Last week we discussed the Inca ruins in South America in our survey of ancient American civilizations. Let's now turn to North America. The oldest known buildings on this part of the continent were just recently discovered in the mountains near Denver, Colorado. Found during the construction of a pipeline, the hardened mud brick structures are approximately five thousand years old, compared with four thousand seven hundred years for the oldest Egyptian Pyramids. Although mud structures usually disintegrate after two hundred years, those in Colorado happened to have caught fire. The fire was instrumental in baking the mud, creating a durable material resembling pottery. Since the buildings found appear to be permanent dwellings, anthropologists are now reconsidering the original theory that the area's ancient inhabitants were nomadic hunters-gatherers who took up residence in the mountains only during the summer.