OCT., 1809.1 TRANSITIOX ZONE. 55 



To the soiitb the Transition zone tills the McCloud and Pitt Kiver val- 

 leys, embraces the canyon of the Sacramento, and stretches onward 

 along tlie flanks of the Sierra all the way to southern California. To 

 the west it overspreads the wild mountain region between Shasta and 

 the Pacitic Ocean, changing gradually from Arid Transition to Humid 

 Transition, and surrounding the Upper Sonoran bottoms of Scott and 

 Hoopa valleys, and the Boreal summits of Salmon, Trinity, and Siski- 

 you mountains. It covers the lower slopes and eastern part of the 

 Siskiyous, and passes around the southern ends of the Salmon and 

 Trinity mountains continu(»usly to the sea. 



On the tianks of Shasta the Transition zone forms a broad continuous 

 belt covering the basal slopes on the northwest, west, and south, but 

 interrupted on the cold east and nortbeatt sides by the Boreal, which 

 here pushes down to the actual base of the mountain, crowding the 

 Transition out to the east around a group of low volcanic liills. 



On the southwest and west it pushes up on ordinary slopes to 

 5,500 or 0,000 feet, rising on stee]) pumice canyon slopes a couple of 

 thousand feet higher, and everywhere eaibracing tongues of Canadian 

 which descend along the cold streams and on the cold easterly slopes 

 of ridges. 



On Shasta the study of the Transition zone is complicated by strong 

 local differences of soil-moisture and humidity — differences that exert 

 a profound effect on the distribution of plants, and to a less degree on 

 that of animals also. It has been shown elsewhere (Life Zones and Ciop 

 Zones of the United States, p. 28, September. 1898) that in some places 

 the Arid Transition of the Rocky Mountains and Great Basin passes 

 gradually into the Humid Transition of the Pacific coast. On Shasta 

 similar changes occur in such small compass that they may be studied to 

 excellent advantage. Thus, near the south end of Shasta Valley the 

 dominant types of vegetation are Pinus ponderom, (^hiercKs calij arnica, 

 Artemisia tridentata, Kmizia tridentata, Aretostapliylos painJa, Chryso- 

 thamnua oceidentalis, Rhus irllohata^ Garrya fremoHti, and rrniius sab- 

 cordata, all characteristic Arid Transition species. On moister soils 

 near by, particularly in shady canyons, the dominant types are Pseu- 

 dotsiKja mucronata, Abies concolor lowiaria, Acer glabrnm, Cornus mittalli, 

 Ruhns parvifloriis {=^niitlanus Auct.) Spircea douglasi and other Humid 

 Transition species. 



In this connection it is important to bear in mind that the extreme 

 bottom of the west slope of Shasta, between the Sacramento Elver and 

 Shasta Valley (elevation about 3,4C!J feet), is decidedly cooler and more 

 boreal than the middle part of the slope 1,000 or 2,000 feet higher. 

 The reason is twofold: The bottom ])art of the west slope, from the 

 head of the Sacramento Canyon northward, lies close to the cast base of 

 Mount Kddy and the Scott Mountains, by which it is sliielded from the 

 late afternoon sun, and consequently receives less heat than higher 

 parts of the same slope. Furthermore, it is well watered, and the rapid 

 evaporation caused by the dry atmosphere tends to lower the tem- 



