214 A. JR. Hunt — Vein- Quartz and Sands. 



temperature water-vapour would cease to act as a solvent, whereas 

 according to subsequent experiments it appears to do so.^ This, of 

 course, is a most important point as regards the consolidation of 

 minerals other than quartz ; but fortunately all the authorities appear 

 to agree that the quartzes of granites consolidated under 342° ; and 

 probably no one would maintain that the quartzes of veins represent 

 a temperature as great as that of granitoid rocks. If all this be true, 

 the question of vein -quartzes is very much simplified. Liquid water 

 will dissolve a variety of minerals which water-vapour under 342° C. 

 will reject and deposit. "With water-vapour over 342° C. we need 

 not ex hypothesi concern ourselves — fortunately for the length of 

 this paper. 



Writing of the variation in the proportional contents of cavities. 

 Dr. Sorby has this very weighty passage, viz. : " The very great 

 variation in the relative amount of water and liquid carbonic acid in 

 the cavities clearly proves that very great changes in the surrounding 

 circumstances sometimes took place, even during the growth of one 

 single crystal ; and there is good reason to suspect that there may 

 often have been considerable variations in temperature and pressure, 

 as well as in the relative amount of water and gas " (" Critical Point," 

 etc., E. Micr. Soc, 1876). In 1889 and 1890 I wrote two papers 

 on the granite question, and Dr. Sorby, when acknowledging receipt 

 of one or other of them, remarked, " 1 very much agree with what 

 you have said in your paper, and it now seems to me that the 

 conditions when granite was consolidated were very complex in 

 some cases, even more so than I urged in my paper." This view 

 has been wonderfully confirmed by the rock Trowlesworthite, which 

 indicates fluoric, boracic, and phosphoric acids, a regular jumble of 

 water and brine, and two tourmalines, one probably above and one 

 certainly below the critical temperature of water. 



With respect to the evidence borne by cavities that the temperature 

 was over 342° C, we may cite Mr. Hartley : — " In a colourless and 

 clear topaz there were discovered thousands of perfectly cylindrical 

 tube-like cavities, round at each end. In the case of fifty-two 

 cavities .... they each contained the same proportions of carbonic 

 acid liquid, carbonic acid gas, and water. Hence at the time they 

 were enclosed in the mineral, these fluids must have existed in the 

 state of a homogeneous vapour. This of necessity places the 

 temperature of formation of the mineral somewhere above 342° C, 

 the critical point of water" (Eep. Brit. Assoc, 1877, p. 236). 

 The general conclusion seems to be as follows : — 

 Fluid inclusions with deposited crystals are clear proof that the 

 fluid was entangled in the quartz or other mineral, under 342° C. 



Water inclusions with variable proportions of water and vacuity 

 were also formed under 342° C. 



Groups of inclusions with proportionate amounts of water and 

 liquid carbonic acid prove a temperature exceeding 342° C. ; but with 

 disproportionate amounts of water and acid they indicate a tem- 

 perature below 342° C. 



1 J. B. Hanuay : Proc. E.S., 1881, p. 321. 



