474 Reviews — Sir W. Thomson's Geological Dynamics. 



cannot be reasonably urged that a hotter sun is not a probable 

 explanation of tbe supposed warmer climate of the palseozoic 

 ages." Whilst willingly agreeing with Sir W. Thomson that meta- 

 morphic action at depths beneath the surface may have been 

 accelerated in the earlier, or pre-Cambrian times, still we cannot 

 suppose it possible that such rocks now remain to tell of such 

 increased telluric action. As regards the assumed warmer climate 

 in paleeozoic times, we see no palseontological grounds for such an 

 inference. 



It was suggested indeed by a pseudo-geological writer, some time 

 ago, that the bony armour of the Devonian and Silurian fishes was 

 given to protect them from the Ixot water of those early seas ; but 

 as the writer was neither a chemist nor naturalist, his opinion will 

 not assist Sir W. Thomson. 



Sir W. Thomson replies at some length to Prof. Huxley's exami- 

 nation of his arguments. The Professor, after pointing out that 

 tidal retardation can be checked and overthrown by temporary 

 conditions, asks "What becomes of the confident assertion, 

 based upon the assumed uniformity of tidal retardation, that 

 10,000,000,000 years ago the earth must have been rotating more 

 than twice as fast as at present, and, therefore, that we geologists 

 are 'in direct opposition to the principles of natural philosophy,' 

 if we spread geological history over that time." Sir W. Thomson 

 answers " that tidal retardation cannot be permanently overthrown 

 by temjDorary conditions ; that its true amount may be considerably 

 greater than that which we have estimated from the theory of the 

 moon's motion ; and that from million of years to million of years it 

 must always be a positive retardation." 



We could have wished for a more explicit reply to Prof. Huxley's 

 objections, which appear to us to remain practically unanswered. 



Professor Huxley asks if the cooling of the earth has been 

 viniform, considering an affirmative answer to be necessary to the 

 validity of the calculations on which Sir W. Thomson lays so much 

 stress ? To this Sir W. Thomson gives a negative answer, stating 

 that investigation shows the greater rate of conduction of heat 

 outwards in past times, and demonstrates a much closer limit for 

 the whole time during which the earth has been solid, and con- 

 tinuously cool enough at its surface to be habitable without break of 

 continuity of life, than can be estimated without taking into account 

 the deviation from uniformity which he (Sir W. Thomson) asserts. 



His calculations, he acknowledges, depend only on the assump- 

 tion, that through geological history the temperature of the upper 

 surface of land and water has been suitable for such life as now 

 exists on the earth. 



Although the limitation of time proposed by Sir W. Thomson 

 may not of itself cause so great a revolution in geological specula- 

 tion as some of the other considerations he has brought into notice, 

 the general tendency of his observations is to support the teaching 

 of Kant, upheld by Professor Huxley — " He reasons back to a be- 

 ginning of the present state of things ; he admits the possibility of 



