168 
IOWA ACADEMY OF SCIENCES. 
The section shows for the J ornada plain a simple synclinal 
structure, with profound faults on the opposite side of the 
marginal mountain ranges in either direction. 
On the Rio Grande side of the Caballos range repeated 
faulting is indicated — the river occupying the minor fault- 
blocks next the great fault-block of the main mountain 
ridge. 
The highest part of the Caballos exhibits very clearly the 
evidences of a profound thrust-plane the geological age of 
which greatly antedates the period of normal block faulting 
which gives the present characteristic aspects to the region. 
The Carboniferous limestones for a distance of three-fourths 
of the distance to the summit of the range stand nearly 
vertically. Erosion has bevelled the stratification planes 
at angles of about 35 degrees — the slope of the eastern 
side of the range. This imparts to all except the upper 
part of the mountain face a remarkably contorted 
appearance, with horizontal beds of the same com- 
position capping the summit. The effect is almost inex- 
plicable until the position of the thrust-fault is recognized. 
Immediately east of the western rim of the bolson and 
between the Sierras de los Caballos and Era Cristobal are 
a number of small lava cones 300 to 400 feet high, each of 
which send out a basaltic flow for several miles in all 
directions from its center. These basalt flows appear to 
cover some of the earlier mesa gravels. They are quite 
recent — probably early Pleistocene in age. 
^ The gentle syncline of the Jornada is perhaps its most 
characteristic structural feature. It is to be noted, however, 
that this region is not a simple trough but a syncline which 
has experienced repeated, or rather continued, upturning 
