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, illustrate 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.
