DAVIS: MOUNTAIN RANGES OF THE GREAT BASIN. 169 
During this pause, the definition of the mountain base would have been 
much less distinct than it is to-day ; indeed the base line would have 
been obliterated along a considerable fraction of the mountain front. Re- 
newal of uplift has made the base line well defined to-day in the southern 
division of the range, but the streams on the aggraded- floor of Alvord 
valley do not share in the revival by which the same streams on the 
mountain flanks have trenched the old graded slopes. A considerable 
period of time must have elapsed since the original uplift of the block 
began, for the crest of the range is now worn two or three miles back 
from the eastern base. 
The middle division of the Stein mountains, Figure 17, is a mono- 
cline of so gentle a westward dip that it possesses a broadly rolling 
Figure 17. 
Cross-section of Mid-Stein mountains, looking north. 
upland surface, limited on the east by the strong escarpment which 
falls 2,500 or 3,000 feet to the Alvord trough, by a lower escarpment 
which overlooks the monoclinal ridges and valleys on the south, and by 
a strong escarpment again on the west. I believe all these escarpments 
have been worn back from fault lines, and there is some reason for 
thinking that the fault between the two divisions of the range, trend- 
ing west-northwest, is older than the meridional faults on the east 
and west; but I will leave the discussion of this point to some one 
who can treat it in greater detail. As in the southern block, the crest 
line of the eastern block is now worn back two or three miles from 
base, indicating a long period since the block was first uplifted. 
The face of the eastern escarpment presents many graded slopes 
between the ledges of more resistant lavas. The spurs between the 
obsequent ravines by which the escarpment is dissected usually descend 
with concave slopes toward the plain; but a short convex profile is 
often seen as the spur reaches the base line. The lower parts of many 
spurs exhibit well-defined graded slopes on the interstream surfaces, 
but the spurs are now rather sharply separated by the ravines, and 
thus indicate a prolonged pause followed by a subrecent uplift. 
