CONCLUSIONS. 417 



often seems to influence the deposition of tlie ore. Cinnabar occurs along 

 faulted surfoces in wliolh' unaltered Neocomian rocks, but tliis is rarely the 

 case. Unchanged sandstones and shales are more likely to be distorted than 

 to break, while the metamorphos'ed rocks are brittle and are intersected by 

 innumerable partially cemented cracks. Adjoining masses of metamorphosed 

 and unaltered rocks will present different amounts of resistance to disloca- 

 tion, and a tendency to the formation of fissures is most likely to manifest 

 itself near the junction of such areas. Knoxville and New Idria are excel- 

 lent examples of this fact, which it is worth the while of prospectors to note. 

 Of course it does not debar the formation of fissures in extensive metamor- 

 phosed areas 



Relations of deposits to volcanic rocks. — Thc rclatlou of the Cjuicksilver de[)Osits to 

 volcanic rocks is indirect, yet close. The ore deposition seems to have been 

 immediately dependent upon the existence of hot sulphur springs, which 

 were probably in all cases of volcanic origin. Such springs are most likely 

 to occur at a very moderate distance from lava, but this is no invariable rale, 

 several miles sometimes separating such springs from the nearest volcanic 

 vents. So far as is known, basalt is the lava usually associated with the de- 

 posits, but the most important series of deposits in the State seems to have 

 been induced by a rhyolite eruption. Some little deposits of cinnabar exist 

 in andesite, and it is possible they are due to hot springs following the erup- 

 tion of this rock. Some other deposits are also nearer to andesites than to 

 basalts. I know of no reason to doubt that cinnabar deposits were formed 

 by springs which owed their temperature and composition to volcanic activity 

 of the andesitic period, but I have no unquestionable evidence that this was 

 the case. 



Age of the ore deposits. — So far as Is kuowu, volcaiuc activity in the Coast 

 Ranges began in the Pliocene, and the main outbursts of the andesite seem 

 to have closed that period. The age of the cinnabar deposits is, then, lim- 

 ited to Post-Miocene times, and there is little doubt that nearly all the ore 

 has been deposited since the end of the Pliocene. 



Future of quicksilver mining — I cauuot Say that tlic futurc of tho quicksilver 

 industry on the Pacific slope seems to mo very hopeful. The trouble is not 



MON XIII 27 



