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. 
