384 Transactions. — Geology. 



mainly derived from the sun, and may, though variable and 

 irregular, be laid down with some degree of certainty ; that the 

 heat thus derived extends to a depth of from 60 to 90 feet, and 

 that below this stratum the temperature increases at such a 

 rate, that a temperature must soon be reached sufficient to keep 

 in fusion the most refractory rock-substances ; that this high 

 internal temperature is apparently the cause of hot springs, 

 volcanoes, earthquakes, and other igneous phenomena which 

 make themselves known at the surface. In another passage he 

 says : " Looking at the comparative thinness of the solid crust, 

 one can readily conceive how much it would be affected by any 

 commotion in the interior zones, or by any contraction or expan- 

 sion of the entire mass. Hence the tremors, the undulations, 

 the upheavals and subsidences occasioned by earthquakes and 

 volcanic convulsions ; and hence, also, the fissures and fractures 

 which everywhere traverse the rocky crust, whether they may 

 have arisen from the efforts of local forces, or from the opera- 

 tions of some unknown but general law of secular contraction." 

 I do not propose to enter into a discussion of the causes 

 which may have brought about the present figure of the Earth, 

 because, except in so far as that figure adds strength to the 

 view of its original fluidity from heat, it does not materially 

 affect the question under consideration ; but I propose to make 

 some observations on this subject in the sequel, in order to 

 show its connection with the special matter dealt with in this 

 paper. It is curious, however, that amongst physicists who 

 have accepted the nebular hypothesis as a sound cosmological 

 theory, considerable differences of opinion have been expressed 

 as to the mode in which the cooling of our globe commenced. 

 As mentioned by Sir Charles Lyell in his " Elements of 

 Geology," Poisson controverted the doctrine of the present high 

 temperature of the central nucleus, and declared his opinion 

 that, if the globe had ever passed from a fluid to a solid condition 

 in consequence of the loss of heat by radiation, the cooling and 

 consolidation of the surface would have begun at the earth's 

 centre, or, in other words, that the aggregation was so slow as 

 to admit of the dynamical heat generated in the act being 

 radiated into space as fast as it was generated. Other physicists 

 treat the cooling as having commenced at and extended down- 

 ward from the surface of the completely aggregated mass, and, 

 whilst admitting that the nucleus may still be in a fluid state, 

 have assigned a very groat thickness to the solidified crust. 

 Hopkins has fixed this at from 800 to 1,000 miles at the least, 

 whilst others have treated the fact, that the mean density of 

 the earth exceeds that of the rocks which compose the known 

 portions of the crust by 2^ to 3, as justifying the assumption that 

 the nucleus consists chiefly of solidified metallic substances. Dr. 

 Page has, however, given the most conclusive reasons against 



