CIRCULATION OF CASEOUS SOLUTIONS. 1019 



CIRCl'LATIOX IN BELT OF CEMENTATION. 



In the belt of cementation the conditions are very different. Here the 

 openings in the rocks are normally occupied by aqueous solutions. How- 

 ever, adjacent to igneous masses gaseous solutions may be produced either 

 as emanatious from the igneous rocks or by the heating of the solutions of 

 the intruded rocks above their critical temperature, or by both combined. 

 The surrounding rocks, being occupied by aqueous solutions, serve the pur- 

 pose of condensers. The conditions are those which would obtain between 

 steam generated in a boiler connected by many openings with condensers 

 containing water. Almost as fast as the steam is developed it is liquefied in 

 the condensers, thus heating the contained water. Thus gaseous solutions 

 are condensed to liquids almost as rapidly as formed. It is possible that in 

 the zone of fracture immediately adjacent to a great mass of intrusive 

 igneous rocks there may be a narrow border area in which the water may 

 be maintained as a gas and rise toward the surface, because it is driven by 

 its expansive force from places of high temperature to places of low tempera- 

 ture, but evidence is necessary upon this point 



In attempting to arrive at the probability of the importance of gaseous 

 solutions in the zone of fracture it is necessary to consider the manner 

 in which igneous rocks solidify. It is well known that in this zone the 

 intrusive igneous rocks ordinarily show sahlbands, giving evidence of 

 quick chilling and solidification along their borders. From the exteriors 

 crystallization extends inward. It is therefore plain that the surface of 

 crystallization is ever a changing one. It should also be remembered that 

 rocks are poor conductors of heat. It has further been pointed out that as 

 crystallization and condensation continue openings are likely to form along 

 the contacts, and thus give ready passages for water. The question is 

 whether in the zone of fracture, along' the borders of the intrusive masses, 

 where there is free circulation, the temperature would be long maintained 

 above the critical temperature of water. It seems to me that the prob- 

 ability is strongly against this. This probability is confirmed by observa- 

 tion, which shows that minerals produced along- the contacts of igneous rocks 

 in the zone of fracture are those characteristic of the belt of cementation, 

 and therefore the same as those known to be produced by aqueous solutions. 

 There is therefore no good reason for supposing that ores adjacent to the 

 igneous rocks in the belt of cementation are commonly precipitated from 



