396 QUICKSILVER DEPOSITS OF THE PAOIFIC SLOPE. 



serpentine contained mercuric sulphide, it seems strange that it should not 

 have crystallized out in the silicified mass from mere relief from pressure 

 and diminution of temperature. Either, then, the silicification was effected 

 before the solutions became charged with mercuric sulphide or the cinnabar 

 was precipitated in the crevices before the solutions made their way into 

 the rock. It seems fairly certain that a large part of the opalization took 

 place before the main deposition of tlie ore ; but, as the solutions at the 

 actual time of ore deposition were certainly siliceous, a portion of the 

 opalization was doubtless contemporaneous with the precipitation of cin- 

 nabar, and, although the earlier solutions may have Ijeen poor in mercury 

 compared with the later ones, it does not seem to me probable that they 

 were entirely barren at any time. I can only conclude that the cinnabar 

 was separated from the solutions remaining in the fissures when the siliceous 

 fluid permeated the rocks. Some mechanical process, more or less anal- 

 ogous to dialysis, seems to be the only natural explanation of such a sepa- 

 ration. There are a number of phenomena in mineral chemistry which 

 seem to require some such hypothesis as this for their adeqiuxte ex])lanation ; 

 but I am not prepared to off"er any positive evidence in favor of it and 

 it is suggested only as a logical possibility. 



Pseudomorphism and substitution. — Tlic Iiypothcsis tluxt any orc has been de- 

 posited by substitution for country rock is equivalent to the hypothesis 

 that the ore has replaced the mineral constituents of the rock molecule for 

 molecule. The ore minerals must therefore be capable of forming pseudo- 

 morphs after such of the component minerals of the rock as are crystalline, 

 and of replacing, without essential change of form, dense masses of these 

 minerals or of components which are not crystalline. When it is known 

 that any mineral, wliether an ore or not, forms pseudomoi'phs after other 

 substances, it is not unreasonable to assert that it may replace rock masses 

 consisting of these substances. Thus talc is known to occur pseudomor- 

 pliically after a great numl)er of minerals, and to assert that it may replace 

 whole masses of rocks, composed, for exanqile, of pyroxene, dolomite, and 

 (juartz, is onl}' to maintain tliat the physical conditions under whidi it may 

 form pseudomorphs after all three of these minerals are the same. But 

 when it is asserted that an ore replaces rocks composed of minerals after 



