116 DESCRIPTIVE GEOLOGY. 



Cretaceous are well represented in the North Park. The Dakota beds 

 present identical topographical and lithological features with the corre- 

 sponding horizon along the Colorado Range, — a prominent ridge or wall of 

 yelloAvish-brown sandstone, frequently so well compacted as to form a hard 

 quartzite, with well-marked bedding-planes, and with occasional layers of 

 fine cherty conglomerate, so characteristic in many places of the Dakota 

 sandstone. This lower division of the Cretaceous is estimated at 350 feet 

 in thickness. Of the Colorado group, all the subdivisions, — the Fort Ben- 

 ton, Niobrara, and Fort Pierre beds, — which have been recognized east of 

 the Colorado Range and on the Laramie Plains, may be recognized here, 

 but not always so sharply defined. . Beds of pure clays and limestones, 

 although present, and giving character to the Middle Cretaceous rocks, 

 are usually less thickly developed, being more arenaceous throughout 

 the series, which would naturally tend to obscure all divisions. Never- 

 theless, the darlc fissile clays, with the ferruginous layers of the Fort 

 Benton, passing up into fine argillaceous limestones and variegated white, 

 blue, and yellow marls, associated with gypsum and selenite, which mark 

 the Niobrara, and in turn pass up into bluish clays, and ssijidy argil- 

 lites of the Fort Pierre, may all be observed at numerous localities. At the 

 junction between the Fort Benton and Niobrara beds, the limestones attain 

 their greatest development, but are rarely more than 20 feet in thickness, 

 and are marked by a fine, almost impalpable, texture, with sufficient silica 

 in their composition to give them a conchoidal fracture when broken by the 

 hammer. They emit a strong bituminous odor. A peculiarity of the hori- 

 zon is, that while in many localities the overlying marls are easily recog- 

 nized, in others the beds pass so rapidly into sandstones as to lose all lith- 

 ological characteristics. 



The uppermost members of the Cretaceous series represented in the 

 North Park consist of heavy beds of yellowish-white sandstones, with a 

 somewhat friable texture, and generally roughly bedded. They have all 

 been included in the Colorado group, although the great thickness of nearly 

 pure sandstones Avould suggest still later horizons, and it is not impossible 

 that in a few localities they may prove to belong to Fox Hill strata. The 

 reference, however, of the entire series to the Colorado grcrup rests partly 



