BROWN'S PARK. 223 



opposite side of the valley along the clififs which form the southern face of 

 the Owi-yu-knts Plateau. 



At the base of the eastern portion of these cliffs, the white strata of the 

 Brown's Park Tertiary are found resting against the quartzites, inclined at 

 an angle of 25° to the southward. The suddenness of this change of angle 

 from the approximately horizontal position of the beds in the middle of the 

 valley suggests a local displacement and sinking of the beds at the base of 

 the cliffs since Tertiary times. This line of fault corresponds with that 

 developed in the neighborhood of Elk Gap, in which the downthrow is also 

 to the south. Owing to the obscurity which involves the correspondence of 

 horizons in these Tertiary beds, the amount of displacement cannot be esti- 

 mated with accuracy. The relative difference of level between the Tertiary 

 beds of the western end of Brown's Park and the beds of the Green River 

 group in the valley of Red Creek is not less than 3,000 feet; and, if the 

 latter extended southward at their present angle to a direct superposition, 

 this amount would be doubled. It is probable, however, that in this region 

 of the valley the displacement has been much greater than farther eastward. 



At no one point were the exposures of the Brown Park Tertiaries suffi- 

 ciently continuous to give a definite idea of the thickness of the formation. 

 We roughly estimate about 1,000 to 1,200 feet still remaining above the low- 

 est beds exposed. The best sections are obtained in the deep cuts made by 

 Vermillion Creek at the eastern end of the Park. The most characteristic 

 strata are composed of an extremely fine, white siliceous silt, enclosed in 

 beds of coarse grayish-drab sandstone. A chemical analysis of the former 

 was made by Mr. B. E. Brewster, with the following result : 



Silica 62.431 



Alumina 14.566 



Lime 1.859 



Magnesia 1.409 



Soda and potassa 4.900 



Water and organic material 14.836 



100.001 

 The drab sandstones consist of a loose aggregation of rounded grains 



