244. BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
regions the soft formations were reduced to a plain which truncated 
the strata at an elevation determined by the local base-level. The 
harder, more resistant formations stood up as terraces or plateaus where 
they were horizontal, and as ridges or cuestas where they were tilted. 
In the most elevated regions the relief was very marked. In a highly 
resistant mountain massif, such as the Pine Valley range, standing at 
an elevation of eight thousand feet, there were fairly deep valleys which 
nevertheless did not penetrate far into the centre of the lava mass. In 
a plateau such as that of Colob, where relatively soft strata stood at 
an elevation of from five to seven thousand feet, advanced dissection 
was the rule, and we find great valleys penetrating far into the interior 
and having a relief of several thousand feet. 
Two rather unexpected results arise from this restoration of the 
country to its pre-faulting condition, and for neither of them can we offer 
an adequate explanation. One is that the Virgin river, if it was then 
located where it now is, flowed along the broad, flat arch of a gentle 
anticline. It is possible that the river was shifted to its present position 
after the anticline was flattened out, but of this we have no evidence. 
Another point that attracts attention is that the anticline lies directly in 
line with the most massive portion of the Pine Valley lava mass, and is 
almost a continuation of the line where the folds of an earlier date flatten 
out into simple monoclines. Whether these three things are causally 
connected or not, is wholly unknown. Their relation suggests that the 
lava of the mountains acted as a great buttress, around which the sedi- 
mentary strata have been bent and plicated, just as in a tidal estuary ice 
is bent around the pier of a bridge. 
Before leaving the subject of the condition of the country just prior to 
the second faulting, let us get a bird’s-eye view of the whole region. 
From the top of the lofty Pine Valley mountains an observer would have 
seen to the southwest, west, and north, the low ridges of the southern 
part of the Basin Range province — ancient mountains, well dissected and 
mature, and presenting nearly the same appearance as to-day. To the 
east were the High Plateaus, lying considerably lower than at present, 
and everywhere carved into broad valleys, from which rose rounded hills 
of horizontal strata to a height of two thousand feet, more or less. 
The cliffs and canyons which are now so striking did not then exist, 
but in their places were long, gentle slopes, covered deeply with soil 
and probably with vegetation. Still farther around the horizon from 
the east to the southwest, the observer would have seen a smooth 
monotonous expanse, the Mohave peneplain, sloping slightly away to 
