THE CASPIAN SEA AND ITS NEIGHBORS 341
the founding of Baku in its present site in the fifth or sixth century; but Bruckner, on the basis of architectural resemblances, considers it of Arab origin, dating from the twelfth century. However this may be, the walls of Aboskun and Derbent are sufficient to establish the fact that about 500 A. D., the level of the Caspian Sea was as low or lower than it is to-day. The climate must then have been at least as dry as it now is.
During the succeeding Middle Ages, there is unmistakable evidence that the level of the Caspian Sea again rose, though not to such an extent as formerly. So far as the water-spread of the medieval sea is concerned, it is probably possible that the expansion of the sea may have been due to the deflection of the Oxus and Jaxartes from the Sea of Aral to the Caspian. There is evidence that such deflection took place, or, at least, that part of the water of the rivers was so deflected. Hence it is necessary to proceed with the greatest caution, in order to ascertain whether the epochs of high level agree with those of deflection, or whether the expansion of the lake occurred independently of the river. The chief evidence, one way or the other, is found in the works of various Arab and Persian authors quoted by Humboldt and Rawlinson. I shall assume that the oriental authors state the facts correctly, unless there is clear reason for doubting their information.
During the Dark Ages, in the seven and a half centuries between the times of Ptolemy and of Istakhri, A. D. 920, war and confusion prevailed in the Aralo-Caspian region. The only addition to our knowledge of the two salt lakes is Menander's mention of the existence of the Sea of Aral as a
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