And he said that the river that passes through Cairo, which we call the Nile, they call the Gyon. And he said that it comes from that country, passing in between two mountains, and he says this because one can find it written that it comes from the terrestrial paradise, just like the Tigris and the Euphrates which, saving the grace of those who say it, it is from there that all four [rivers] come. However, he said that the Nile passes between these two mountains and there is only one small river, and part of a great canyon. And near to this passage, Prester John has had two large towers constructed and a large chain from one side to the other, so that no one can see into that cave, because he said that people used to go in and that, after anyone went in, he would never return. The cause of this, he told me, is that once one is inside it, one hears a very sweet song that makes you never want to leave.
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And he also told me that if it pleased Prester John, he could easily move the river to another course. But he left it be because there are many Christians living on the aforesaid River Nile.