DETAILED DESCRIPTION OF SLIDES. 115 



Slide 346. Sxtro Tunnel, south branch, 3,960 feet from fork. 



Hornbiendic diabase. — Macroscopicallj this Tock looks much like those already 

 described, except that it contains a considerable number of clearly recog- 

 nizable hornblendes. Three or four of these occur in the thin section. 

 They are bright brown in color, and decomposition has scarcely set in. 

 Far more numerous are the augite sections, which, though wholly decom- 

 posed to uralite and chlorite, retain their characteristic outline. In some of 

 these the conversion of chlorite into epidote may be traced. The relations 

 of the porphyritical crystals to the groundmass in this slide are precisely 

 those met with in the ordinary diabases of the District. 



Slide 396. Yellow Jacket shaft, 2,299 feet from surface. 



Diabase containing epidote. — Thls is a grecnish-grav grauular rock, somewhat 

 unusual in color for Washoe diabase. In most cases in this Disirict grains 

 of epidote may be observed under the microscope, developed in the chlorite 

 formed by the decomposition of the augite; but this change seems to cease 

 almost as soon as begun. In the more decomposed rocks the chlorite is 

 seen passing into calcite and quartz, while the epidote grains are replaced 

 by an opaque substance, which is probably iron oxide. In this slide, how- 

 ever, it is plain that augite has passed into uralite, this into chlorite, and 

 that a great part of the chlorite has been converted into epidote. All the 

 stages can be observed here, as in the McKihbcn Tunnel diorite, and, as in 

 that rock, the crystals of epidote ai'e seen eating their way into the chlorite. 



Slide 134. Hierra Nevada, 1,450, iiorth drift, 217 feet north of shaft. 



Diabase containing diaiiage. — Macroscopically tbis is a dark, fine-grained rock, 

 which looks more like some of the dark diorites than it does like diabase. 

 Under the microscope it is seen to be composed of rather uniform grains of 

 plagioclase, diaiiage, and hornblende. 



The plagioclase is broad-banded, contains fluid inclusions, and gives 

 the angles of extinction of labradorite. The hornblende is bright brown. 

 The diaiiage, which is much in excess of the hornblende, is dark gray and 

 feebly diaphanous. In general it is disposed in irregidar patches between 

 the feldspars, but there are a few sections with the augite outline, and 



