382 GEOLOGY OF THE COMSTOCK LODE. 



occupied tlie position now covered by Virginia City. Underground it is 

 extensively developed from the Overman to the Sierra Nevada, and from the 

 Lode to the Combination shaft, as is seen in the cross-section on the Sutro 

 Tunnel line, Atlas-sheet VI. Its great importance is due to the fact that all 

 the important bodies of the Comstock have been intimately associated with 

 it, as are many of the other famous silver mines of the world. This diabase 

 is of a rather unusual character, being more than commonly porphyritic, and 

 containing comparatively little augite — a trifle less than twenty per cent. 

 In appearance it is often not dissimilar to the andesites, but the resemblance 

 does not extend to details. Almost the whole of this diabase is greatly 

 decomposed, and has hitherto escaped recognition on that account.^ 



Between the east-country diabase and the west wall of the Comstock 

 occurs a thin dike, which has long been known as " black dike." It is only 

 in the lower levels that fresh occurrences of this material have been met 

 with. The "black dike" appears to be identical with the Mesozoic diabases 

 of the Eastern States, from which it is scarcely distinguishable macroscopic- 

 ally, microscopically, or chemically. This younger diabase forms a remark- 

 ably thin and uniform dike, nowhere more than a few feet in thickness, 

 extending from the Savage southward to the Overman, and then l)ranching 

 off to the southwest as far as the Caledonia shaft. This is the only dike 

 known in the District, excepting one of diorite in diorite, in spite of the 

 prevalence of eruptive rocks. Its presence shows that the fissure on which 

 the Comstock Lode afterwards formed was first opened in pre-Tertiary 

 times, and its uniform thickness indicates that its intrusion antedates any 

 considerable dislocation on the contact. This inference receives strong 

 confirmation from the evidence already adduced that the faulting is a 

 comparatively recent phenomenon. 



The occurrence of the two diabases also goes a long way toward 

 demonstrating the nature of the fork in the vein, which has always been a 

 mysterious point in the geology of the Lode. The prolongation of the 

 " black dike" beyond Gold Hill is toward American Flat, whereas the older 

 diabase extends in the direction of Silver City. 



Andesites. — Mucli tlic larger part of the surface of the District is occupied 



' Though diabase is the most important east-couutry rock, it by no means coincides in position 

 either below ground or above with the rocks which have been regarded as propylite. 



