The whole aspect of the place is southerly, yet the climate of Constantinople has its rigour. The cold winds of the steppes of Russia freely penetrate through the strait, and the thermometer has been known to fall four degrees below zero in the winter. The neighbouring sea renders the climate more equable than it would otherwise be; but as the winds, from whatever direction they blow, meet with no obstacle, sudden changes of temperature are frequent. The average temperature varies very considerably in different years. Sometimes it sinks to the level of that of Pekin or Baltimore, at others it is as high as that of Toulon or of Nice. In exceptional cases the Bosphorus has become covered with ice, but thaws always set in rapidly, and then may be witnessed the magnificent spectacle of masses of ice striking against the walls of the Seraglio, and floating away across the Sea of Marmara. In A.D. 762 these masses of ice were so stupendous that they became wedged in the Dardanelles, and the tepid waters of the Ægean Sea then assumed the aspect of a bay of the Arctic Ocean. * * * * * The geological features of the coast region of the Sea of Marmara differ essentially from those of the rest of Turkey. Low ranges of hills rise close to the coast, increasing in height towards the west, until they attain an elevation of 2,930 feet in the Tekir Dagh, or “holy mountains,” the grey slopes of which, covered here and there with patches of shrubs or pasturage, are visible from afar. A narrow neck of land joins the peninsula of Gallipoli—the Thracian {105} Chersonesus of the ancients—to this coast range. This peninsula is composed of quaternary rocks, which differ in no respect from those met with on the shore of Asia opposite. Anciently a huge fresh-water lake covered a portion of Thracia and more than half the area now occupied by the Ægean Sea. When the land first emerged above the waters, the Chersonesus formed an integral portion of Asia. Subsequently the waters of the Black Sea, which had forced themselves a passage through the Bosphorus, likewise found their way through the Hellespont into the Ægean Sea. The geological formation of the country and the configuration of the sea-bottom prove this to have been the case, and this irruption of the waters was attended, probably, by volcanic eruptions, traces of which still exist on the islands of the Sea of Marmara and near the mouth of the Maritza, the former to the east, the latter to the west of the peninsula. [Illustration: Fig. 32.—THE HELLESPONT, OR DARDANELLES, AND THE GULF OF SAROS. Scale 1 : 1,220,000. The dark shading expresses a depth exceeding 55 fathoms.]