For a short distance, the army kept along the narrow tongue of land that divides the Tezcucan from the Chalcan waters, when it entered on the great dike, which, with the exception of an angle near the commencement, stretches in a perfectly straight line across the salt floods of Tezcuco to the gates of the capital. It was the same causeway, or rather the basis of that, which still forms the great southern avenue of Mexico.[264] The Spaniards had occasion more than ever to admire the mechanical science of the Aztecs, in the geometrical precision with which the work was executed, as well as the solidity of its construction. It was composed of huge stones well laid in cement, and wide enough, throughout its whole extent, for ten horsemen to ride abreast. They saw, as they passed along, several large towns, resting on piles, and reaching far into the water,--a kind of architecture which found great favor with the Aztecs, being in imitation of that of their metropolis.[265] The busy population obtained a good subsistence from the manufacture of salt, which they extracted from the waters of the great lake. The duties on the traffic in this article were a considerable source of revenue to the crown. Everywhere the Conquerors beheld the evidence of a crowded and thriving population, exceeding all they had yet seen. The temples and principal buildings of the cities were covered with a hard white stucco, which glistened like enamel in the level beams of the morning. The margin of the great basin was more thickly gemmed than that of Chalco with towns and hamlets.[266] The water was darkened by swarms of canoes filled with Indians,[267] who clambered up the sides of the causeway and gazed with curious astonishment on the strangers. And here, also, they beheld those fairy islands of flowers, overshadowed occasionally by trees of considerable size, rising and falling with the gentle undulation of the billows. At the distance of half a league from the capital, they encountered a solid work or curtain of stone, which traversed the dike. It was twelve feet high, was strengthened by towers at the extremities, and in the centre was a battlemented gateway, which opened a passage to the troops. It was called the Fort of Xoloc, and became memorable in after-times as the position occupied by Cortés in the famous siege of Mexico.