WGNBOOS ROCKS OF TE SIERRA NEVA DA, 397 
Silica - - 73.62 per cent. to 79.41 per cent. 
Lime ~ a iy .09 ee se ee 2.55 oe oe 
Potassa BS fs 2.23 ce “ce “ec 4.27 oe ce 
Soda = e a 2.46 oe “ec “cc 3.78. “ec “ec 
As there is some carbonate in minute particles in these rocks, 
it is quite likely that in the fresh rock the lime never reaches as 
small a quantity as .og per cent. Although containing too much 
lime and too little potassa for a typical quartz-porphyry, the 
variation is not sufficient to exclude the rock from the quartz- 
porphyry group, and it will be so called here.’ 
Overlying the quartz-porphyry is a sedimentary lens, partly 
a dark siliceous schist and partly coarser material, dipping east- 
erly at angles of from 50° to 60°. In this lens is the quartz 
vein of the Mountain mine. At one point in the sedimentary 
mass is a layer of quartz-porphyry about 50 feet thick which may 
be an intruded sheet or a contemporaneous flow. Radiolarian- 
like remains were noted in the sections of a dark, fine-grained 
breccia that forms part of this clastic series. This sedimentary 
“since it feathers out both to 
the north and south. Other lenses, usually entirely made up of 
dark siliceous schist and slate, occur in the Sierra Buttes, from 
mass has been designated a “‘lens’ 
one of which at the Phcenix mine an ammonite was collected.” 
Some of these lenses are entirely enclosed in the quartz-por- 
phyry, and it is probable that the entire series, the quartz-por- 
phyry as well as the overlying formations, is Jura-Trias in age. 
Superimposed with apparent conformity on the clastic series 
just described is a breccia containing abundant angular microlitic 
fragments and angular quartzes which are in part idiomorphic. 
This series is designated in the section a quartz-porphyry breccia, 
overlying which is a huge bed of altered augitic pyroclastic rock 
without free quartz. Moraine material covers the lower part of 
the slope, but one mile further south in the bed of the North 
Fork of the North Fork of the Yuba River, this augitic tuff is 
™CROsS gives analyses of two quartz-porphyries from Leadville that contain very 
similar silica, lime and alkali ratios to those given above. Monograph XII., U.S. 
Geol. Survey, p. 326. 
2 Am. Geol., Vol. XIII., p. 232. 
