A number of years ago, Dr. A. E. Douglas, a professor of Astronomy and Research Fellow of the Carnegie Institution first made public a method of reading weather data from tree rings in sequoias. This was followed by his work with ponderosa pine, principally on the Colorado plateau. This latter phase of his work led to the dating of old Pueblos and Cliff dwellings by comparison of the rings in old house timbers with those of already known dates in the series worked out in meteorological studies. In the continuation of that work a definite calendar of building dates for many of the old ruins has been worked out for a period of nearly two thousands years. The article focuses on the process of studying tree rings.