A. R. Hunt— The Crystallimtion of Granite. 401 



exhaustive! Dr. Sorby attempted to deduce the weight of once 

 superincumbeiit strata from the uniformity of the fluid inclusions. 

 It was, however, soon noticed that fluid inclusions are often not 

 uniform. Petrologists at once ran off with the idea that Dr. Sorby's 

 theory had collapsed. Dr. Sorby, however, accepting the teaching 

 of the variable inclusions, pointed out that variable incrusiona 

 indicated varying conditions during crystallisation, a fact infinitely 

 more important than the deduction that uniform fluid inclusions 

 might supply a tape measure for measuring the thickness of strata.^ 

 If we are to credit chemists and physicists. Dr. Sorby is correct in 

 his premisses, and these premisses go to prove that at any rate the 

 final consolidation of most granites was at a temperature below the 

 critical temperature of water ; and this critical temperature is as 

 important a mark for the petrologist on the physical thermometer as 

 that of blood heat is to the doctor in his clinical thermometer. Now 

 what would the world say if, knowing as the world does the value 

 of the clinical thermometer, the whole medical profession were to 

 discard it without so much as discussion ? Yet this is precisely 

 what the petrological profession has done with its own physical 

 thermometer. And it has done it in the most authoritative way. 

 For many years past petrologists have been quietly treating granite 

 as a high-temperature rock. They have even tacitly accepted 

 Lord Kelvin's theory that granite was the rock primaeval, whose 

 minerals crystallised out of and gravitated through a basalt lava. 

 They now accept without demur the doctrine that quartzes, con- 

 taining both gases and fluids with deposited crystals, can have 

 crystallised at high volcanic temperatures. Well, be it so, but they 

 must not expect golfers and the rest of the uneducated public ta 

 follow them, simply because the geological doctrines are self- 

 destructive. Otherwise the said public would have no right to form 

 an independent opinion. 



With regard to the origin of plutonio rocks, what we want done 

 is to satisfy both chemists and geologists. For instance, the geologists 

 say that crystals are permeable. The chemists declare that at any 

 rate many crystals will withstand gas pressure of 1,155 pounds to 

 the square inch for geological epochs. I have one set of carbonia 

 acid inclusions which have been imprisoned since Devonian times^ 

 yet they will in a thin section undertake their 1,155 lbs. pressure 

 as often as desired. The geologists say that crystals of chlorides 

 can be deposited by water or water-vapour at a temperature 

 approaching 1,200° C. The chemists say " Tush ! " or think so. 



Here is an important fact. Professor Hartley tells us that when 

 carbonic acid inclusions are present, it is easy to ascertain the 

 temperature of crystallisation of the enclosing mineral.^ He 

 observes incidentally that such inclusions occur in tourmaline,^ 

 but as tourmaline is not in the line of his enquiry he omits to 

 state the crystallising temperature of tourmaline ; yet that is one 

 of the geological desiderata. 



1 Eep. Brit. Assoc, 1877, p. 236. 



2 Loc. cit., p. 233. 



DECADE IV. — VOL. X. — NO. IX. 26 



