The Cambridge History of India, Vol. I: Ancient India v1.c15.159

v1.c15.159

and once fruitful lands have been converted into impassable deserts. Both Irān and Turkestān, the two reservoirs from which the streams of migration flowed into the Indus valley, have been affected by this desiccation of the land. Archaeological investigations in Seistān and in Chinese Turkestān have brought to light the monuments of ancient civilisations which had long ago passed into oblivion. Especially valuable from the historical point of view are the accounts given by Sir Aurel Stein of his wonderful discoveries in Chinese Turkestān. From the chronological evidence, which he has so carefully collected from the documents and monuments discovered, we are enabled to ascertain the dates, at which the various ancient sites were abandoned because of the progressive desiccation during a period of about a thousand years (first century b.c. to ninth century a.d.). We may thus realise how it has come to pass that a region which once formed a means of communication not only between China and India, but also between China and Europe, has now become an almost insuperable barrier. The same causes have tended to separate India from Irān. The last irruption which penetrated to Delhi, the heart of India, through the north-western gateway was the Persian expedition of Nādir Shāh in 1739.