The _chinampas_, that archipelago of wandering islands, to which our attention was drawn in the last chapter, have, also, nearly disappeared. These had their origin in the detached masses of earth, which, loosening from the shores, were still held together by the fibrous roots with which they were penetrated. The primitive Aztecs, in their poverty of land, availed themselves of the hint thus afforded by nature. They constructed rafts of reeds, rushes, and other fibrous materials, which, tightly knit together, formed a sufficient basis for the sediment that they drew up from the bottom of the lake. Gradually islands were formed, two or three hundred feet in length, and three or four feet in depth, with a rich stimulated soil, on which the economical Indian raised his vegetables and flowers for the markets of Tenochtitlan. Some of these _chinampas_ were even firm enough to allow the growth of small trees, and to sustain a hut for the residence of the person that had charge of it, who with a long pole, resting on the sides or the bottom of the shallow basin, could change the position of his little territory at pleasure, which with its rich freight of vegetable stores was seen moving like some enchanted island over the water.[309] The ancient dikes were three in number. That of Iztapalapan, by which the Spaniards entered, approaching the city from the south. That of Tepejacac, on the north, which, continuing the principal street, might be regarded, also, as a continuation of the first causeway. Lastly, the dike of Tlacopan, connecting the island-city with the continent on the west. This last causeway, memorable for the disastrous retreat of the Spaniards, was about two miles in length. They were all built in the same substantial manner, of lime and stone, were defended by draw-bridges, and were wide enough for ten or twelve horsemen to ride abreast.[310]