DAVIS: MOUNTAIN RANGES OF THE GREAT BASIN. 133 
backward slope of the block, cx, is also partly buried in a plain of waste, 
FJ, which meets another waste plain from a third faulted block, pK. 
Certain features shown in the figure are essential to the type in the 
stage of erosion here considered. The fault-bounded block, ace, must 
present a back-sloping surface, crE, whose form before the faulting oc- 
curred is now more or less modified by erosion in its exposed part, cr, 
and buried under waste in its depressed part, aB. The lower part, as, 
of the faulted face, apc, is buried under the waste derived from the 
exposed and more or less dissected part, Bc. Blocks which stand so 
high that the trough between them is now dissected instead of aggraded 
Fiecure 1. 
Diagram of a tilted block; youthful stage. 
are not here considered. Examples of this kind are described in the 
northern Sierra Nevada by Diller (1886, 12-16). 
Other features of the type are extremely variable. The size, structure, 
and form of the block are entirely undefined. Its upper surface may 
have been in the prefaulting period, a peneplain worn down on ancient 
schists ; a mountain area of folded or faulted strata more or less sub- 
dued by erosion ; a series of horizontal and slightly dissected aqueous or 
igneous strata; or anything else. The block faulting may be on a large 
or small pattern ; of regular or irregular arrangement ; reaching over an 
extensive or a restricted area ; of great or little displacement ; and with 
much or little tilting. 
The displacement may be slow or rapid, uniform or variable in rate, 
of brief duration or long continued, of remote or reeent beginning and 
ending ; it may vary greatly in amount along the fault line, diminishing 
