124 



CLIMATIC CYCLES AND TREE-GROWTH 



This relationship of western cycles only appeared in recent work 

 and is still provisional. It may be real, but, on the other hand, there 

 may be some preferential selection by the analyzing instrument or the 

 observer, in spite of great effort to get rid of such errors. It should be 

 added that the cj'cle given as 20.5±1, really covers the interval from 

 19 to 21, and could have interpretations at 19, 20, or 21 years. The 

 brief study, given later, of solar records in the long Flagstaff tree- 

 records, throws a little more light on this.* 



5 6 7 3 10 II 12 13 K 15 16 17 18 19 20 2; 22 23 24 25 U 27 28 29 30 31 32 33 34 3b ?G 37 38 39 40 



5 6 7 8 3 10 II 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 3940 



Cycle length in years 

 Fio. 15 — Cycles in western zones 



Arizona zone — The Arizona zone is distinguished by the absence 

 of 8, 10, 11, and 17 year cycles and the great dominance of 14 and 20 

 years. Its double sunspot cycle averages a little over 23 years. 



Rocky Mountain zone — Cycles of 10, 11, and 14 years are largely 

 lacking. The 8+ and 17 year cycles have more prominence here than 

 in the other zones, but the 20 and 23 year cycles are the strongest in 

 the zone. 



♦Recent independent tests sustain these results. 



