238 GYMNOSPERMS 



valuable, because the large amount of late wood gives extreme 

 strength, while the early wood prevents brittleness. Where the 

 amount of spring wood is relatively large, as in Finns strobus, the 

 timber is light and not so strong. 



The growth-ring in a year with abundant rainfall will be larger 

 than in a dry year. Sometimes there are dry periods of several 

 years, and, consequently, trees over great areas show the small rings; 

 and subsequent, wet or moderate conditions are similarly recorded. 



Fig. 245. — I'iiiu. i,.; ,L ,l;/.>v. .il luiicy I'iiild, Caliluinia, willi tlic Pacific Ocean in 

 the background. 



By cutting living trees in such localities, one can count back antl 

 determine the dates of various wet and dry periods. It is claimed 

 that the dates on which the timbers for some of the prehistoric 

 buildings in New Mexico were cut can be determined with certainty; 

 and the claim seems well founded. A log of Pseudotsuga taxifolia, 

 586 years old, was felled in 1 260 a.d. In this way it was determined 

 that the Pueblo Bonito, in northwestern New Mexico, was under 

 construction as early as the ninth century a.d. The differences in 

 annual rings are not hard to recognize (fig. 246). 



In a Sphagnum bog 7\su^a canadcnsc grows very slowly, with very 

 narrow rings of small, thick-walled cells; but, when a bog is drained, 



