148 GEOLOdY OF THE COMSTOCK LODE. 



'J'liesc jirisins iire coloiles.s near the center, but green near the border. The 

 figure sliows one of a vast numbei- within the same crystal outline, the 

 sliadcd portion representing green. No change in the angle of extinction 

 is i)ro(ince(l by the alteration. The rock is nietaniorphic diorite from the 

 Aviazoii mine. Magnitled 1270 diameters. 



Fir. 12. Slide 29.j'''*-'. Colorless hornblende passing into a green variety of the same 

 mineral, longitudinal section. Mo longitudinal section so perfect as the 

 cross-section shown in Fig. 11 has been met with. Many, however, like 

 that portrayed in Fig. 12, show colorless libers encroa('he<l upon by the 

 green mineral. This section also contains a little chlorite, shaded a 

 deeper tint than the remainder of the section. The rock is nietamorphic 

 diorite from the Arutizon mine. Magnified 40 diameters. 



PIjATE III. 



Fig. 13. Slitle 20''-'''. Zonal feldspar. The kernel and the outer zone extinguish light 

 when the ]irincipal plane of the Nieols is inclined at an angle of about 

 14° to the twinning plane, and the fine reversed lamelhe are blackest 

 when the angle measures about 14° in the oi)posite direction. The inter- 

 niediate zone extinguishes at an angle of 5° in the same sense as the 

 other zones. .Inst within the outer zone is a belt of nearly ojtaciue inclu- 

 sions whicli connects with the groundmass of the rock at tln^ top of the 

 figure. Tlie rock is hornblende-andesite from the quarry 1,000 feet west 

 of the Yellow Jaclcet east shaft. Magnified .'JO diameters. 



Fig. 14. Fortieth Parallel collection, slide 284"*'. Feldspar with rectangular glass 

 kernel. The two halves of this cryst.al extinguish light at angles of 24° 

 and 2(io to the twinning plane, and minute twin lamella are visible at 

 the lower end of the section. Magnified 140 diameters. 



Fig. 15. Slide 349'^-'". Augite section showing discontinuous twin lamellre. These 

 are shaded dark gray. Two included crystals of iron ore are indicated 

 in black, and some chloritic patches in light gtay. The rock is diabase 

 from the Stdro Tunnel north branch, 50 feet south of Ophir connection. 

 Magnified 40 diameters. 



Fig. 16. Slide 42S'^-'". Augite with contorted twin-lamellai, which are shown in black. 

 The rock is an augite-andesite from near the Sutro Tunnel air-shaft (beyond 

 the limits of the niap). Magnified 70 diameters. 



Fig. 17. Slide 450>''--'. Fragment of brown hornblende with black border on the frac- 

 tured surf^ie.e, as well as on the crystal faces, and a second jjarallel internal 

 belt of magnetite. The figure is from a hornblendic andesite from a cut 

 1,000 feet east of the raib-oad station at the Silver City switch. Magni 

 fied CO diameters. 



