124 Scientific Proceedings, Royal Dublin Society. 



last the frequency may become so great that the intervals are 

 almost imperceptible until the fissure reaches the surface, and the 

 igneous matter finds a vent for its expansion. 



Of course the sheet of igneous magma may solidify at any part 

 of its journey towards the surface in consequence of — 



(a) Loss of heat from conduction away by the surrounding 



rocks. 



(b) Eaising the acquired water to the mean temperature of 



the solution of silicates in which it is dissolved. 



(c) By loss of heat in consequence of expansion during the 



extension of the fissure. 



(d) By gradual escape of water in the form of steam or 



vapour through fissures so supplying fumaroli. 



(e) By convection currents of waters forming Greysers or 



thermo -mineral springs. 



It is a common fact that the water-bearing qualities of different 

 rock strata are widely different, and we also know that an igneous 

 dyke may traverse an alternation of more or less permeable strata. 

 Where the supply of water was greatest, conductivity and other 

 things being equal, there would take place the greatest amount of 

 diffusion of that liquid through the igneous magma. Were this 

 latter a perfect fluid, and non- viscous, the more aquifarous, pro- 

 bably lighter, part would soon diffuse itself in all directions, render- 

 ing the whole a homogeneous mass. We know, however, that all 

 lavas are exceedingly viscous, especially the more highly silicious 

 ones, and therefore such diffusion would take place very slowly. 

 This would be aided by the upper part of the column being lighter, 

 from that portion being placed under the more favourable condi- 

 tions for absorbing water. 



The more porous the strata the greater the tendency will be 

 for the conduction away of the heat of the magma, either directly 

 or by the aid of convexion, currents of water, or by the conversion 

 of tlie latter into vapour, where "pressure is so low as to permit it. 



Extrusion or Eruption of the Igneous Magma into the Atmosphere. 

 — If we suppose, simply for the sake of brevity of argument, 

 the lava canal to be a tube of uniform size between the source of 



