44 CLIMATIC CYCLES AND TREE-GROWTH. 
1835, and only the earlier part, ending in 1834, was used. Extra- 
polated values for the missing part were derived in the usual way. 
A few short, apparently doubtful, regions of rings required careful 
study and it was found that well-adjusted illumination of the rubbings 
was very necessary to their correct reading. When the ring impres- 
sions were deep in the paper, the end of the rubbing showing the tree 
center was held toward the source of light in order that the elevation 
corresponding to the beginning of the spring growth might be brightly 
illuminated. When the impressions were shallow and faint, it was 
noted that the rings became very distinct if the rubbing was held 
between the eyes and the light, thus giving a very faint and perfectly 
even illumination. If this did not bring out the individual rings, the 
rubbing was not used. 
The location in which these trees grew was visited in 1918 and 
general contours were noted. The hills are low and comparatively 
flat-topped, with disintegrated rocks showing in railroad cuttings. 
The sides of the hills are steep, and the valley bottom is narrow and 
usually has a wash near its center. In general the drainage is toward 
the east, but there is no high and sharp ridge between this region and 
the ocean on the west. The situation is far enough north to have a 
good snowfall in winter. It is about 800 feet above sea-level. 
The tabular matter giving the results of the measures on the 17 
Douglas firs of Oregon will be found on page 117. The plotted values 
appear in figure 11. 
THE SEQUOIA GROUP. 
In 1911, after examining the writer’s results obtained on the yellow 
pines, Huntington made an extensive series of measurements on the 
big tree, Sequoia gigantea. He did this work on the stumps themselves 
by direct counting from the outside. This introduced errors of begin- 
ning due to removal or injury of outer rings, and errors of omission 
which of course could not be checked. In order to correct for large 
errors of omission, he worked out an approximate correction on the 
grounds of probability which depended upon a comparison between 
two or more radii of the tree, and in that way many errors were com- 
pensated. In the vast majority of cases, his measures were not of 
individual rings but of successive groups of ten. I have collected seven 
of his trees, and after complete cross-identification verify his centers 
as shown in table 4. 
But Huntington’s method of working directly on the stump enabled 
him to get data from a very large number of trees, some 450, in a way 
that served his purpose very admirably. He was searching for general 
effects, and accuracy to a year or two was less essential. He wished to 
approximate absolute values of rainfall in past climates, in contrast 
with which my chief aim is to get relative and periodic values. These 
