392 A. B. Hunt — The Crystallisation of Granite. 



The Pecten quadricostatus, Sowerby, and the Exogyrce are by far 

 the commonest shells, the Pectens being mostly fragmentary ; but 

 I have found nearly perfect ones, which, however, have invariably 

 fallen to pieces on being removed. 



All the fossils are siliceous, many, especially the flat fragments of 

 Pecten, showing the curious concentric structure peculiar to Beekite. 



The obscure fossil remains in beds 3 and 4 are casts in sand, 

 which at once fall to pieces on removal ; but none of those found 

 could be identified. Possibly they are the ' impressions ' mentioned 

 in the Survey Memoir.^ 



Half a mile further westwards, where the Foxmould cliff recedes 

 some two hundred yards from the road, and immediately beneath 

 the Golf Links, another fossil-bed occurs. This, like the last, lies 

 within a foot of the capping of chert detritus, but is at a higher 

 level, the cliff rising here to about 500 feet. This has yielded 

 fragments of P. quadricostatus, Sowerby, and from it in September, 

 1902, I obtained some fine specimens of Exogyra conica, Sowerby. 



My best thanks are due to Dr. F. L. Kitchin and Mr. E. T. 

 Newton, F.R.S., of the Museum of Practical Geology, for their 

 kindness in helping me to identify the specimens. 



III. — Some Disputed Points in the Crystallisation of the 

 Constituent Minerals op Granite. 



By A. R. Hunt, M.A., F.G.S. 



IN writing my little paper on vein-quartz I was particularly 

 anxious not to introduce controversial matter, but to be strictly 

 orthodox throughout. After the paper was published I was much 

 perturbed to find that I had unwittingly come into collision with 

 two incidental remarks in General McMahon's most interesting 

 paper on the Satlej granite, which was the subject of his address to 

 Section C at Belfast. My oversight arose owing to the said remarks 

 being incidental, and no stress having been laid upon their paramount 

 importance. 



To explain, the modern theory of granite is a chemico-physical 

 one, founded on the critical temperatures of carbonic acid and of 

 water. According to the physical evidence, ordinary granites 

 crystallised about the critical temperature of water ; some minerals 

 possibly above, and others below ; or all above, or all below. The 

 critical temperature of water I have taken as 342° C. (Eep. Brit. 

 Assoc, 1877, p. 236). The chief witnesses are the various inclusions 

 of gas, water, and other liquids, and the deposited crystals contained 

 in the water inclusions. 



In my paper I assumed it to be an accepted fact that deposited 

 crystals were proof positive that the mineral containing them was 

 crystallised below the critical temperature of water, and that groups 

 of inclusions with proportional amounts of carbonic acid and water 

 were evidence of a temperature above the critical temperature of 

 water. I also assumed that above the critical temperature of water, 

 1 Jukes - Browne : ibid., p. 186. 



