16 CLIMATIC CYCLES AND TREE-GROWTH. 
may be divided into two classes: first, local errors of identity in small 
groups of rings in a few individual trees, which simply flatten the 
curve without affecting the final count; second, cases in which a given 
ring, in spite of attempts at cross-identification, is still in doubt, 
showing clearly in perhaps half of the trees and not in the other half. 
Such cases affect the final count, but do not flatten the curve. They 
leave a question of one year in the dating of all the earlier portions of 
the curve. Only two cases of this latter kind have been noted. One 
was the year 1822 in the Flagstaff pines (of which there is very little 
doubt) and the other is the ring 1580 in the sequoias, which was finally 
decided by material gathered in the special trip of 1919. 
CROSS-IDENTIFICATION. 
Apart from care in measuring the rings, the details of which will be 
given in Chapter IV, the most fundamental and essential feature of the 
method of studying tree-growth is the cross-identification of rings 
among a group of trees. The ease and accuracy with which this can 
be done in a fairly homogeneous forest is remarkable. A group of 13 
tree sections collected along a distance of a quarter of a mile in the 
forest of Eberswalde, near Berlin, show almost identical records. Two 
to ten rings in every decade have enough individuality to make them 
recognizable in every tree. A group of 12 sections from Central 
Sweden show such agreement that there is not a single questionable 
ring in the last 100 years or more. Especially marked combinations 
of rings can occasionally be traced across Europe between the groups 
hereafter mentioned. In Arizona the identification across 70 miles 
of country is unquestioned, and even at 200 miles the resemblance 
is apparent. 
The value and accuracy of cross-identification was first observed 
in 1911 in connection with the Prescott trees. After measuring the first 
18 sections, it became apparent that much the same succession of rings 
was occurring in each; therefore the other sections were examined and 
the appearance of some 60 or 70 rings memorized. All the sections 
were then reviewed and pinpricks placed in each against certain rings 
which had characteristics common to all. For example, the red ring 
of 1896 was nearly always double, while the rings of 1884 and 1885 were 
wider than their neighbors. In the 60 years investigated several 
obvious details in each decade appeared in every tree. After this 
success it was evident that the process should be applied to the Flag- 
staff trees which had been previously collected. Of the 25, however, 
only 19 had been preserved. A minute comparison was made between 
these with complete satisfaction: Since then this process has been 
applied with great care to every group. 
After the Flagstaff set was finished, it was compared with the Pres- 
cott group. It was interesting to find that the Flagstaff ring records 
