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 



