356 DESCRIPTIVE GEOLOGY. 



Plate XV, whicli represents a spur on the north side of the canon running 

 out from the base of Twin Peak, gives a good representation of these feat- 

 ures of its mass-structure, the general slope of the bedding-planes to the 

 "west, and the tendency of large masses to flake off, leaving rounded sur- 

 faces, which give the appearance of having been worn smooth by glacier- 

 action, as they doubtless have in part. From the immense boulders, often 

 as large as a moderate-sized house, which are scattered through this part 

 of the canon, the rock has been quarried, of which the foundations of 

 the Mormon temple at Salt Lake are built. It is a remarkably beautiful 

 stone, white and rather coarse-grained, dotted here and there with round 

 black spots, where there has been a concentration of the dark-green horn- 

 blende, which is a prominent constituent of the mass. This granite belongs 

 to the class of dioritoid granites, which are of frequent occurrence along the 

 Fortieth Farallel; tliatis, itis a compound of quartz, biotite, hornblende, ortho- 

 clase and plagioclase feldspars, and titanite, with a relatively large proportion 

 of plagioclase, hornblende, and titanite. The crystals of titanite can be 

 readily seen by the naked eye, and are frequently quite large and of very 

 perfect form. By the aid of the microscope, pale-green hornblende micro- 

 lites are found within all of these crystals; apatite is found to be abundantly 

 disseminated through the mass, and, in some varieties, plates of red oxide of 

 iron ; the larger hornblendes are also seen to be made up of accumulations 

 of single prisms, and the quartz to be comparatively poor in liquid-inclu- 

 sions. An analysis of this granite made by Prof. Thomas M. Drown gives 

 the following constituents: 



Silica 71.78 



Alumina * 14.75 



Ferrous oxide 1.94 



Manganous oxide 0.09 



Lime 2.36 



Magnesia 0.71 



Soda -■ 3.12 



Potassa 4.89 



Ignition 0.52 



100.16 



