104 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



between the precipitation at Prescott and the annual tree-growth nearly 70 miles distant 

 seemed evident. Later studies have confirmed this conclusion and show that the agree- 

 ment between tree-growth and rainfall is fairly close when the two are measured at approxi- 

 mately the same place. For more distant localities an agreement in individual years is 

 not to be expected, but averages of three or more years show strong similarity, even in 

 places so far apart as Prescott and the Californian Coast, 500 miles to the west. As soon 

 as it became evident that the method under consideration gave genuine results further 

 measurements were made. Lists of the sizes of individual rings of each of 25 trees were 

 prepared. The trees were divided into three groups consisting of: A, 6 trees from 3 miles 

 south of Flagstaff; B, 9 trees from about 11 miles southwest of Flagstaff; C, 10 trees a 

 mile west of the last group. A comparison of the three groups clearly reveals the general 

 character of the longer periodicities hereafter to be discussed and shows many lesser 

 variations common to the three groups. Interesting differences also appear corresponding 

 to the location in which the trees grew. Group A dropped to strong minima in 1780 

 and 1880 more promptly than the others. This appears to be due to the fact that it grew in 

 a porous Umestone soil lying upon rocks full of crevices. The other groups grew on recent 

 lavas, very compact and unbroken and covered with rather a thin layer of claj-ey soil. 

 In the region where group A grew, the rain passed quickly through the soil and was not 

 so well conserved as in the other groups where the water could find no convenient outlet. 



Other interesting facts came to light. It was especially noticeable that a given year of 

 marked peculiarities could be identified in different trees with surprising ease. For instance, 

 this is illustrated in Plate 4, where shavings from three of the Flagstaff trees have been 

 photographed, and the photographs have been enlarged to such a scale that the distance 

 from the ring for 1898, indicated by the upper hne of black crosses, to 1851, the lower line 

 of crosses, is equal in all cases. The other lines of crosses indicate the noticeably broad rings 

 of 1868 and 1878. An examination of the photographs shows that the most characteristic 

 feature is a group of narrow rings about the years 1879 to 1884. These can be identified in 

 practically every tree, and an examination of stumps, which were not measured, showed 

 that it was easy to pick them out wherever one chose. Striking verification of this was 

 found in the case of a stump near town which had been cut about 20 years previously. 

 By finding this group of rings the writer was able to name the year when the tree was felled 

 and the date was verified by the owner of the land. In the more recent work this same 

 group shows conspicuously among Prescott trees, and in general 95 per cent of these 

 trees have rings so characteristically marked that the identification of the same series of 

 rings can be made with little doubt, whether at Flagstaff or at Prescott. 



As a rule, the thickness of a given ring is not uniform on all sides of the tree. It varies 



for accidental reasons, and also according to the points of the compass. In the 19 trees 



of groups B and C the maximum growth occurs a little to the east of north. The average 



variation between the maximum growth in the northerly direction and mininnun growth 



to the south is 12 per cent. The explanation of the increased growth to the north is in the 



increased amount of moisture on that side, due to the slower melting of snow and the 



decreased evaporation in the shade. For nearly all these trees, also, the ground had a 



gentle slope toward the south, so that moisture working downhill would come to the north 



side first. All of these facts agree in pointing to moisture as the factor of greatest influence 



in tree growth. 



THE DATING OF RINGS. 



In comparing the growth of trees and the rainfall over long periods of years, it is essen- 

 tial that the date of formation of any individual ring shall be certain. There is little 

 danger that two rings will coalesce, for the cold winters at an elevation of 7,000 feet cause 

 the seasonal growth to be sharply defined. The mean temperature of 29° F. in January 

 is so different from that of 65° F. in July that the ring of one year is nearly alwajs clearly 



