The evidences of a phase of culture that had developed and was on the decline before the interior of North America was explored by Europeans are nowhere better shown than in southwestern Colorado, New Mexico, Arizona, and Utah, the domain of the Cliff-dwellers, or the cradle of the Pueblos. There flourished on what is now called the Mesa Verde National Park, in prehistoric times, a characteristic culture unlike that of any region in the United States. This culture reached its apogee and declined before the historic epoch, but did not perish before it had left an influence extending over a wide territory, which persisted into modern times. Through the researches of archeologists the nature of this culture is now emerging into full view; but much material yet remains awaiting investigation before it can be adequately understood. The purpose of this article is to call attention to new observations bearing upon its interpretation made by the author, under the auspices of the Bureau of American Ethnology, on brief trips to Colorado and Utah in 1917 and 1918. The peculiar cliff-dwellings and open-air villages of the Mesa Verde are here shown to be typical of those found over a region many miles in extent. They indicate a distinct culture area, which is easily distinguished from others where similar buildings do not exist, but not as readily separated from that of adjacent regions where the buildings are superficially similar but structurally different. In order to distinguish it from its neighbors and determine its horizon, we must become familiar with certain architectural characteristics. As our knowledge of the character of buildings in this area is incomplete, the intention of the author is to define the several different types of buildings that characterize it.