168 DESCRIPTIVE GEOLOGY. 



trachytes, containing generally large, well-defined crystals of this feldspar 

 in a rough, porous, and mostly crystalline groundmass, are characterized 

 by the presence of a large amount of free quartz. They have, nevertheless, 

 a physical habit which is not rhyolitic, but decidedly trachytic. The 

 quartz occurs in rounded grains, which have a dull, greasy lustre, and are 

 cracked and riven, like a glass which has cooled suddenly. While this 

 quartz is evidently an nnessential and accessory constituent, like tridymite 

 in other trachytes, occurring only in grains large enough to be distinguished 

 by the naked eye, and not, therefore, forming part of the groundmass or 

 influencing the general aspect of the rock, the fact that it frequently contains 

 glass-inclusions proves that it is a primary product, and not the result of later 

 secretion. These trachytes, besides the normal constituents, sanidin, horn- 

 blende, and mica, contain also a relatively large proportion of augite, and 

 in some cases considerable amounts of olivine, generally, however, where 

 the rock has a decidedly basic character. 



The basalts of the region are no less remarkable than the trachytes, 

 being the only representatives of the group of nepheline-basalt found within 

 the limits of our exploration. 



The trachyte outflow forms, as we have seen, a north and south ridge, 

 nearly parallel with the line of outcrop of the Archasan rocks of the Park 

 Range, which, at the northern end of the ridge, it comes in contact with and 

 partially covers. The dense forest, which covers it almost continuously, 

 renders its exploration somewhat difficult. Its forms are generally rounded 

 and dome-shaped ridges, with sharp conical peaks, in striking contrast with 

 the prevailing flat-topped peaks and ridges of the basaltic outflow. 



North of the east and west line of elevation, the trachyte-flows have 

 been more deeply eroded, leaving sharp, jagged peaks, of which the most 

 prominent is Hantz Peak, a comparatively regular cone, 10,906 feet in 

 height, while a number of similar shaped peaks of less height form the pro- 

 jecting summits of Steves Ridge. In the angle of the two ridges, a line of 

 sharp ridges and narrow dikes extend out in a northwest direction, partly 

 co% ered by the more recent basalt flows, of which Crescent Peak and the 

 Skelligs Ridge present the most striking topographical features. To the 

 southward, the trachyte ridge has generally bi'oad, gentle slopes. Its crest 



