200 GEOLOGY OF THE COMSTOGK LODE. 



hornblende rock, as seen in the tunnel, are parallel to one another and to 

 the Lode. 



Even on the surface, indications of the parallelism of the contact be- 

 tween the two andesites and the Lode are observable. If a line is drawn 

 at a distance of 4,500 feet east of the vein, it will fall very close to the 

 easternmost edges of the earlier hornblende-andesite, and include only one 

 considerable tract of the augite rock between it and the Lode. The For- 

 man shaft is nearly at the center of this tract, and the section through it 

 shows that hornblende-andesite e.xists below the surface. The contact be- 

 tween these two rocks in depth is therefore probably nearly parallel to the 

 Lode throughout the whole length of the latter. 



Besides the area of earlier hornblende-andesite to the east of the Lode, 

 it covers a large extent of country to the west of the diorite. Before the 

 fault occurred the top of the range was probably about on a level with the 

 east wall, and it seems probable that the whole exposure of earlier horn- 

 blende-andesite is ascribable to a single eruption, or an unbroken series of 

 eruptions. I can find no indication of bedding, nor of the distinct lava 

 streams which give evidence of intermittent action in the neighborhood of 

 modern volcanoes. At first the andesite most likely buried the diorite com- 

 pletely, but the latter must have been reexposed by erosion before the 

 fault took place. The hornblende-andesite, as well as the diabase, is di- 

 vided into sheets by a system of parallel fissures. If the conclusions drawn 

 in Chapter IV. are correct, this fissure system was developed by faulting at 

 a comparatively recent period, but tlje tendency to parallelism in the struct- 

 ure of the country was first exhibited as far back as the earlier hornblende- 

 andesite eruption. 



The area north of Silver City is remarkable for the unusual develop- 

 ment of hornl)lende crj'stals, which are frequently an inch and a half in 

 leng-th, and occasionally more. In tliis area, too, there are several sharp 

 cones one or two hundred feet in height, which suggest volcanic vents, but 

 no craters are traceable ; and the evidence of degradation opposite Virginia, 

 especially the flatness of the surface previous to the fault, as is proved from 

 the present regular character of the fault-curve, makes it improbable that 

 distinguishable relics of craters or cones of eruption should remain. In the 



