290 ANNUAL OF SCIENTIFIC DISCOVERY. 



each pole a circular area of from eighty to ninety miles upon which the sun 

 would never set, and a much larger area within which, at the midnight of a 

 very short night, there would always be the light and heat of summer twi- 

 light; whilst in all other parts of the earth there would be equal clay and 

 night, and no distinction of summer or winter, a condition obviously 

 favorable to the production of a very uniform flora; whilst at each of the 

 supposed intervals of change, the long, cold arctic winters of the present 

 time Avould be gradually introduced, with a corresponding change in the 

 different regions of the earth. 



2d. If these changes in the climate were the only facts to be accounted for, 

 this supposed change in the direction of the axis of the earth would explain 

 them, although we are unable to assign any cause for such a change. 



But we have also to account for the fact, that the now cold, barren arctic 

 regions had at one time the flora of a temperate climate, as in earlier periods 

 it had that of a tropical climate. Again, we have proof that here in Great 

 Britain, where we enjoy a temperate climate, we at one time had an arctic 

 climate, and at another and earlier period a tropical climate; we can also 

 prove that in certain parts of the present tropical regions there was formerly 

 a temperate climate. 



Now these facts cannot be explained on the first hypothesis ; but they may 

 be explained if we suppose that, instead of a simple change in the direction 

 of the axis of the earth, the change was produced by the evagation of the 

 poles that is, that the north pole might at one time, for example, have been 

 in the position of the magnetic pole, and that it has successively occupied 

 several other positions till it reached its present position and by the inclina- 

 tion of the axis being changed at each period. 



Under this hypothesis we could explain the uniformity of the climate in 

 the first period, the diversity of climate in the different regions of the earth 

 as at present, and also the diversity of climate in the same parts of the earth 

 in the intermediate periods. 



3d. But if such changes have taken place in the position of the poles, and 

 the earth has revolved in successive periods upon different axes, we ought to 

 have evidence of great corresponding changes in the crust of the earth, 

 arising from the movement of the protuberance of the equatorial regions 

 into new positions corresponding to the movements of the poles. 



Now, this is precisely what we do observe; for the crust of the earth is 

 thrown into undulations or corrugations in lines parallel to the movements 

 of the equatorial regions ; that is, in lines either parallel to a great circle, 

 or in lines analogous to loxodromic lines, or in curves such as would be pro- 

 duced if the poles travelled by a succession of movements along curved 

 lines into their present positions. 



The corrugations of the surface of the earth have, in fact, an arrangement 

 something like the engine-turned lines iipou the back of a watch ; and there 

 are particular sets of these corrugations corresponding to the epochs of 

 those great changes in climate and organic life which have been referred to. 

 Thus, for example, we have in this country the strata thrown into systems of 

 undulation, which extend from Cape "Wrath to the Ise of Wight; the undu- 

 lations of the older strata being in the north in lines which cross the meridian 

 at an angle of about forty-live degrees from northeast to southwest (that is, 

 at right angles to a line drawn in the direction of the magnetic pole), and in 

 the south in undulations running nearly east and west. Humboldt, in his 

 Essai sur le Gisemtnt (ks lloclies, page 57, says: "The lines of direction of 



