-ab delta are irregularly 
pronged and have the aspect of a change or shrinkage of alluviating area, upon 
whose abandoned parts sand-dunes are drifting. In a general way it resembles 
a long leaf about 30 miles across, with two prongs—the left-hand one longest, 
and main axis pointing northwest to follow a general slope of the Turkoman Trough 
towards the Caspian. The convexity of alluviation is beautifully emphasized by 
its radiation of distributary veining and indicates a permanence of the present 
position of the delta for many centuries. 
At present it is impossible to indicate the whereabouts of Murg-ab’s most 
ancient oascs. Knowing that for some thousand years all Central Asia has been 
undergoing desiccation, our first thought is to look north beyond the limits now 
attained by water. There is, however, no reason to doubt that a climate even 
