530 THE FRESH-WATER LOCHS OF SCOTLAND 
sixteenth centuries this overflow actually occurred, and it would 
appear that this was a period of comparatively high rainfall in 
Eastern Europe and Western Asia. As far as may be judged from 
the evidence that has been collected, it seems that from about 400 b.c. 
to 400 A.D. the climate of the Aralo-Caspian basin was damper and 
cooler than now. From 400 to 800 a.d. there was a transition to a 
warmer or drier epoch than that of to-day ; this was succeeded in 
the course of the n^xt few centuries by somewhat damper or cooler 
conditions of the fluvial period, and in turn there has been a change 
to our modern drier period. 
The Caspian Sea and the Aral Sea are salt lakes, which owe their 
saltness to their having been originally part of the ocean, from which 
they were separated, in the opinion of Russian geologists, by under- 
ground movements or warping of the eartVs crust at a comparatively 
recent geological period. These movements, according to Helmersen,^ 
are still in progress, and this has been given as one reason for the 
desiccation of the Central Asiatic area. The Tertiary deposits of the 
north of India show that elevation must have gone on in Central Asia 
continuously during the Tertiary period, and at the present time the 
same process is being steadily continued. It is interesting to note 
in this connection that the western portion of North America has 
similarly been undergoing a steady elevation, and at the same time 
a continued desiccation has been in progress. 
The molluscs living in the waters of the Caspian Sea are very 
much like those living in the Black Sea, and banks of similar shells 
may be traced between the two seas. This and other evidence, 
together with the fact that many salt lakes and marshes are found in 
the district, indicate that the Caspian Sea was formerly connected 
with the Black Sea, and that a great firth running up between Europe 
and Asia stretched completely across what are now the steppes and 
plains of the tundras, till it merged into the Arctic Sea. 
Caspian Sea, the largest inland body of water in the world, 
is 180,000 square miles in extent, and has a maximum depth of about 
3200 feet, which makes it rank as the second deepest lake in the 
world (a sounding of 5413 feet has been taken in Lake Baikal). 
Its basin is naturally divided into three portions, of which the 
northern is the shallowest (maximum depth 120 feet), and is being 
gradually silted up by the deposits of the Volga, the Ural, and the 
Terek. A depression, half of which has a depth of more than 300 
feet and reaches a maximum depth of 2526 feet, occupies the middle 
portion of the sea, and is separated from the southern and deepest 
portion of the Caspian by a submarine ridge, a continuation of the 
main Caucasus range. The Caspian receives the drainage of the 
1 Cited by Geikie, Text -hook of Geology, ed. 2, p. 383, London, 1885. 
