Prof. Johnsion-Lavis — Formation of Fluorides, etc. 309 



cannot, of course, increase the sum of heat in the earth. Therefore 

 the increased heat of the sediments by rise of isogeotherms, must be 

 taken from someiohere else. Is it taken from below ? Then the 

 radius [or rather crust] below must contract as much as the 

 sediments expand, and therefore there will be no elevation. Is it 

 taken from the containing sides ? Then the sides must lose as 

 much as the sediments gain, and therefore must contract and make 

 room for the lateral expansion, and therefore there would be no 

 folding and no elevation." 



It appeared to me possible, however, that the sediment, owing to 

 a less conductivity, might check the transmission of heat through it 

 more than the surrounding crust, and that there might consequeiitly 

 be some, though certainly a small, relative elevation due to the 

 cause invoked. This I now believe to be an error, for I was not 

 then aware how greatly the presence of water can increase the 

 conductivity of sand and rocks. That it does so is evident from 

 the experiments of Professors Herschel and Lebour.^ They find, 

 for instance, that the conductivity of quartzose sand is -00105 when 

 dry and -00320 when wet, that of New Red Sandstone is increased 

 from -00250 when dry to -00600 when wet, and that of clay from 

 •00250 to -00350. Now, considering that the average conductivity 

 of crystalline and volcanic rocks is -00519, of schistose and slate 

 rocks -00531, of different kinds of sandstone -00734 and -00323, 

 of limestones -00561, and of argillaceous strata -00242, it is evident 

 that I was wrong in supposing that the conductivity of saturated 

 sediment might be small enough for it to act as a relative check on 

 the passage of heat from the interior. I conclude, therefore, that 

 the force of the objection, great as it was before, is increased by 

 this additional consideration. 



YII. — On the Formation at Low Tempkkatures of certain 

 Fluorides, Silicates, Oxides, etc., in the Pipebnoid Tuff 

 of the Campania. 



By Prof. H. J. Johnston-Lavis, M.D., F.G.S., etc. 

 With a Note on the Determination of some of the Species. 

 By Prof. Pasquale Franco, M.D., etc, 



WHEN such minerals as mica, pyroxene, nepheline, fluorite, 

 and hematite are mentioned to us, we can hardly avoid 

 associating their genesis with very high temperatures, and if 

 amphibole be added we are equally bound to imagine also the 

 existence of high pressure. So deeply rooted is this, I might 

 almost say, superstition that few petrographefs, when they find such 

 minerals lining the fissures or cavities in rocks, fail to immediately 

 conclude that great heat, and sometimes pressure, is indicated by 

 such an occurrence. No doubt that in the vast majority of cases 

 they would be right, yet I hope to show in these notes that such 

 minerals have occasionally been produced under little or no pressure 



1 Brit. Assoc. Eep. 1881, pp. 130-135; quoted by Prestwich, "Controverted 

 Questions of Geology," pp. 240-241. 



