292 PHYSIOGRAPHY OF CENTRAL-ASIAN~DESERTS AND OASES. 
To begin with, the mountains directly bordering it have contributed but 
a small portion of the sediments spread before them. The Amu, for instance, 
brings most of its load from many hundred miles behind the border ranges, whereas 
these border ranges probably had the benefit of rising to give way for sinking of 
most of that load, if the differential movements of sinking plains of deposition 
and rising of worn-down mountains resulted from such a displacement. Another 
deviation from the ideal type is a serious difference in height of mountains from 
east, where they rise to over 20,000 feet, to west, where the Kopet Dagh attain a 
maximum of only 9,000 feet. And this difference appears to have been no less in 
Tertiary time, for no crystallines have yet risen to view in the Kopet Dagh, though 
the Pamir and Tian Shan have weathered to their granite cores. More sediments 
were, therefore, loaded onto the plains in the east, and we find that mountains 
there were uplifted higher and more often than the Kopet Dagh, where only one 
really great erosion cycle appears to have followed the peneplain stage, though 
the second and third uplifts are recorded on a small scale. Besides all this, the 
plains have been seriously warped, a fact evidenced by channels of the Oxus and 
Zerafshan and other streams, probably as a result of the great difference of deposi- 
tion load over different areas. 
Perhaps even more serious than these internal organic peculiarities must have 
been the climatic complications arising from external sea connections. A recent 
connection between the Caspian and Black Seas through the Manitch, north of 
the Caucasus, and a more remote overflow of an Aralo-Caspian Sea through Siberia 
to the Arctic, limit the possibility of climatic calculations based on determinations 
of sea-water areas. The moment an interior sea either overflows or receives an 
influx from some other basin or of tide-water, its surface area ceases to be a function 
of climate. And, on top of all this, the river Don, behaving in the uncertain 
manner of a large aggrading river, may have flowed alternately into the Caspian 
and Black Seas. It is, however, hoped that a more complete study of the past and 
present marine fauna and flora of the Aralo-Caspian Basin may decipher most of 
these remarkable hydrographic variations. 
THE LACUSTRIAN ZONE (ARALO-CASPIAN SEA EXPANSIONS). 
Its lacustrian or marine history, though involved, affects such a large area 
and throws so much light on its climate, especially during archeological time, 
that it becomes of first importance. Russian and other geologists have naturally 
directed special attention to this aspect of the basin, but their work has so far 
been lacking in quantitative results. After the early Tertiary upheavals of its 
peripheral mountain ranges, a Tertiary sea, perhaps at first connected with the 
Mediterranean and Arctic, appears to have shrunken till in late Tertiary time it 
washed the oldest shores indicated on Konshin’s map (fig. 467). From that it withered 
into an Aral and Caspian with shores below those of to-day. This fact, recognized 
first by Davis, is demonstrated on both sides of the Caspian, where valleys dissect- 
ing its high-level Tertiary sediments are contoured up to 200 feet and higher by 
shore-lines of its later Quaternary expansions, but extend down under water or 
