38 



GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



generally much decomposed, and shows no megascopic mica. This por- 

 phyry forms a bench north of the stream which extends back to the base of 

 the cliffs where Dakota conglomerates occur. In the stream channel a 

 brownish-gray sandstone speckled with black, and belonging to the Dakota 

 series, is overlain to the east by the andesite-porphyry, forming the two 

 9,000-foot hills shown on the map. These higher beds, forming the west 

 end of Little Quadrant, are tilted up by the porphyry intrusion. The 

 following section, made by Mr. Wright, shows the sequence and thickness 

 of the beds exposed in ascending the creek, the beds being given in descend- 

 ing order. The thicknesses are estimated and approximate, and are given 

 in feet: 



Section on north side of head of Fawn Greek Valley. 



Num- 

 ber. 



Feot. 



Dakota .. 



Ellis ■ 

 sandstone. 



Ellis 

 limestone. 



Teton 

 limestone. 

 Quadrant 

 quartzite. 



300 



{Mica-hornblende porphyry . . . ~| 

 Limestone > ■ 

 Mica-hornblende-porphyry ... J 



Sandstone 50 



Conglomerate 50+ 



i Interval with no exposure 50 



(Sandstone Few. 



Hornblende-porpliyry, in sloping belt across an interval 165 



Sandstone, very calcareous 



Limestone, very arenaceous 



Interval with no exposure— steep slope 



15 



25 



365 



Mica-hornblende-porphyry 40+ 



Interval with no exposure 100 



Cherty, arenaceous, and calcareous beds, 35+ 



Interval with no exposure 



100 



Sandstone in bed of Fawn Creek. 



Total thickness of section 1, 295+ 



Total thickness (approximate) of intruded porphyry 505 



Total thickness of sedimentary rocks and intervals 790 



The slopes on the northern side of the west end of the mountain show 

 a succession of beds very similar to that of the section just given. The meas- 

 urement made of the series from the Dakota conglomerate to the summit 

 of the ridge showed the 100 feet of conglomerate and sandstone, which 

 was assigned to the Dakota, overlain by 45 feet of compact gray limestone 

 considerably altered by intrusive sheets of porphyry,, which occur both 

 above and below the bed and also split it in half. The overlying sand- 

 stone, which forms the summit of the Dakota series, is a pure white, rather 



