120 Scientific Proceedings, Royal Dublin Society. 



Even in granite, although there is no vitreous salband which 

 would he incompatible with its coarse, well-crystallized structure, 

 Naumann describes granite dykes in which the grain is finer 

 towards the margin. 



It seems probable that where intrusion takes place into rocks, 

 the cleavage planes of which are nearer the horizontal in direction, 

 the loss of heat will take place slowly, and we should expect to find 

 coarse-grained granites and trap rocks ; whereas, the more the 

 cleavage planes approach the vertical the greater will be the 

 rapidity of cooling. This is a question well worth inquiring into, 

 for Jannettaz 1 has shown that the major axis of the isothermic 

 ellipsoid in crystals is parallel to the principal planes of cleavage, 

 and in rocks with the planes of schistosity. 2 



Intrusion of Igneous Matter into Porous Aquiferous Strata. — The 

 same results as in the last case may be looked for, but we shall see 

 that superposed upon them there is another series of far greater 

 importance. Let us suppose the fissure formed, injected, and that 

 a salband has solidified. The water in the immediate neighbour- 

 hood will tend to increase in temperature until it arrives at the 

 same degree as that of the lava, since in most cases the enormous 

 superincumbent pressure will have proportionally raised the boiling 

 point. Then again, as the water exists bound up, as it were, within 

 the pores of some permeable rock, little convexion circulation is 

 permitted, at the same time that expansion to the gaseous condition 

 furthermore is resisted. This shell of superheated water is only 

 separated from the igneous magma by the salband, which according 

 to varying circumstances may differ very much in thickness, and 

 so will act as a more or less imperfect porous septum between the 

 igneous matter and superheated water. Although probably not 

 possessing exactly the same physical characters as the porous septum 

 in dialysis, nevertheless it no doubt would permit diffusion to go 

 on between the two fluids which it separates, or even the porous 

 rocks themselves may play that part. Besides, we have another 



1 Memoires sur la propagation de la chaleur dans les corps cristallises (Ann. Ch. et 

 Ph., 4° serie, t. xxix., p. 5 ; Bull de la Societe Geologique de France, 3° serie, t. I" 

 et suit.) . 



2 As the stratification of strata in general approaches nearer to the horizontal than 

 the vertical, the conditions will be most favourable for the retention of the internal 

 heat of our globe. 



