REVIEWS 443 



Tuolumne river. The low ridges that formed the sides of this Neocene 

 river valley have since that time been worn away by the erosive agencies, 

 leaving the river channel with its hard basaltic capping standing above 

 the surrounding country. At many points tunnels have been run in 

 under the lava capping to penetrate the gravels beds, which have 

 proved highly auriferous at some points. Rhyolite lavas which are so 

 abundant in the Sierra Nevada to the north of the Sonora quadrangle 

 have been found here only in small amount intercalated in the beds 

 of the lone formation. The andesitic tuffs and breccias, which, at one 

 time, must have covered a large portion of the northern half of the 

 quadrangle, have largely disappeared. The Table Mountain basalt is 

 evidently younger than a portion of the andesite, as it overlies andes- 

 itic tuffs and gravels. 



Economic geology. — The Sonora quadrangle comprises a portion of 

 the southern part of the mother lode gold district. This remarkable 

 linear system of gold quartz veins extends across the quadrangle in a 

 northwest-southeast direction, and lies partly within the eastern belt 

 of the slates of the Mariposa formation and partly to the east of this 

 belt. Many of the mines are now being operated with good results. 

 Profitable gold quartz veins are also found in the bedrock series, 

 both to the east and west of the mother lode district. The mines in 

 the granodiorite east of Sonora have been worked with varying success 

 for many years. The limestone of the Calaveras formation, which 

 extends from Sonora to the southeast, has been largely metamorphosed 

 into marble, much of which is valuable for building purposes. Chrome- 

 iron deposits occur within the serpentine areas. The sandstones of 

 the lone and Tejon formations, along the borders of the great valley, 

 are valuable for building stone. The forest zone of the northeastern 

 portion of the quadrangle furnishes valuable timber for the mines and 

 for building purposes. 



