Following the lines of social evolution and architectural types considered in the preceding pages, we come now to a classification of buildings in the Southwest. Passing over the earliest expression of architecture, where a hut or dugout shows few peculiar features but practically is universal among a seminomadic people, we come to durable houses built of clay or stone. Even in these small buildings we recognize two types of rooms—circular and rectangular. We find two distinct types of village communities, one occupying the area extending from Utah to the inhabited pueblos on the Rio Grande. This group may be known in prehistoric culture by circular ruins and circular kivas. Here probably arose the original terraced form of building. The purest expression of its architecture occurs in cliff-dwellings like Cliff Palace and Spruce-tree House in the Mesa Verde National Park, but its extensions west and south are modified as the distance from the place of origin increases. The second type of buildings in the Southwest arose in the Gila valley, and is best illustrated by Casa Grande in southern Arizona. From this nucleus extensions of architectural forms were carried northward and eastward to the pueblos now inhabited by Hopi and Zuñi Indians. A characteristic feature of this type is the massive-walled buildings surrounded by a rectangular wall or compound. The circular kiva and circular ruin do not exist in present forms of this type. Ruins in southern Arizona, belonging to this type, often have very much modified forms, especially as the type extended northward and came in contact with extensions of the pueblo culture. Architectural characters and other features of this type show marked affinities with the corresponding culture of prehistoric peoples of Mexico.