152 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the valleys will widen without much deepening at their mouths, the 
spurs will be narrowed, and the truncating terminal facets will in time 
be so far consumed that the spurs will become pointed, as in Figure 10. 
The further erosion progresses into maturity, the farther will the points 
Figure 10. 
Diagram of tapering spurs between open valleys in a late mature stage of a tilted block; 
same scale as Figs. 6-8. 
of the wasting spurs withdraw from the fault line, and the more perfect 
will be the relation of structure and form ; but as old age is reached this 
relation is more and more suppressed. It is evident that late maturity 
or early old age will introduce the system of interlocking valleys and 
spurs already described as characteristic of subdued residual mountains. 
Spurs and Terminal Facets of the Wahsatch Range. —The Spanish 
Wahsatch, opposite the villages of Springville and Mapleton, presents a 
group of forms that resembles to a singular degree those represented in 
Figures 8 and 9. The mountain base has already been referred to as 
shown in Plate 1, B, an examination of which will now discover the pro- 
files of a series of basal spur-facets, sloping at an angle of 38° or 40°, 
and possessing remarkably systematic forms which correspond closely to 
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