PORTION OF CLIFF PALACE, Mesa Verde National Park. In the shelter of great caves in canyons cut deep into the plateau that the Spanish conquistadores named the "green table," pre-Columbian Indians built their homes, the ancestors of the modern apartment house. Cliff Palace, largest known of these dwellings, is shown by tree-ring chronology to date from the 13th century. In its heyday it is estimated to have had more than 200 living rooms, plus ceremonial chambers, storage rooms, and lived-on terraces—the latter on the roofs of lower levels, in modern penthouse fashion. Apparently begun on a single-story basis, with population increases, second-, third-, and even fourth-story rooms were superimposed on the original structure—for land then, as now in modern cities, seems to have been at a premium. (Photograph by George A. Grant.)