VOLCANIC ROCKS OF THE PINON RANGE. 559 



ward, the stratified rocks are overlaid and cut off by irregular flows, which 

 score the base of the range frona its northern end to Pine Nut Pass. Not- 

 withstanding these broad areas of volcanic rocks, it is noticeable that the 

 Palaeozoic ridge is never traversed by dikes or shattered by massive 

 eruptions, forming prominent Tertiary peaks within the range ; and 

 although rhyolite hills in the region of Pinon Pass rise to considerable 

 height, they lie to the eastward of the main uplift, and do not appear to have 

 disturbed the broad anticlinal fold. From the region of Dixie Pass, the 

 eastern foot-hills for about 12 miles are covered by successive outflows of 

 trachyte, which, directly south of the pass, forpi rolling hills which abut 

 against the ends of the Ute-Pogonip limestone, and trend southward until 

 they flank the Cambrian quartzites, then continuing southward overflow the 

 latter limestone on the east side of the anticlinal and also the Ogden Quartz- 

 ite. This belt of trachyte widens out directly south of Dixie Valley for 

 about six miles, and forms the material of a group of conical hills which, 

 from their peculiar outlines, are conspicuous anywhere from the valley of 

 Dixie Creek. 



The overlying sheets and masses of trachyte, as well as all the promi- 

 nent conical hills, are formed of a brownish and reddish-brown sanidin- 

 trachyte, in which the characteristic vitreous crystals of sanidin are riven 

 with a multitude of cracks like those in the typical trachyte of Washoe. The 

 groundmass is very coarse and rough in texture, and is formed of sanidin 

 and brown magnesian mica. In a number of places where erosion has laid 

 bare the lower members of the outflow, there is seen a gray trachyte rich in 

 hornblende, and carrying a high proportion of plagioclase. This has a 

 distinct bedding and a decided dip to the east, or away from the flanks of 

 the range. In these gray hornblendic trachytes, the base is more com- 

 pacted, and the rock has far more the habit of bedded andesites. It is not 

 unlike the rock of the cross- spur at Washoe, and only differs from it in 

 having rather smaller feldspar crystals. Mica, although present, is always 

 in less quantity than the hornblende, and forms large black crystals, with 

 a tendency to decomposition into brown earthy material. The less 

 altered crystals cleave with very smooth and brilliant black faces. It is 

 interesting to observe here, as in so many other places, noticeably in 



