372 Revieius — Dr. Felix Osiuald's Mail of the Caucasus. 



The chain of the Caucasus falls naturally into three main divisions : 

 (1) the Central Caucasus, between Elburs and Kazbek; (2) the 

 Western Caucasus, from Elburs westward to the point where the 

 main axis runs into the Black Sea ; (3) the Eastern Caucasus, from 

 Kazbek to a point where the main axis runs into the Caspian. There 

 are areas of Kecent and Middle Tertiary beds (well-known to contain 

 petroleum) at either end of the chain, which are characterized by the 

 presence of mud- volcanoes. The western or Taman district lies in the 

 north, while the eastern or Apsheron district lies to the south of 

 the main axis. 



The Central Caucasus is not only the loftiest part of the chain, but 

 erosion has carved out such deep valleys that the structure is here 

 most clearly revealed ; its delimitation by Elburs (18,523 feet) and 

 Kazbek (16,546 feet) is no arbitrary selection, but the natural 

 expression of tectonic structure, for these volcanic masses lie on two 

 transverse N.-S. fractures along which volcanic activity has been 

 very pronounced, while they are still the seat of violent earthquakes. 



The "Western Caucasus becomes progressively simpler in structure, 

 the core of gneiss and crystalline schists disappearing at Eisht beneath 

 a cover of Jurassic and Cretaceous deposits ; the main axis then 

 diminishes rapidly in height, from over 9,000 to less than 3,000 feet, 

 and consists for the last 200 miles of its course of only Upper 

 Cretaceous strata. 



The Eastern Caucasus, on the other hand, widens out to 140 miles 

 in Daghestan. This district is essentially a succession of parallel 

 chains in the south ; but the northern part is a wide undulating 

 plateau of Jurassic and Cretaceous beds. 



The Daghestan plateau is broken off on the east by a nearly 

 meridional line of fracture, a fall of 12,000 feet being thereby 

 produced in a distance of only 10 miles. A fracUire of similar magnitude 

 parallel to the main axis delimits the Eastern Caucasus along its 

 southern side, and at the point of intersection of these two great 

 fractures the basalts and andesites of the Mai Dagh have been 

 erupted, mineral springs occur, and earthquakes are frequent. 



The account of the evolution of the Caucasus is of much interest. 

 The main axis in the central division consists of gneiss, flanked by 

 crystalline schists, which have been acutely folded into a geanticline 

 displaying fan-structure, so that the schists dip beneath the gneiss 

 and overlie in turn the reversed Palaeozoic schists. The upraising 

 of the Caucasus in the Permian period continued into the Trias,' 

 excepting in the western division, which was partly covered by the 

 waters connecting the Alpine sea with the Armenian and Indian seas. 

 In the Lower and Middle Jurassic periods there was extensive 

 submergence with oscillations allowing the local formation of coal. 

 Intense folding again took place in the Upper Jurassic, with 

 a Tithonian transgression on the north of the chain. The chief and 



^ In 1907 an important addition was made by the Eussian geologist Mr. J. 

 Worobiev to our l^nowledge of the geology of the Caucasus by the discovery 

 of beds of Upper Triassic age on the Little Laba Eiver at Psebai, and along 

 the Khods Eiver. Dr. Oswald gives an account of this in the GEOLOGICAL 

 Magazine for .1909, pp. 171-3. 



