56 DESOKIPTIVE GEOLOGY. 



logical and structural features that have already been noted north of La- 

 porte. The bluish-gray argillaceous limestone at the base of the Niobrara, 

 serves as an excellent geological horizon, as it crops out so characteristi- 

 cally and is so persistent. 



A specimen of this rock, a fine-grained variety, in which were im- 

 bedded numerous specimens of the genus InoceramuSj was submitted to 

 chemical analysis, yielding — 



Carbonate of lime — . 65. 93 



the residue indicating a fine blue clay. The lower members of the Col- 

 orado group maintain approximately the dip of the Dakota beds. An 

 observation made just below Laporte gave, for the yellow chalk marls, an 

 inclination of 16^, while the upper, or Fort Pierre beds, rapidly flatten out, 

 and occur with as gentle a dip as 3° to 5°. The upper member of the Fort 

 Pierre beds consist largely of arenaceous material, which renders it diffi- 

 cult to separate them from Fox Hill beds. The line which has been fol- 

 lowed, however, lies at the base of a long, low ridge of red ferruginous 

 sandstone, which stretches far out upon the plain. In these lower beds, 

 which have an earthy yellowish-brown color and loose texture, were found: 



Scaphites nodosus. 

 Pinna restituta. 

 Inoceramus Barrabini. 

 Inoceramus incurvus. 

 Callista Deweyi. 

 Anisomyon sexsulcaius. 

 Ostrea f 



characteristic Fort Pierre fossils, which have likewise been found on the 

 Upper Missouri Eiver near the boundary line between the Fort Pierre and 

 Fox Hill formations. 



Several miles before reaching the Big Thompson Creek, a long spur 

 or ridge of Archaean schists projects out, with a slight angle to the south- 

 west, away from the main body of the Colorado Range, becoming gradu- 

 ally lower, finally passing away under the plain. As the great body of 

 Archaean rocks still lies to the westward, this spur causes a sort of depres- 



