440 DESCRIPTIVE GEOLOGY. 



low range of hills called the Ti'averse Mountains, which form a partial con- 

 nection between the Wahsatch Range and the Oquirrh Mountains. These 

 hills rise only about 2,000 feet above the valley, and show but few good 

 exposures of the rocks which compose them, their slope being generally 

 covered with gravel and detrital material. They seem to be composed 

 mainly of tracliyte, the flows of which extend close up to the flanks of 

 the bounding ranges, and on the western side extend along the foot-hills of 

 the Oquirrh as far as the mouth of Bingliam Canon. 



It is probable that this trachyte-flow was preceded by an ejection of 

 andesite, since among the specimens brought in from this range was one 

 which a microscopic examination has proved to be andesite, though, owing 

 to the fact that its locality cannot be well determined, this rock has not been 

 indicated upon the map. It is a dark-gray rock, having a decidedly tra- 

 chytic feel, the groundmass being very porous, and containing a few micro- 

 scopical crystals of hornblende, with an occasional mica. Under the micro- 

 scope, it is seen to be composed of plagioclase, with a little sanidin-feldspar, 

 a large amount- of dark-brown hornblende in crystals, which show little 

 alteration, and a small quantity of augite, while the groundmass, which is 

 rich in gray glass, consists of an interwoven mass of microlites, which is 

 characteristic of the andesites of the Wahsatch region. 



The trachyte of the eastern end of the Traverse Mountains is a dark 

 bluish-gray, sometimes reddish, crystalline rock, made up of large crystals 

 of sanidin-feldspar and mica, with such a small proportion of groundmass 

 that at a little distance it might be taken for granite. While the larger 

 feldspars are almost all sanidin, a considerable amount of plagioclase- 

 feldspar can also be detected in the mass. It is also rich in hornblende, 

 which is comparatively unaltered. Under the microscope, the fresher 

 feldspars are seen to have a zonal structure, and to be rich in glass- 

 inclusions containing large bubbles. Considerable greenish-yellow augite 

 and apatite are also detected, but no quartz. The groundmas is made 

 up of felsitic microlites and magnetite grains inclosed in a glassy base. 

 Near the point of contact with the granite mass of Lone Peak, there is 

 found a greenish-white, earthy, decomposed rock, in which the only traces 

 of crystallization left are white irregular spots of partially kaolinized feld- 



