GEANITE-PORPHYET. 121 



white vitreous rock inclined at angles seldom exceeding 10 and frequently 

 horizontal. The country offers, in places, broad table-topped masses, and 

 ; i gain in others is roughly accidented, caused by numerous minor faults 

 and small displacements, producing picturesque mural-like cliffs that serve 

 to break the otherwise monotonous scenery. A measurement of the thick- 

 ness of the quartzite is impossible. These displacements, although fre- 

 quent, are seldom sufficient to bring the underlying limestones to the surface. 

 The greatest thickness observed in any vertical wall is about 300 feet, which, 

 however, fails to take into account the amount carried off from the surface 

 by denudation. A section across the vertical cliff just west of Castle 

 Mountain will be found on page 56. Near the base of the quartzite cross- 

 bedding has been detected in one or two localities, indicating shallow water 

 deposits; it appears, however, to be wanting in all the higher beds that 

 present a singularly uniform body of quartz grains free from impurities. 



Castle Mountain is capped by 200 feet of Lone Mountain limestone 

 overlying the quartzite, and from here extends in a narrow belt in a south- 

 east direction for over 2 miles. Here, as in many other localities, the Lone 

 Mountain limestone is devoid of fossils, and not until Stromatopora, Chcetetes, 

 and Atrypa reticularis appear in beds generally regarded as Devonian, have 

 organic forms been recognized. The country is monotonous in the extreme, 

 dazzling to the eyes, waterless, and for the most part treeless. The lime- 

 stone shows no lines of stratification. 



Granite-porphyry. To the northwest of Bellevue and White Cloud Peak, 

 in the region of the granite-porphyry dikes, the simple structural features 

 of the Fish Creek Mountains are lost by the intrusion of large bodies of 

 granite-porphyry. It occurs in two distinct masses with a few outlying 

 smaller dikes and knolls, the two principal bodies being separated by a 

 belt of limestone scarcely 300 feet in width. 



The largest exposure of granite-porphyry presents an irregular body 

 lying between Fish Creek Mountain and Mahogany Hills on the extreme 

 western edge of the District. The smaller body occurs as a prominent 

 north and south dike, which, breaking through Pogonip limestone, appears 

 at the surface as an offshoot from the larger mass. From this massive dike 



