72 CLIMATIC CYCLES AND TREE-GROWTH. 
trees is probably not as good in details as Huntington’s samples from 
young and sensitive trees. His material is well worth cross-identifying 
and dating with care, and then comparing with any records of snowfall 
which can be obtained from the sequoia groves. It is greatly to be 
regretted that Fresno, 65 miles away and at 5,000 feet lower elevation, 
is the nearest point where precipitation records can be obtained for a 
period long enough to be of value. 
Future work.—It will be very interesting to find whether the charac- 
teristics of the correlation at Prescott are general in arid climates and 
dry soils and whether practical formulas for conservation in moist 
soils or climates can be worked out. When this is done the significance 
of the study of annual rings will be greatly increased. 
METEOROLOGICAL DISTRICTS. 
The study above described raised emphatically the question as to 
the extent of the region or district from which comparative rain 
records should be selected. Such a meteorological district could be 
defined as one in which homogeneous weather elements are found. 
But we immediately ask ourselves the questions: must all weather 
elements be alike in it or is it sufficient to have only rainfall (for 
example) essentially the same throughout; will the district remain 
constant through indefinite time or will it change; is the district for 
short-period weather changes the same as the district covered by 
secular changes. In the present discussion I have understood by 
meteorological districts such regions as may show similar or identical 
variations in some one weather element. It seems likely that a region 
which may show unity in small or rapid variations may not do so in 
large and slow variations, or more likely may be a small fraction of a 
region which will show unity in large variations. 
Meteorological districts and growth of trees.—The_ cross-identifica- 
tion of trees over large areas has already suggested the use of annual 
rings as a possible aid in delineating meteorological districts. This 
function of the rings has received some exemplification in the present 
study. For instance, the pine trees of Norway differed in such a way 
that it was necessary to divide them into two classes, one of which came 
from the outer coast near sea-level and the other from the inner fjords 
and mountains. The trees from these different regions show strong 
reversal with reference to each other. Again, the trees from the low- 
lands about the Baltic Sea show marked similarity in their variations 
and indicate, as we would expect, a homogeneous district. Further- 
more, groups from near the Alps show strong differences from the 
other European groups, as we might expect from our experience with 
the five groups from the mountainous country about Prescott. A 
rugged and mountainous region is very difficult to divide satisfactorily 
