280 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



sandstones and granular metaraorpliics, as well as some of the serpentinized 

 rocks, show a similar injection. The purer serpentines are seldom inter- 

 sected by quartz veins, apparently only because this rock, though soft, is 

 tough and not easily fissui-ed. Where the net of quartz and more or less 

 serpentinoid rock coexist, as is the case at many localities in this district, it 

 seems certain that the silicification followed serpentinization ; for, while the 

 angular fragmei:ts surrounded by quartz veins are green, the veins them- 

 selves are not thus tinged. Had they existed prior to serpentinization they 

 iiiust have been attacked like the sandstones, and, since the quartz veins 

 are permeable by solutions, they must have acquired a green tint. This 

 silicification is a common characteristic of the metamorphosed rocks of the 

 Knoxville group throughout the Coast Ranges. It is substantially coex- 

 tensive with metamorphism and, as explained in a former chapter, seems 

 to represent the last phase of that series of alteration processes. A more 

 intense and different silicification occurs at Knoxville and elsewhere in the 

 neighborhood of ore bodies, to which reference will be made in describing 

 the deposits of the district. 



Basalt — The area to be mapped was selected in such a manner as to 

 include all the ore deposits of the neighborhood and before anything was 

 known of the distribution of volcanic rocks. It would almost seem, from 

 an inspection of the map, as if the basalt area, which is the only one near 

 Knoxville, had been purposely selected as the central feature. This coin- 

 cidence is not meaningless. The basalt occnpies portions of the crest and 

 flanks of a low range of hills which forms the boundary between Napa and 

 Yolo Counties. The range itself is composed of metamorphic rocks and it 

 is evident from the topography that the basalt forms a thin sheet. It is 

 altogether jn-obable that the eruption of this small basalt sheet took place 

 at two or more points along the crest of the ridge and flowed down on both 

 sides. Not only does the disposition of the rock answer to this natural 

 supposition, but a dike of the lava is said to have been met by a tunnel 

 from the Lake mine run at right angles to tlie crest of the ridge from the 

 spot called Johntowu. Lithologically this basalt presents no peculiarities, 

 excepting that it carries a rather small quantity of olivine. Near the basalt 

 field is a small area of tufa, undoubtedly a portion of the basaltic erudition 



