THE SECRET OF THE BIG TREES. 11 
have been kept varies in harmony with the amount of precipitation. 
Other investigators have since done similar work elsewhere, and it 
is now established that in regions with cold winters and dry summers 
the thickness of the annual layers of growth gives an approximate 
measure of the amount of rain and snow. 
Obviously the best trees upon which to test the theory of climatic 
changes are the big trees of California. They grow at an altitude 
of 6,000 or 7,000 feet on the western slope of the Sierra Nevadas. 
Abundant snow falls in winter and there is a fair amount of rain 
up to about the 1st of June, but the rest of the warm season until 
the end of September is dry. Hence the conditions are highly 
MARIPOSA GROVE, YOSEM1TE NATIONAL PARK. 
Tbotograph by I’illsbury Picture Co. 
