270 
ELDRIDGE. 
beds were then deposited upon this fold, closing the first 
four periods of history discussed above. 
5th. The readjustment of forces by which the structure of 
post-Niobrara and Laramie times was changed to that of the 
present day. —At the close of the events detailed in the last 
paragraph there began a readjustment of the major forces 
acting against the range, by which the fold of pre-Montana 
age and its cap of horizontal strata gradually gave way to 
the structure of later times. The results of this readjust¬ 
ment may have been developed prior to the time of the in¬ 
clusion of the affected area within the general uplift of the 
range, but were more probably synchronous with it. 
The complex movement which brought about these results 
may properly be resolved into two chief components. The 
first of these includes the movement by which the strata 
composing the pre-Montana fold were brought from their 
position, as represented in profile IV, to that which they 
hold in the natural section given by the outlines on the 
map. The effect of this movement can be seen in diagram¬ 
matic representation, shorn of all complicated details, by com¬ 
paring figure 1, which shows the conditions previous to the 
movement, with figure 2, which shows those subsequent to 
it. In this movement the strata resting horizontally upon 
the pre-Montana fold of necessity followed the recession of 
the beds beneath, assuming the position of the synclinal 
depression d in Fig. 2, or the highly curved position—the 
result of the synclinal position—which they hold in the sec¬ 
tion on the map. The second component is the movement 
specially involved in the elevation of the range, by which 
the strata were brought into the highly inclined position 
they hold along its base at the present time. 
6th. Readjustment of the forces accounted for. —The read¬ 
justment of the forces effecting such important structural 
changes can be accounted for by relief from the compression 
to which the strata had been subjected, brought about be¬ 
yond the immediate region here considered. The exciting 
cause may have been elevations in other areas, or even an 
increase in the force of the general lateral thrust to the north 
