Terek, has its sources, not in the main ranges of the Caucasus, but in an outlying group of mountains near Pyatigorsk, the highest summit of which, Besh-tau, does not exceed 4600 ft. But its waters become absorbed in the sands of the desert steppes before they reach the Caspian. Of the streams that carve into chequers the elevated plateau or highland region of Daghestan four are known by the common name of the Koisu, being distinguished inter se as the Andian Koisu, the Avarian Koisu, the Kara Koisu and the Kazikumukh Koisu, which all unite to form the Sulak. The only other stream deserving of mention in this province is the Samur. Both rivers discharge their waters into the Caspian; as also does the Zumgail, a small stream which drains the eastern extremity of the Caucasus range in the government of Baku.
Volcanic Evidences.—Ancient, but now extinct, volcanic upheavals are pretty common at the intersections of the main range with the transverse ranges; of these the most noteworthy are Elbruz and Kasbek. The town of Shemakha, near the eastern end of the system, was the scene of volcanic outbreaks as late as 1859, 1872 and 1902; while in the adjacent peninsula of Apsheron mud volcanoes exist in large numbers. All along the northern foot of the system hot mineral springs gush out at various places, such as Pyatigorsk, Zhelesnovodsk, Essentuki and Kislovodsk; and the series is continued along the north-eastern foot of the highlands of Daghestan, e.g. Isti-su, Eskiendery, Akhta. In this connexion it may also be mentioned that similar evidences of volcanic activity characterize the northern border of the Armenian highlands on the southern side of the Rion-Kura depression, in the mountains of Ararat, Alagöz, Akmangan, Samsar, Godoreby, Great and Little Abull, and in the mineral springs of Borzhom, Abbas-tuman, Sleptzov, Mikhailovsk and Tiflis. (J. T. Be.; P. A. K.)
Geology.—The general structure of the Caucasus is comparatively simple. The strata are folded so as to form a fan. In the centre of the fan lies a band of crystalline rocks which disappears towards the east. Beneath it, on both sides, plunge the strongly folded Palaeozoic and Jurassic schists. On the northern flank the folded beds are followed by a zone of Jurassic and Cretaceous beds which rapidly assume a gentle inclination towards the plain. On the south the corresponding zone is affected by numerous secondary folds which involve the Sarmatian or Upper Miocene deposits. In the eastern part of the chain the structure is somewhat modified. The crystalline band is lost. The northern Mesozoic zone is very much broader, and is thrown into simple folds like those of the Jura. The southern Mesozoic zone is absent, and the Palaeozoic zone sinks abruptly in a series of faulted steps to the plain of the Kura, beneath which no doubt the continuation of the Mesozoic zone is concealed.
The geological sequence begins with the granite and schists of the central zone, which form a band extending from Fisht on the west to a point some distance beyond Kasbek on the east. Then follow the Palaeozoic schists and slates. Fossils are extremely rare in these beds; Buthotrephis has long been known, and doubtful traces of Calamites and ferns have been found, but it was not until 1897 that undoubted Palaeozoic fossils were obtained. They appear to indicate a Devonian age. Upon the Palaeozoic beds rest a series of Mesozoic deposits, beginning with the Lias and ending with the Upper Cretaceous. Whether the series is continuous or not is a matter of controversy. F. Loewinson-Lessing states that there is a more or less marked discordance between the Lias and the Upper Jurassic and between the latter and the Cretaceous; E. Fournier asserts that there exists a very strongly marked unconformity at the base of the Tithonian, and other writers have expressed other views. In general the Upper Jurassic beds are much more calcareous on the north flank of the chain than they are on the south. The Mesozoic beds are followed by the Tertiary deposits, which on the north are nearly horizontal but on the south are in part included in the folds—the Eocene and Miocene being folded, while the later beds, though sometimes elevated, are not affected by the folding. The final folding of the chain undoubtedly occurred at the close of the Miocene period. That there were earlier periods of folding is almost equally certain, but there is considerable difference of opinion as to their dates. The difference in character of the Jurassic beds on the two sides of the chain appears to indicate that a ridge existed in that period. The last phase in the history of the Caucasus was marked by the growth of the great volcanoes of Elbruz and Kasbek, which stand upon the old rocks of the central zone, and by the outflow of sheets of lava upon the sides of the chain. The cones themselves are composed largely of acid andesites, but many of the lavas are augite andesites and basalts. There seem to have been two periods of eruption, and as some of the lavas have flowed over Quaternary gravels, the latest outbursts must have been of very recent date.
Near the northern foot of the Caucasus, especially in the neighbourhood of the hot mineral springs of Pyatigorsk, a group of hills of igneous rocks rises above the plain. They are laccolites of trachytic rock, and raised the Tertiary beds above them in the form of blisters. Subsequent denudation has removed the sedimentary covering and exposed the igneous core. (P. La.)
Bibliography.—Of the older works the following are still useful: A. von Haxthausen, Transkaukasia (2 vols., Leipzig, 1856); A. Petzholdt, Der Kaukasus (2 vols., Leipzig, 1866–1867); M. G. von Thielmann, Travels in the Caucasus (Eng. trans., 2 vols., London, 1875); F. C. Grove, The Frosty Caucasus (London, 1875); G. Radde, Reisen im mingrelischen Hochgebirge (Tiflis, 1866) and Vier Vorträge über den Kaukasus (Gotha, 1874); E. Favre, Recherches géologiques dans la partie centrale de la chaîne du Caucase (Geneva, 1875); Batsevich, Simonovich and others, Mat. dlya geologiy Kavkaza (Tiflis, 1873 seq.); O. Schneider, Naturwissenschaftliche Beiträge zur Kenntnis der Kaukasusländer (Dresden, 1879), and J. Bryce, Transcaucasia (London, 1878). The more important amongst the more recent books are D. W. Freshfield, Exploration of the Caucasus (2nd ed., 1902, 2 vols., London); A. F. Mummery, My Climbs in the Alps and Caucasus (London, 1895); H. Abich, Geologische Forschungen in den kaukasischen Ländern (3 vols., Vienna, 1878–1887), Aus kaukasischen Ländern (2 vols., Vienna, 1896), and “Vergleichende Grundzuge des Kaukasus wie der armenischen und nordpersischen Gebirge,” in Mém. Acad. Sc. St-Pétersb. (sér. 6, Math. et Phys., vii. 359-534); R. von Erckert, Der Kaukasus und seine Völker (Leipzig, 1887); E. Chantre, Recherches anthropologiques dans le Caucase (4 vols., Lyons and Paris, 1885–1887); C. von Hahn, Aus dem Kaukasus (Leipzig, 1892), Kaukasische Reisen und Studien (Leipzig, 1896), and Bilder aus dem Kaukasus (Leipzig