46 j. E. iWlSDEN ON POSSIBLE DISPLACEMENTS 



so enormous a protuberance as 125 miles over a belt 20° in width, 

 should be necessary in order to displace the polar axis by 20°, when 

 the present equatorial protuberance was only about one tenth of 

 that height. 



Moreover the probability is that the earth, instead of being a 

 rigid solid, is to a certain extent viscous or plastic, and that such, 

 should be the case seems quite in accordance with geological facts. 

 If the globe were a viscous body, with a solid crust of moderate 

 thickness, elevations such as those suggested in the Address might 

 well suffice to bring about a shifting of the crust, either by sliding 

 on the fluid or viscous interior, or by causing it to undergo a certain 

 amount of gradual deformation. The thinner the crust, provided it 

 were sufficiently rigid to support the elevations once made upon it, 

 the more readily would its geographical position be changed with 

 regard to the poles. With regard to the thickness of the crust at 

 the present time, he did not despair of astronomers at last conceding 

 a less thickness than that assigned by the late Mr. Hopkins and Sir 

 "William Thomson. He was glad to find that the latter, in his 

 Address to the Mathematical Section of the British Association at 

 Glasgow, was willing not merely to admit, but to assert as highly 

 probable, that the axis of maximum inertia of the earth and the 

 axis of rotation, always very near one another, may have been in 

 ancient times very far from their present geographical position, and 

 may have gradually shifted through 10, 20, 30, 40 or more degrees, 

 without at any time any perceptible sudden disturbance of either 

 land or water. 



Mr. George Darwin, also no mean mathematician, in his J)aper 

 recently communicated to the Royal Society, agrees as to the proba- 

 bility of large geological changes affecting the position of the poles, 

 and regards the effects of such changes as possibly cumulative. 



Mr. Evans felt that the Society was much indebted to Mr. Twis- 

 den for having likewise investigated the question, in which, of 

 course, he was personally much interested. 



Prof. Hughes presumed that those who in this discussion had 

 suggested changes in the position of the earth's axis to account for 

 difference of climatal conditions in past times, contemplated only a 

 change in the geographical position of the point which we now call 

 the pole, and not a shifting back and fore of the earth's axis of 

 rotation to explain the phenomena of Miocene and Glacial times. 



The argument, he pointed out, rested entirely upon the analogy 

 between the ancient and recent forms of life, and that it assumed 

 that the climatal conditions required by the ancient plants and 

 animals must have been exactly or nearly the same as those under 

 which their nearest living analogues chiefly flourish. But, he said, 

 we were continually multiplying examples to prove the fallacy of 

 this reasoning. Some creatures, as e. g. several common species of 

 mice, are lively through the winter; others closely allied, as the 

 dormouse, hibernate. We are continually hearing of the discovery 

 in Arctic regions of forms of life we should have expected only in 

 subtropical or temperate regions, as e. g.- many Echinoderms. 



