100 GEOLOGY OF THE DENVER BASIX. 



may have been developed prior to the time of the inclusion of the affected 

 area within the general uplift of the range, hut were more probably syn- 

 chronous with it. 



The com] ilex movement which brought about these results may prop- 

 erly 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, Pl.X, 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 diagrammatic representation, shorn 

 of all complicated details, by comparing fig. 4, which shows the conditions 

 previous to the movement, with fig. 5, 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. 5, or the highly curved 

 position — the result of the synclinal position — which theA'hold in the section 

 on the map. Hie 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. 



Readjustment of the forces accounted for. Tlie readjustment of the 1'olVeS effecting 



such important structural changes can lie accounted for by relief from the 

 compression to which the strata had been subjected, brought about beyond 

 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 and south of the field, accompanied by a variation 

 from its normal western direction to directions diagonal to the range and 

 divergent as this is approached, by which the original north-and-south 

 compressive forces would have been compensated by the components of the 

 later one acting in the reverse direction respectively (1>, fig. 5). Equilibrium 

 having been restored over the area in question, and a portion of the affected 

 region having become involved in the general uplift of the range, which 

 still continued, subsequent erosion and the formation of the plane surface 

 of the present day exposed the underlying strata in the superficial section 

 now existing. 



Relation of the basalt eruption to the above events. TllC CrUptioU of tile 'Faille MlUlll- 



tain basalt took place early in the period of the Denver formation, 



