236 DOUGLAS WILSON JOHNSON 
on this point; but it may be said that both the contours of the 
Klamath topographic quadrangle and such glimpses as we secured 
of the back slopes agree in suggesting that the prefault topography 
was moderately rugged. The back slope of neither block appears 
to be as featureless as it would be had the region subjected to fault- 
ing consisted of a young lava plain on the one hand, or, on the 
other hand, of a volcanic region reduced by long-continued erosion 
to a peneplain. Dr. Gilbert writes that the reconstruction of the 
prefaulting relief would be a difficult task, because the great blocks 
have been ‘‘intricately sliced and dislocated on a small scale; and 
one of the marvelous features of the region is the association of 
major faulting with elaborate contemporaneous minor faulting.”’ 
West of the southern end of the Modoc Point block is a sub- 
sidiary fault block called Plum Hills or Plum Ridge. Near the 
northern end of the steep, undissected scarp which bounds this . 
ridge on the west there is a magnificent exhibition of slickenside 
surfaces visible from the passing train. Here we have preserved 
that rare phenomenon, a portion of the actual fault plane of a fault- 
block mountain. The perfection of the slickenside surfaces may 
be inferred from the fact that one of them has been used as a 
“bill board”’ upon which are painted the advertisements of certain 
business houses, as shown in Fig. 5. 
