118 Scientific Proceedings, Royal Dublin Society. 



Intrusion of Igneous Matter into Dry, or nearly Dry, Rocks, but 

 not reaching the Surface. — Should a fissure opening downward to 

 the volcanic magma be formed by secular cooling or other means, 

 we should expect that it would be simultaneously filled by the 

 oozing in of the igneous magma. This mass of fused silicates, at 

 a very high temperature, will now undergo a series of changes, 

 which we will attempt to trace. The first thing will be the cool- 

 ing of a layer of the magma, which is in actual contact with the 

 walls of the fissure ; and should that substance be in a purely 

 vitreous condition, a pitchstone salband of variable thickness will 

 result. Now, should the conductivity of the surrounding strata 

 be great, or should the temperature of the magma be near solidi- 

 fication point, then that process will continue from the salband 

 inwards through the whole mass, and a blind dyke will result. 

 On the other hand, should the surrounding strata be bad con- 

 ductors, already heated, and the magma at a very much higher 

 temperature than that of its solidification, so that its heat might 

 be given out quicker than the surrounding rocks could absorb, 

 any salband that might at first have been formed would be re- 

 fused, and such re-fusion might extend some distance into the 

 surrounding rocks, continuing to do so until the supply of heat of 

 the injected material was exhausted. Should the surrounding 

 rocks be infusible, a chemical interchange would take place be- 

 tween the igneous and solid matter, resulting in the metamor- 

 phism of the former, and a corresponding change in the latter. 

 Although I am not personally acquainted with many examples in 

 illustration of this condition, probably some of those dykes which 

 are so abundant in the Western Isles and Highlands of Scotland, 

 described by Jameson and others who have followed him, will 

 serve. If the intrusion of the igneous magma takes place in solid 

 rocks, which themselves are at a high temperature from pressure — 

 crushing or conduction upwards from below — three things will 

 probably result. First, the magma, from the small absorption of 

 its heat by the surrounding rocks, would require a very long time 

 to cool, and that would also occur in a very gradual and uniform 

 manner, so that an extremely course crystalline structure would 

 result. This is the case in a great number of pegmatite granite 

 veins. Secondly, no salband will be formed, and partial fusion of 

 the fissure walls may occur, so that in gneisose rocks the line of 



