into the wall of the cathedral, in the Plaza Mayor. It is sculptured on the face of a single block of basalt, which weighs between twenty and thirty tons, and must have been transported thirty miles by Mexican labourers, for the stone is not found nearer than that distance from the city; and this transportation was, of course, managed by hand-labour alone, as there were no beasts of burden.
We know pretty well the whole system of Mexican astronomy from this calendar-stone and a few manuscripts which still exist, and from the information given in the work of Gama the astronomer and other writers. The Aztecs and Tezcucans who used it, did not claim its invention as their own, but said they had received it from the Toltecs, their predecessors. The year consisted of 365 days, with an intercalation of 13 days for each cycle of 52 years, which brought it to the same length as the Julian year of 365 days 6 hours. The theory of Gama, that the intercalation was still more exact, namely, 12½ days instead of 13, seems to be erroneous.