498 General McMahon — Further Remarks on Granite. 



a volcano. But the case of an active volcano in the immediate 

 vicinity of the sea is very different from that of a heated magma 

 of a plutonic rock like granite, buried at great depths in the bowels 

 of the earth, far removed from the sea. 



The possibility of the access of sea-water to the root of a volcano 

 like Krakatoa may be readily conceded, as this would explain in 

 a reasonable way the tremendous explosion which took place, but 

 Mr. Hunt, in his theory of the crystallisation of rocks, goes much 

 further. He contends that the access of water would liquefy a rock 

 that had consolidated and already cooled considerably at a dry heat 

 (p. 403) ; and he argues on the same page that the pressure of 

 water at 3,000 fathoms being 9,000 pounds to the square inch, 

 water may have gained access through fissures to highly heated 

 though consolidated rocks, and must have resulted in " liquefaction, 

 reconstruction, recrystallisation, and metamorphosis in almost every 

 variety." 



If by these remarks Mr. Hunt contemplates the case of a granitic 

 magma buried at plutonic depths, I doubt if he will find many 

 geologists able to go with him, and still fewer to believe that if 

 sea- water could gain access through open fissures to a granitic 

 magma, it would lead to the quiet liquefaction and recrystallisation 

 of the minerals of the granite. 



In my Belfast address I said that Sorby had proved " that the 

 liquid contained in the inclusions in granite is water, and showed 

 that it was caught up during the formation of the crystals, and 

 was not introduced subsequent to the consolidation of the rock." 

 I think, if my critic wishes to convert geologists to his theories 

 about the formation of granite, he should begin by showing that 

 Sorby's facts are erroneous and that his verdict based upon them 

 is unsound. 



I have only time, in conclusion, to notice very briefly another 

 adverse criticism by Mr. Hunt. He expresses his disbelief in the 

 permeability of rocks, and adduces as counter evidence the case of 

 inclusions of liquid carbon-dioxide. He has specimens, he tells us, 

 which have been imprisoned since Devonian times, and although 

 they are now contained in thin microscopic sections they have not 

 escaped, though the pressure exerted by liquid carbon-dioxide is 

 1,155 pounds to the square inch. 



When considering the question of the porosity and the permeability 

 of rocks, one adverse case is not sufficient in my opinion to dispose 

 of a good many instances which show that chemical reagents have, 

 as a matter of fact, penetrated into rocks and minerals. 



The permeability of one substance by a liquid or gas depends, 

 among other things, on the relative size of the molecules of the 

 invader, as compared with the size of the interspaces between the 

 molecules of the crystal invaded. To take a homely simile, a sieve 

 of small meshes will allow sand to readily pass through them, but 

 will effectually bar the passage of half-crowns. Some crystals, 

 therefore, may be permeable to certain chemical reagents, but not 

 to others. I have heard on good authority that the permeability 



