ELKHEAD MOUNTAINS. 169 



rises from the Yampa River northward, finding its culminating point in 

 Whitehead Peak, a dome-shaped mountain-mass, abruptly escarped on its 

 eastern side, whose summit has an elevation of 10,817 feet above sea-level. 



Teachytic Region. — The trachyte of Whitehead Peak is one of the 

 most interesting and curious of the remarkable group of trachytes of this 

 region. It is a grayish-drab rock, having a tendency to split into thin 

 laminae, from half an inch to an inch in thickness. It is formed of crystals 

 of sanidin, hornblende, and augite, with large rounded masses of cracked 

 quartz, in a purplish-gray, fine-grained groundmass of a rough porous 

 texture. Besides the hornblende and augite, it contains a few sparse grains 

 of bronze-colored mica, while some portions of the rock are filled with 

 reddish-brown spots, which the microscope shows to be half-serpentized 

 olivine, "a mineral which," Professor Zirkel remarks " has never before been 

 observed in a sanidin rock". He suggests, with regard to the occur- 

 rence of this mineral in connection with free quartz, that it almost seems as 

 if the unusual secretion of free silica had been counterbalanced and neu- 

 tralized by the introduction of so basic a mineral as olivine. Large sani- 

 din crystals are frequently found in this rock measuring an inch or more 

 in diameter, and showing a tendency to zonal decomposition. At White- 

 head Peak, there is also an extremely local occurrence of granite-porphyry, 

 which is too small to be indicated on the map. It contains both orthoclase 

 and plagioclase-feldspars, with black mica and a large amount of black horn- 

 blende prisms, in a grayish felsitic groundmass. On the bold eastern escarp- 

 ments of Whitehead Peak, the trachyte is seen to be underlaid by a very 

 considerable thickness of white fine-grained sand-rock, often quite thinly 

 bedded and shaly, with somewhat of the appearance of a volcanic ash. 

 Under the microscope, however, the rock is seen to be made up of rounded 

 grains, largely of quartz, with some colored jasper, and black grains, which 

 may be magnetite. No angular crystals can be detected in the rock, and 

 it may possibly be a remnant of some later Tertiary formation, which cov- 

 ered this region before the trachyte overflow. 



To the south of Whitehead Peak, the trachyte-flow forms a high ridge, 

 dividing Elk River from Elkhead Creek, on the spurs of which it spreads 

 out, covering the underlying Cretaceous beds, nearly to the banks of the 



