378 QUICKSILVER DEPOSITS OF THE rACIFIC SLOPE. 



Solano County. — The iiiost southei'ly of the mines nortli of tlie Bay of San 

 Francisco is the St. Jolin's, four miles nortlieast of Vallejo. It is situated in 

 an isolated ridge of metamorphic rock trending from northwest to southeast 

 and lying in Sulphur Spring Valley. The sarrounding region is unaltered. 

 In the metamorphic area also there are portions of little modified rock, and 

 at two points on the ridge Mr. Turner found Aucella mosquensis. One of 

 these localities is in the main tunnel of the St, John's mine, and the shales 

 in which the fossils occur seem beyond doubt the same which, at a di.stance 

 of 150 feet from the fossils, are indurated and contain cinnabar. The 

 second AucdJa locality is three-quarters of a mile southeast of the mine on 

 the same ridge. At this point the shells occur in calcareous nodules in 

 shale. These fossils of course detemnine the rocks as belonging to the Neo- 

 comian formation. 



The ore was found at the croppings and is said to have been discovered 

 as early as 1852. The ore body exposed at the croppings extended down- 

 ward about four hundred feet and furnished most of the metal which this 

 mine has produced. Much of this ore was inclosed in a white, greatly 

 decomposed rock, which is probably a metamorphosed sediment, further 

 altered by the action of sulphuric acid and other reagents. At one point it 

 crosses the tunnel very like a dike. There is nothing suggesting the pres- 

 ence of erui)tive rocks on the surface. According to Mr. Neate, the owner 

 of the property, an open fissure exists in proximity to the main ore body, 

 but the position of this fissure is now inaccessible. Like most of the quick- 

 silver mines, the St. John's carried some bituminous matter. In the west- 

 ern mine workings cinnabar occurs in highh- siliceous, apparently chalce- 

 donic rocks, and close by is serpentine, forming a wall to the ore-bearing 

 ground. As appears from the table of production, this mine has yielded 

 8,598 flasks of metal. 



Mt. Diablo. — Cinnabar is found on the eastern slope of the nortli peak of 

 Mt. Diablo, associated with the usual black opal and chromic iron ore. It 

 is said that some thousands of dollars' worth of the metal was extracted 

 from this locality. In the ravine just below the mine is a sulphur spring, and 

 farther down the slope is another mineral spring which must formerly have 

 been very active, for it has deposited a large quantity of calcareous sinter. 



