MINK CREEK. 199 



definitely determine their age as belonging to the Ellis formation: Campto- 

 nectes platessiformis, C. pertenuistriatus, G. bellistriatus, Cyprina iclclingsi, 

 Neritina i vyomingensis. 



Both the red beds of the Teton and the limestones and sandstones of 

 the Ellis dip steeply down the mountain sides, and are exposed along Pacific 

 Creek below the mouth of Mink Creek. Here they are abruptly cut off by 

 the Snake River fault. On both sides of Pacific Creek the limestone bluffs 

 are covered by the basic breccias. 



Mink creek. — Undoubtedly the light-colored limestones exposed by the 

 deep trench cut by Mink Creek are connected with those of Pacific Creek, 

 the continuity at the surface being broken by andesitic breccias. All along 

 Mink Creek the abrupt walls of light-colored Madison limestone are over- 

 lain by the somber breccias; the latter, extending to the top of the plateau, 

 afford a most impressive view. On the divide between Fox and Mink 

 creeks the limestone is well shown along the fault just southeast of the 

 small lake indicated on the map. Lithologically the beds resemble the 

 upper members of the Madison limestone, passing into coarse crystalline 

 beds of reddish limestone, characteristic of the summit of the formation. 

 Near the top of the ridge the limestones yielded well-preserved specimens 

 of Spirifer cenlronatus, which has, however, a wide geographical distribution 

 and an extended vertical range throughout the Madison limestone. 



Immediately north of the small lake just mentioned rises a bold and 

 isolated hill 100 feet in height and one-half mile in length, in striking 

 contrast to the surrounding country. It is formed of light-colored pyroxene- 

 andesite, which in its general aspect possesses a remarkable resemblance to 

 rhyolite. The top of the hill is smooth and polished and beautifully 

 glaciated, and the flat-topped summit is marked by long parallel furrows 

 frequently 20 inches in width and a foot deep, the result of ice movement. 

 In the cavities of the porous rock Professor Iddings has determined the 

 presence of .tridymite 



As already mentioned, the Madison limestone and Teton formation are 

 exposed along the west base of Two Ocean Plateau. The sedimentary 

 rocks come to the surface from beneath the breccias along the edge of 

 the plateau in the deeply eroded valley of Plateau Falls. Thence they 

 stretch northward in an unbroken line for nearly 10 miles, finally dis- 

 appearing beneath the plateau breccias north of Grouse Creek. In the 



