EHYOLITE. 313 



Ttjon and tlie Miocene are conformable. Hence there is no room for an ex- 

 tensive and violent disturbance of this region between the beginning of the 

 Chico and the close of the Pliocene, and tlie convulsions attending the met- 

 amorphism of the Neocomian beds must liave preceded the Chico. The 

 period is still more closel}' determined by the discover}- of the Wallala 

 beds, which are much earlier than the Ciiico and are referred b}- Dr. W'liite 

 to the Middle Cretaceous. These beds, described in Chapter V, also contain 

 metaniorphic pebbles and show that tlie metamorphism and uplieaval which 

 accompanied it occurred soon after the close of the Knoxville period. 



Not all the areas laid down as Miocene on the map are fossil iferous, 

 but the similarity in litliological character and in the disturl)ance wliich 

 they exhibit is sufficient to justify their reference to the same series. The 

 Miocene beds were raised and folded at the close of that period, as was 

 shown by Professor Whitney from evidence in this part of the countrv. 

 The axis of upheaval nearl}- coincided with that of the present range. 



Besides the Tertiary rocks laid down there exists along the border of 

 the Santa Clara Valley, at the northern edge of tlie map, a small ([uantity 

 of conglomerate, composed of metamorphic pebbles embedded in an arena- 

 ceous matrix, which is similar to the Miocene sandstone. The surface here 

 is covered with a Quaternary soil, and none of the conglomerate could be 

 found in place, nor was any fragment of a fossil detected in it. Tlie rock, 

 which is of no great importance, may be a remnant of Miocene ; but, were 

 it so, it would be strange that portions of it should not have been found at 

 greater altitudes, raised with the sandstones by the Post-Miocene upheaval. 

 It more probably represents the Pliocene, but I do not dare to map it as 

 such in the absence of positive evidence. 



Rhyoiite. — When the earliest examinations of this part of the country 

 were made, now many years since, the serpentine was supposed to be 

 eruptive, and at later dates some of the granular metamorphic rocks have 

 been hastily classed as trappean ; but at the time of my examination true 

 eruptive rocks had not been detected near New Almaden. This is not so 

 strange as it might seem, for it happens that the long and tolerably regular 

 dike of rhyoiite, which many observers must have crossed in visiting the 

 district from San Jose, is for the most part a light-yellow, finely porous, 



