24 CLIMATIC CYCLES AND TREE-GROWTH. 
process was recognized in 1911, the 19 sections of which samples had 
been preserved were compared and a complete cross-identification 
carried through. Thus the errors of identity in the former tabulation 
were found (published in 1914*) and a complete new set of tables and 
averages made from the original measures. For a time it was thought 
that an error of one year might exist in the period of the great drought 
of 1820-23, but the various checks made upon identity lead easily to 
the belief that there are no errors of identity in this 500-year series. 
Subgroups.—The trees of this group were divided into three sub- 
groups consisting of (1) 6 trees from 3 miles south of Flagstaff; (2) 9 
trees from 11 miles southwest of Flagstaff; (3) 10 trees from a point 
1 mile west of the last subgroup. A comparison of the 3 subgroups 
clearly reveals the general character of the longer periods hereafter 
to be discussed and shows lesser variations to be common to all. 
Interesting differences, depending on the location in which the trees 
grew, have been mentioned. 
First suggestion of cross-identification—Other interesting facts came 
to light. It was especially noticeable that a given year of marked 
peculiarity could be identified in different trees with surprising ease. 
This is illustrated in plate 2, where shavings from 5 of the Flagstaff 
trees have been photographed; the photographs have been enlarged 
to such a scale that the distance from the large ring 1898 (indicated 
by the upper line of black crosses) to the small ring 1851 at 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 a very 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 many 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 charac- 
teristically marked that the identification of the same series of rings 
can be made with little doubt, whether at Flagstaff or at Prescott. 
The Flagstaff 500-year record.—Figure 3 shows the Flagstaff tree 
record from 1385 to 1906 A. D., a period of 522 years. The table of 
measures from which the curve was plotted will be found on page 
112. To give the record from 1503 to 1906, 5 trees are used, and com- 
parisons showed that these 5 gave as accurate a record as a larger 
number whose inclusion would have shortened the record or made 
awkward breaks in it. The earlier part of the record is from 2 trees 
