4 REPORT 1876. 



with marvellous frequency and pertinacity, that at depths of from 30 to 100 kilo- 

 metres the temperatures are so high as to melt all substances composing the earth's 

 upper crust ? It has been remarked, indeed, that if observation showed any dimi- 

 nution or augmentation of the rate of increase of underground temperature in great 

 depths, it would not be right to reckon on the uniform rate of 1 ° per 30 metres or 

 thereabouts down to 30 or 60 or 100 kilometres. " But observation has shown 

 nothinty of the kind ; and therefore surely it is most consonant with inductive philo- 

 soph}' to admit no great deviation in any part of the earth's solid crust fi-om the 

 rate of increase proved by observation as far as the greatest depths to which we 

 have reached ! " Now I have to remark upon this ai'gument that the gi-eatest depth 

 to which -we have reached in observations of underground temperature is scarcely 

 one kilometre ; and that if a 10-per-cent. diminution of the rate of augmentation of 

 underground temperature downwards were found at a deptli of one kilometre, this 

 would demonstrate * that within the last 100,000 years the upper siwface of the 

 earth must have been at a higher temperatui-e than that now found at the depth 

 of one kilometre. Such a result is no doubt to be found by observation in places 

 which have been overtiowu by lava in the memory of man or a few thousand 

 years further back ; but if, without going deeper than a kilometre, a 10-per-cent. 

 diminution of the rate of increase of temperature downwards were found for the 

 whole earth, it would limit the whole of geological history to witliin 100,000 

 years, or, at all events, would interpose an absolute barrier against the continuous 

 descent of life on the earth from earlier periods than 100,000 years ago. There- 

 fore, although search in particular localities for a diminution of the rate of aug- 

 mentation of underground temperature in depths of less than a kilometre may be 

 of intense interest, as helping us to fix the dates of extinct volcanic actions which 

 have taken place within 100,000 years or so, we know enough fiom tlioroughly sure 

 geological evidence not to expect to find it, except in particular localities, and to 

 feel quite sure that we shall not find it imder any considerable portion of the earth's 

 surface. If we admit as possible any such discontinuity within 900,000 years, we 

 might be prepared to find a sensible diminution of the rate at three kilometres 

 depth ; but not at any thing less than 30 kilometres if geologists validly claim as 

 much as 90,000,000 of years for the leng-th of the time with which their science is 

 concerned. Now this implies a temperature of 1000° Cent, at the depth of 30 kilo- 

 metres, allows something less than 2000° for the temperature at 60 kilometres, and 

 does not require much more than 4000° Cent, at any depth however great, but does 

 require at the great depths a temperature of, at all events, not less than about 4000° 

 Cent. It would not take much " hurrying up " of the actions with which they are 

 concerned to satisfy geologists with the more moderate estimate of 60,000,000 of 

 years. This would imply at least about 3000° Cent, for the limiting temperature 

 at great de]ith8. If the actual substance of the earth, whatever it may be, rocky 

 or metallic, at depths of from 60 to 100 kilometres, under the pressure actually there 

 experienced by it, can be solid at temperatures of from 3000° to 4000°, then we 

 may hold the former estimate (90,000,000) to be as probable as the latter 

 (50,000,000), so far as evidence from undergi-ouud temperature can guide us. If 

 4000° would melt the earth's substance at a depth of 100 kilometres, we must reject 

 the former estimate though we might still admit the latter ; if 3000° would melt 

 the substance at a depth of 00 kilometres, we should be compelled to conclude that 

 50,000,000 of years is an over-estimate. Whatever may be its age, we may be 

 quite sure the earth is solid in its interior ; not, I admit, throughout its whole 

 volume, for there certainly are spaces in volcanic regions occupied by liquid lava ; 

 but whatever portion of the whole mass is liquid, whether the waters of the ocean 

 or melted matter in the interior, these portions are small in comparison with the 

 whole ; and we must utterly reject any geological hypothesis which, whether for 

 explaining underground heat or ancient upheavals and subsidences of the solid 

 crust, or earthquakes, or existing volcanoes, assumes the solid earth to be a shell 

 of 30, or 100, or 50^, or 1000 kilometres thickness, resting on an interior liquid 

 mass. 



* For proof of this and following statements regarding underground heat, I refer to 

 " Secular Cooling of the Earth," Transactions of the Eoval Society of Edinburgh, 1SG2; 

 and Thomson and Tail's ' Natural PhiloEophy,' Appsndix D, 



