214 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



{Limestone conglomerates and thinly bedded limestones, with trilobite remains.. •, 

 Massive belt of dark-green mottled limestone, resembling the same formation > 850 

 in the Gallatin Range ' 



Flathead Shales. 



This section was made on the high limestone bluff at the north 

 base of the north spur of Abiathar Peak. At the northwest base of the 

 spur the strata dip to the southwest. The dark-colored, coarsely mottled 

 limestone belt which is the base of the Gallatin formation is again exposed, 

 overlain by finely bedded limestones bearing trilobites and by limestone 

 conglomerates. Higher up the slope the massive belt of light-colored 

 limestone belonging to the Jefferson formation weathers in an almost 

 insurmountable cliff extending along' the valley wall for a long distance. 

 The top of this bed is about 1,000 feet above the creek. It is overlain by 

 a stratum of broken limestones, followed by light and dark limestone beds 

 without noticeable fossils up to 200 feet above the gray belt, At this point 

 a purplish layer carrying gastropods overlies a brown earthy and clayey 

 layer of limestone conglomerate with a fossiliferous ledge 20 feet above. 

 Fossils collected from these beds show that it belongs to the Three Forks 

 formation. The talus slope for 200 feet above these Devonian strata shows 

 no exposures until we reach the base of a massive limestone about 1,500 

 feet above the cliff of Jefferson limestone. The limestones composing this 

 upper belt are indurated and not fossiliferous, but the strata exposed above 

 it, at 1,675 feet above the stream, contain numerous fossils. The limestones 

 extend 150 feet higher, where the eroded surface is overlain by dark-colored 

 basic breccias. 



The limestone bluffs extend along both sides of the Soda Butte Valley 

 to Cook City and continue up Republic Creek for 2 miles. Crystalline 

 schists are exposed on the road 100 feet above the Republic Creek road, 

 and also on the lower slopes of Mount Henderson, the gneiss forming 

 obscure outcrops. The Flathead shales are exposed higher up the slopes, 

 at 700 feet above the stream bottom. Above them the dark-colored and 

 mottled Gallatin limestone is exposed, but the rock is much altered and 

 mineralized. Still higher on the south slope of Mount Henderson, east of 

 the road, are altered shales and limestones with hornblende-mica-andesite- 

 porphyry well exposed. The summit of Mount Henderson consists of 

 heavily bedded mottled limestone, broken through by intrusive rock, which 

 cuts across the bedding and also forms intrusive sheets. 



