DAVIS: MOUNTAIN RANGES OF THE GREAT BASIN. tSs 
As long as the faulting and tilting continue, strong relief may be 
maintained ; but after displacement ceases, erosion will advance without - 
more hindrance than is offered by the resistance of the rocks; it will 
slowly subdue the earlier relief to rounded forms, and still more slowly 
widen the valleys and consume the intervening hills as the forms of old 
age, Figure 3, are realized. Ina late stage of degradation, the mountain 
mass will be invaded by numerous flat-floored, branching valleys between 
low, rounded forking spurs. The valleys will then be largely adjusted 
to the weaker rock structures, while the fading ridges will stand longest 
where upheld by the resistant structures. The mountain base, an 
Figure 3. 
Diagram of a tilted block; old stage. 
irregular line, will have no close relation to the path of the fault, and 
the slope between the mountain base and the fault line will carry a 
thin and discontinuous veneer of waste on a planed rock floor. It would 
probably be impossible to distinguish the residuals of tilted and lifted 
blocks in a late stage of erosion. 
Place and Value of Deduction. — It is important here to emphasize 
two general considerations. First, the details of form appropriate to 
any desired special case under the ideal type should be deduced with as 
much completeness and definiteness as possible. As long as the details 
of a theoretical form are vaguely conceived, the observer will be unable 
to give his theory a rigorous test ; its consequences will be so indefinite 
that he can hardly say whether they are confirmed or contradicted when 
he confronts them with the appropriate facts of observation. It is particu- 
larly important that deduction should not be postponed till after the 
field work is “completed ” and after the field is left. The two processes 
