Another circumstance mentioned is not without significance. It is said, in connection with this account of the Toltec migration, that Huehue-Tlapalan was successfully invaded by Chichimecs, meaning barbarous aboriginal tribes, who were united under one great leader. Here is one statement (a little condensed) touching this point: “There was a terrible struggle, but, after about thirteen years, the Toltecs, no longer able to resist successfully, were obliged to abandon their country to escape complete subjugation. Two chiefs guided the march of the emigrating nation. At length they reached a region near the sea named ‘Tlapalan-Conco,’ where they remained several years. But they finally undertook another migration and reached Mexico, where they built a town called ‘Tollanzinco,’ and later the city of Tullan, which became the seat of their government.” This is substantially what is told of the defeat and migrations of the Toltecs. The history of Ixtlilxochitl adds doubtful modifications and particulars not found in the “Codex Chimalpopoca.” (See Quatre Lettres, etc.) This Chichimec invasion of Huehue-Tlapalan is placed at a period which, in the chronology of the native books, was long previous to the Christian era, and is mentioned to explain the beginning of the Toltec movement toward Mexico; but the account of it is obscure. To find a system of chronology in these old books is not surprising when we consider that even the Aztecs of Montezuma’s time knew enough of astronomy to have a correct measure of the year. The Aztecs adopted the methods of astronomy and chronology which were used by their predecessors. They divided the year into eighteen months of twenty days each; but, as this gave the year only three hundred and sixty days, five supplementary days were added to each year, and a sixth day to every fourth year. The bissextile is known to have been used by the Mayas, Tzendals, and Quichés, and it was probably common.