26 BOTANY. 



On the limestone rocks near Camp Bowie were Cevallia sinuata and 

 Macrosiphonia brachysiphon, plants that we found to be by no means common. 

 The Artemisias no longer formed the predominant feature of the plains 

 landscape, as they had north of the Mogollon Mesa. 



The immense stretch of plain from Camp Grant south, gradually be- 

 comes lower, until at Tucson it is but 2,400 feet above the sea. This plain 

 as far as the San Pedro — say thirty-five miles north of Tucson (though, as 

 1 >ef< >re stated, in the main dry) — is covered with a luxuriant growth of grasses 

 of nutritious character, wherever, as at Sulphur Spring, moisture is found in 

 sufficient quantity in the soil. The immediate slopes of the San Pedro 

 Vhlley are densely covered with Atriplex, Sarcobatus, Suceda, etc., while 

 the malarial cursed flats along the river produce heavy crops of the ordinary 

 cereal grains and garden vegetables. Thence to Tucson the country be- 

 comes more sandy, and even the Chenopodiacea give way largely to Larrea 

 and various species of Cadacece. 



From Tucson south the plain again rises until at Tubac it is again at 

 least 3,000 feet, and east of the Santa Rita Mountains Old Camp Crit- 

 tenden stands at an elevation of 4,74!) feet. Here we leave the area of the 

 Colorado Eiver drainage, and enter another, sloping toward Mexico. 



To generalize : we may say that from the Gila south almost to the 

 Sonora line (along our route of travel), the country may be regarded as a 

 plain with a gradual slope to the south, more or less barren and dry save 

 along the river-banks, and in the immediate vicinity of springs; Avith the 

 Pinaleno, Caliuro, Santa Catalina, and Chiricahua Ranges, and Dragoon 

 and Santa Rita Mountains rising above the general level to a height of from 

 6,000 to 10,400 feet, the middle altitudes or mesas shading off into plains 

 below and leading to mountain elevations above, with in neither case a 

 clear line of demarcation between. 



Indeed, we may go a step further and consider the entire country from 

 South Park south to the Mexican line as a series of continental swells and 

 depressions, illustrating still this southward slope. 



