NANNIE'S PEAK. 603 



mentary rocks around a core of granite and granite-porphyry, wliicli in 

 Tertiary times Las been deluged by flows of rliyolite to such an extent 

 that but little can be seen at present of the original structure of the range. 

 Nannie's Peak, which is the central and most elevated point of this range, 

 is a sharp, crescent-shaped ridge of granite, wrapped around by steeply- 

 dipping beds of the Wahsatch limestone, which are intersected and covered 

 by flows of rhyolite. The granite, though distinctly an eruptive rock, has 

 remarkably regular cleavage or bedding-planes developed in the strike of 

 the ridge, whose summit is formed of projecting beds from 50 to 100 feet thick, 

 dipping at an angle of 80° to the westward, in conformity with the overlying 

 limestones on the western flanks. It also shows a tendency to split up into 

 narrow bands from one to three inches in thickness, which have a general 

 conformity with this larger system of bedding. 



The granite is a gray, coarsely crystalline rock, made up of quartz, 

 plagioclase, and orthoclase, and large crystals of mica, but containing no 

 hornblende. The orthoclase feldspar predominates over the plagioclase, and 

 is generally decomposed and opaque, showing a zonal structure, while the 

 latter remarkably is fresh and well preserved. Under the microscope, the 

 quartz is seen to contain numerous fluid-inclusions, with well-defined salt 

 cubes and sometimes grains of apatite. A tendency is observed, in the 

 fine-grained aggregations of small quartz and feldspar crystals, to form a 

 groundmass. Its chemical composition, as determined by Prof Thomas M. 

 Drown, is: 



Silica 70.77 



Alumina 15.22 



Ferrous oxide 2.65 



Manganous oxide 0.1 1 



Lime 2.33 



Magnesia .'. '. . . . 0.71 



Soda 3.75 



Potassa 4.58 



Ignition 0.52 



100.64 



