Three Forks 

 and 

 Jefferson. 



58 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



of the Gallatin shows an excellent exposure of the contact between the 

 andesite-porphyry and the Cambrian shales, the latter being altered by 

 contact metamorphism for a few feet from the andesite-porphyry. The 

 Cambrian shales are overlain by limestones in which there are intruded 

 several sheets of andesite-porphyry, and are capped by cliffs of a heavily 

 bedded white limestone of the Madison formation, with basic intrusions, 

 near the summit of the mountain. The following partial section shows the 

 series found immediately above the laccolith: 



Section east of Gallatin River, below Fan Creel;. 



Feet. 

 Madison. Limestones carrying corals, thickly bedded, of a dense texture, drab or dark-gray 



colored, and holding black chert. 

 Limestone shale of pink, red, buff, and purplish colors, carrying a few fossils and 



underlain by dark-blue (almost black) limestone 200 



Andesite-porphyry, poorly exposed 25 



Granular, dark-brown and black limestone, sometimes banded, and of Silurian 



aspect 15 



Thinly bedded and fissile light-gray limestone, dense and not crystalline, impure 



aud carrying argillaceous matter 5 



Dark-colored granular limestone, carrying Obolella 12 



Limestone, thinly bedded and with a knotty texture, dark blue in color, of typical 



Cambrian aspect, and evidently of shallow-water origin 30 



Limestone shale, blue and olive gray in color 5 



Limestone, dense in texture and dove colored 5 



Limestone, becoming shale: dip 50° to the east 5 



Shales, green or olive colored, seldom exposed 15 



Limestone, mottled, of typical Cambrian aspect 15 



Shale 5 



Andesite-porphyry laccolith. 



West of the Gallatin River the mountain slopes show andesite-porphyry 

 extending up nearly to the summit of the flat-topped mountain, but the 

 stratified rocks are seen both to the north and to the south, forming great 

 curved plates, with dip away from the intrusion in every direction. A 

 stream from the west has cut its valley in the dome, exposing the sedi- 

 mentary rocks on the valley walls. 



The mountain opposite the mouth of Fan Creek is composed entirely 

 of Paleozoic strata, which are not affected by the laccolithic uplift, but dip 

 to the east and northeast, awa)^ from the axis of the Madison Range. The 

 lower slopes show Cambrian beds, which are overlain by the Silurian 

 rocks, of which the most prominent strata are quartzitic in nature and form 

 heavy, massive beds that cap the summit of the mountain and extend east- 



Gallatin. 



