Notwithstanding the labours of the various authorities who have from time to time written upon the ancient topography of the lower Indus Valley, the subject remains in much need of further investigation. Attempts to identify places mentioned by the historians of Alexander's Expedition, by the author of the Periplus of the Erythræan Sea, the geographer Ptolemy, the Chinese pilgrim Hiouen Thsang, the historians of the Arab conquest, and by later writers, have not usually been attended with the happiest results. Nor, when the difficulties which beset investigation are considered, is it at all surprising that this should be so. It is hard enough to have to contend with the vagueness, inconsistencies and contradictions of old writers; but these are as nothing compared with the obstacles which the physical characteristics of the country itself oppose to the inquirer. For ages the Indus has been pushing its bed across the valley from east to west, generally by the gradual process of erosion, which effectually wipes out every trace of town and village on its banks; but, at times also, by a more or less sudden shifting of its waters into entirely new channels, leaving large tracts of country to go to waste, and forcing the inhabitants of many a populous place to abandon their old homes, and follow the river in search of new settlements.