BLACK BUTTE. 739 



Rawling's Peak, "Wyoming. The plagioclase crystals have in some places 

 the same structure as the labradorite feldspars from Paul's Island on the 

 Labrador coast. They contain black needles and grains and brown laminae 

 in the same arrangement, which is an unusual occurrence in the presence of 

 so much quartz as is found in this rock. 



Southwest from Winnemucca Peak, along the banks of the river, are a 

 number of obscure outcrops of light-gray limestones, considerably broken 

 up by intrusions of basalt and partially buried in sand, which have on the 

 map been colored as belonging to the Star Peak formation, although it is 

 by no means improbable that they belong to the lower beds of the Jurassic. 



Black Butte derives its name from the color of the prevailing slates 

 upon its summit. It forms an isolated mass, roughly circular in outline, 

 between 3 and 4 miles in diameter, and rises over 2,000 feet above the 

 valley of the Humboldt, presenting steep slopes on all sides, with deeply- 

 eroded canons. On the south side of the summit occurs a body of medium- 

 grained, light-gray crystalline rock, which, with some hesitancy, has been 

 classed as a syenite. Where examined, it was found to be considerably 

 decomposed, but appeared to carry both monoclinic and tri clinic feldspars; 

 the orthoclase of a flesh-red color, however, being the prevailing form. 

 The hornblendes are usually quite small and of a greenish tinge. 



The slates on the summit of the ridge strike north 50° east, and dip 

 from 50° to 55° to the northwest, and, although along the spurs the beds 

 show considerable variation in dip, they are always inclined at high angles. 

 It is evident that there are exposed here between 3,000 and 4,000 feet of 

 conformable strata, mainly composed of thinly-bedded slates and shales, 

 with occasional layers of compact brown sandstone and some bluish-gray 

 limestone. In the slates may be recognized grains of pyrites, while the 

 microscope reveals, under a high power, pale-greenish chlorite surround- 

 ing the quartz. 



Eugene Mountains. — The Black Butte is connected with the Eugene 

 Mountains by a low pass, in which are a number of obscure exposures of 

 slates, standing at high angles and dipping northwest, serving to show the 

 continuity of strata between the two main uplifts. The mountains extend 

 from the Humboldt River for 12 miles, with a northeast and southwest 



