As I said before, when speaking of the stuccoed floors near Teotihuacán, the accumulation of alluvial soil goes on very rapidly and very regularly all over the plains of Mexico and Puebla, where everything favours its deposit; and the human remains preserved in it are so numerous that its age may readily be seen. We noticed this in many places, but in no instance so well as between Tezcuco and the hacienda of Miraflores. 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. The only place where any calculation can be based upon its thickness is on the banks of the Nile, where its accumulations round the ancient monuments may perhaps give a criterion as to the time which has elapsed since man ceased to clear away the deposits of the river.[14] [14] The researches instituted by Mr. I. Horner in the alluvium near Heliopolis and Memphis _(Philos. Transact._, 1855 & 1856), although very elaborate, still leave much to be desired before we can arrive at definite conclusions. As an instance of the tendency of alluvial deposits to entomb such monuments of former ages, I must mention the temple of Segeste, which stands on a gentle slope among the hills of northern Sicily. I had heard talk of the graceful proportions of this Doric temple, built by the Greek colonists; and great was my surprise, on first coming in sight of it, to see a pediment supported by two rows of short squat columns, without bases, and rising directly from the ground. A nearer inspection showed the cause of this extraordinary distortion. The whole slope had risen full six feet during the 2500 years, or so, that have elapsed since its desertion; and the temple now stands in a large oblong pit, which has lately been excavated. As we left the spot, and turned to see it again a few yards off, the beautiful symmetry of the whole had disappeared again.