TOPOGRAPHIC FEATURES DUE TO LANDSLIDES. 481 



slide in the "White Mountains in 1826, and the still greater one which 

 descended into Biraki Ganga River, near Gohna, India, in 1893, and 

 gave origin to Gohna Lake, are instances of this nature. A large 

 number of landslides, however, are due to geological rather than 

 climatic causes, and it is to this class that I wish to invite attention. 



It frequently happens that a layer of hard rock rests on softer or 

 more easily soluble beds. When steep escarpments are formed of 

 two strata having this arrangement, conditions are produced which 

 favor the breaking off of masses of the hard upper layer and their 

 descent to the foot of the escarpment in landslides. There are other 

 conditions, such as the thickness of the hard layers and the manner 

 in which they are jointed, and the consistence of the soft beds — 

 whether slippery clays, loose volcanic lapilli, etc. — which modify 

 the process. The main or controlling conditions referred to are fur- 

 nished at hundreds of localities in the region occupied by what is 

 known as the Columbia lava, and drained by Columbia River. In 

 that region sheets of basalt, ranging from two to five hundred feet 

 and more in thickness, alternate with or overlie sheets of clay, shale, 

 volcanic tuff, etc., which in many instances are hundreds of feet 

 thick. The basalt was poured out in a molten condition and sj)read 

 over the land in horizontal sheets. On cooling, these sheets acquired 

 a columnar structure due to joints, usually at right angles to the top 

 and bottom surfaces. This columnar structure facilitated the break- 

 ing away of great masses of basalt, when, for various reasons, por- 

 tions of the sheets form the summits of bold escarpment. 



In part, the Columbia lava, and the softer beds interleaved with 

 it, have remained undisturbed, and are now practically horizontal 

 over thousands of square miles. In adjacent areas of a great extent 

 the beds have been broken by extensive fractures and the blocks thus 

 formed varioush^ tilted. Over still other extensive regions, particu- 

 larly in the Cascade Mountains, the originally horizontal sheets, 

 aggregating several thousand feet in thickness, have been raised into 

 dome-shaped uplifts, at least one of which is nearly circular, while 

 others are more nearly elliptical, the major being several times as 

 long as the minor axis. Some of these domes, if unaffected by ero- 

 sion, would have an altitude above their immediate bases of from 

 five to eight thousand feet. 



Where the Columbia lava is still essentially horizontal, as in 

 southeastern Washington, it has been dissected by streams which 

 now flow in magnificent caiions with clifflike walls from a thousand 

 to four thousand feet high. In the region referred to as having been 

 extensively fractured, cliffs have been produced by the tilting of the 

 blocks thus produced. The dome-shaped uplifts have been broken 

 by weathering and by the work of streams in such a manner as to 



TOL. LIII. — 34 



