494 General McMahon — Further Remarks on Granite. 



that " the potential energy of water held in a fluid state by pressure 

 must have been great. When, therefore, in the course of the 

 earth-movements which accompany or in some cases are caused by 

 the intrusion of eruptive igneous masses, pressure was temporarily 

 relieved by the rupture and faulting of rocks, the superheated water 

 contained in the magma was ready to flash into steam with almost 

 explosive violence." 



Mr. Hunt calls attention to the expressions used in the above 

 passage that water was held in a fluid state by pressure, and he 

 also refers to expressions used in other passages in my address, 

 where I speak of water at a red heat, and considers that the 

 views indicated by these expressions are not onl}^ inconsistent with 

 the old physical theory of granite, but entirely subversive of them. 

 I will take the first remark objected to into consideration before 

 proceeding to the other. 



In his previous paper on vein-quartz (Geol. Mag., May, 1903, 

 p. 212) Mr. Hunt reminds us that Dr. Sorby in 1876 estimated 

 the critical temperature of water to be 412° C., but that Professor 

 Hartley in 1877 estimated it at 342° C, which latter figure Mr. Hunt 

 accepts as the true one. From Mr. Hunt's own showing, therefore, 

 a margin of at least 242° C. exists between tlie boiling-point of 

 water, 100° C, and the critical temperature, 342° C. It follows, then, 

 that throughout a range of 242° of Centigrade temperature water 

 must be held in a fluid state in the molten magma by pressure, 

 so that for this range of temperature, at all" events, the expression 

 used by me is strictly and literally true. As regards the phrase 

 water " at or above a red heat," I was following so good an authority 

 as Serope, who, as quoted in my address, speaks of "red-hot water 

 or steam in a state of extreme condensation and con sequent tension." 

 When I spoke of water being at or above a red heat (550° 0.) 

 I thought experts would understand that I had water in a gaseous 

 state in my mind. For the purposes of my argument the clearer 

 the fact was realized by my hearers that we were considering water 

 in its gaseous state, the better for my purpose, which was to impress 

 upon them the potential energy and the kinetic motion exercised 

 by water at this heat. 



At the commencem.ent of my address I warned my hearers that, 

 the time at my disposal being brief, I would use great simplicity 

 of language, free from technicalities. I wished to avoid, therefore, 

 a dissertation on such elementary topics as the critical temperature 

 of water, and of water existing in three states as a solid, a liquid, 

 and a gas, and to confine the attention of my audience to the 

 consideration of the potency and energy of a molten granite 

 intruded into other rocks, and thought that any digression to 

 consider objects outside the central idea of my paper would have 

 put it a little out of focus. 



If an artist has painted a landscape with the intention of drawing 

 attention to the beauty and poetry of Nature as exhibited by various 

 forms of rocks and foliage with fleeting lights and shadows, it 

 would ruin the broad effect of his picture to introduce into his 



