T. Mellard Reade—The Age of the World. 149 



cannot be right, and, further, that any estimate of age from the 

 thermal condition of the crust may be further vitiated by our 

 ignorance of the nature, constitution and materials of the globe 

 underlying the outer strata. 



These, I think, are the nature and character of the errors incidental 

 to any calculation of the actual age of the Earth from the outer 

 crust of sedimentary rocks. Possessing as they do vai-ying con- 

 ductivities, and being of various thicknesses, the average con- 

 ductivity of the whole has not improbably been overestimated, and 

 the average increase of heat overstated, while the combined resistance 

 of strata of varying conductivities may be greater to the passage of 

 heat across them than what would be inferred, even from a correct 

 average, as was shown to be the case in my second theoretical 

 example. 



If, in addition, we assume that our crust rests upon a basement 

 of rocks analogous to granite possessing high conductivities, it is 

 evident the more non-conducting envelope would be retained at a 

 high increment of temperature for a longer period than if the whole 

 mass of the Earth underneath possessed only the same conductivity 

 as the crust. Nor should it be lost sight of in this inquiry, that the 

 strata we are acquainted with are not cooling down from a state of 

 fusion, but are deposits laid down upon the surface and gradually 

 heated from below. This, again, obscures the results, and, indeed, the 

 question becomes so exceedingly complicated, that it is beyond my 

 powers to unravel ; but it is not inconceivable that an initial incre- 

 ment of temperature might be retained in this way for a lengthened 

 period. 



The cumulative results of these possible errors might then at 

 least quadruple the maximum age of the Earth as inferred from 

 calculation. At all events, I trust that I have shown the extremely 

 untrustworthy character of averages in such an investigation. But 

 these are not by any means all the arguments that can be urged 

 against the method — the data, as ascertained by experiment, being of 

 a complicated and contradictory character. 



Before dealing with the purely geological aspect of the question, 

 it will be well sliortly to examine the nature and extent of the data 

 forming the vital elements of the mechanical problem. 



Sir W. Thomson states the variation of the increase of temperature 

 to be from -^\-q of a degree Eahr. in some localities, to as much as i^g- 

 in another per foot of descent, but -5V is accepted as a rough mean. 



The Eeports on Underground Temperature by the British Asso- 

 ciation show most remarkable differences in localities not very far 

 apart. Thus, according to experiments made by Mr. Fairbairn at 

 Astley Pit, Duckinfield, Cheshire, the average rate of increase was 1° 

 every 80 feet for a depth of 685 yards ; while similar experiments at 

 Eose Bridge Colliery, Ince, near Wigan, for a depth of 815 yards, 

 gave an average increase of 1° in every 54*3 feet.^ Again, the 

 rate of increase in some of the wells in Paris is 1° for every 50 to 60 

 feet; while the great well of La Chappelle at Paris showed, at a depth 

 1 Britisli Association Eeport on Underground Temperature, 1870, pp. 31-2. 



