SANDSTONE DIKES OF UTE PASS. 133 



Exactly at the mouth of South Cheyenne Canon, where 

 the carriage road crosses the creek and the burro trail from 

 the terminus of the electric railroad joins it (21), a large 

 and entirely typical sandstone dike outcrops on both sides 

 of the creek. On the west side it is sixty, if not seventy- 

 five feet wide and both walls can be located. The north 

 wall is clearly exposed by the roadside and shows sand- 

 stone penetrating and enclosing the coarse red granite ; 

 while on the south side the sandstone is involved with 

 a dark green, fine grained, crushed igneous rock. Crossing 

 the end of the mountain toward North Cheyenne Creek, 

 I found a good outcrop in a prospect hole seventy-five to 

 one hundred feet above the creek. This excavation does 

 not expose either wall ; but it does show an inclosed mass 

 of the compact greenish igneous rock. It is dike-like in 

 form, eighteen to twenty-four inches thick, and hades 

 southwest 45^^ ; but it is also clearly traversed by dikelets 

 of the sandstone one-fourth inch to two inches in diame- 

 ter. Hence the altered trap must be older than the sand- 

 stone dike. The dike sandstone is quite friable in part, 

 and some of it rather coarse, even containing pebbles of 

 quartz and granite. Appearances indicate a wide dike 

 here, and it is readily traced down the slope to North 

 Cheyenne Creek and up the west slope of the valley. 

 Inunediately north of the dike is a large but ol)scure out- 

 crop of a soft or friable white sandstone which weathers 

 bufi", yellow, red and purple (mainly yellow). It agrees 

 very well with the Dakota sandstone of Bear Creek, while 

 it is entirely difterent from the adjacent dike sandstone. 



The great shear plane so conspicuously exposed on the 

 west side of South Cheyenne Canon (22) demands atten- 

 tion here. It rises from the creek level near the toll gate, 

 passes around the end of the mountain at a height of 

 about two hundred feet, and appears to be traceable east 



ESSEX INST. BULLETIN, VOL. XXVII 13 



