50 



University of California Publications. 



[Geology 



of the later Pliocene overlies the Franciscan, and on the same 

 ridge, two miles to the south, large petrified redwoods, ten feet 

 in diameter, and fully 500 years old, as shown by their rings, 

 are lying on a Franciscan surface covered with pumicious tuff. 

 Of some eight or ten trees observed, all lay Avith their roots 

 pointed toward the northeast, the natural inference beim>' that 

 they were uprooted by a. blast of air from some volcano to the 

 northeast, and subsequently buried in the pumicions ashes in 

 which they now lie. Another argument in favor of this ridge 

 having been an elevated portion of the land during Pliocene 

 times is the absence of andesite on both of its Hanks, when two 

 miles to the west, and five miles to the east, thick flows are found 

 beneath the tuff. 



A third small area of Franciscan is exposed on the southwest 

 flank of Mt. St. Helena, low down on the divide between Knight's 

 Valley and the upper end of Napa Valley. It is exposed here 

 by the faulting which has tilted the block forming Mt. St. Helena, 

 and is quite limited in extent. It is largely made up of serpen- 

 tine. 



A fourth area lies on the northeast Hank of St. Helena, along 

 St. Helena Creek at Mirabel and Mhldletown, extending north- 

 west toward Cobb Mountain. 



A fifth large area is near the last mentioned one. It occurs 

 at the Oat Hill Quicksilver Mine, and extends to Knoxville, and 

 occupies a large area west of Berryessa Valley. 



Quicksilver Deposits. — The last two areas mentioned contain 

 important deposits of cinnabar. These deposits occupy the west- 

 ern part of the two areas last mentioned in a general northwest-- 

 erly and southeasterly direction along the eastern flank of Mt. 

 St. Helena and its outliers, Twin Peaks and Round Valley Peak. 

 The cinnabar deposits are invariably associated With serpentine, 

 and usually occur at contacts between serpentine and sandstone, 

 or crashed shale ("alta"), or rarely radiolarian chert. The 

 veins are not true veins at all, but merely zones of silicified ser- 

 pentine. The silica is in the form of opal, and has largely 

 replaced the serpentine, the opal containing the cinnabar and 

 metacinnabarite. The opalized areas are from a few feet to 

 over two hundred feet in width, and very irregular in shape. 



