I I 1 J. ]■:. MILLS — ROCKS OP THE SIERRA NEVADA OE CALIFORNIA. 



place at the time of the tilting and metamorphism of these rocks, and 

 thai possibly a part of it took place during their deposition. 



It has been shown that the pre-Mesozoic rocks were raised above sea- 

 level and a part of them had undergone the quartzitic alteration before 

 the Mesozoic rocks were deposited. They were probably also more or 

 less fissured while being uplifted and altered, and the fissures may have 

 been at that time filled with quartz containing pyrite and gold. It is 

 entirely probable that a part of the gold of the Sierra is of pre-Mesozoie 

 age. and it is certain that a large part of it is of Mesozoic age. 



A Large proportion of the gold product of the Sierra has been obtained 

 from Tertiary and Quaternary and Recent gravels, ami is of Tertiary. 

 Quaternary and Recent age. in the sense of having been detached, con- 

 centrated and deposited by streams of those times ; hut whether gold has 

 been deposited in veins within the Sierra proper since the Mesozoic uplift 

 has not been certainly proved or disproved. Professor Whitney saw a 

 vein of chalcedonic quartz traversing Tertiary gravels* and silica is not 



infrequently found forming a cement of such gravels, and silicified w 1 



is not uncommon in them. There is chalcedony, evidently deposited by 

 a now extinct hot spring, near the edge of a lava flow near Inde- 

 pendence, smith of the South fork of the Mokelumne. The fragments 

 of chalcedony, resting on partially kaolinized slate, have been moved 

 and washed for gold, hut whether the gold was from the chalcedony or 

 from the bed-rock on which it rested I could not learn in the short time 

 spent there. 



It is certainly not improbable that some gold-bearing quartz was de- 

 posited by the solfataric action that accompanied and followed the great 

 Tertiary outflowing of lava : but the greater part of the gold-bearing 

 quartz was deposited in veins older than the Tertiary lavas, for debris of 

 such veins underlies the oldest of them. 



* i teology of California, vol. i. I860, p.. 276; Auriferous Gravels of the Sierra Nevada, 1879, p. 330. 



