1936] MAMMALS AND LIFE ZONES OF OREGON 5 



sheer cliffs and abrupt rimrock effects so common over the eastern 

 part of the State. 



The great peaks of the Cascades, from Mount McLoughlin (Pitt) 

 and Mazama on the south to Jefferson and Hood on the north, are 

 all old craters from which the Cascade Kange has been augmented 

 by enormous flows of lava that reach both slopes and spread out 

 over the valleys. While the ice-clad peaks, some of them world- 

 famous spectacles, are of special interest in their fauna and flora and 

 of great value as sources of water, the broad basal slopes of the 

 range, bearing dense forests of valuable timber, are especially impor- 

 tant in human economy. 



The configuration of surface, various types of erosion, color and 

 character of soils, and to some extent, the character of forest and 

 other vegetation forming the ground cover, are modified by the gen- 

 eral volcanic base so widely spread over the State. The general fer- 

 tility of the valley soils is in part due to their volcanic origin, while 

 even the resulting dark colors are in some cases an advantage in 

 added ability to absorb light and heat from the sun. It is well known 

 that species of plants and animals range to higher altitudes on dark 

 soil than on light and that other high-ranging species range lower 

 down on light-colored soils. 



The Paulina, Yamsay, Yanax, and Klamath Mountains, lying 

 east of the Cascades, are of the same volcanic type, but are scattered 

 in lower groups or buttes. Their height reaches generally only to 

 about 7,000 feet, but this is sufficient to give them cold upper slopes 

 with an extra amount of precipitation and relatively heavy growths 

 of timber. 



Still farther to the eastward other types of ranges rise abruptly 

 from the desert valleys. While composed mainly of volcanic mate- 

 rials, they appear as elongated ridges with one or both sides broken 

 and pushed up along more or less well-defined fault lines. These 

 include the Winter Eange, Warner and Hart Ranges, and the higher 

 and still more striking Steens Mountains. 



The Steens Mountains, a boldly tilted uplift from the lava plains 

 of southeastern Oregon, are largely volcanic from near the base to 

 the summit. Their sharply faulted eastern escarpment rises abruptly 

 5,500 feet above the valley bottom, attaining a total altitude of about 

 9,354 feet in the highest parts (pi. 3, A). Of them Eussell (1903, 

 p. 19) 1 says in effect: 



The eastern slope of that splendid mountain is composed of the broken and 

 eroded edges of sheets of basalt, which dip westward at an angle of about 

 3 to 4 and present an aggregate thickness of not less than 5,000 feet, while 

 the lacustrine sediments beneath their base reach a thickness of at least 1,000 

 feet. Thin layers of sandstone separate some of the laval flows but even those 

 which rest directly upon each other are easily counted in cross section and 

 some 30 or 100 sheets, averaging about 60 feet thick, are more or less exposed 

 in this escarpment. 



Deep canyons are guttered into the sides of the range, and canyon 

 walls expose heavy stratifications of successive lava flows through 

 which the mountains have been pushed up. At the northern end of 

 the range several broad U-shaped canyons show evidence of glacial 

 erosion, and several beautiful examples of hanging valleys along the 



1 Citations in italic in parentheses refer to the bibliography, p. 394. 



