356 QUIGK«ILVEK DEPOSITS OF THE PACIFIC SLOPE. 



ner of the map this thin sheet is in contact with a plateau of basalt, in which 

 the rock is of much greater thickness. This mesa is about a mile square 

 and is bounded on all sides by precii)itous walls. To the north of tlie mines 

 is a larger area of pyroxene-andesite, a tongue of which enters the region 

 mapped. 



Deposits of the Napa Consolidated. — Thc deposits of OatliiU arc tlic Mcrcury vein, 

 the Manzanita vein, and tlie Accidental vein, which are the property of the 

 Napa Consolidated Mining Company, and the Eureka claim, which contains 

 a deposit similar to those of the Napa. The principal mine is upon the two 

 veins first mentioned. 



The Mercury and Manzanita deposits are on two nearly parallel fissures 

 in the unaltered sandstones' of Oathill. They strike northwest-soutlieast 

 magnetic and dip at a high angle, usually more than 4,5°. The strata, both 

 in the mine and outside of it, are nearly horizontal, excepting close to the 

 fissures. Here the faulting which has taken place between the walls of the 

 fissures has flexed the strata. Those of the hanging wall are bent upward 

 toward the fissure and those on the foot-wall are flexed downward, thus 

 indicating the direction of tiie movement. The walls of the fissures are 

 almost everywhere well marked by slickensides and the interval between 

 them is chiefly filled with products of their attrition, sometimes in the form 

 of clay.' Often also fragments of sandstone and shale, showing the original 

 stratification, are found betv.een the walls. The attrition mixture is im- 

 pregnated with silica, calcite, pyrite, and cinnabar. The silica is found 

 mainly in stringers, which intersect the vein matter in surfaces parallel to 

 the walls and sometimes give a cross-section of the vein a stratified appear- 

 ance. A portion of the pyrite is oxidized, and iron oxide, ferrous sulphate, 

 and magnesium sulphate have resulted from this process. The seains car- 

 rying most ore are often marked by iron stains. 



At a number of points the cinnabar has followed the stratification of 

 the inclosing sandstone away from the veins, forming horizontal chambers, 

 which are sometimes 100 feet in length and 50 in height. In the Mercury 

 several such chambers occur in the foot-wall. The only one accessible in 



' It may be well to call atteotiou to the fact tbat the clay of miaes very frequently contaius little 

 or no kaolin. Any soft, tough mass is called clav by luiners (see Geology of the Comstock Lode, 

 p. 217). 



