DETAILED DESCRIPTION OF SLIDES. 131 



same remark applies to the mica. Some of the horn])lende crystals, too, 

 are decomposed, while the majority are perfectly fresh. This is probably 

 due to the structure of the rock, which must admit liquid currents more 

 easily on certain lines than on others. The groundmass is thoroughly 

 crystalline and microlitic, and consists of feldspar microlites and magnet- 

 ite. This is the most important of the so-called trachytes of the District, 

 and was therefore selected for illustration. Plate V., Fig. 32, shows a 

 characteristic field. 



Slide 474. Quarry 2,000 feet east of Occidental Mill. 



A similar rock.— This is a rcddish porphyry similar, except in color, to that 

 described under slide 472, and also used for building purposes. Under the 

 microscope it is remarkable for its intensely dichroitic hornblende, which 

 shows an extremely light yellowish-brown tint, when parallel to the long 

 section of the analyzer, and a bright red-brown when at right angles to this 

 position. The slide also contains considerable poorly crystallized augite. 

 The mica gives the same decidedly biaxial interference figure as that in 

 slide 472. The feldspars also are similar to those in that sUde. This is 

 the rock separated by Dr. Hawes by Thoulet's method, and found to con- 

 tain no sanidin. 



Slide 230. Quarry above Utah mine. 



More compact, gray rock. — A bluish-gray rock of manifestly loose texture, 

 showing both mica and hornblende. Under the microscope plagioclase, 

 mao-netite, and some brown glass are also visible. The feldspars are beau- 

 tifully fresh. Extremely few lack stripes, and these are not in determin- 

 able zones. Several of the larger feldspars show nearly square sections and 

 pericline as well as albite twinning. These give angles of extinction of 

 about 30° on each side of the albitic twinning plane. The small elongated 

 feldspars also give labradorite angles in many cases, and I see no reason to 

 suspect any considerable quantity of any other feldspar. The feldspars con- 

 tain great numbers of colorless glass inclusions, most of them entirely fresh, 

 as well as patches of the brown glass and of groundmass with glass. A few 

 of the smaller feldspars seem to consist of negative crystals of brown glass 



