TDE NOETH PAEK. 115 



Range, wliile the calcareous and gypsiferous strata, although they may be de- 

 tected in thin layers, are rarely exposed in sections along the creeks. Near 

 the base, they show, in many localities, beds of fine conglomerate, which con- 

 sist of small, rounded, siliceous pebbles cemented by a fine, ferruginous sand. 

 At the top of the series, the sandstones pass gradually up into lighter-colored 

 beds of Jurassic marls. In thickness, they vary considerably, but appear 

 to have an average development greater than along the east base of the 

 Colorado Range, attaining in several places nearly 1,000 feet. As, over 

 long distances, the Red Beds rest directly upon the Archaean rocks, the 

 great differences in thickness may in part arise from the lower beds failing 

 to reach the surface. Although from the Jurassic beds of the North Park 

 no well-defined organic remains have ever been found, they show too close 

 a resemblance with undoubted Jurassic rocks, lying between the Red Beds 

 and Dakota sandstone, to question their true position. The light- drab or 

 dove-colored cherty limestone, which forms so characteristic a feature of 

 Jurassic beds, seems a very persistent stratum in the North Park, and, as 

 in many other places, stands out prominently a few inches above the surface 

 of the more easily eroded, crumbling marls, which usually possess light 

 cream and ash colors. 



An analysis of this cherty limestone, made by Mr. B. E. Brewster, 

 yielded as follows : 



Silica ; 6.535 



Ferric oxide and alumina , . . 0.921 



Lime - - - . 50.577 



Magnesia - ,,..... 0.358 



Carbonic acid ^ 40.177 



Water 1.498 



100.086 



Combining the carbonic acid with the bases, we have : 



Carbonate of lime 91.11 



Carbonate of magnesia 0.75 



In thickness, the Jurassic beds were estimated at from 200 to 250 feet. 

 As already mentioned, both the Dakota and Colorado groups of the 



