OcT., 1899.] TRANSITION ZONE. ya) 
To the south the Transition zone fills the McCloud and Pitt River val- 
leys, embraces the canyon of the Sacramento, and stretches onward 
along the 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 Tumid 
Transition, and surrounding the Upper Sonoran bottoms of Scott and 
THoopa valleys, aud 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 continuously to the sea. 
On the flanks 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 northeast 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 hills. 
On the southwest and west it pushes up on ordinary slopes to 
5,500 or 6,000 feet, rising on steep pumice canyon slopes a couple of 
thousand feet higher, and everywhere embracing 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 C1op 
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 
sinilar changes occur in such small eompass that they may be studied to 
excellent advantage. Thus, near the south end of Shasta Valley the 
dominant types of vegetation are Pinus ponderosa, Quercus californica, 
Artemisia tridentata, Kunzia tridentata, Arctostaphylos patula, Chryso- 
thamnus occidentalis, Rhus trilobuta, Garrya fremonti, and Prunus sub- 
cordata, all characteristic Arid Transition species. On moister soils 
near by, particularly in shady canyons, the dominant types are Pseu- 
dotsuya mucronata, Abies concolor lowiana, Acer glabrum, Cornus nuttalli, 
Rubus parviflorus (—=nutkanus Auct.) Spirea 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 River and 
Shasta Valley (elevation about 3,400 feet), is decidedly cooler aud more 
boreal than the middle part of the slope 1,000 or 2,000 feet higher. 
The reason is twofold: The bottom part of the west slope, from the 
head of the Sacramento Canyon northward, lies clase to the vast base of 
Mount Eddy and the Scott Mountains, by which it is shielded 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- 
