TREE RINGS AND HISTORY — SCHULMAN 463 



It is evident that matching the outer rings of an old beam with the 

 inner rings in a living tree serves two purposes, namely, to date the 

 old beam and to extend into earlier times the potential climatic chro- 

 nology in the living tree. 



Those acquainted with the great range of variability, which seems 

 to be one of the universal properties of biologic elements, will recog- 

 nize that such simple growth and perfect synchroneity from tree to 

 tree as shown in the figure is quite unlikely to be found, even in trees 

 of one species and within a small locality. 



In many species and individual trees, the rings are so complex and 

 variable that cross-referencing with other rings is not possible. In- 

 deed, the botanist is familiar with so many reasons for such ring 

 irregularity — specific characteristics, environmental influences, acci- 

 dental events, and so on — that close parallelism in ring fluctuations 

 among different trees might well seem the rare exception. It was 

 fortunate for the pioneer work in dendroarcheology that it was ap- 

 plied in the Southwest, where species, climate, site, and wood collect- 

 ing by the ancients all so happily favored the research. It should not 

 be supposed, however, from the foregoing that such cross-dating is a 

 characteristic only of certain Southwestern trees. This property has 

 now been found present in many other regions, though nowhere in 

 such good form in so many trees. 



The tendency of the ring widths in dominant conifers of the South- 

 west — Pseudotsuga menziesii, Pinus ponderosa^ P. edulis — to show ap- 

 proximately the same patterns over a large area made it significant 

 to take broad-scale averages of many trees and thus derive a so-called 

 master chronology. In this way local peculiarities in growth were 

 minimized and the master chronology served as a general standard, 

 against which beams from widely separated localities could be dated. 



The development from living trees and relatively recent house 

 beams of a master chronology which extended back mto the time of 

 the prehistoric Pueblos was not accomplished at once — the dating of 

 the Cliff Dweller ruins, announced by Douglass in the National Geo- 

 graphic Magazine in December 1929, was preceded by over a decade 

 of collection and analysis of archeological wood. A number of float- 

 ing chronologies were developed, built up of ancient beams which 

 cross-dated with each other but which could not be joined to the dated 

 rings of the living-tree master record ; a gap of unknown length had 

 to be bridged. At last beams were obtained which did overlap the 

 inner part of the master chronology, and immediately dates could be 

 assigned to several scores of ruins in the Southwest. The most mag- 

 nificent of these, the apartment houses of the "cliff dwellers," as at 

 Mesa Verde National Park, were among the most recent, principally 

 in the 1200's A. D. 



