312 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



Olivine gadbro. — At vavious poiiits in the district thoroiiglily rounded 

 pebbles of a peculiar, coarse-grained, crystalline, gray rock ot unusual 

 liardness were found, which differ strikingly trom any rock seen in place. 

 They must have come from the higher mountains of the range to the south- 

 east, but the croppings were not identified. This rock, when examined 

 luider the microscope, proved to be an olivinitic gabbro, the only rock of 

 the kind known in the Coast Ranges and the only rock carrying even a 

 trace of olivine detected during tliis investigation, excepting the lavas. 

 The coarse grain and the general aspect of this gabbro suggest for it a very 

 considerable age, but the olivines are very fresh and show onl}' thin seams 

 of serpentine along the cracks. Whether the rock is eruptive or metamor- 

 phic could not be determined from the material available. Olivine does 

 not form a large proportion of the mass and decomposition would not yield 

 a material to which the name of serpentine would apply unless other con- 

 stituents besides olivine were sei'pentinized. As for the serpentine of the 

 New Almaden area, the researches recorded in Chapter III show that they 

 are derivative of sandstone, as is the case in all the districts examined. 



Miocene. — Upou tlio uietamorphic rocks lie some areas of Miocene sand- 

 stones. These are soft, yellowish strata, containing a considerable number 

 of poorly preserved fossils {Pcctcii, Ostrca, Balanus), which, however, taken 

 in connection with structural and lithological characteristics, are amply 

 sufficient to fix the age of the beds. The Miocene rocks are considerably 

 disturbed, though far less so than tlie metamorphic series. The structural 

 relations indicate a non-conformity between the Tertiary and the metamor- 

 phic series, which is also demonstrated by the presence of fragments of the 

 earlier strata in the Miocene beds. Professor Whitney observed sections 

 exhibiting this non-conformity witliin a few miles of New Almaden. As is 

 shown in Chapter V, the non-conformity displayed in an unequivocal man- 

 ner here and elsewhere in the neighborhood, as well as in the San Benito 

 Valley, is really between the Knoxville series and the Chico, or between 

 the Neocoraian and the Upper Cretaceous. Any upheaval between the 

 Neocomian and the beginning of the Miocene would produce the relations 

 existing at Almaden. But at New Idria and elsewhere the Chico and the 

 Ttjon are conformable, and in the Vallecitos Valley and at Mt. Diablo the 



