The core of the Balkan, between the basin of Sofia and that of Slivno, consists of granite, but the terraces which descend towards the Danube present every geological formation, from the metamorphic to the most recent rocks. The cretaceous formation occupies the largest area in Bulgaria, and the rivers rising in the mountains, in traversing it, form picturesque valleys and defiles. Ancient fortresses defend each of these valleys, and the towns have been built where they debouch upon the plain. Tirnova, the ancient capital of the tsars of Bulgaria, is the most remarkable of these old bulwarks of defence. The Yantra, on debouching there from the mountains, winds about curiously; steep cliffs form an amphitheatre, in the centre of which rise two precipitous isolated rocks, crowned formerly by walls and towers. The houses of the town are built on the slopes, and its suburbs extend along the foot of the cliffs. A singular parallelism has been noticed on the northern slopes of Balkan. The elevated mountain saddles, crests of secondary chains, geological formations, the faults which give rise to the meandering of the rivers, and even the Danube itself, all follow the same direction, from west to east. As a consequence, each of the parallel valleys descending from the Balkans offers similar features; the {134} population is distributed in the same manner; and the towns occupy analogous positions. The valley of the Lom offers the only exception to the rule, for its direction is towards the north-west. It debouches upon the Danube at Rustchuk, and its green orchards and gardens are hemmed in by dazzling white cliffs of chalk rising to a height of about 100 feet. The symmetry would be almost complete in Northern Turkey if it were not for the detached arid hills of the Dobruja, which force the Danube to make a wide détour to the north. Rising in the low and swampy delta of the Danube, these hills appear to be much higher than they are. In reality they do not exceed 1,650 feet in height. It is possible that during some very remote geological epoch the Danube took its course to the south of these hills, through the depression which has been utilised for the construction of the first Turkish railway. Trajan, who feared that the Goths might obtain a footing in this remote corner of the Roman empire, constructed one of those lines of fortifications here which are known throughout the countries of the Lower Danube as Trajan’s Walls. Remains of walls, ditches, and forts may still be traced along the banks of the marshes, and on the heights commanding them. This country of the Dobruja is the “savage hyperborean region” where Ovid, exiled from Rome, wept for the splendours of the capital. The port of Tomi, the place of his banishment, is the modern Kustenje.