120 CLIMATIC CYCLES AND TREE-GROWTH 



ZONE CENTERS AND THEIR MEAN CURVES 



The material collected over western areas has opened such a field 

 for immediate development that the contents of this chapter can only 

 be regarded as a transition rather than a conclusion. Such progress 

 and results as have appeared to date will be given, but they must be 

 taken as subject to revision at a later time. 



Cross-identification — Introductory to the comparison of smoothed 

 curves, it should be recalled that cross-identification by individual 

 rings is the exact and reliable method of comparing curves over large 

 or small areas. In the western States it is found to grow easier and 

 more reliable as the climatic stress of the arid regions is approached, 

 that is to say, such dating is highly satisfactory within the Arizona 

 region, which extends to the Rio Grande on the east and the coast- 

 line on the west. It is fairly satisfactory between Arizona and Central 

 California, as also from Arizona to the central Rockies, but the northern 

 States, with a very different tree-record, do not cross-date with Arizona. 

 An electrical instrument is now under construction which it is hoped 

 will reduce this cross-dating by individual rings to mechanical quanti- 

 tative measurement. When that is accomplished it will perhaps be 

 possible to express similarity between groups by a single coefficient. 



Comparison of smoothed curves — The crests of these curves give 

 the phase or epoch of maximum of the various cycles which may not 

 be the same in different regions. Two results appear in this curve 

 comparison, namely, first, a real separation into the three zones, and 

 second, a latitude effect in which there is much more similarity east 

 and west between the zones in their southerly or drier parts, than in 

 the northerly moist latitudes. 



Flagstaff area mean curve — In consequence of the southern sim- 

 ilarity just mentioned, the Arizona area could be regarded as exceed- 

 ing the others in size, for Pine Valley and Charleston Mountains show 

 similarity on the west, and Basin Mountain, Aztec East, and Santa 

 Fe repeat Arizona features on the east. However, the Catalina and 

 Santa Rita Mountain groups near Tucson show marked differences. 

 The Flagstaff area presents an excellent central homogeneous collec- 

 tion of curves from the Grand Canyon to the Rim and Cibecue, a 

 distance of 175 miles (GC, FV, SH, NE, FL, FLU, RL, and J). These 

 curves have been combined together graphically and the mean result, 

 1702 to 1920, is shown in figure 18, upper curve, page 128. This curve 

 is important, because it is probably a better rainfall curve than those 

 of the other zones. We note that shorter periods are largely smoothed 

 out, except parts of a 7-year cycle. A period of 21 years (with lesser 

 14-year effects) strongly dominates, thus agreeing with a result reached 

 in 1908 and referred to in the previous volume (p. 104). The sunspot 

 cycle with its half and double appear in the early parts of the curve, 



