VEGETATION OF THE SANTA CATALINA MOUNTAINS. 11 



truncated at its lower edge by the Rillito, which flows at right angles 

 to the slope of the bajada, and on the western side of the range the 

 Canada del Oro has worn off the lower edges of the detrital slopes in 

 similar matter. The well-developed bajadas which lie between Pima 

 and Ventana Caiions fall at the grade of 150 to 175 feet per mile. 

 Between Ventana and Bear Cafions the uppermost portion of the 

 bajada has been worn away, so that at present a shallow valley lies 

 between the base of the mountain and the lower portion of the old 

 bajada, now cut into isolated and rounded hills. On the northeast 

 side of the Santa Catalinas the bajada which extends down to the San 

 Pedro River exhibits approximately the same grade as the bajada at 

 Pima Canon. Its surface is crossed, however, by so many drainages 

 from the steep northeast face of the mountain that the bajada region 

 consists of a series of rounded ridges extending out from the base of 

 the mountain, very unlike the relatively flat bajadas of the Santa Rita 

 and El Rincon ranges. 



VEGETATION OF THE SANTA CATALINA MOUNTAINS. 



The journey from the base to the summit of the Santa Catalina 

 Mountains brings to the eyes of the observer a constantly changing 

 panorama of vegetation. New types of plants are constantly being 

 encountered with increase of altitude, while types already familiar are 

 being left behind. There is no portion of the mountain, at least below 

 7,500 feet, in which a climb of 500 feet does not materially alter the 

 physiognomy of the surrounding vegetation. The course of the vege- 

 tational panorama is not merely a gradual transition from the open 

 desert of succulents and microphylls to the heavy fir forest which 

 occupies the summit of Mount Lemmon (plate 1). There are inter- 

 posed between these vegetations two distinct belts of plant life through 

 which this tremendous transition takes place. 



The arborescent cacti and the trees and shrubs of the desert give 

 way gradually to evergreen oaks, leaf-succulents, sclerophyllous shrubs, 

 and perennial grasses. This open but arborescent vegetation reaches 

 a full development and then gives way to pine forest, with a distinctive 

 accompanying carpet of herbaceous perennials. The pine forest is 

 then, in turn, invaded by spruce and fir and the heavy stands of these 

 trees are accompanied by still another assemblage of shrubs and her- 

 baceous plants. The striking character of these gradations of vegeta- 

 tion is not due solely to the contrast between the varied vegetation 

 of the open desert and the monotony of the closed coniferous forests, 

 but is quite as largely due to the striking types of plants which are 

 to be found both in the desert and in the region of evergreen oaks. 



A first and most general observation of these vegetational stages will 

 discover the distinctive regions of desert, of park-like semi-desert and 

 of forest. The first is like the desert of the extensive bajada slopes 

 which surround the entire mountain: the second is similar to plant 



