482 POPULAR SCIENCE MONTHLY. 



remove their more elevated portions, and leave their tnmcated bases 

 as a series of cliffs facing inward toward the center of the uplift. 

 Thus, in many ways, and owing to still other conditions noted in part 

 below, the region of the Columbia lava has become a land of great 

 escarpments. 



The fact that the escarpments referred to are formed of the edges 

 of layers of hard basalt, which are traversed by joints at right angles 

 to the planes of bedding, and also the occurrence of layers of soft 

 rocks beneath the hard, cliff-forming layers, furnish conditions un- 

 usually favorable for landslides. In fact, landslide topography, as 

 it may be termed, is nearly as characteristic of the Columbia lava 

 region as are its magnificent cliffs. The topographic features due 

 directly to landslides themselves are probably seldom recognized, 

 while the long lines of frowning escarpments obtrude themselves on 

 the attention of even the least observant. The fact is, however, that 

 the cliffs in many instances have resulted from the breaking away 

 of large rock masses, and will in time be destroyed by the same 

 process. 



The alternation of lava sheets and of lacustral sediments, etc., is 

 not a marked feature of the entire country occupied by the Columbia 

 lava, and for this reasan great variations occur in the extent to 

 which the escarpments of that region have been affected by land- 

 slides. In southeastern Washington, for instance, sedimentary or 

 other soft layers between the lava sheets are relatively unimportant, 

 and throughout scores and even hundreds of miles of canon walls 

 appear to beabsent. In this portion of the field the evidences of 

 former landslides have not been noted. 



In striking contrast with the region just referred to is the great 

 dome from which the Wenatchee Mountains have been sculptured. 

 (The Wenatchee Mountains are situated in the central portion of 

 Washington, and on the eastern flank of the still vaster and much 

 elongated Cascade dome. Mount Stuart and a number of associated 

 peaks composed of dense granite form the center of the Wenatchee 

 dome, and now stand in bold relief, owing to the removal of softer 

 beds from about them.) In this instance the central portion of a 

 dome fully fifty miles in diameter, has been removed and the 

 truncated edges of the hard layers composing it left in prominent 

 escarpments which sweep about the central core of granite in vast 

 irregular curves. At least four sheets of Columbia lava, varying in 

 thickness from three to four or five hundred feet, formerly extended 

 over a large portion and possibly covered the entire region where the 

 Wenatchee dome was upraised, but have been eroded away from 

 its central portion. The imcovering of the region referred to, em- 

 bracing fully one thousand square miles, has been accomplished by 



