THE CHARACTERISTICS OF THE CAUCASUS 31

watershed being transferred to the parallel chain of clay slates—
' pala3ozoic schists ' they have, apparently without conclusive reasons,
been termed abroad— which has followed it from the Black Sea,
attaining on its way the height of 13,400 feet in the Laila, and Hmit-
ing the great parallel basins of the Eion, Ingur, and Skenis Skali.
The how and why of this transfer of the watershed I leave to pro-
fessional geologists. Its historical importance has been great. The
slates, less steep, less lofty than the granite, are also far less formid-
able obstacles to traffic. The Krestovaya Gora — or Cross Mountain
Pass — traverses them at less than 8000 feet ; the Mamison Pass
slips over the grassy ridge that links them to the granites of
Adai Khokh at a height of 9200 feet. These are the natural
highways from the north to Georgia and Mingrelia respectively.
The former has long been the Georgian military highroad; the
latter was opened for wheels in 1889. They are easy passes — so
easy that it was not on the mountain crest, but where, in the
gorge of Darial, the granitic range is cleft to its base, that
Chosroes and Justinian combined to raise and garrison a frontier
fortress between the old civilisations of the world and the legendary
hordes, the Gog and Magog, of northern barbarism. Between
them half a dozen gaps, leading from the head-waters of the
Kur to those of the Ardon, a tributary of the Terek, are frequently
traversed by the Ossetes, whose position astride the chain enabled
them at the beginning of the present century to hand over its
keys to Russia.
