314 QUICKSILVER DEPOSITS OF TOE PACIFIC SLOPE. 



tufa-like substance, which at a little distance is scarcely distinguishable 

 from the ordinary Miocene sandstone. Only in a few small patches is it 

 found in a dense state like ordinary andesitic lavas. The microscope and 

 clieniical tests show that it is quartzose, glass-bearing rhyolite. The im- 

 portance of this dike with reference to the ore deposits is at once evident. 

 It would be entirely unreasonable to suppose that this dike represents the 

 .whole of the lava of this species ejected in the Coast Ranges, though no 

 other mass has yet been detected. If the physical character of other out- 

 bursts is similar to that of this dike, they may have been seen repeatedly 

 at a distance without recognition, by myself as well as by others, in the 

 course of reconnaissance ti-ips, though they could certainly not escape detec- 

 tion in any area subjected to a detailed examination. This dike not only 

 pi%ives the former existence of volcanic activity in this district, but empha- 

 sizes a fundamental structural axis. The character of the metamorphic 

 rocks .shows that the line along which compression and upheaval took place 

 in the early Cretaceous was about west by north, east by south The fold- 

 ing of the Tertiary rocks shows that compression was repeated in the same 

 direction at the close of the Miocene. The position of the rliyohte dike 

 proves that the dislocation which opened a passage for this lava again fol- 

 lowed a similar course. As for the age of the rhyolite, it is certainly Post- 

 Miocene, for had it been earlier it must have shown the effects of the Post- 

 ]\nooene uplift. Had the lava been ejected at tlie time of that uplift, it 

 would probabl}' have been so eroded that the croppings would present a 

 different appearance, for the tufaceous modification is probably superficial. 

 It is possible, however, that its position has in great measure protected it 

 and that during the Pliocene it was covered with sediments. As a rule, the 

 rhyolites of the Pacific slope are younger than the andesites, as was pointed 

 out by Baron von Richthofon, and if this rhyolite is younger than the ande- 

 sites of ilt. Diablo and of Napa County it is Post-Pliocene; but there is no 

 direct evidence tliat this is the case. On the whole, the probabihties are, 

 tlien, that the rliyolite is recent or late Pliocene, but it is certainly known 

 only that it is not older than the PHocene. 



Mine minerals and rocks. — The orcs of thls rcgiou iiro couiposed of the usual 

 association of minerals : cinnabar (sometimes accompanied by a little native 



