102 GEOLOGY OP THE COMSTOCK LODE. 



Slide 197. McKihhen Tunnel, 488 feet from entrance. 



Decomposed dioritic porphyry. — This sHde IS froDi the Same bodj of porphyrite 

 as 194, which it greatly resembles. Fig. 10, Plate III., from this slide, shows 

 a mass of chlorite bounded by the outlines of a former hoi-nblende. A 

 portion of this chlorite has been converted into a mixture of quartz and 

 calcite, accompanied by limonite. This pseudomorph seems to prove that 

 the survival of a border of chlorite at the outer edge of the hornblende sec- 

 tion accompanies the decomposition of chlorite into quartz, etc., as well as 

 the change into epidote. 



Slide 199. McKihhen Tunnel, 488 feet from entrance. 



This slide, from the same specimen as 197, contains a fine hornblende 

 section completely changed into epidote. In this case the formation of 

 epidote appears to have started from points near the edge. It is shown in 

 Fig. 9, Plate III. 



Slide 281. Head of Ophir Eavine. 



Decomposed diorite-porphyry. — This Tock strougly rcscmblcs that from the Mc- 

 Kihhen Tunnel both macroscopically and microscopically. It forms very 

 extensive croppings, different portions of which vary greatly in degree of 

 decomposition and appearance. Where most decomposed it is reduced to an 

 almost uniform dull green color, but in the freshest portions it is granular, 

 greenish gray in tint, displays its feldspars and altered hornblendes in 

 marked contrast, and, in short, betrays its dioritic character. Under the 

 microscope this slide shows the original constituents to have been feldspar, 

 well crystallized hornblende, some augite, magnetic iron, and apatite. 



The hornblende has been completely decomposed, and comparatively 

 little chlorite remains within the hornblende sections, which are mainly filled 

 with epidote. A definite geometrical relation is noticeable here, as in slide 

 194, between the outlines of the hornblendes and the progress of the 

 decomposition. Many of the outlines of hornblende sections are occujjied 

 towards the center by a mass of epidote, between which and the periphery 

 is a band mainly filled by quartz Either, then, the chlorite has been decom- 

 posed from the center into epidote, and simultaneously from the exterior 

 into quartz; or the epidote, after replacing the chlorite, has been decom- 



