146 GEOLOGY OF THE COMSTOCK LODE. 



on the stage, so that when the spot to be hjoated is under the cross-wires 

 the upper left-hand corner of the object glass shall be within the quadrant 

 between the radii, and the sides of the object glass shall l)e parallel to the 

 same, as represented in the figure. The distances, then, of the spot in 

 question from the sides crossing the radii are read from the scales, and rep- 

 resent the rectangular coordinates of that s[)ot referred to the upper left- 

 hand corner of the object glass as an origin, and are recorded thus: 29.'j"'''; 

 the number of the thin section being given, and the coordinates being jdaced 

 after it, with the vertical coordinate preceding the horizontal. The process 

 of finding any spot the coordinates of which are given in this manner needs 

 no further exphmation. 



PLATE II. 



Fig. 1. Slide 252''-'. Brown lioriibleiide passing into clilorite. The small sti])ijled 

 white mass at the right of the cut is secondary quartz. The rock is a 

 porphyritic diorite. Sierra Nevada mine, 1,450-foot level; north drift, 

 289 feet. Magnified 170 diameters. 



Fio. 2. (Slide 326'''^^ Brown hornblende passing into chlorite, wbicli is represented 

 as gray. The white patches are quartz and calcite. The rock is earlier 

 hornblende-andesite from the Hutro Tunnel, 17,100 feet from entrance. 

 Magnified 70 diameters. 



Fig. ;'.. Slide 464-1--''. Greenisb-brown hornblende, in longitudinal section. The cen- 

 tral portion of the veins is chlorite, between which and the solid horn- 

 blende the space is occupied by quartz. The rock is older hornblende- 

 andesite from croppings 1,200 feet northwest of the Geiger Grade toll 

 house. Magnified 45 diameters. 



Fig. 4. Slide Sl"-^'. Pseudomorph of chlorite after augite. The white intrusive mass 

 is feldspar; decomposition has gone on from the surfaces and cracks, pro- 

 ducing a green slightly dichroitic chlorite, which remains nearly black 

 between crossed Nicols. The fragments have also decomposed Irom their 

 centers into a greenish-brown, very fibrous, strongly dichoritic chlorite. 

 The rock is augite-andesite from the Sutro l\innel, 10,055 feet from en- 

 trance. Magnified 95 diameters. 



Fig. 5. Slide 4,65'^-^^. The outline is that of a cross-section of augite. The smooth 

 gray tint represents a felted mass of chlorite, composed of excessively 

 fine fibers. The coarsely granular mineral is epidote, which can be seen 

 sending denticular crystals into the chlorite. In the upper part of the lait 

 epidote has begun to develop from a second center. The rock is augite 

 andesite from Crown Point Eavine. Another ])art of this sliile is lepre 

 sented in Fig. 31. Magnified (15 diameters. 



