LARAMIE FORMATION. 75 



This is particularly the case in the small hill known as the Burnl Knoll, 

 north of the Marshall-Louisville mesa, aboui 3 miles west-northwesl of 

 Louisville; Here the strata have in some instances been fused, developing 

 both flow and vesicular structure, hand specimens being hardly distinguish- 

 able from la\as; in other instances the alteration has been to jasper. A 

 crystalline structure appeal's in some, while elsewhere tin- granular texture 

 dl' the original rock lias been maintained. Main an- coated with hematite 

 from alteration of the iron originally present. Clinker, resembling that 

 from furnaces, is abundant. Columnar structure has been developed in 

 some lit' the clays, probably from evaporation of their water. The colors 

 vary — reds, browns, blues, and blacks prevailing. In many of the rocks, 



even those most altered, leaf impressions are still apparent. 



STRATIGRAPHY, UPPER DIVISION. 



rhe thickness of this divisi owing to uneven denudation from the 



top, varies between 400 and 1,000 feet. The strata are chieflj clays, 

 through which are distributed small, lenticular bodies of sandstone, innum- 

 erable concretionary ironstones from "_' to 1 feel in diameter, and narrow 

 local seams (if impure lignitic material. ( )ne or two beds of lignite are also 

 present in its upper portion, easl of Denver. Especially fine exposures of 

 these clays occur on the slopes south ami east of Rock and Coal creeks, 

 where the strata have a slighl dip east or southeast, forming bluffs looking 

 westward, in which colors, lines of stratification, sandstone concretions, and 

 ironstones are all clearly defined. 



ciays. — These are dark bluish-gray, relieved l>v bright bands of red, 

 yellow, brown, drab, and white. 



sandstones. — The sandstones differ somewhat from those of the lower 



Laramie in containing more or less lime, in their superior hardness, and ill 



their occurrence as lenticular ma>se> from 6 to .'in feet in diameter and 1 to 

 (i feet in thickness. They contain numerous plant remains and a moderate 



amount of iron. In weathering they often scale in concentric layers. There 

 are apparently three or four horizons at which they especially prevail, 

 although they may he found anywhere in the series. 



ironstones. — These form one of the distinguishing features of the upper 



