Mountain Upthrusts. 407 
seem to have thus dragged during their elevation ; while other por- 
tions were sharply severed, as if the displacement had really been 
done by a huge punch acting from beneath, producing an ordinary 
fault there. 
The evidence that the orogenic and epirogenic movements which 
have resulted in the present physical conditions in the Western por- 
tion of our national domain were coéval with each other, seems to 
be unmistakable, so far, at least, as the time-limit of each series of 
movements is concerned, But, considering for the present only the 
orogenic movements, it is evident that the uplifting force has been 
applied along certain lines with great inequality, both as regards the 
amount of force locally exerted and the duration of its application. 
That is, in the region especially referred to in this article there are 
indications that while the orogenic displacements now observable 
there were in progress there were local arrests and accelerations of 
the elevating movement, which produced a final diversity among 
them that did not exist in their inception. 
For example, the present structural condition of the Uinta fold 
Seems to warrant the assumption that it was once, along its entire 
length, in the condition in which its inceptive portion now is, except 
for the presence of the upthrusts. Furthermore, that these upthrusts, 
as well as the main portion of the fold, continued their upward pro- 
gress, while the present inceptive portion remained as it was when 
its elevation was arrested. There are other plain indications of the 
arrest and acceleration of uplifting force in that region; but the 
upthrusts rising upon the inceptive fold constitute the most striking 
examples, 
The elevating force was not only strangely concentrated in the 
case of the two upthrusts, but it seems to have been applied in an 
unusual manner, especially when we consider the position of the 
longer axis of each with relation to that of the other, and also to 
that of the Uinta fold. It has been mentioned that the longer axis 
of the Junction upthrust lies in a northwest and southeast direction, 
and that of the Yampa upthrust in a northeast and southwest direc- 
tion. Viewing these upthrusts only in relation to the Uinta fold 
proper, and regarding them as nearly or quite isolated portions of 
e same, one would naturally expect to find their longer axes coin- 
ciding with a line projected from the axis of the main fold, and he 
would also expect to find the intervening strata along that line to 
