In England, I think, we have no example of this; but the Dead Sea, the Caspian, the Great Salt Lake of Utah, and even the Mediterranean, have various salts accumulated in solution in the same way. It seems to me, that, by taking into account the proportion of soluble material contained in the water that flows down from the mountains, the probable quantity of water that flows down in the year, and the proportion of salt in the lake itself, some vague guess might be made as to the time this state of things has been lasting. I have no data, unfortunately, even for such a rough calculation as this, or I should like to try it.
In spite of the splendid climate, a great portion of the Valley of Mexico is anything but fertile; for the soil is impregnated with salt and soda, which in many places are so abundant as to form, when the water evaporates, a white efflorescence on the ground, which is called tequesquite, and regularly collected by the Indians. Some of it is stopped on its way down from the higher ground, by the evaporation of the water that was carrying it; and some is left by the lake itself, in its frequent floodings of the ground in its neighbourhood. So small is the difference of level between the lake and the plain that surrounds it, that the slightest rise in the height of the water makes an immense difference in the size of the lake; and even a strong wind will drive the water over great tracts of ground, from which it retires when the gale ceases. It must have been this, or something similar, that set Cortes upon writing home to Spain that the lakes were like inland seas, and even had tides like the ocean. Of course, this impregnation with salts is ruinous to the soil, which will produce nothing in such places but tufts of coarse grass; and the shores of the lake are the most dismal districts one can imagine. All the lakes, however, are not so salt as Tezcuco; Chalco, for instance, is a fresh-water lake, and there the fertility of the shores is very great, as I have already had occasion to notice.