ORES DEPOSITED BY GASEOUS SOLUTIONS. 1057 



In this connection it ma}" be recalled that beautiful crystals of the 

 variety of garnet known as spessartite, fayalite, and topaz occur in litho- 

 physse in recent volcanic rocks, especially in the openings in them. The 

 formation of these openings in the lavas and the development in them of 

 these minerals naturally suggest the action of gaseous solutions. It has 

 also been pointed out (p. 685) that so far as the heavy anhydrous minerals 

 have been reproduced artificially under conditions where water is present 

 high temperature and pressure have been necessary. For instance, Chrust- 

 schoff obtained from water solutions heated to a temperature of 550° C. 

 amphibole, pyroxene, quartz, and adularia." 



It thus appears that both observation and experiment strongly faA'or 

 the idea that such minerals as topaz, garnet, and amphibole, so far as sepa- 

 rated from solutions, are deposited by gaseous solutions above the critical 

 temperature of water. If this be so, it follows that those sulphides which 

 were deposited simultaneously with the development of the heavy anhydrous 

 minerals were precipitated from gaseous solutions. 



It is evident that it is difficult to make a certain case in favor of 

 gaseous solutions. It must be shown, where the ores occur in connection 

 with the heavy minerals supposed to be formed under conditions of gaseous 

 solutions, that the metals have not been introduced at a subsequent stage 

 when aqueous solutions were the active agent. There should be excluded 

 also the possibility that the ores were dejDosited by aqueous solutions in the 

 belt of cementation and were later transferred to the zone of anamorphism, 

 where the heavy minerals developed by later alterations in that zone. 

 Apparently in some of the cases above cited the latter possibility is excluded, 

 as all the evidence seems to be in favor of denudation since deposition rather 

 than deeper burial. 



Under normal conditions it might be supposed that ore deposits would 

 be produced by gaseous solutions in the deep-seated zone above the critical 

 temperature of water; that above this zone in the belt of cementation ores 

 would be deposited by aqueous solutions, and that above the level of ground 

 water other ore deposits would be produced in the belt of weathering by 

 gaseous solutions. The latter ores would be those to which the terms 

 fumarolic and solfataric are naturally applicable; but, so far as I know, 

 the actual development of ore deposits by gaseous solutions above the 



" Chrustschoff, K. von, Ueber ktinstliche Hornblende: Neues Jahrb., 1891, Bd. II, pp. S6-90. 

 MON XLVII — 04 67 



