82 
GEOLOGY 
are seen in the trap. The sienite and trap alternate from Rough-and-Ready to Grass-valley, 
where the trap disappears entirely, giving place to the sienitic granite, which extends indetermi- 
nately to the east. 
The veins of quartz commence near Grass-valley, and are found chiefly at the line of contact 
of the veins of trap and sienite, also running from north to south in the general direction of the 
chain. The gold is scattered in very fine particles through the quartz , and is seldom visible to 
the naked eye. One of these veins near Nevada-City has been and still is worked with profit ; 
I will describe it in a few words, as an example of tbe mines in general, as they differ very 
little from each other. This vein, called Canada-Hill mine, on account of its discovery by French 
Canadians, runs from liorth to south, dipping to the west at an angle of 30 degrees; it is from 
six to eighteen inches thick, the rock forming the wall of the vein is the common sienitic granite 
of the Sierra, formed of crystals of common feldspar and hornblend. The orthoclase decomposes 
easily under the combined influence of atmospheric and magnetic actions, and is transformed into 
the true kaolin or porcelain clay. 
The gold quartz of Canada-IIill has a cavernous structure, with cells containing oxyd and 
hydrous-oxyd of iron, which often shows thin laminm of gold and branching threads of this precious 
metal visible to the naked eye. Frequently greenish-yellow iron pyrites occur crystallized in 
dodecahedrons cubes, whoso faces are striated. This auriferous quartz is less hard than usual, on 
account of its cellular structure ; when broken it is irregular and scaly, and its color is a thick 
milky white, often yellow from the quantity of iron it contains. The exterior surface of the vein 
is yellow and black, and there are no striae or other marks showing that the vein has been rubbed 
against the sides of the sienitic mass forming the walls. 
This summary description of the gold-bearing quartz of Canada-Hill applies, with little varia- 
tion to the other quartz mines of California. The quartz is rarely found entirely impregnated with 
gold. One of the finest fragments , called by tbe miners a big lump of gold , was taken from the 
Lafayette and Ileloetia mine, at Grass-valley ; it weighed 150 pounds, and contained the value of 1200 
dollars in gold. The vein of this mine is at the line of contact of the sienite and greenstone trap ; 
it is three feet thick, and at one point even five feet: it is very productive, and is the first vein 
that was discovered and worked. 
The principal mines of auriferous quartz are in Nevada, Sierra, Ruttes, Eldorado, Calaveras 
and Mariposa Counties ; they are always situated in the sienitic granite or the trap , and the richest 
veins are found where these two rocks meet. In the spring of 1854 there were forty mines of 
auriferous quartz in California, more or less profitably worked. 
The valleys of the Sacramento and San-.Ioachin rivers contain a sort of muddy clay or lehtn, 
that changes, with the elevation of the Sierra Nevada, into a true drift containing rolled pebbles, 
sand, gravel, and plastic clay. This drift covers more or less all the eruptive rocks at the base 
of the Sierra Nevada ; and as the greatest number of the California placers (gold diggings) are in 
this deposit. I will describe a section of it. Its greatest thickness in the region of the northern 
mines is one hundred and fifty feet ; generally it is from forty to one hundred feet thick. The 
following is the description of the Quaternary drift in the placers of Nevada-City, at the western 
entrance to the town; the section is from the top down. At the summit a yellowish-white plastic 
