Level-of-no-Strain in a Cooling Globe. 211 



confess I was surprised at this particular chapter escaping 

 the notice of my reviewers, who, as a rule, dealt very fairly 

 and openly with the ideas put forth, so that I was fully pre- 

 pared for a long continuance of indifference towards this 

 interesting problem. 



The able papers of Mr. Davison and Prof. Darwin, and the 

 exhaustive mathematical investigation of Mr. Osmond Fisher* 

 have agreeably dispelled this fear. 



It is very satisfactory that each of these mathematicians, 

 though working on an independent basis, and all holding 

 different views as to the geological consequences of the dis- 

 covery, are in singularly close agreement in numerical results. 

 Mr. Davison puts the level-of-no-strain after the lapse of 

 174 million years since consolidation at 5 miles; Prof. Darwin 

 at 2 miles in 100 million years; and Mr. Fisher, taking the 

 present temperature gradient as 1° F. in 51 feet, arrives at 

 the conclusion that the level-of-no-strain is now at a depth of 

 2"1361 miles. I may add that my own numerical results, 

 computed in an entirely different manner, approximate closely 

 to those of Mr. Fisher and Prof. Darwin. 



My present intention is not, however, to dwell upon the 

 interesting nature of these mathematical investigations of the 

 properties of a cooling globe, but from a geological stand- 

 point to speculate upon the results that flow from the dis- 

 covery of the existence of a level-of-no-strain situated not 

 many miles beneath our feet. 



Volcanic Energy. — Mr. Mallet has based a complete theory 

 of the origin of volcanic energy upon the heat developed by 

 the crushing and compression of the rocks of the outer crust 

 of the earth while following the contracting nucleus. Many 

 objections to this theory have been pointed out from time to 

 time by practical geologists, and I have dealt with the ques- 

 tion elsewhere f. The existence of a level-of-no-strain within 

 a few miles of the surface, if admitted, is absolutely fatal to 

 Mallet's hypothesis. Not only is the number of cubic miles of 

 rock % crushed quite insufficient to account for the necessary 

 heat, but the limitation of the crushing to what I have called 



* " On the Distribution of Strain in the Earth's Crust," by C. Davi- 

 son, Phil. Trans, of Royal Soc. 1887, p. 231. "Note on Mr. Davison's 

 paper," by Prof. G. H. Darwin, Ibid. p. 242. " On the mean height of 

 the Surface-elevations and other Quantitative Results of the Contraction of 

 a Solid Globe through Cooling " (Pbil. Mag., Jan. 1888), by Rev. O. 

 Fisher. 



t Pp. 3-5 and chap. xxi. ' Origin of Mountain-Ranges.' 

 \ Mr. Fisher estimates the amount of rock displaced by compression 

 during all geological time as equal to a spherical shell the diameter of the 

 Earth and 19 feet thick. 



