We have seen at the close of the last chapter that the Deveh Boyun consists of a composite ridge, thrown across a narrow portion of the northerly depression, and dividing it into two. It is due to an outbreak of lava--a hard trachyte--which has pursued a direction almost at right angles to the general structure of the country, its elevation being nearly meridional. Similar outbreaks are readily recognisable in the northern border heights of the plain of Pasin; but those ridges are of little geographical importance, losing themselves on the confines of the plain. On the other hand the Deveh Boyun, the most westerly of the series, determines the drainage of the great basin. From its eastern slopes the waters flow to the Araxes, and from those on the west to the Euphrates. On the one hand lies Pasin, and on the other the plain of Erzerum. The height of the pass over the parapet is not more than some 500 to 800 feet above the level of the adjacent plains. But the ridge is defended by a line of modern forts; and, if these were captured, the invading army would find itself enclosed within a space which, while it can scarcely exceed a width of about four miles, can be swept by the fire from heights on the north and heights on the south. These positions, which have all been fortified since the last Russo-Turkish war, rest against the slopes of the parallel walls of mountain, confining the depression on either side. There does exist, I believe, a narrow passage through an irregular valley between the Deveh Boyun main ridge and the northern wall. But this approach by the flank is commanded by some of the forts already mentioned. Nor would the fate of Erzerum be necessarily determined if both the ridge and the works which protect it had been occupied by the enemy after a series of frontal attacks and great loss of life. There would remain the defences of the Top Dagh, a hill mass, or, as they would say in South Africa, a series of kopjes, separated from the Deveh Boyun by the valley of a small tributary to the Euphrates derived from the wall of mountain on the south. The Top Dagh bristles with forts, of which the most conspicuous are Forts Mejidieh and Azizieh. It immediately abuts on the enceinte of the city which it screens from attack from the east. The city lies with its head upon the talus or accumulated rubble which fans out from the heights on the south. Its feet touch the floor of the plain.