The great peculiarity of the lake of Tezcuco is that it is a salt lake, containing much salt and carbonate of soda. The water is quite brackish and undrinkable. How it has come to be so is plain enough. The streams from the surrounding mountains bring down salt and soda in solution, derived from the decomposed porphyry; and as the water of the lake is not drained off into the sea, but evaporates, the solid constituents are left to accumulate in the lake.
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