28 DESCRIPTIVE GEOLOGY. 



from the lowest strata of Palaeozoic rocks to the Dakota sandstone of the 

 Cretaceous is well shown, while in Colorado the conformity from the Red 

 Beds of the Triassic to the summit of the Laramie beds, at the top of the 

 Cretaceous, seems equally evident. 



The thickness of the beds of the various horizons changes considerably 

 at different localities, but the average thickness of the series may be esti- 

 mated with some degree of accuracy, at least for the strata exposed from 

 the base of the Palaeozoic to the top of the Cretaceous marls and clays of the 

 Colorado group. Above the Colorado group, the Fox Hill and Laramie 

 beds which extend out upon the plains with very low angles, accompanied by 

 gentle folds, are usually so covered with soil and grass as to render any 

 exact measurement of their thickness somewhat difficult. The thickness of 

 the entire series may be estimated as at least 6,000 feet. It must be borne 

 in mind, however, that this is not a continuous section, for nowhere within 

 this region do we find both the lower and upper members of the series exposed 

 in the same locality. Moreover, where the Palaeozoic strata are best devel- 

 oped, the Triassic and Jurassic appear quite thin. And again, where the 

 Colorado group is seen to the best advantage, the underlying and overlying 

 beds do not appear so clearly defined. 



In the following general section is given the estimated thickness for 

 each of the formations represented in the series : 



Feet. 



Laramie 1, 500 



Fox Hill 1, 500 



Colorado 1, 000 



Dakota - - 300 



Jurassic 200 



Triassic 800 



Palaeozoic 850 



Cretaceous < 



6,150 



Wherever this .conformable series rests upon the older rocks, along the 

 front of the Laramie Hills, the Palaeozoic strata may always be found at 

 the base. But just north of where Box Elder Creek leaves the mountains, 



