There a long ditch, some five feet deep, had just been cut in anticipation of the rainy season.

As yet it was dry, and, as we walked along it, we found three periods of Mexican history distinctly traceable from one end to the other.

First came mere alluvium, without human remains.

Then, just above, came fragments of obsidian knives and bits of unglazed pottery.

Above this again, a third layer, in which the obsidian ceased, and much of the pottery was still unglazed; but many fragments were glazed, and bore the unmistakable Spanish patterns in black and yellow.
It is a pity that these alluvial deposits, which give such good evidence as to the order in which different peoples or different states of society succeeded one another on the earth, should be so valueless as a means of calculating the time of their duration; but one can easily see that they must always be so, by considering how the thickness of the deposits is altered by such accidents as the formation of a mud-bank, or the opening of a new channel,--things that must be continually occurring in districts where this very accumulation is going on.