442 QUICKSILVER DEPOSITS OF TOE PACIFIC SLOPE. 



morphic series and in the Chico; and in San Luis (Jbispo County, appar- 

 ently in jMiocene sandstones. 



Possible sources of the ores. — In tliB two cases just mentioned, in wliicli cinnabar 

 has been found in basalt, this lava forms a thin sheet covering earlier rocks, 

 and in each case the ore is found below as well as in the basalt. The ore 

 certainly is not derived from basalt. Leaving- the interesting but vei'y small 

 deposits in andesite out of the question, it appears that all the other deposits 

 mnst have been derived from granite, or from rocks composed of granitic 

 detritus, or from some source below tlie granite. This is manifestly equiv- 

 alent to the statement that quicksilver prior to its solution must either liave 

 formed a constituent of the granite or must have dissolved below the granite 

 and have traversed the entire thickness of that rock witliout being pre- 

 cipitated. 



Observation affords no clew to the material which underlies the granites 

 of California. Professor Whitney is of tlie oi)inion that a portion of these 

 granites is comparatively modern and I am by no means prepared to con- 

 trovert this assertion, but it is certain that long before the Post-Neocomian 

 upheaval granite formed the bed rock of a great part of that State and of 

 western Nevada, as it still does. The fact that neither in California nor else- 

 where do we know anything from observation of what underlies the granite 

 on which the older strata rest shows that the massive rock is of enormous 

 thickness, if indeed granite and granitoid rocks did not, as elder geologists 

 supposed and as is maintained in Chapter IV, form the original crust of the 

 earth. Before undertaking to consider whether it is more probable that the 

 cinnabar and accompanying minerals were derived from the granite or that 

 they came from a source inferior to it, it seems desirable to allude briefly 

 to the general theories held by geologists with regard to the origin of ore 

 deposits. 



Brief statement of the fheories of the genesis of ore deposits. Five dlstinCt tllCOricS liaVe 



been maintained in geological memoirs respecting tlie methods by which 

 the ores occurring in an unstratified condition (as veins, stocks, and the 

 like) reached the positions in which they are found. Tliese are known as 

 the theories of simultaneous formation, descension, injection, ascension, 

 and lateral secretion. The first two have been abandoned for many years. 



