720 VICTOR ZIEGLER 



in the Archean some distance west of the foothills, as for example 

 west of Colorado Springs and also west of Loveland, furnishes 

 good reasons for the belief that the formations underlying the 

 Dakota also extended over the Archean, or at least for a considerable 

 distance westward. In areas undisturbed by special local condi- 

 tions, the Fountain, Lyons, Lykens, and Morrison show as close an 

 agreement in dips and strikes with the overlying formations as 

 would be expected for conformable rock series; and all show 

 beautiful parallelism in folding with the younger rocks. This 

 would be a rather peculiar coincidence, not to be expected except 

 in rocks simultaneously folded. 



The accompanying diagrams show, drawn approximately to 

 scale, reconstructions of the original monoclinal fold worked out 

 from dips and strikes at the localities noted. In each case faults 

 and other local irregularities which would unnecessarily complicate 

 the fold have not been indicated. Their possible effect, due to dis- 

 placements of parts of the fold and consequent introduction of 

 anomalous dips and strikes, has been carefully considered in each 

 case, and, where necessary, corrections have been made in the 

 flexure of the monocline. These reconstructions are merely in- 

 tended to show the variation in the shape and curvature of the 

 foothills monocline in its major outlines (Fig. i, a, b, pp. 718-19). 



It is interesting to note that in older interpretations of the 

 monoclinal fold in this region the overturn is located underneath 

 the surface and not above the surface, as the present writer has 

 drawn it. Considering no more than the present dip and strike 

 relationship along an east-west line at such localities as Golden 

 or Boulder, and considering these to be located on the same hori- 

 zontal plane on the monoclinal fold, no other logical interpretation 

 is possible. If, however, the variations in dip be carefully plotted 

 along the axis of the fold, and if we carry in mind the fact that the 

 dips as observed at Golden and Boulder represent different hori- 

 zontal planes on the monocline due to the disappearance of thou- 

 sands of feet of strata, the type of overturn postulated by the 

 writer becomes necessary in order to explain observed conditions. 

 This will perhaps be clearer after a discussion of the structure at 

 Golden. 



