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



water inclusions and chlorite, and is associated with other hydrous 

 minerals. Then, as to quartz : in a slightly altered Devonshire 

 diabase we find, besides chlorite in abundance, secondary quartz- 

 granules which contain fluid inclusions and deposited crystals. No 

 one would attribute much heat to these rocks at the time of 

 alteration ; and, as for the diabase, its accompanying Devonian 

 slates do not even reach the dignity of phyllites. Then, on 

 Dartmoor we find felspar, quartz, and tourmaline crystallising at 

 temperatures too low to produce a granitic structure. Although the 

 rule there, is, that the felspar crystallises before the quartz and 

 greenish tourmaline, I have detected one case in which, after the 

 breaking down and partial solution of a felspar by quartz-liquor 

 containing tourmaline-belonites, the felspar repaired damages at the 

 cost of the quartz, and enclosed some of the belonites. This might 

 possibly be explained by the process of cooling being checked, and 

 by a slight reheating first dissolving the felspar and then permitting 

 the recrystallisation of that mineral. But felspar, quartz, and 

 tourmaline were clearly in this case crystallising in the wet way. 

 Dry-fusion temperatures would not assist us in the least ; indeed, 

 they could only mislead. 



With regard to the permeability of rocks, although, as we have 

 seen, many minerals in mass will resist the pressure of a gas at 

 1,155 pounds to the square inch for countless ages, and in the 

 thinnest section for an indefinite time, the same minerals are 

 liable to minute and successive fractures ; so that a series of water 

 inclusions may traverse a plane of water and carbonic acid inclusions. 

 Although the fluids were originally introduced through planes of 

 fracture, the mineral may be so completely restored that no evidence 

 may remain of the original fissures. In one specimen water 

 inclusions, carbonic acid inclusions, and inclusions with deposited 

 crystals occur near together. It is obvious on inspection that the 

 carbonic acid inclusions are later than the crystal, and that the 

 water inclusions are later again. The relative age of the inclusions 

 with deposited crystals is not apparently indicated. The great 

 charm of the physical theory of granite is that it afi"ords possible 

 solutions for many enigmas, such, for instance, as that of augite- 

 granite, where we find a true high-temperature mineral associated 

 with those of low temperature. 



Just at present tourmaline is exercising the minds of petrologists. 

 Nearly forty years ago I took some lessons in mineralogy from 

 Mr. Tennant in the Strand, and I learned that tourmaline had no 

 oleavage, but only a pseudo-cleavage. In after years the absence 

 of tourmaline proved to be the great test of the non-Dartmoor 

 origin of the blocks trawled in the English Channel. Later still 

 I bought a Dartmoor farm, where I can never step outside the door 

 without trampling under foot the tourmaline ' waterstones,' or 

 river stones, which pave the garden path. Many years ago Professor 

 Bonney was good enough to give me his paper on Luxullianite, 

 wherein he derives tourmaline from mica and felspar. I hunted 

 Dartmoor for some years in vain for any evidence that the Dartmoor 



