442 REVIEWS 



the mother lode. The Palaeozoic sediments are called the Calaveras 

 formation. The only evidence of the age of this formation in the 

 limits of the quadrangle consists in rounded crinoid stems found in 

 limestone on Mormon Creek, but the formation is stratigraphically 

 continuous with the Calaveras formation of the Jackson and Yosemite 

 quadrangles, in both of which Carboniferous fossils have been found. 

 The pre-Cretaceous rocks in general are very similiar to those of other 

 portions of the Gold Belt, but two rock types occur here that are rare 

 elsewhere. These two types are certain soda-feldspar dike rocks, called 

 soda-syenite, and a hornblende-pyroxene rock. The soda-feldspar 

 dikes occur chiefly along the mother lode and at many points have 

 been altered by mineral solutions which have deposited calcite, 

 dolomite, pyrite and some gold and silver. East of Jacksonville, at 

 Kanaka Creek, and east of Moccasin Creek, these dike rocks have 

 been extensively exploited for gold. The soda-feldspar dikes (soda- 

 syenite) of the Sonora quadrangle are practically the same as the soda- 

 syenite which forms the lode of the Treadwell mine on Douglas Island 

 in Alaska. The hornblende-pyroxene rock, above referred to, is 

 chiefly remarkable as being apparently unique. The hornblende 

 forms porphyritic crystals in a finer ground mass of augite and horn- 

 blende. 



The superjacent series is composed of Eocene beds, Miocene beds, 

 andesitic sandstone, auriferous gravels, and various lavas. All of the 

 rocks of the bedrock series were greatly eroded during Cretaceous 

 and early Tertiary time, and upon this old surface of erosion, or 

 approximate peneplain, the river gravels and lavas of the superjacent 

 series were deposited. During Cretaceous and a large portion of 

 Tertiary time the San Joaquin Valley was underwater. The sediments 

 deposited at that time, which have been preserved in the Sonora 

 quadrangle, consist of sandstone of Eocene age (Tejon formation), 

 shale, sandstone, clay and rhyolitic tuff of supposed upper Miocene age 

 (lone formation), and coarse andesitic sandstones and conglomerates 

 which are presumably of Pliocene age. The gravels deposited by the 

 rivers of this period are called the Auriferous river gravels. These river 

 deposits have very largely disappeared through erosion, but are still pre- 

 served at some points underneath the lavas of late Tertiary. The best 

 preserved river channel is that underlying the Tuolumne Table Moun- 

 tain west of Sonora. This mountain owes its table character to a dark 

 basaltic rock {latite) which flowed down the valley of the Neocene 



