186 



FORESTRY 



these fragments was one with attached 

 bark, the outer ring of a.d. 46 providing 

 the earliest precise culture date pres- 

 ently available for the Southwest. 



It will be evident from the fore- 

 going that, in general, the successful 

 application of tree-ring analysis to 

 archaeological dating requires two prin- 

 cipal favoring factors which are, un- 

 fortunately, by no means widely found: 



1. One or more living species must 

 exist in which, on at least some type 

 of site, the annual rings are sharply 

 defined, show fairly high year-to-year 

 changes in ring-width, vary in es- 

 sentially parallel fashion along dif- 

 ferent radii and from tree to tree, 

 and provide centuries-long se- 

 quences. 



2. Available archaeological beams must 

 be of the datable species and from 

 the datable types of sites, in gen- 

 eral must overlap the time range of 

 the master chronology for at least 

 50 years, and must have sufficiently 

 high ring sensitivity to provide un- 

 qualified dating. (Since the ring 

 chronologies in any region tend to 

 be much alike over broad areas, the 

 preceding restriction is not so se- 

 vere as it might otherwise be.) 



For specially favored localities of 

 the Pueblo area, ring chronologies in 

 some species are so simple and consis- 

 tent that reliable archaeological beam- 

 dating, given a sufficiently long master 

 chronology, is an absurdly simple mat- 

 ter. But this is far from true in general. 

 Definitive beam-dating requires, first, 

 a professionally secure solution of all 

 problems, such as those presented by 

 false annual rings, and second, an 

 identification which is not merely 

 probable but absolute with a selected 

 segment of the master chronology. 



Absolute identification is possible 

 by the forecast-and-verification method. 



Given a tentative matching of test 

 specimen and master chronology by 

 some ring characters, corresponding 

 additional characters (locally-absent 

 rings, check-segments of the ring se- 

 quences outside the test-dated interval, 

 etc.) are sought; with a sufficient num- 

 ber of verifications the probability of 

 chance correlation becomes vanishingly 

 small. Since such verification depends 

 on fairly close congruence of both ring 

 sequences, such parallelism should ap- 

 pear in the compared growth curves 

 and in the correlation coefficients, es- 

 pecially of the check or forecast in- 

 tervals. 



The unqualified archaeological 

 dating to the year which tree-ring 

 analysis makes possible under favoring 

 circumstances is, from a world-wide 

 point of view, of highly limited appli- 

 cation. Even in the Southwest, ruins 

 yielding only juniper or hardwood 

 beams cannot be directly dated; in 

 many other regions the absence of 

 favorable species makes the construc- 

 tion of sufficiently long master chro- 

 nologies extremely difficult or quite im- 

 possible. This deficiency has been met 

 in a most unexpected way. 



In the last few years a new method 

 has been devised, by W. F. Libby of 

 the Institute for Nuclear Studies of 

 the University of Chicago, for the 

 dating of wood and other materials by 

 the measurement of the amount of 

 decay in the radioactive isotope of 

 carbon, C^*, which these materials 

 contain. This ver\' elegant and powerful 

 technique, now in process of active de- 

 velopment, is applicable to a wide 

 range of organic and other material 

 and appears able to provide dates from 

 about 1,000 to some 30,000 years in 

 the past with a probable error that is 

 satisfyingly small. By supplying an 

 absolute time-scale for regions where 

 the construction of a master tree-ring 

 chronology anchored in the present is 



