BLACK HILL. 171 



torted, which is probably due to the original compression of the beds, and not 

 dependent on the outflow of rhy olite. The lines of strike, so far as observed, 

 run continuously through the hill, and do not curve round it. Almost 

 the whole surface of the hill is covered with loose fragments, detached 

 through frost and atmospheric action, but its south and southeast faces pre- 

 sent steep cliffs. On the lower northeastern slopes of the hill are two or 

 three large bowlders of coarse reddish granite, half buried in the soil, in 

 company with quartzites and sandstones, which are evidently erratics and 

 . show that at one time the glacier from Twelve-Mile Creek reached down as 

 far as this. 



The rhyolite of Black Hill is remarkably uniform in general character. 

 It has a delicate pinkish-gray color, a conchoidal fracture, and shows in the 

 unaltered specimen white glassy feldspars, fresh black mica, and some horn- 

 blende, with prominent and rather smoky quartz in a distinctly marked 

 groundmass. The existence of this groundmass makes a marked distinction 

 from the rhyolite of Chalk Mountain, which is seenmacroscopically to be made 

 up entirely of crystalline elements. To the naked eye it is apparent that 

 the quartz contains many bays of the groundmass. Under the microscope 

 the groundmass is seen to be entirely microcrystalline, being composed 

 mainly of quartz, with some rather cloudy feldspars. The large feldspars are 

 plagioclase in part and contain a few gas pores and some fluid inclusions, 

 which often carry cubes of a mineral like salt. Undoubted glass inclusions 

 are not visible, but there are some dihexagonal in form, which are either 

 devitrified inclusions or represent the character of the groundmass at a time 

 prior to its complete crystallization. In decomposition the feldspars seem 

 to tend more to a kaolin substance than to nmscovite. 



Twelve-Mile Creek. In the region between Sheep Ridge and the main crest 

 of the range are the valleys of Twelve-Mile and Sheep Creeks. The sur- 

 face is covered with outcrops of Weber Grits formation, or, in its lower por- 

 tion, with surface gravels, either actual moraines or rearranged drift mate- 

 rial. At the head of Sheep Creek, near the south base of Lamb and Sheep 

 Mountains, is a little valley or park, bounded on the north and east by steep 

 talus slopes of debris from these peaks, and by forest-covered spurs of 

 Weber. Grits on the south and west. On the broad ridge between Horse- 



