382 The American Geologist. June,1896 
that of the Shaw mine was al once noted, and chemical and 
optica] analyses made since prove beyond doubt that some of 
them are essentially the same and may be called soda-syenite 
porphyries or albitite-porphyries. The same association of 
these soda feldspar dikes with gold deposits noted with the 
Eldorado county occurrences obtains with some of the dikes 
in the Sonora sheet area. Indeed the field and subsequent 
laboratory investigations make it reasonably probable that 
some of the quartz veins along the Mother lode have been 
formed by replacement of the feldspar of these dikes with 
silica. This is best seen in the neighborhood of the Tuolumne 
river, where these dikes accompany the lode at most points 
and are often tilled with secondary quartz veinlets, dolomite 
and specks of iron-di-sulphide. This is finely shown at the 
north end of the croppings of the Bachelor quartz vein on the 
north bank of the Tuolumne river, two miles southeast of 
Jacksonville. The vein lies at the contact of a body of ser- 
pentine on the west and a body of clay slate or schist, prob- 
ably of Carboniferous age, to the east. The slates dip N. 30° 
E., at an angle of about 65°. The vein itself is composed of 
white quartz, dolomite and mariposite, a micaceous mineral 
often of a bright green color. Just east of the vein, in the 
clay schists within a width of 30 feet, are six or eight dikes, 
which usually run parallel with the strike of the schists, but 
at two points cut across the schistosity. Such a series of 
dikes might be called a multiple dike, following Lawson,* as 
it is reasonably certain that at some depth below the surface 
they all come together. The dikes vary from two inches to 
two feet in width. Quartz veinlets, one with a convoluted 
course, cut both the schists and the dikes. Between the dikes 
and the ledge is a broken up mass of the dike rock of a red- 
dish-brown color, penetrated by quartz veinlets and seams of 
dolomite and apparently in a fair way to form a quartz vein, 
like that immediately west, if the alteration should go far- 
ther. This mass seemed a friction breccia and would indi- 
cate movement and faulting along the vein. A microscopic 
examination of this breccia showed it to be made up of frag- 
ments of the dike rock cemented by dolomite and quartz. 
Throughout the rock, as well as in the dikes just east, isscat- 
* American Geologist, vol. xin, p. 293. 
