568 THE JOURNAL OF GEOLOGY. 



viz., that of circumferential contraction and that of radial descent, 

 intersect, is the level of no strain. 



Now laborious calculations have been made by Davison, Dar- 

 win and Fisher to determine the depth of this level of no strain. 

 All make it very superficial. Davison, taking an initial tempera- 

 ture of 7000 F., makes it five miles below the surface. Fisher, 

 on the same data, only two miles, and with an initial tempera- 

 ture of 4000 only o.j of a mde. It is easy to see that if this be 

 true, the amount of lateral thrust must be small indeed. 



Now undoubtedly there is a true principle here which must 

 not hereafter be neglected, but it is almost needless to say that 

 these quantitative results are in the last degree uncertain. The 

 calculations are, of course, based on certain premises. These are 

 a uniform initial temperature of say 7000 F., a time of cooling, 

 say 100 or 200 millions of years, and a certain rate of cooling 

 under assumed conditions. The depth of the level of no strain 

 increases with the time and is still going downward. In a word, 

 in a question so complex both mathematically and physically and 

 in which the data are so very uncertain, every cautious geologist, 

 while freely admitting the soundness of the principle, will 

 withhold assent to the conclusions. Huxley has reminded us 

 that the mathematical mill, though a very good mill, cannot 

 make wholesome flour without good wheat. It grinds indiffer- 

 ently whatever is fed to it. It has been known to grind peas 

 cods ere now. It may be doing so again in this case. Let us 

 wait. 



But besides withholding assent and waiting for more light, I 

 may add that these calculations, of course, go on the supposition 

 that the whole contraction of the earth is due to loss of heat ; 

 but as we have already said, it may be due also to loss of consti- 

 tuent water. This would put an entirely different aspect on the 

 subject. 



ALTERNATIVE PHYSICAL THEORIES. 



I have given the objections to the contractional theory frankly 

 and I think fairly. They are undoubtedly serious. Let us see 

 what has been offered it its place. 



