CORRELATION WITH RAINFALL. 73 
into meteorological districts. Yet, in spite oflocal differences, mountain 
regions may be alike in major characteristics, for all the Prescott 
groups, though differing among themselves, cross-identify excellently 
with the Flagstaff trees 60 miles away. The sequoias also cross- 
identify perfectly in mountain localities 50 miles apart, showing that 
there is enough similarity in different parts of the high Sierras to cause 
the trees to agree in many variations. 
Arizona and California——Fully 450 miles intervene between the 
sequoias of California and the pines of Arizona, yet there are strong 
points of identity between them in the last 300 years. The dates of 
notably small rings are much alike in each. The details of this com- 
parison have not yet been fully studied, but they support the idea long 
since expressed (1909) that Arizona and California, especially its 
southern half, form parts of a large district which has similarity in 
certain variations. A long acquaintance with this region throws light 
on the details of this similarity. The winter precipitation, which is 
largely in the form of snow at the altitude of the trees studied, has the 
major influence on tree-growth, for it is largely conserved near the 
trees, whereas the summer rains are usually torrential and the water 
quickly flows away. The winter storms moving in an easterly direction 
reach the coast region first and after about 24 hours are felt in Arizona. 
Thus, in spite of the coast range of mountains and the intervening 
low-level deserts, each winter storm passes over both regions and 
causes an evident similarity between them. Ina large view they belong 
to a single meteorological district. 
Meteorological districts and solar correlation—In searching for a 
link of connection between solar variation and meteorological changes, 
we must bear in mind the effect of possible reversals in neighboring 
meteorological districts, such as noted above in Norway. It may be 
the lack of such precaution which has caused many meteorologists to 
- condemn at once the suggested connection between the distant cause 
and the nearby effect. We must remember that districts may be small 
in area, andin combining many together we may neutralize the result 
for which we are in search. Some illustration of correlation found in 
small districts will be given in the final chapter. 
