The valley of the city of Mexico, which lies to the south of these hills, is also of an oval shape, but is not more than twenty miles in extent. The surface-water with which it is saturated is in part fresh, and in other parts _tequisquite_; that is, where the waters have a current, they are fresh; but where they remain from year to year discharging their volume only by evaporation, then they become infused with the saline properties of the soil,[31] and all about them is marked with barrenness. If the process of evaporation was less intense than it is,[32] all vegetation would die from the extreme humidity of the soil; as the gardener's phrase is, it would rot. Even in the city of Mexico itself, a couple of feet of digging in its alluvial foundation brings you to the water-level in the dry season, and seventy or eighty yards of boring does not carry you beyond the perceptible influence of _tequisquite_.[33] The effects of this law of evaporation puzzled the Aztecs, who were, of course, ignorant of all philosophical principles, and could only account for the disappearance of the immense mass of water that fell in the valley in the wet season, upon the hypothesis that the Tezcuco had a leaky bottom, or that there was a hole in the lake--an idea that thousands in Mexico credit to the present day. This was the origin of that absurd story which Cortez repeats in his letters, that this lake communicated with the sea, and had its daily tides. There could not have been a much greater volume of water in this marshy valley in the time of Cortez than at present, if the whole accumulations of each year were to be carried off by evaporation alone from so small a surface as is here presented for the sun to act upon. But as the volume of water is the turning-point in the history or fable of the conquest, I must adduce the proofs and arguments that are at hand to establish this statement. The level of the water could not have been higher, it is clear, for in that case neither Mexico, Mexicalzingo, or Iztapalapan could have been inhabited. Cortez's account of deep waters has often been made plausible by adding the hypothesis that the accumulating mud of centuries has filled up the lakes, so that they now are only shallow ponds. But this by no means removes the difficulty, for then, as now, the waters of the southern laguna flowed into Tezcuco, conveying with them the infinitesimal infusion of _tequisquite_ that had instilled itself into the Chalco. Had the volume of Chalco and Xochimulco been increased several feet, then the slight Indian barriers and the long grass would no longer have been able to retard the progress of the water till evaporation had diminished its quantity, but, precipitating itself in a mass into the Tezcuco, it would have overwhelmed the town of Tezcuco and all other villages upon the shores, and established an equilibrium of surface in the two ponds.