10 CLIMATIC CYCLES AND TREE GROWTH 



anyone who will take the trouble to test it in a favorable locality, such as 

 the Flagstaff region. 



Cross-identity was observed in 1904 and recognized as a fundamental part 

 of tree-ring work in 1911. From the start it was realized that chronological 

 identification can be carried from tree to tree by grouping of the rings them- 

 selves as described above. Cross-identification, therefore, depends on a 

 succession of specialized rings which, as a group, do not occur in other parts 

 of the long chronological ring record. Thin rings are found more useful in 

 Arizona in this identification process than wide rings, because ring records 

 from different trees show better correlations in thin rings than in wide ones. 



Thus a short series of 10 or 20 rings is not usually datable — that is, it 

 can not convincingly establish a cross-identity, but if seemingly recognized 

 it may serve as a guide and when these rings are joined with twenty on each 

 side also recognized the series of fifty usually becomes datable. Thus, com- 

 monly, fifty years is considered desirable for cross-dating. Occasionally 

 sequences with some conspicuous grouping of the rings may be considered 

 datable if containing twenty or twenty-five years. Often a sequence of a 

 hundred can not be dated because it does not contain enough deficient rings 

 to render identification possible by comparison with other trees. 



A highly typical case of cross-dating is shown in Plate 7, which represents, 

 above, a series of pine rings near the outside of JPB-17 from Pueblo Bonito, 

 and, below, a Douglas fir BK-2 from Betatakin, 125 miles away, giving rings 

 near the center of the tree. In spite of these differences and the great distance 

 apart, the spacing of the deficient rings as indicated is identical in the two 

 trees. 



Compact groups of rings which appear almost identical in many different 

 trees are sometimes called signatures or fingerprints. Plate 8 gives a series of 

 rings showing a compact group from A.D. 611 to 620 which was recognized 

 as a specialized group many years before its date was ascertained. It ap- 

 peared in M-179, a beautiful Douglas fir specimen collected by Mr. E. H. 

 Morris in 1927. Then it was recognized in 1931 as a part of the JCD (John- 

 son Canyon Dating) series from the upper La Plata River near Mesa Verde 

 National Park, and it has come to be called the JCD signature. It was 

 recognized in 1932 in very beautiful form by Dr. Glock in the MLK series 

 from Broken Flute Cave west of Shiprock. In the latter part of 1934 it 

 was found in a charcoal section, FR-20, collected at Allantown, Arizona, by 

 Mr. C. F. Miller under the direction of Dr. F. H. H. Roberts Jr., for the 

 Smithsonian Institution. In early 1935 it was found on a charcoal piece, 

 F-3992 (Plate 9), from the Baker Ranch Ruin north of the San Francisco 

 Peaks, Flagstaff, collected under the direction of Dr. H. S. Colton for the 

 Museum of Northern Arizona and handed to me by Mr. J. C. McGregor, 

 who does the tree-ring work for that Museum. These locations form a tri- 

 angle whose three sides are approximately 95 miles, 150 miles, and 160 miles. 

 Plate 8c shows the center of MLK-179, whose central microscopic ring grew 

 in the year A.D. 536 and corresponds to the 15th ring from the center of 

 MLK-127, shown in the frontispiece. 



