ocr: islands that are intensively farmed. Indeed, it is possible that this form of agriculture, which is much better known as it was practiced in Aztec times near Tenochtitlan, was invented centuries earlier by the people ofTeotihuacan. The valley had major deposits of obsid- ian, the volcanic glass used all over ancient Middle America to make cutting and scraping tools and projectile points. Obsid- ian mining in the valley was apparently most intensive during the city's early years. Later the Teotihuacanos appear to have gained control of deposits of obsidian north ofthe Valley of Mexico that were ...
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