lii PEOCEEDINGS OF THE GEOLOGICAL SOCIETT. 



energy radiated from year to year was supplied from year to year — 

 a doctrine which would have suited Hutton perfectly. jBut the fact 

 that so eminent a physical philosopher has thus recently held views 

 opposite to those which he now entertains, and that he confesses his 

 own estimates to be " very vague," justly entitles us to disregard 

 those estimates if any distinct facts on our side go against them. 

 However, I am not aware that such facts exist. As I have already 

 said, for anything that I know, one, two, or three hundred millions 

 of years may serve the needs of geologists perfectly well. 



III. The third line of argument is based upon the temperature of 

 the interior of the earth. Sir "W. Thomson refers to certain investi- 

 gations which prove that the present thermal condition of the interior 

 of the earth implies either a heating of the earth within the last 

 20,000 years of as much as 100° ¥., or a greater heating all over 

 the surface at some time further back than 20,000 years, and then 

 proceeds thus : — 



" Now, are geologists prepared to admit that, at some time within 

 the last 20,000 years, there has been all over the earth so high a 

 temperature as that ? I presume not ; no geologist — -no modern 

 geologist — would for a moment admit the hypothesis that the present 

 state of underground heat is due to a heating of the surface at so 

 late a period as 20,000 years ago. If that is not admitted, we are 

 driven to a greater heat at some time more than 20,000 years ago. 

 A greater heating all over the surface than 100° Pahrenheit would 

 kill nearly all existing plants and animals, I may safely say. Are 

 modern geologists prepared to say that all life was Mlled oif the 

 earth 50,000, 100,000, or 200,000 years ago? For the uniformity 

 theory, the further back the time of high surface-temperature is put 

 the better ; but the further back the time of heating, the hotter it 

 must have been. The best for those who dj^aw most largely on time 

 is that which puts it furthest back ; and that is the theory that the 

 heating was enough to melt the whole. But even if it was enough 

 to melt the whole, we must still admit some limit, such as fifty million 

 years, one hundred million years, or two or three hundred million 

 years ago. Beyond that we cannot go." (L. c. p. 24.) 



It will be observed that the " limit " is once again of the vaguest, 

 ranging from 50,000,000 years to 300,000,000. And the reply is, 

 once more, that, for anything that can be proved to the contrary, one 

 or two hundred million years might serve the purpose, even of a 

 thorough-going Huttonian uniformitarian, very well. 



But if, on the other hand, the 100,000,000 or 200,000,000 years 

 appear to be insufficient for geological purposes, we must closely cri- 

 ticise the method by which the limit is reached. The argument is 

 simple enough. Assuming the earth to be nothing but a cooling 

 mass, the quantity of heat lost per year, supposing the rate of cooling 

 to have been uniform, multiplied by any given number of years, will 

 give the minimum temperature that number of years ago. 



But is the earth nothing but a cooling mass, "like a hot- water jar 

 such as is used in carriages," or *' a globe of sandstone "? and has its 

 cooling been uniform ? An affirmative answer to both these ques- 



