168 GEOLOGY OF THE HIGH PLATEAUS. 



They all lie near the ancient shore line of the great Eocene lake, and cases 

 of unconformity, not only with the underlying series, but among themselves, 

 are not uncommon. Their physical characters are, in general, indicative of 

 an epoch of gradual displacement in the several tracts which they occupy. 

 It would be obviously extremely difficult to correlate such a group with 

 any such formations as those which are found on both flanks of the Uintas, 

 forming the comparatively regular and systematic strata of the Upper Green 

 River series, though general considerations may warrant a provisional 

 reference of these local deposits to that period. 



The unconformities just spoken of are probably in some cases apparent 

 rather than real. It is easy to see that while deposits are accumulating 

 along the slope of a flexure which is in process of formation, the two going 

 on pari passu, there may result a want of parallelism in successive layers 

 as well as other irregularities which produce collectively the appearance of 

 unconformity. This differs, however, from that type of real unconformity 

 which is usually relied upon as proof of an interval of time between con- 

 tiguous formations in which the record is interrupted by a blank of unknown 

 duration. Where the exposures are satisfactory the apparent and real occur- 

 rences may be distinguished, but in a majority of cases the distinction is 

 not easy to find. 



The thickness of the formation is highly variable, ranging from 300 to 

 750 feet. It consists of alternating marls and sandstones, the latter being 

 sometimes coarse-grained, with here and there a patch of conglomerate. 



