A. R. Hunt — Age of the Dartmoor Granites. 105 



existence of an ancient granite, a suiScient explanation suggests 

 itself, both of the presence of the imported tourmaline and of the 

 imported chlorides. If a piece of granite be heated in an ordinary 

 fire, even to a bright red heat, and cooled, its general appearance 

 is not much affected; it is still rigid enough to be sliced for the 

 microscope, but differential expansions in its dissimilar minerals have 

 rendered it permeable by fluids, as can be readily tested by a dye, 

 such as aniline. Now, if a deep-seated granite were exposed to 

 alterations in temperature sufficient to set up such differential ex- 

 pansions, the most compact rock would be minutely cracked through- 

 out, and be rendered permeable by liquids and gases, especially 

 under pressure. The doctrine of the rise and fall of the earth's 

 internal heat through the crust, by the deposition and denudation 

 of sediments, has long been accepted by geologists. All we require 

 is an application of this doctrine to Dartmoor, so that our granite, 

 before or during Carboniferous times, should have suffered sufficient 

 alterations of temperature to render it porous, and so lay it open to 

 the invasion of solvent liquids or gases. Indeed, many granites are 

 so rotten and decomposed, from whatever cause, that they suck up 

 fluids like a sponge. 



Now a porous granite being granted, there is little further diffi- 

 culty ; for although some modern geologists decline to entertain the 

 hypothesis that sea-water could gain access to heated granite and 

 set up chemical changes therein, the weight of the evidence is 

 against them. Years before a section was cut, or the chlorides in 

 liquid inclusions discovered by Dr. Sorby, the almost omniscient 

 De la Beche, with well nigh incredible foi'esight, pointed out the 

 probability of saturated solutions of salt being accumulated in 

 granite, by sea-water obtaining access to the rock when highly 

 lieated.^ De la Beche also pointed out the important chemical 

 effects which the various salts dissolved in sea-water might be 

 expected to have on the granite itself.^ 



Given a porous granite, covered by sedimentary rocks thick 

 enough to bring the internal heats within reach of a superincumbent 

 ocean, and we have all the machinery requisite for the solution of 

 quartz^ and silicates; and, with the addition of fluorine and boron, 

 for their redeposition in a variety of forms such as quartz, tourma- 

 line, topaz, fluor spar, and triclinic felspars ; all of which occur as 

 introduced minerals in the west country granites. 



The late Mr. J. A. Phillips has recorded the occurrence of saline 

 water in Huel Clifford mine, 13;i0 feet below the sea-level.* Prof. 

 Le Conte has calculated that at a depth of 10,000 feet below the 

 surface the temperatui-e would be 230 F.,® a very moderate depth in 

 terms of ocean soundings, being 1666 fathoms. But even 230 F. 

 is a temperature not to be neglected, seeing that the glass tubes 

 of the water guages of ordinary locomotives become corroded. 



1 Eeport on the Geology of Cornwall, Devon, etc., p. 378. ^ Zoc. cit. p. 387. 

 ^ Mr. J. B. Hannay has kindly informed me that he has dissolved quartz in super- 

 heated steam and produced transparent quartz crystals thereby. 

 * Phil. Mag. 1873 (2), p. 32. ^ Loc. cit. p. 45. 



