Mountain Upthrusts. 403 
The inceptive portion of the Uinta fold is, of itself, by no means 
a conspicuous geological feature—first, because a broad valley or 
basin, which I have called Axial Basin, is formed along the greater 
part of its length by the erosion of the strata which have been up- 
lifted there; and second, because the uplift is comparatively slight. 
A transverse section (Fig. 2) across Axial Basin shows the character 
of this portion of the fold, and it also shows the formations which 
are involved in it. 
Fig. 2.—Transverse section across Axial Basin. 
A. indicates the axis of the inceptive fold; B., the north base of 
Danforth Hills and a part of that uplift ; C., Yampa River. Carb. 
indicates Carboniferous strata; J. T., Jura-Trias; D., Dakota 
Group ; Col., Colorado Group; F. H., Fox Hills Group ; L. Lara- 
mie Group; W., Wasatch Group. The line xx indicates propor- 
tionally the longer diameter of the Yampa Mountain upthrust, and 
~ also its position with reference to the inceptive axis. 
Horizontal scale: 4 miles to the inch. 
Vertical scale: 1-12 inch to 1,000 feet. 
This comparatively slight fold becomes of great importance be- 
cause of its evident relation to the great Uinta fold, and especially 
because of the presence upon, jts axis of the two upthrusts which 
have been already referred to, the presence of each of which is marked 
by an isolated mountain which rises abruptly out of Axial Basin. 
These are, Junction and Yampa Mountains, which are plainly 
outlying, isolated members of the Uinta Range. 
The main portion of the Uinta fold terminates at its eastern end 
by a dip of the uplifted strata which is quite as abrupt as that at 
either side of it, and which carries them far beneath the surface of 
the adjacent lowland. Going now, only two or three miles east- 
ward from this eastern terminus—where we have seen the later 
formations dip so suddenly from view—we come to the western 
border of Junction Mountain upthrust. Here we find the same 
Strata to rise again, even more suddenly than they disappeared ; and 
