620 * DESCRIPTIVE GEOLOGY. 



bedding-. Lithologically the granite is composed of quartz, both monoclinic 

 and trichnic feldspars in almost equal proportion, magnesian mica, a great 

 deal of hornblende, and a little apatite. Under the microscope, the ortho- 

 clase is seen to contain fluid-inclusions. This rock seems alnxost to occupy 

 an intermediate position between normal granites and diorites, which con- 

 tain quartz, hornblende, triclinic feldspars, and, exceptionally, monoclinic 

 forms. It is very similar to the granite from the region of Ravenswood 

 Peak, and only differs from it by caiTying' a less proportion of mica and 

 hornblende. 



Shoshone Peak, which here forms the culminating point of this por- 

 tion of the range, is an immense outburst of gray and green dacite. It is 

 the grandest and most elevated body of dacite to be found within the limits 

 of the Fortieth Parallel Survey. Lithologically it is an interesting rock, from 

 the large size of its included quartz-grains, which are present in the form of 

 irregular globes, sometimes exceeding an eighth of an inch in diameter, and 

 sometimes as rude dihexahedral crystals. Under the microscope, they show 

 numerous inclusions of nearly colorless glass with a dark bubble. The qnartz 

 also contains hornblende-microlites, a rare phenomenon in dacites. The 

 hornblende occurs as distinct green crystals, and as a fine earthy green 

 element, intimately mixed with plagioclase to form the groundmass. Much 

 of the dacite consists of a well-individualized groundmass, with large, partially 

 decomposed plagioclase-feldspars, arfd pellucid grains of quartz, and small 

 angular fragments of a finer-grained, less quartzitic dacite. There are 

 passages where these included fragments become so numerous as to give to 

 it strictly the character of a breccia, and again there are considerable 

 regions where, in hand-specimens, there will be only one or two included 

 fragments, They are always sharply angular and always of dacite, as is 

 proved under the microscope by the presence of hornblende in glass 

 cavities of the quartz, The dacite contains well-defined vertical jointing 

 planes, and, in some instances, betrays a rude columnar structure. Crown- 

 ing as it does £t group of considerable elevation, Shoshone Peak has acted 

 as a condenser of the snows of the Glacial period, and there is ample 

 evidence of the presence of actual glaciers, both in amphitheatre-structure 

 and in actual glacial debris, as well as the striated surfaces at the base 



