In places where vanished towns were in existence as late as the eighteenth century (for instance, in the Helmund valley below Rudbar), debris of pottery may be found literally in tons. In other places, still living, where generations of cities have gradually waxed and waned in successive stages, each in turn forming the foundation of a new growth, it is very difficult to derive any true historical indication from the debris which is to be found near the surface. Nothing but systematic and extensive excavation will suffice to prove that the existing conglomeration of rubbishy bazaars and ruined mosques is only the last and most unworthy phase of the existence of a city the glory of whose history is to be found in the world-wide tradition of past centuries. And so it happens that, moving in the footsteps of these old mediæval commercial travellers, with the story of their travels in one's hand, and the indications of hill and plain and river to testify to the way they went, and a fair possibility of estimating distances according to their slipshod reckoning of a "day's journey," one may possess the moral certainty that one has reached a position where once there stood a flourishing market-town without the faintest outward indication of it. Without facilities for digging and delving, and the time for careful examination, there must necessarily be a certain amount of conjecture about the exact locality of some even of the most famous towns which were centres of Arab trade through High Asia. Some indeed are to be found still under their ancient names, but others (and amongst them many of great importance) are no longer recognizable in the place where once they palpitated with vigorous Eastern life. The area of Asia which for three or four centuries witnessed the monopoly of Arab trade included very nearly the whole continent. Asia Minor may be omitted from that area, and the remoter parts of China; but all the Indian borderland was literally at their feet; and we can now proceed to trace out some of their principal lines of route and their chief halting-places in those districts of which the mediæval geography has lately become known. It is not at all necessary, even if it were possible, to follow the records of all the eminent Arab travellers who at intervals trod these weary roads. In the first place they often copied their records from one another, so that there is much vain repetition in them. In the second place they are not all equally trustworthy, and their writing and spelling, especially in place-names, wants that attention to diacritical marks which in Eastern orthography is essential to correct transliteration. It is perhaps unfortunate that the most eminent geographer amongst them should not have been a traveller, but simply a compiler.