28 GEOLOGY OF TOE DENVER BASIN. 



MONTANA FORMATION. 



The lower subdivision of the Montana formation, the Pierre, consists 

 mainly of plastic clays, generally gray in color, with lenticular bodies of 

 limestone, and an arenaceous /one from 100 to 300 feet thick about one- 

 third way up in the formation. Its aggregate thickness is taken at about 

 7, Too feet, no part of which is supposed to have crossed the Golden arch. 

 The clays contain a few clay-ironstone concretions, and are impregnated 

 with gypsum and alkaline salts. The formation has a characteristic mol- 

 luscan fauna, whose remains are usually found in the lenticular bodies of 

 impure limestone. 



The upper subdivision of the Montana, the Fox Hills, has an average 

 thickness in this field of 800 to 1,000 feet, only the upper 500 feet of 

 which is assumed to have been deposited over the crest of the Golden arch. 

 The formation is characteristically more arenaceous than the Pierre, and is 

 generally of a yellowish or buff color. The arenaceous shales in the 

 lower part carry some ferruginous concretions and gypsum, and lenticular 

 limestones are also found in subordinate quantity, which contain a charac- 

 teristic molluscan fauna and some plant remains. The most characteristic 

 and persistent stratum in the formation is a sandstone at the very summit, 

 which in this field is about 50 feet thick, of greenish-yellow color, and 

 carries an abundant and typical marine fauna. It consists of quartz 

 grains, with a little muscovite and biotite mica and disseminated iron. 



LARAMIE FORMATION. 



Lithologically in close connection with the Fits. Hills is the Laramie 

 formation, which consists of a series of basal sandstones up to 200 feet in 

 thickness, above which are in this field 400 to 1,000 feet of clays, with small 

 lenticular bodies of sandstone. 



The basal sandstones of the Laramie and the sandstone at the top of 

 the Fox Hills often form a single sandstone bluff. The fossil horizon 

 forms a sharp division between these horizons, and the Laramie sandstone 

 is usually white in color, consisting mainly of quartz, with minute grams 

 of black chert and but few other impurities. The Laramie sandstones 



