94 PROCEEDINGS OF THE AMHERST MEETING 



thrusting mass must be conceived as emerging from beneath the surface at B 

 and as being separated from the surface crust, BA, by another slanting sur- 

 face, KL, similar to the undersurface, MN. It is particularly with the phe- 

 nomena of underdrag, where the root UU of the advancing mass is driven 

 along beneath the passive crust, AB, that we are here concerned ; the under- 

 drag ought to break up this part of the crust into slabs and blocks on oblique 

 fractures, and each block ought to be dragged along and more or less displaced 

 and tilted by the advance of the root of the overthrust; and such a displace- 

 ment and tilting of the blocks might give rise to the Basin ranges, if they are 

 really characterized by horizontal extension instead of by vertical upheaval. 

 This is of course deeply fantastic, as there is no direct warrant for accepting 

 the existence and movement of such a mass as the root of an overthrust, UU, 

 between the passive masses below and above it; but inasmuch as the occur- 

 rence of overthrusts at the surface is now a well established fact, and inas- 

 much as Gilbert's latest interpretation of the Basin-range displacements sug- 

 gests the operation 6f what is here called underdrag, it is perhaps permissible 

 to inquire into the possible connection of the two phenomena by continuing 

 the overthrust backward underground toward its root. It is manifest, how- 

 ever, that the ultimate root or source of the overthrust mass is not reached in 

 figure 2. 



But there is another interpretation of the displaced Basin-range blocks 

 which involves vertical upheaval, as Gilbert originally thought, rather than 

 horizontal extension, as he later thought. It may be imagined that the spur- 

 end facets, which have been regarded as parts of moderately inclined block- 

 mountain fault-planes, are really landslide surfaces of much less declivity than 

 that of the true fault-planes between the mountain blocks, and that great slabs 

 of the steep-faced blocks slipped down these surfaces into the intermont de- 

 pressions while the displacement was going on; for if the bounding fault of a 

 mountain block be nearly vertical, and if the upheaval of the block be rela- 

 tively rapid, the upper edge of the block might not be able to sustain itself 

 unsupported, and great slabs of it would therefore break off and slide down 

 on the depressed block. The uniformity of inclination in the spur-end facets 

 would at first thought seem to discountenance this idea ; but if the landslide 

 slabs be conceived as 2, 5, or 10 miles in length along a range front, their 

 surface of motion might be fairly regular. The rarity of visible landslides at 

 the base of the ranges may be explained in part by their burial ; but besides 

 buried slides, the rarity of described slides may be explained in part by inat- 

 tention to such slides as are still visible. 



The best evidence in favor of this second view is found in the recent land- 

 slides described by Russell as occurring at the base of very young fault-block 

 mountains in central Washington. Rattlesnake Mountain is described as "one 

 of the most typical monoclinal mountains. . . . Its northeast escarpment is 

 an exceeding bold bluff rising nearly 3,000 feet above the plain . . . and 

 is formed of the broken edges of the strata of Columbia lava, which dip gently 

 to the south and southwest and give the mountain its long, gentle, southern 

 slope. Owing to the fact that many landslides have fallen from its precipitous 

 northeast face, the outcrops of the beds on that side have been covered with 

 debris and greatly obscured." Similarly in the steep scarp of Horse-heaven 



