14 INTRODUCTION. 



The unobstructed topography of the plateau where the trees were 

 collected is without doubt a very favorable feature. This leads to 

 very similar conditions for the trees over many miles of country and 

 doubtless greatly assisted in producing concordant tree-records. On 

 the other hand, the San Francisco Peaks, 10 miles north of town, illus- 

 trate how meteorological data may vary in rugged localities. The west 

 slopes of these mountains are exposed to the winter westerly storms 

 and have an immense snowfall. Springs abound and all favorable 

 localities are taken up as ranches. East of the mountain, however, 

 the land is dry and barren, and long distances intervene between 

 watering-places. 



In a very rugged country like that about Prescott similar differences 

 between east and west mountain slopes must constantly occur. This 

 is the reason of an early difficulty with the Prescott groups. Nearly 

 60 trees from various localities were measured before a group was 

 found close enough to Prescott to be compared minutely with records 

 of precipitation at that place. 



