the Uzboi trench for 100 m. from the latter sea. How far they reach up the present courses of the Oxus (Amu-darya) and Jaxartes (Syr-darya) is not known. Hence, it is plain that in late Tertiary, and probably also in Post-Tertiary, times the Aralo-Caspian Sea covered a vast expanse of territory and embraced very large islands (e.g. Ust-Urt), which divided it into an eastern and a western portion, communicating by one or two narrow straits only, such as on the south the Sary-kamysh depression, and on the north the line of the lakes of Chumyshty and Asmantai. More than this, the Caspian was also, it is pretty certain, at the same epoch, and later, in direct communication with the Sea of Azov, no doubt by way of the Manych depression; for in the limans or lagoons of the Black Sea many faunal species exist which are not only identical with species that are found in the Caspian, but also many which, though not exactly identical, are closely allied. As examples of the former may be named—Archaeobdella, Clessinia variabilis, Neritina liturata, Gmelina, Gammarus moeoticus, Pseudocuma pectinata, Paramysis Baeri, Mesomysis Kowalevskyi and M. intermedia, Limnomysis Benedeni and L. Brandti, and species of the ichthyological fauna Gobius, Clupea, and Acipenser; while as illustrating the latter class the Black Sea contains Dreissenia bugensis (allied to D. rostriformis and D. Grimmi), Cardium ponticum (to C. caspium), C. coloratum (to Monodacna edentula), Amphicteis antigua (to A. Kowalevskyi) and Bythotrephes azovicus (to B. socialis).
In the opinion of Russian geologists the separation of the Caspian from the great ocean must have taken place at a comparatively recent geological epoch. During the early Tertiary age it belonged to the Sarmatian Ocean, which reached from the middle Danube eastwards through Rumania, South Russia, and along both flanks of the Caucasus to the Aralo-Caspian region, and westwards had open communication with the great ocean, as indeed the ancient geographers Eratosthenes, Strabo and Pliny believed it still had in their day. This communication began to fail, or close up presumably in the Miocene period; and before the dawn of Pliocene times the Sarmatian Ocean was broken up or divided into sections, one of which was the Aralo-Caspian sea already discussed. During the subsequent Ice Age the Caspian flowed over the steppes that stretch away to the north, and was probably still connected with the Black Sea (itself as yet unconnected with the Mediterranean), while northwards it sent a narrow gulf or inlet far up the Volga valley, for Aralo-Caspian deposits have been observed along the lower Kama in 56° N. lat. Eastwards it penetrated up the Uzboi depression between the Great and Little Balkhan ranges, so that that depression, which is strewn (as mentioned above) with Post-Tertiary marine deposits, was not (as is sometimes supposed) an old bed of the Oxus, but a gulf of the Caspian. After the great ice cap had thawed and a period of general desiccation set in, the Caspian began to shrink in area, and simultaneously its connexions with the Black Sea and the Sea of Aral were severed.
Fauna.—The fauna of this sea has been studied by Eichwald, Kowalevsky, Grimm, Dybowski, Kessler and Sars. At the present time it represents an intermingling of marine and freshwater forms. To the former belongs the herring (Clupea), and to the latter, species of Cyprinus, Perca and Silurus, also a lobster. Other marine forms are Rhizopoda (Rotalia and Textillaria), the sponge Amorphina, the Amphicteis worm, the molluscs Cardium edule and other Cardidae, and some Amphipods (Cumacea and Mysidae,), but they are forms which either tolerate variations in salinity or are especially characteristic of brackish waters. But there are many species inhabiting the waters of the Caspian which are not found elsewhere. These include Protozoa, three sponges, Vermes, twenty-five Molluscs, numerous Amphipods, fishes of the genera Gobius, Benthophilus and Cobitis, and one mammal (Phoca caspia). This last, together with some of the Mysidae and the species Glyptonotus entamon, exhibits Arctic characteristics, which has suggested the idea of a geologically recent connexion between the Caspian and the Arctic, an idea of which no real proofs have been as yet discovered. The Knipovich expedition in 1904 found no traces of organic life below the depth of 220 fathoms except micro-organisms and a single Oligochaete; but above that level there exist abundant evidences of rich pelagic life, more particularly from the surface down to a depth of 80 fathoms.
Fisheries.—No other inland sea is so richly stocked with fish as the Caspian, especially off the mouths of the large rivers, the Volga, Ural, Terek and Kura. The fish of greatest economic value are sturgeon (four species), which yield great quantities of caviare and isinglass, the herring, the salmon and the lobster. The annual catch of the entire sea is valued at an average of one million sterling. Some 50,000 persons are engaged in this industry off the mouth of the Volga alone. Seals are hunted in Krasnovodsk Bay.
Salinity.—The proportion of salt in the water of the Caspian, though varying in different parts and at different seasons, is generally much less than the proportion in oceanic water, and even less than the proportion in the water of the Black Sea. In fact the salinity of the Caspian is only three-eights of that of the ocean. In the northern section, which receives the copious volumes brought down by the Volga, Ural and Terek, the salinity is so slight (only 0.0075% in the surface layers) that the water is quite drinkable, its specific gravity being not higher than 1.0016%. In the middle section the salinity of the surface layers increases to 0.015%, though it is of course greater along the shores. The concentration of the saline ingredients proceeds with the greatest degree of intensity in the large bays on the east side of the sea, and more especially in that of Kara-boghaz, where it reaches 16.3°% (Spindler expedition). The bottom of this almost isolated basin is covered for an area of 1300 sq. m. with a deposit of Epsom salts (sulphate of magnesia), 7 ft. thick, amounting to an estimated total of 1,000,000,000 tons. While the proportion of common salt to sulphate of magnesia is as 11 to 1 in the water of the Black Sea and as 2 to 1 in the Caspian water generally, it is as 12.8 to 5.03 in the Kara-boghaz. The salinity of the surface water of the southern section of the Caspian averages 1.5%.
Climate.—The temperature of the air over the Caspian basin is remarkable for its wide range both geographically and seasonally. The January isotherm of 15° F. skirts its northern shore; that of 40° crosses its southern border. But the winter extremes go far below this range: during the prevalence of north-east winds the thermometer drops to -20°, or even lower, on the surrounding steppes, while on the Ust-Urt plateau a temperature of -30° is not uncommon. Again, the July isotherm of 75° crosses the middle section of the Caspian, nearly coinciding with the January isotherm of 25°, while that of 80° skirts the southern shore of the sea, nearly coinciding with the January curve of 40°, so that the mean annual range over the northern section of the sea is 60° and over the southern section 40°. The former section, which is too shallow to store up any large amount of heat during the summer, freezes for three or four months along the shores, effectually stopping navigation on the lower Volga, but out in the middle ice appears only when driven there by northerly winds.
The prevalent winds of the Caspian blow from the south-east, usually between October and March, and from the north and north-west, commonly between July and September. They sometimes continue for days together with great violence, rendering navigation dangerous and driving the sea-water up over the shores. They also, by heaping up the water at the one end of the sea or the other, raise the level temporarily and locally to the extent of 4 to 8 ft. The currents of the Caspian were investigated by the Knipovich expedition; it detected two of special prominence, a south-going current along the west shore and a north-going current along the east shore. As a consequence of this the temperature of the water is higher on the Asiatic than on the European side. The lowest temperature obtained was 35°.24 on the bottom in shallow water, the highest 70°.7 on the surface. But in March the temperature, as also the salinity, was tolerably uniform throughout all the layers of water. Another interesting fact ascertained by the same expedition is that the amount of oxygen contained in the water decreases rapidly with the depth: off Derbent in the middle section of the sea the amount diminished from 5.6 cc. per litre at a depth of 100 metres