150 T. Mellard Reade—The Age of the World. 



of 600 metres, an average increase of 1° in 94-3 feet.^ In experiments 

 made at Przibram, in Bohemia, by Herr Johann Grimm,^ Director of 

 the School of Mines, in a depth of 1900 feet, there was an increase of 1° 

 in every 141 feet. The experiments at the Mont Cenis Tunnel show 

 an increase of 1° in 93 feet, without correction for the convexity of 

 the ground. 



If, however, there exist this variation in the average increase of 

 temperature of each total depth of rock penetrated, there is still more 

 variation in the rate of increase in different parts of the same bore, 

 the variation in the Bohemian mine being from 1° in 45 feet to 1° in 

 1400 feet. 



On the other hand, if we refer to the conductivity of certain rocks, 

 we find, according to the revised scale in the experiments made by 

 Herschel and Lebour,^ the thermal conductivities vary from ■008&2 

 for opaque white quartz to -00065 for cannel coal. I understand 

 Sir W. Thomson's average was founded upon the conductivity of 

 only two descriptions of rock, viz. Calton trap rock and Craigleith 

 sandstone, estimated respectively by him, if reduced to British Asso- 

 ciation units, at -00266 and •00689." 



Without going further, I think sufficient has been said to show on 

 what an insecure basis this tremendous superstructure of inference 

 has been built. From experiments only made in a few places, and 

 necessarily upon rocks, geologically speaking, subject from time to 

 time to great fluctuations of temperature, and in mere scratches in 

 the Earth's surface, through all manner of variations, the first average 

 necessary to the calculation has been educed, viz. a ratio of increase 

 of heat of 1° per 50 feet. 



The average conductivity is a still greater assumption, because, in 

 the first place, the relative amount of each class of rock composing 

 the crust of the Earth and its conductivity should be calculated, 

 which is not done, and would prove very difficult to even approxi- 

 mately estimate. Neither are laboratory experiments on conductivity 

 conclusive, as a mass of rock through its structure may behave in a 

 different way to a small piece, while the impossibility of testing in 

 any way the average conductivity of the materials of the interior of 

 the Earth, subject to such enormous pressure and heat, is apparent. 

 It is also not at all improbable that frequent variations of strata, 

 possessing different conductivities, may create interference in the 

 transmission of heat ; for it is proved that laminations increase the 

 non-conducting property of rock across the planes of lamination. 



If we turn from this most complex and difficult problem, to study 

 the history of the Earth, and form our views by induction from 

 observation, we find that from the earliest periods to the present 

 time the known crust of the Earth has been in continual process of 

 construction, or reconstruction, by sedimentary and volcanic deposits, 



1 Ibid. 1873, p. 253. 2 jii^_ 1875, p. 15. 



^ British Association Report on Thermal Conductivities of certain Eocks, 1875, p. 57. 



* The estimate is really made in terms of the thermal capacity of the unit of 

 bulk, which is averaged at 400 ; the unit in length being a British foot, and unit of 

 time a year. 



