516 KEPORT— 1886. 



necessary to start it. But if this is so, then, when the earth has come to depart 

 both internally and externally from the equilibrium condition a flow of solid will 

 set in, and will continue until a near approach to the equilibrium condition is 

 attained. 



'When we consider the abundant geological evidence of the plasticity of rock, 

 and of the repeated elevation and subsidence of large areas on the earth's surface, 

 this view appears to me more probable than Sir William Thomson's. 



On the whole, then, I can neither feel the cogency of the argument from tidal 

 friction itself, nor, accepting it, can I place any reliance on the limits which it 

 assigns to geological history. 



The second argument concerning geological tune is derived from the secular 

 cooling of the earth. 



We know in round numbers the rate of increase of temperature, or temperatura 

 gradient, in borings and mines, and the conductivity of rock. These data enable 

 us to compute how long ago the surface must have had the temperature of melting 

 rock, and when it must have been too hot for vegetable and animal life. 



Sir William Thomson, in his celebrated essay on this subject,^ concludes from 

 this argument that ' for the last 96,000,000 years the rate of increase of tempera- 

 ture underground has gradually diminished from about ith to about ^th of a 

 degree Fahrenheit per foot. ... Is not this, on the whole, in harmony with geo- 

 logical evidence, rightly interpreted ? Do not the vast masses of basalt, the general 

 appearances of mountain ranges, the violent distortions and fractm-es of strata, the 

 great 2}revalence of metamorpliic action (which must have taken place at depths of 

 not many miles, if so much), all agi-ee in demonstrating that the rate of increase of 

 temperature downwards must have been much more rapid, and in rendering it 

 probable that volcanic energy, earthquake shocks, and every kind of so-called 

 plutonic action, have beeu, on the whole, more abundantly and violently operative 

 in geological antiquity than in the present age ? ' 



Now, while I entirely agree with the general conclusion of Sir William Thom- 

 son, it is not unimportant to indicate a possible flaw in the argument. This flaw 

 will only be acknowledged as possible by those who agree with the previous criti- 

 cism on the argument from tidal friction. 



The present argument as to the date of the consolidation of the earth reposes on 

 the hypothesis that the earth is simply a cooling globe, and there are reasons why 

 this may not be the case. The solidification of the earth probably began from the 

 middle and spread to the surface. Now is it not possible, if not probable, that 

 after a firm crust had been formed, the upper portion still retained some degree of 

 Tiscosity ? If the interior be viscous, some tidal oscillations must take place in it, 

 and, these being subject to friction, heat must be generated in the viscous portion ; 

 moreover the diurnal rotation of the earth must be retarded. Some years ago, in a 

 paper on the tides of a spheroid, viscous throughout the whole mass,- 1 estimated the 

 amount and distribution of the heat generated, whilst the planet's rotation is being 

 retarded and the satellite's distance is being increased. It then appeared that on 

 that hypothesis the distribution of the heat must be such that it would only be 

 possible to attribute a very small part of the observed temperature gradient to such 

 a cause. Now, with a more probable internal constitution for the earth in early 

 times, the result might be very difierent. Suppose, in fact, that it is only those 

 strata which are within some hundreds of miles of the surface which are viscous, 

 whilst the central portion is rigid. Then, when tidal friction does its work the 

 same amount of heat is generated as on the hypothesis of the viscosity of the whole 

 planet, but instead of being distributed throughout the whole mass, and principally 

 towards the middle, it is now to be found in the more superficial layers. 



In my paper it is shown that with Thomson's data for the conductivity of rock 

 and the temperature gradient, the annual loss of heat by the earth is one 260 

 millionth part of the earth's kinetic energy of rotation. 



Also, if by tidal friction the day is reduced from D^ hours to D hours, and the 



• Eepublished in Thomson and Tait's Natural Philosophy, Appendix D. 



* Phil. Tra7is. Part II. 1879 



