300 QriCKSILVKR DEPOSITS OF THE PACIFIC SLOPE. 



mass of tlic Cliico, and tlie dip is in some places reversed near the contact, 

 giving- this line g-reater irregularity than it would possess had the strata 

 been more nnitorm in pli\\sical character. Towards the east end of the 

 contact tliere has been a certain amount of disturbance, seemingly pro- 

 duced by a longitudinal stress. It has affected both series of beds, and 

 here as elsewhere there is evidence of conformity. Near the western end 

 of the contact a tongue of Chico rock is shown projecting into the Tcjon 

 area. This tongue is suj:erficially composed of sandstone rubble, probabh' 

 brought down from the more elevated area by floods. There is no reason 

 to doubt that the contact of rock in ])lace follows the dotted line drawn on 

 the map across this tongue. 



Although the greater part of the Tc'jon beds are nearly white, some of 

 them contain dark, ferruginous concretions of a kind similar to those noted 

 in the Chico. Tiiese nodules arc less abundant and less regular in the 

 Eocene. In passing it may be noted that Miocene rocks in the Vallecitos 

 Canon are thickly studded with .spherical concretions of the same kind. 



The Tejon is the coal-bearing formation at Mt Diablo. At Xew Idria 

 also there is a coal seam near the San Carlos Creek, just north of the limits 

 of the map. Fuel was supplied to the mining company for the bliicksmith- 

 shop and other pui-poses for many years from this seam, but it was of purely 

 local importance. 



The thickness of the Chico-Tejon strata is very great. The thickness 

 of the least disturbed portion represented upon the map, when measured 

 perpendicular to the stratification, is about seven thousand feet, but the 

 entire series is not included in the area mapped. There cannot be less than 

 ten thousand feet of the complete series How such enormous accumula- 

 tions, consisting almost exclusively of sandstone, can have been foruied is 

 a mystcrv. There are great thicknesses of Miocene beds also within a few 

 miles of New Idria on both sides of the range, and these too are substan- 

 tially composed of sandstone. 



Absence of lavas. — No cruptive rocks are known to exist nearer to New 

 Idria than about ten miles. There is a considerable area of basalt to the 

 northwest of Vallecitos Canon, or to the southeast of the Cerro Bonito mine. 

 Pebbles of lava are also to be found along the lower portion of San Carlos 



