Of all the ruins in Mesa Verde National Park only 28 have been named and only 30 excavated. No survey of the unexcavated sites has been made, and the total number of ruins is unknown. Several hundred cliff dwellings have been discovered, and new ones will probably be found in the more remote canyons. The surface pueblos outnumber the cliff dwellings, and a careful search would reveal many that are now hidden by a thick growth of underbrush. The earth lodges of the Late Basket Makers are so common that hundreds will be brought to light by careful search. Dozens of them can be located in a half-hour walk over any of the mesas. DATES FOR MESA VERDE RUINS ESTABLISHED BY THE TREE-RING CHRONOLOGY[2] Dr. A. E. Douglass, director of Steward Observatory, University of Arizona, established the tree-ring chronology for dating Southwestern ruins. This chronology is based upon the facts that solar changes affect our weather and weather in turn the trees of the arid Southwest, as else-where, and that such affects are recorded in the variation of tree-ring growth during wet and dry years. Thus the tree-ring record of living trees has been extended into the past by arranging beams from historic pueblos in their proper sequence so that the inner rings of one match the outer rings of its predecessor, and in turn match the rings of the living trees. After completing the series from living trees and pueblos, of known dates, the record has been continued through the cross-sections of prehistoric beams of fir and pine that were chopped with the stone axes. The continuation of this chronology is only limited by the finding of earlier beams than those used in the established chronology. The National Geographic Society tree-ring expedition took, in all, 49 beam sections from ruins within Mesa Verde National Park. During 1932 and 1933 further tree-ring research was carried on in this area and additional dates have been secured. Presuming that the year of cutting the timber was the year of actual use in construction, the following dates have been established for the major cliff dwellings: Mug House, A. D. 1066 Long House, A. D. 1204-11 Cliff Palace, A. D. 1073-1273 Square Tower House, A. D. 1204-46 Oak Tree House, A. D. 1112-84 Spruce Tree House, A. D. 1230-74 Spring House, A. D. 1115 New Fire House, A. D. 1259 Hemenway House, A. D. 1171 Ruin No. 16, A. D. 1261 Balcony House, A. D. 1190-1272 Buzzard House, A. D. 1273 Since considerable tree-ring material from these ruins remains yet to be examined, the dates given above are not final. On the basis of present evidence, Cliff Palace, the largest and most complex cliff house within the park, shows an occupancy of 200 years. It is an interesting fact that all of the dates fall just short of the beginning of the great drought, which the tree-ring chronology shows commenced in 1276 and extended to 1299, a period of 24 years.