398 A. B. Hunt — The Crystallisation-of Granite. 



willingly stake my reputation as a golfer that this crystal has 

 not the most distant connection with mica. So far as I have 

 been able to judge from the few sections I have seen, the Cornish 

 granites differ from the Dartmoor granites. In Cornwall there 

 seem to be more pressure and fewer chlorides in the main granites. 

 I have noticed bent mica, which is a very significant clue. My 

 CJornish elvan-slides, too, are not so simple as those from Dartmoor ; 

 there is more evidence of re-solution. Thus it is quite likely that 

 a good many things occurred to the Cornish granites which did 

 not happen to the more simple and less metalliferous Devonshire 

 rock, and it may not be safe to judge the one by the other. I gather, 

 for instance, that the schorl-veins in Cornwall are sometimes 

 micaceous. Those on the east of Dartmoor seem never to be so. 

 I have a specimen of tourmaline granite sharply in contact with 

 and invading a micaceous granite, in which the tourmalines in the 

 one and the micas in the other, closely adjacent, are absolutely 

 characteristic, and keep strictly to their own rocks. 



As a very rough working hypothesis I would suggest the 

 following explanation of the eastern el vans and non - granitoid 

 felspar-quartz-tourmaline veins. We must assume the previous 

 cooling of the main granite, because both elvans and non-granitoid 

 veins invade it. The elvans have the true granitic structure, the 

 non-granitoid veins have not ; so we take the elvans to represent 

 a hotter stage of the intruding minerals. These elvan intrusions, 

 whether in the granite or in intrusions a quarter of an inch wide 

 in the Culm slates, contain characteristic idiomorphic crystals of 

 tourmaline, which have crystallised first ; and the veins, both 

 granitic and non-granitoid, usually contain no trace of mica. When 

 we get down to the non-granitoid veins, in which the felspar, 

 tourmaline, and quartz have crystallised in succession, we find the 

 tourmaline often to be the fan-like variety, while occasional compact 

 crystals are not specifically characteristic, as they have no transverse 

 pseudo-cleavage. The crystals here seem by nature gregarious, 

 whereas in the granitic veins they are characteristically solitary. 

 In the non-granitoid veins we have proof that the tourmaline 

 occasionally crystallises after the quartz ; in the granitic veins 

 never. 



If there is any truth in the doctrine of critical temperatures, 

 there is no shadow of a doubt that the non - granitoid veins 

 crystallised out of liquid, below the critical temperature of water. 

 The highest temperature of the elvan-veins is not so certain, but 

 their quartzes almost invariably prove that the rock had cooled 

 down below the critical temperature of water before they, the last 

 mineral in those rocks, crystallised. We have this apparent anomaly, 

 viz., that in the granitic elvan-veins the tourmaline was the first 

 mineral to crystallise, whereas in the non-granitoid veins the 

 tourmaline occasionally crystallises after some of the quartz. But 

 these two tourmalines differ so much in character that no one 

 would guess they were the same mineral unless credibly so informed. 

 Between these extremes of tourmalines there are many inter- 

 mediate stages. 



