ELKHEAD MOUNTAINS. 171 



development of bronze-colored mica. The large quartz grains and crystals 

 of feldspar are somewhat less frequent than in the Whitehead rock. 



Crescent Peak is a high, sharp ridge, having a somewhat curved out- 

 line, which is isolated from the main mass of the trachyte hills by the val- 

 leys of Slater's and Steves Forks. It is composed, however, of a trachyte, 

 which is in every way analogous to the main body of Whitehead Peak and 

 Steves Eidge. The rock of the peak itself has the same sherdy habit as 

 that of Whitehead Peak. It consists of a light-gray groundmass, in which, 

 to the naked eye, only crystals of sanidin, with occasional hornblendes and 

 micas, and the peculiar rounded grains of cracked quartz, are visible. The 

 microscope detects a few yellowish-brown augites, and around the quartz 

 grains a peculiar greenish ring, made up of an interwoven mass of micro- 

 lites. The quartz contains glass-inclusions, and the groundmass is made up 

 of feldspar-microlites, with small prisms of augite and hornblende, and bio- 

 tite plates in a brown, globulitic, amorphous base. To the north of Cres- 

 cent Peak is a curious dike, called Skelligs Ridge. It is a wall of semi- 

 columnar trachyte, in which the columns are arranged horizontally, from 

 20 to 50 feet in width, rising vertically out of the soft grassy slopes to a 

 height of from 50 to 100 feet, and extending in a northwest direction for 

 several miles. Its walls, especially on the southwest side, are almost per- 

 fectly perpendicular. The surface of this rock presents a peculiarly rough 

 appearance, from the holes or cavities left by the weathering-out of the 

 quartz grains. It resembles mineralogically the rock of Crescent Peak, 

 but is more massive in habit, and is remarkable for the fine definition of its 

 crystalhne constituents, particularly the hornblende and mica. In this, as 

 in all the other quartziferous trachytes, no more quartz can be detected by 

 the microscope than by the naked eye. 



On the low saddle and ridge, which extends to the northeast from the 

 base of Crescent Peak, is found a rock of rather a different habit. It has, in 

 general, a rather homogeneous groundmass, in which no crystalline ingre- 

 dients are visible, but which contains still these same cm-ious grains of 

 cracked quartz. This rock has often a shaly texture, and weathers with an 

 earthy-brown surface, so that, at first glance, it might be mistaken for a 

 sedimentarv rock. The quartz grains are frequently colored brown, appar- 



