Lavis — On the Structure of Rocks. 121 



striking resemblance to the process of dialysis, for the igneous 

 magma is in a vitreous state, which we may take as the representa- 

 tive of the colloids, 1 whilst the superheated water in all probability 

 may still be regarded as a crystalloid. In consequence of this we 

 should look for endosmosis as the principal function, although the 

 metamorphism of surrounding rocks, which in the case of the 

 existence of salbands is comparatively slight, would indicate some 

 amount of exosmosis. In the case of the blind fissure being con- 

 verted into a channel through which the igneous magma circulates, 

 then no doubt the salband would, in most cases, be refused or 

 carried away by other means, and the permeable rocks would then 

 play the part of the septum. In fact, even in a blank fissure we 

 can comprehend that no salband may exist. 



The rapidity of this endosmosis of water, and its diffusion 

 through, or solution in, the colloidal-like magma, will obviously 

 depend upon a variety of circumstances, such, for instance, as the 

 composition of the magma, the form of fissure, and therefore 

 amount of surface exposed, pressure, &c. This we see portrayed 

 in the illustrations we chose ; for if carbonic anhydride is in contact 

 with the calm surface of water solution takes place very slowly. 

 A knowledge of this fact is practically made use of in the seltzer- 

 water machine, in which a number of lashers revolve with great 

 rapidity in a chamber filled with water and the carbonic anhydride, 

 so that a very large surface of each is brought into contact by the 

 churning motion, and consequently solution takes place with very 

 great rapidity. 



But to return to the main question, this absorption of water 

 will go on at the expense of heat to the igneous magma, which, 

 however, will only lose so much as will raise the amount of water 

 absorbed to its own temperature. This loss will not, of course, be 

 very great, since there is no conversion of a liquid into a gas. 

 Nevertheless this loss of heat, combined with that due to the con- 

 duction away by the surrounding rocks, may be so great that the 

 igneous magma may reach its point of solidification, and further 

 action will be prevented by the fissure being now filled by a cooled 

 rock mass. 



1 At any rate as far as the silica, and probably the alumina and iron oxides, are 

 concerned. 



