This is parted into four first divisions, that is to say, into four rivers. The name of the first is Pheison, which is the Indian Ganges; the name of the second is Geon, which is the Nile flowing from Ethiopia down to Egypt: the name of the third is Tigris, and the name of the fourth is Euphrates. There are also very many other mighty rivers of which some empty themselves into the sea and others are used up in the earth. Thus the whole earth is bored through and mined, and has, so to speak, certain veins through which it sends up in springs the water it has received from the sea. The water of the spring thus depends for its character on the quality of the earth. For the sea water is filtered and strained through the earth and thus becomes sweet. But if the place from which the spring arises is bitter or briny, so also is the water that is sent up Basil, Hom. 4 in Hexaëm.. Moreover, it often happens that water which has been closely pent up bursts through with violence, and thus it becomes warm. And this is why they send forth waters that are naturally warm.