150 GEOLOGY OF THE COMSTOCK LODE. 



abscissae. In seeking a mineral, it is convenient to lay a card, or rectan- 

 gular slip of paper, on the illustration with its edges parallel to those of 

 the figure, but intersecting the graduated edges of the latter at the given 

 distances. The corner of the card will then coincide with the point sought. 

 This method is capable of any desired degree of exactness and permits of 

 the indefinite multiplication of references. 



Tig. 25. Slide 2i;i"'-"'. (Trauular dioiite from Bulliou Raviue at Water Company'.s 

 flume. Nicols crossed. Magnified 30 diameters. 



Green, fibrous hornblende: 20-22; 27-28; 22-1.3. 

 Labradorite : 12-15 ; 14-28, and most of the unspecified grains. 

 Quartz: 8-14; 15-23; 17-18. The quartz carries fluid inclusious, 



some of which show active bubbles. 

 Magnetite: 19-10; 25-27. 

 At 19-20 epidote is developing in a patch of chlorite, but cannot be well 

 observed with crossed Nicols or with so low a power. 



Fig. 20. Slide 421"'-'". Porphyritic diorite from the center of Cedar Hill ridge. Mcols 

 crossed. Magnified 30 diameters. 



Greenish-brown hornblende: 20-20; 20-27; .30-21; 10-22, etc. 

 A small feldspar is inclosed in the large hornblende, and chlorite 

 in small quantities is developing along the cleavages of the lat- 

 ter, producing with crossed Nicols the broad black marking.s 

 noticeable in the drawing. 



Feldspars: The porphyritic feldspars in this slide, as at 10-18, ap- 

 pear to be labradorite. Some of the inicrolites give oligoclase 

 angles of extinction. The greater part of the small feldspars 

 are granular. 



Magnetite: 8-24; 20-23, and many grains too small to appear indi- 

 vidually on this scale. The apatites are also too minute to be 

 .shown. 



Epidote developing out of cbloi-ite occurs at 18-5, but requires a 

 higher power and different light for study. 



Fig. 27. Slide 354""'''. Quartz-porphyry 1,000 feet southwest of Lawso^i's Tuniicl. 

 Nicols at 45°. Magnified 30 diameters. 



UrthoclasE: 22-5; 20-25; 20-23; 17-15; 15-10. 



Quartz: 25-10; 15-25. The quartz contains bays of groundmass 



and numerous fluid inclusions with moving bubbles. 

 Mica: 1.5-20; 5-24. The mica is wholly decomposed and replaced 

 by linionite and other secondary i)roducts. 



