III. COLLECTION OF SECTIONS. 
The material upon which the discussion of climatic cycles and tree- 
growth is based has been derived from 230 trees collected in the 15 
years from 1904 to 1918. The regions drawn upon comprise chiefly 
Arizona with its yellow pine, the Baltic drainage area of north 
Europe with its Scotch pine, and the high Sierras of California with 
their great sequoia. Two small collections come respectively from 
the northeast and northwest coast of the United States. The col- 
lections have been made in small, convenient groups as opportunity 
offered, to each of which a name has been given which will appear 
below. 
The relative dimensions of the various groups may be expressed in 
terms of the number of measures of rings. In the first Flagstaff group 
there were about 10,000. In the second Flagstaff group of 1911 only a 
few hundred. The Prescott groups included about 4,000; the 9 
European groups about 9,000. The Vermont group had between 
2,500 and 3,000, and the Oregon group about the same. The first 
collection of sequoias in 1915 had about 25,000 measures and the col- 
lection in 1918 embraced about 22,000. 
Throughout the whole study it was desired to get as long records 
as possible and old trees were therefore selected. In nearly every case 
this meant large trees also. Apart from this no special selection of 
trees was made at any time, save only in the Christiania group, in which 
so many of the logs showed a “complacent” habit, with long succes- 
sions of equal rings rather large in size, that some effort was there made 
to find the logs which showed variations in ring-size. A complacent 
ring-record without doubt means that the environment of the trees was 
well adapted for its best development. 
THE FIRST FLAGSTAFF GROUP OF TWENTY-FIVE SECTIONS. 
The plan of using tree-rings for the general purpose of a check on 
astronomical and meteorological phenomena was first formulated in 
1901. The first measurements were made in January 1904, on a huge 
log in the yards of the Arizona Lumber and Timber Company at Flag- 
staff. This method of measuring was extremely inconvenient and the 
succeeding 5 sections were cut from logs and sent to town for more 
careful examination. Hence the exact location of these first 6 was 
never visited. The remaining 19 trees were selected in 1906 by myself 
in the forest while the logs were yet lying near their stumps, and I was 
able to mark on each section the points of the compass and otherwise 
describe the location. The measurements were completed in 1907 and 
published in the Monthly Weather Review of June 1909. They had 
not been subjected to cross-identification and, when the value of this 
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