THE REGION ABOUT BOULDER. 109 



of Boulder is deduced. < >f these, one occurs between the Trias and Jura; 

 the other between the Dakota and Benton. 



The existence of the unconformity between the Trias and Jura is 

 established, (a) by the gradual disappearance over tin- middle portion of 

 the area of the upper member of the Trias, in which the order of disappear- 

 ance is from the top downward as the center of the region is approached, 

 the lowesl beds of the series, as, for instance, the characteristic limestones 

 near the base, being the longest retained; (l>) by divergence in strike 

 shown in tin- inward or westward curve of the Jura and overlying beds, 

 while the Triassic beds continue their uormal trend, parallel to the general 

 direction of the foothills; (c) by the direcl contact of the Jura with the 

 lower member of the Trias, the Red Beds; (d) by the fact that the Jura, 

 w hich i> partially presenl over the entire area, is. where thinnest, recognized 

 in its upper beds, showing conclusively the uondeposition of it- basal 

 members over the central portion of the affected region. 



The deductions regarding the unconformity between the Dakota and 

 overlying formations are less firmly established That an unconformity 

 exists is evident (a) from the gradual thinning of the Dakota, which took 

 place from above as the center of the region is approached, the lower beds 

 of the tun iia ti< m holding completely across the affected area: and (b) by the 

 complete disappearance of the Benton, with probably also the very base ol 

 the Niobrara. The only doubl that can arise in the deductions as sketched 

 in Profile III, PI. XII, rests upon the question whether the strata of the 

 Benton retain a parallelism with the Dakota or with the Niobrara. In 

 the former case the unconformity would have occurred at the close of the 

 Benton; in the latter, a1 the close of the Dakota. This question the outcrops 

 fail tu answer definitely, although the preponderance of evidence is in favor 

 (if the occurrence at the closeof the Dakota. The fad that the very lowest 

 beds •'!' the Niobrara are wanting argues nothing, as the pre-Niobrara hill 

 mighl readily have been of a height to prevent their deposition whether 

 nf Dakota entirely or partly of Dakota and partly of Benton. 



That no other breaks in the continuity of deposition exist in this series 

 of lieds is evidenced liv direct observation of conformability at the remain- 

 ing; horizons. 



