SUBSTITUTION. 395 



in contact vvitli cinnabar tlie ore does not penetrate it, but forms crusts ; and 

 the contact surface preserves the same geometrical character as freshly fract- 

 ured rock of the same kind presents. Had an active process of replacement 

 gone on, one would expect to find angular masses of ore containing rounded 

 kernels of rock, just as in the partially serpentinized rocks globular masses 

 of sandstone or pseudodiabase are found coated by less regular layers of ser- 

 pentine. I liave never met with cinn'abar bearing this relation to serpentine 

 or to other rocks. When the rocks are porous, or practically when they 

 consist of sandstone of loose texture, tlie ore does, indeed, permeate them and 

 the interstitial space is filled up with cinnabar and cpiartz ; but neither in such 

 specimens nor in the slides doesany evidence present itself that replacement 

 has occurred. So far as I know, the case most nearly sim'dating replace- 

 ment is the deposit at Steamboat, where the cinnabar is chiefly found dis- 

 seminated in decomposed granite. But liere much of tlie granite is reduced 

 to a loose, gravelly mass, in which there is no ore, and this material in every 

 respect resembles that in wliich the ore is found. I can form no other con- 

 clusion than tliat the decomposition and impregnation witli cinnabar were 

 independent plienomena. The precipitation of cinnabar took 23lace in the 

 granite only after space was made to receive it, and the deposition of the ore 

 was not a condition of the solution of the granite mass. So, also, in the 

 ba.salt of Sulphur Bank cinnabar is found partially filling- crevices, which 

 have manifestly first been enlarged from mere fissures by sulphuric acid or 

 by other means independent of the precipitation of cinnabar. Were the ore 

 deposited by replacement, there would also be a closer relation than seems 

 to exist between the chemical character of the rock and tlie ricluiess of the 

 deposits. In the metamorphic series phtlianites, pseudodiabase, serpentine, 

 and altered sandstone seem indifferently to limit rich ores, and such ores 

 are also found in contact with basalt. The amount of disturbance (except 

 in the case of porous sandstones), and not the qualify of the rock, is approxi- 

 mately proportional to tlie amount of ore. 



The absence of ore in the opal mysterious. 1 OanUOt SatisfaCtOrily aCCOUUt for tllC 



fact that, while various rocks adjoining ore bodies become silicified, the 

 cinnabar is either absolutely or substantially confined to fissures, in which 

 it is usually associated with quartz. If the solutions wliich opalized the 



