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.

As soon as the novelty of this kind of travelling had worn off, we
began to find it dull, and retired under our awning to breakfast and
bitter beer; which latter luxury, thanks to a suitable climate and an
English brewer, is very well understood in Mexico, and is even accepted
as a great institution by the Mexicans themselves.

We were just getting into a drowsy state, when an unusual bustle among
the crew brought us out of our den, and we found that three hours of
assiduous poling had taken us half-way across the lake, just six
miles--a good test of the value of the Aztec system of navigation. Here
was