Midway across the continent, somewhat nearer the Pacific than the Atlantic Ocean, at an elevation of nearly seven thousand five hundred feet, is the celebrated Valley of Mexico. It is of an oval form, about sixty-seven leagues in circumference,[12] and is encompassed by a towering rampart of porphyritic rock, which nature seems to have provided, though ineffectually, to protect it from invasion. The soil, once carpeted with a beautiful verdure and thickly sprinkled with stately trees, is often bare, and, in many places, white with the incrustation of salts caused by the draining of the waters. Five lakes are spread over the Valley, occupying one-tenth of its surface.[13] On the opposite borders of the largest of these basins, much shrunk in its dimensions[14] since the days of the Aztecs, stood the cities of Mexico and Tezcuco, the capitals of the two most potent and flourishing states of Anahuac, whose history, with that of the mysterious races that preceded them in the country,[15] exhibits some of the nearest approaches to civilization to be met with anciently on the North American continent. Of these races the most conspicuous were the Toltecs. Advancing from a northerly direction, but from what region is uncertain,[16] they entered the territory of Anahuac,[17] probably before the close of the seventh century. Of course, little can be gleaned with certainty respecting a people whose written records have perished, and who are known to us only through the traditionary legends of the nations that succeeded them.[18] By the general agreement of these, however, the Toltecs were well instructed in agriculture and many of the most useful mechanic arts; were nice workers of metals; invented the complex arrangement of time adopted by the Aztecs; and, in short, were the true fountains of the civilization which distinguished this part of the continent in later times.[19] They established their capital at Tula, north of the Mexican Valley, and the remains of extensive buildings were to be discerned there at the time of the Conquest.[20] The noble ruins of religious and other edifices, still to be seen in various parts of New Spain, are referred to this people, whose name, _Toltec_, has passed into a synonym for _architect_.[21] Their shadowy history reminds us of those primitive races who preceded the ancient Egyptians in the march of civilization; fragments of whose monuments, as they are seen at this day, incorporated with the buildings of the Egyptians themselves, give to these latter the appearance of almost modern constructions.[22]