FOX HILLS FORMATION. 71 



ZONE TRANSITIONAL TO POX BILLS. 



Between the Pierre and overlying Fox Hills formation there is a 

 change from the pure clay of the one to the arenaceous shales of the 

 other. Limestones and small ferruginous nodules, similar to those already 

 described, are presenl throughout this transitional zone, extending well into 

 the Fox: Hills. Fossils also occur, bul the life of the zone is marked by 

 the sudden increase in the members of the genus Mactra, a genus which 

 below has onlj been occasionally met with, hut which from this up is 

 frequently found. 



POX BILLS FORMATION. 

 -I R IWGRAPHT. 



The Fox Hills formation has a normal thickness of between 800 and 

 1,000 feet, falling In 'low this only al Golden, where its decrease to 500 feet 

 is attributable t<> the uondeposition of it- lower portion. 



The formation consists mainly of soft, friable, arenaceous shales, with 

 occasional interstratified bands of claw At the summit is a persistenl and 

 characteristic sandstone, usually about 50 feet thick. The entire formation 

 has a yellowish cast, but while the shales are generally of a grayish-yellow 

 the sandstone itself has a pronounced tint of green. The composition of 

 both shales and sandstone:- is very uniform. 



shaie. — The shales carry a small amount of ferruginous matter in concre- 

 tions and seams, and a minor quantity of gypsum. Limestone c -retioii- 



resembling those of the Pierre are numerous, but less abundant than in the 



older formation; like the shale- in which they occur, they usually contain 



more or less -and, those near the summil resembling a calcareous quartzite. 

 All are fossiliferous. Line carbonaceous matter is generally present. 

 Cone-in-cone structure is frequent. 



sandstone. — The sandstone at the summit of the formation i- noteworthy on 



account of its position a- cap to the great mass of Cretac is clays, from its 



wide occurrence overthe W est. from the fossil remains in its upper stratum, 

 aud from the marked difference displayed in its materials from those of the 



basal sandstones of the Laramie which overlie it. Its composition is chiefly 

 quartz, hut it carries an appreciable amount of biotite and muscovite, and 



