50 THE PLANT WORLD. 



Geological record shows the higher portions to be verv old ; 

 they constitute an integral part of the Cordilleran uplift, but 

 the lower portions of the Territory are more recent and to about 

 the 3,000 foot level were washed by the waves of Tertiary seas.* 

 It is not surprising, therefore, that, with so extensive a varia- 

 tion in altitude, with a portion of its territorv constituting a 

 part of a great plant highway, and with a situation so far south, 

 31.3° to 2)7° ' the plant covering of Arizona should be so diverse 

 in character, as, without doubt, it also is in origin. 



Plant Zones in the San Franciscan Region. 



Perhaps in no region does the succession of plant zones occur 

 in so short a distance as in the northern portion of Arizona. 

 From the summit of the San Francisco mountains, an altitude 

 of 12,600 feet, to the bottom of the Grand Canyon, which at 

 Bright Angel creek is about 2,400 feet above the sea, the distance 

 in a straight line is probably not more than forty miles. This 

 great difference in altitude is accompanied by an equally great 

 variation in temperature. Merriamf assumes the mean tempera- 

 ture during the period of reproduction at the altitudes of the 

 various plant zones in this region to be as follows : 



Arctic-alpine zone, 11,500 to 13,000 ft 39-2° F. 



Subalpine or timber line zone, about 10,500 to 11,500 ft... 44.6° 

 Hudsonian or spruce zone, about 9,000 to 10,500 ft. ..... . .50° 



Canadian or fir zone, 8,000 to 9,000 ft 554° 



Neutral or pine zone (transition ?), about 7,000 to 8,000 ft. 60.8° 



Pinon or ceder zone, about 6,000 to 7,000 ft 60.2° 



Desert zone, about 4,000 to 6,000 ft 71.6° 



Thus, in so short a distance as forty miles as the crow flies, a 

 vertical one of nearly two miles is crossed during which climatic 

 changes and plant zones are encountered which correspond to 

 the temperatures and the plant zones of all of North America 

 outside of the tropics and the extreme north. 



* Privately communicated to tbe writer by Professor W. P. Blake, Uni- 

 versity of Arizona. 



t Results of a Biological Survey of tbc San Francisco Mountain Region 

 and the Desert of the Little Colorado, Arizona. U. S. Dept. of Agric, 

 Div. of Ornithology and Mammalogy, North American Fauna, No. 3. 

 page 32. 



