168 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The monoclinal structure of the southern division has a strike in its 
western part to the west-northwest, with a southerly dip of 15° or 20°. 
Erosion ‘has developed a number of well-defined ridges and valleys, and 
the generally accordant heights of the ridges as they rise gradually east- 
ward suggests that the monoclinal mass had been much eroded previous 
to the uplift whereby the present dissection was initiated. As the ridges 
\ 
\ 
AE: YSTALL! N 
Fievure 15, 
Cross-section of the Pueblo mountains, looking north. 
approach the crest of the range, which lies about three miles from its 
eastern base, the strike of the monocline turns to the southeast or south- 
southeast ; the harder beds in the monoclinal ridges rise eastward to 
form the peaks, while the valleys of the monocline may be traced upward 
to the notches in the serrated mountain crest. On the eastern slope, the 
harder beds form benches that descend obliquely southward toward the 
eastern mountain base, where they are successively and evenly cut off. 
HIGH DOME or PUEBLO 
MOUNTAINS 
HOLUS SPUR 
°F PMID-STEINS 
are 
FIGURE 16. 
View of the rorth end of the Pueblo mountains, looking south over the Hollis spur of 
the Mid-Stein mountains. 
This form can be easily explained as a block of a baselevelled monoclinal 
mass, lifted and somewhat tilted to the west, and maturely eroded ; but 
I find it dificult to explain it in any other way. A subrecent pause in 
the uplift of the block is indicated by the occurrence of well-defined 
graded basal slopes, independent of structure, now raised several hun- 
dred feet above the Alvord plain and dissected by numerous streams. 
