NEW IDKIA MINES. 301 



Creek. Tlie absence of eruptive rocks close to the mines of New Idria, 

 however, does not preclude the former existence of hot springs here. It 

 simply leaves the question to be decided by analogy. 



Gc.e.a>descHptionofu,eNewidHa._The deposits of the Ncw Idriu mme are 

 substantially inclosed in rocks of the metamurphic series. Of these, ser- 

 pentine is represented only to a small extent, tlie prevailing rocks being 

 shales and sandstones in various stages of alteration. An important por- 

 tion of the shales have been converted into phthanites, while some of the 

 sandstone has almost escaped induration. The deposits are near the con- 

 tact between the metamorphic rocks and the adjoining Cluco sandstones; 

 so also is the San Carlos mine to the east. At a distance of about three 

 thousand feet westward fro.n the New Idria, however, the limitn.g hue ot 

 the mctan...rphic area bends in a southerly direction, and in tho bay thus 

 formed is a mass of somewhat indurated Chico sandstone, in which cinna- 

 bar has been found. This occurrence, known as the Washington crop- 

 pings, shows at least that the period of ore deposition is later than the 

 Chico, or that it is Post-Cretaceous. 



The New Idria mine has disclosed important deposits of three distinct 

 types: reticulated masses, or stockworks, impregnations, and true fissure 

 ;eins-in short, all the principal classes of original ore deposits are rep- 

 resented There is no question whatever as to the fact of a connection be- 

 tween these deposits and a system of fissures, but this system is unusually 

 complex in some particulars which are of the greatest importance m rela- 

 tion to the economic value of the property. A portion only of the deposits 

 was accessible at the time of my visit. 



The upper part of the deposits, and particularly the northeastern por- 

 tion, consisted of irregular stockworks. Only the excavations close to the 

 croppino-s are now accessible. They show siliceous shales and phthanites, 

 containi°ig a few carbonized plant remains, with small seams and "paints" 

 of cinnabar. The ore, accompanied by pyrite and quartz, has sometimes 

 tilled cracks across metamorphic strata and has frequently also followed 

 the bedding exactly as it is observed in innumerable mines and prospects 

 throuo-hout'the Coast Ranges. There is nowhere in these dense rocks any 

 indicrtiou of impregnation. Tlie descriptions of the great ore bodies of the 



