1868.] 



Geology of the Gold-fields of California. 



295 



In many cases, however^ laminse of the enclosing slates divide the vein 

 into distinct bands ; and in such instances it will be observed that the thick- 

 ness of the interposed fragments of slate is sometimes not greater than 

 that of a sheet of tiie thinnest paper. Cavities or druses containing 

 crystals of quartz occur in all the auriferous veins of the country ; and a 

 certain amount of crystallization ma}'- also not unfrequently be remarked 

 along the lines of junction of the several bands of which a vein is composed. 

 In addition to ordinary quartz, in a more or less crystalline form, amor- 

 phous hydrate d silica, or semiopal, and chalcedony are occasionally met 

 with : in some instances the opal is interfoliated between layers of true 

 quartz, and is sufficiently auriferous to repay the expenses of treatment. 



The metallic minerals enclosed in the gangue of auriferous veins are 

 ordinary iron pyrites^ blende, and galena, and, less frequently, arsenical 

 pyrites, magnetic and copper pyrites, and cinnabar. These sulphides 

 invariably contain gold ; and veins in which some one or more of them 

 does not occur, in considerable amounts, are not regularly and lastingly 

 productive. 



Near the surface the iron pyrites and other sulphides become decom- 

 posed by the action of air and the percolation of meteoric water through 

 the mass, staining the quartz of a red or brown colour, and leaving the 

 gold in a free state. Under such circumstances numerous cubical moulds 

 of iron pyrites are found in the veinstone ; and although this mineral has 

 been entirely removed by chemical action, the cavities left contain finely 

 divided gold, obviously liberated by the decomposition of pyrites. 



Beneath the line of natural drainage of the country the sulphides remain 

 undecomposed ; but if rock containing pyrites be placed in nitric acid the 

 sulphide becomes dissolved, and finely divided, crystalline, or filiform 

 gold will partially occupy the resulting cavities. 



In one of the detrital beds in the vicinity of the village of Volcano in 

 the County of Amador, and elsewhere, distinctly marked quartz veins 

 may be observed cutting through the gravel, and evidently formed by the 

 action of water holding silica in solution. 



Attention has also been recently directed to bands of auriferous slate 

 found in the copper-bearing belt west of the main gold-belt of the State, 

 and in the foot hills of the Sierra. In this locahty the gold, instead of 

 being obtained from a well-defined vein, chiefly composed of ordinary 

 quartz, is enclosed in a band of siliceous slaty rock, extending north-west 

 and south-east, and dipping in conformity with tlie other strata of the 

 district. 



The number of fluid-cavities contained in the veinstones of the auriferous 

 lodes of California is seen under the microscope to be exceedingly limited ; 

 and in order to obtain sections affording good examples, even of small size, 

 it is necessary to select such bands as may be more than ordinarily crvs- 

 talline, or to operate on thin fragments of crystals sometimes found 

 lining the interior of drusy cavities. In the more opaque and generally 



