Page:Popular Science Monthly Volume 53.djvu/500

482 remove their more elevated portions, and leave their truncated 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 unusually 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 reasanreason [sic] great variations occur in the extent to which the escarpments of that region have been affected by landslides. 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 beabsentbe absent [sic]. 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 uncovering of the region referred to, embracing fully one thousand square miles, has been accomplished by