44 J. E. TWISDEN ON POSSIBLE DISPLACEMENTS 



the north pole in some one direction, possibly on a meridian in a 

 longitude of about 90° E. 



23. The next point to be considered is, that so long as a given 

 distribution of land and water exists there will be no great change 

 in the position of the earth's axis of figure, even supposing that so 

 long as that distribution lasts the denudations do not produce 

 effects which neutralize each other, but have on the whole a predo- 

 minating tendency to shift the pole in some determinate direction. 



24. It will be well to define what I mean by a given distribution 

 of land and water. Take the land as it is at present, and suppose 

 the process of denudation to go on, making amongst other changes 

 such a change as this — that the whole continent of JSTorth America 

 is removed bit by bit and formed into a new continent side by side 

 with South America. By the time such a result has been brought 

 about, it may, I think, be fairly reckoned that a new distribution of 

 land and water has been effected. JSTow, while this change and the 

 other contemporaneous changes are going on, let us suppose that 

 their whole resultant effect on the position of the earth's axis of 

 figure is represented by the effect produced by the removal of the 

 North- American continent ; i. e. the other tendencies neutralize each 

 other, and leave this as the residual effect of the whole. On this 

 supposition, taking the quantity of matter moved as a volume three 

 miles thick with an area equal to -^y of the area of the globe, and 

 supposing the whole moved as a point from lat. 45° N. to 20° S., the 

 north pole would be shifted through about three quarters of a degree 

 during the continuance of this one distribution of land and water. 



25. This reasoning leads directly to the following conclusion : — In 

 order to produce by denudation such an effect as the geographical 

 displacement of the pole through an angle of 15° or 20°, we must 

 make several suppositions, thus : — (a) We must suppose that a large 

 number of redistributions of land and water have been effected one 

 after another, a redistribution being a change on a scale as large as 

 I have endeavoured to indicate ; (6) that during each of the epochs 

 during which any one of the distributions has lasted there has been 

 a prevailing tendency to shift the pole in one determinate direction ; 

 and, further, (c) that the prevailing tendencies throughout the suc- 

 cessive distributions have been such as to add up and produce a 

 large total, or, in other words, that they have not been such as to 

 neutralize one another. 



26. If, then, a great geographical displacement of the pole is to 

 be effected by denudation, it becomes necessary to allow that geologists 

 have at their disposal a considerable number, say ten, fifteen, or 

 twenty consecutive distributions of land and water. If this be 

 allowed, it may also be allowed that it is not improbable that during 

 each of them severally there would be a prevailing tendency to shift 

 the pole in some determinate direction. Each successive distribution 

 of land and water would gradually change into that which succeeds 

 it, and its prevailing tendency to shift the pole would insensibly 

 change into that of its successor. It is very hard to imagine that 

 these successive prevailing tendencies to move the pole should re- 



