TZ EVOLUTION OF THE ENVIRONMENT OF CENTRAL-ASIAN CIVILIZATIONS. 
process the surface rises slowly during the centuries to form great thicknesses of 
the soil we call loess. Look back again over the region. While the sand, from 
which was separated the dust we have just seen deposited to form loess, lags still 
scores of miles behind in its advance, we see the grassy plains bordered by a sea 
of high sand-dunes. They, too, have been arrested in their overwhelming progress 
by the slight growth of grasses and plants that are compatible with a soil of 
sand, under the slight precipitation near the border zone. Both the loess and 
the dunes grow continually in height. 
We have seen a cycle of geological activity quite different from that which 
takes place on the periphery of a continent where the silts are distributed by ocean 
currents over great submarine areas. Here, on the contrary, the waste from the 
degrading mountains, which was spread by rivers over the plains, is returned by 
the winds to pile up on the piedmont zone, and this is obviously true not only 
of the solids, but of the soluble alkaline and earthy salts as well. 
All this conforms strictly to Richthofen’s theory that loess was a product 
of deflation of desert-surfaces, wind-borne till it found protection on the grass- 
covered zone. Here, however, we see that water intervened as an earlier trans- 
porting agent, and that on the plains evaporation restored to the fine silts the salts 
that had been leached out. 
Let us return to our panorama. It is still that of many thousand years ago, 
and the grassy steppes across all Central Asia teem with herds of wild ruminants 
and horses and other animals that during early glacial and interglacial time were 
common to the Eurasian continent. I will ask the reader to look, at the same 
time, toward the edge of the plains. At short intervals we see streams emerging 
from the mountains through canyons on to the plain, where they spread out evenly 
over large fan-shaped deltas that slope radially outward from the apex at the 
canyon mouth. ‘These are the delta-oases, of which I shall have more to say. 
Casting our eyes along the southern border of the plains, from the Caspian Sea 
eastward we see grassy loess-plains fringing the southern mountains, and filling 
out the great embayments between the spurs of the Tian Shan ranges in the east. 
But everywhere both these plains and the deltas are hemmed in by the sea of dunes. 
During our foreshortened time-scale, our present glance sees also the effects 
of later climatic oscillations. It is perhaps a period of diminishing regional pre- 
cipitation. The zone of vegetation narrows, the scant protecting plant life disap- 
pears from the dunes, and they advance over the edge of the loess-belt, and encroach 
also on the shrinking delta-plains. With a period of renewal of precipitation, 
vegetation resumes its former area, and the loess deposits expand over the dunes. 
The processes which we have reviewed have been operating with fluctuating 
intensity since Tertiary time. The maximum of intensity existed probably as a 
consequence of the glacial period. Glacial epochs were accompanied by swollen 
rivers with broad flood-plains, expansions of the seas with extensive marshes, and 
by great extent of loess-steppes. During interglacial epochs the conditions were 
reversed, and subsequent to the last glacial epoch there began the general trend 
