26 



NORTH AMERICAN FAUNA 



[No. 55 



In the middle section of the Cascades around Mount Jefferson 

 (fig. 2) the zone extends from about 3,500 to 5,500 feet on the north- 

 east slopes and from about 4,500 to 6,500 on the southeast slopes, and 

 around the Three Sisters Peaks (fig. 3) there is a noticeable eleva- 

 tion of the zone to about 4,500 to 6,000 feet on the northeast slopes, 

 and a little under 5,000 to 7,000 on the southwest slopes in con- 

 formity with the more elevated base level of the upper Deschutes 

 and Willamette Valleys on the two sides of the range. 



s.w. 



10.352 feet- 



N. E 



Sea /eve/ 



FIGURE 3. Life zones on Three Sisters Peaks from northeast to southwest exposures, 

 showing approximate elevation above sea level and effect of elevated base level on 

 northeast slope. 



In the southern Cascades the zone extends generally from about 

 5,000 to 7,000 feet in altitude on the cold, or northeast slopes, and 

 from about 5,500 to 7,500 feet on the warmer southwest slopes, except 

 on Mount McLoughlin, where it shows little variation from 5,000 

 to 7,000 feet clear around the mountain (fig. 4). In many places 

 west of the mountains with the low base level of nearby valleys, 



sw. 



N.E 



So no ran 



Sea level 



FIGURE 4. Life zones on Mount McLoughlin (Pitt) from northeast to southwest expo- 

 sures, showing effect of elevated base level on northeast side in neutralizing the usual 

 difference in zone levels on sunny and shady slopes. 



the lower edge of the zone falls 500 to 1,000 feet lower, or to 4,000 

 or 4,500 feet in altitude, without corresponding change in its upper 

 limit. 



