XI
1
2
You have still a difficulty, you say. If the causes giving rise to rivers and fountains are constant, why are their waters sometimes dried up? and why do they sometimes appear in places where they did not exist before? Their routes, I should reply, are often disturbed by earthquakes; the channel is cut off by a fall of rock or earth, and the water being held back seeks a fresh exit, which it forces with a certain measure of violence; or merely by the earth’s vibration the course is shifted from one place to another. On the surface of the earth one may observe that rivers that have lost their channels are first of all dammed back, but afterwards, in lieu of the course they have lost, force a new one. Theophrastus affirms that an incident of the kind took place in the Corycian Mount,52 where, after a slight shock of earthquake, a fountain burst out from a fresh source.
3
But some writers are of opinion that other causes too are at work to call up water in other ways, or to drive or turn it from its course. Mount Haemus was once destitute of water; but after a tribe of the Gauls, being hard pressed by Cassander, took refuge there, and felled the woods, an immense supply of water appeared. No doubt the woods had attracted it for their nourishment previously. When they were uprooted, the moisture, ceasing to be used up by their roots, overflowed. Theophrastus affirms that the same thing happened near Magnesia.
But with all respect to Theophrastus, this is not a very likely story. Everything that is most shady tends most to gather water. But that would not be the case if trees drained off water. Roots draw their nourishment from their immediate vicinity; but the volume of river water flows from recesses far down, and is derived from a source deeper than roots can penetrate. Besides, when trees are cut down, more moisture than before is required; the stumps suck up a supply, not merely for life, but for new growth.
4
5
Theophrastus tells us, too, that round Arcadia, which was a city in the island of Crete, the wells and lakes disappeared, because the land ceased to be tilled after the destruction of the city; but after it had got back its tillers, it recovered its waters also. He sets down as the cause of the dryness, that the earth had got hidebound and quite hard, and not being stirred could not transmit to the underground reservoirs the rain that fell. But if this is true, how comes it that we see springs in great plenty in the most desert ground? In fact, one finds a great deal more ground that began to be tilled on account of the abundance of water than that began to have an abundant supply of water because it was tilled. You may be quite sure that it is not mere rain water that is carried down in our greatest rivers, navigable by large vessels from their very source,53 as is proved by the fact that the flow from the fountain-head is uniform winter and summer. Rainfall may cause a torrent, but it cannot maintain the steady, constant flow of a full river. Rains cannot produce, they can only enlarge and quicken, a river.