Fisher— Ages of the " Trail" and " Warp." 195 



Age of the trail.— li this deposit be of Grlacial origin, but do not 

 date back to the Glacial epoch as Mr. Dawkins suggested, nor yet 

 to the later Glacial era, which Mr. Maw supposes to be evidenced by 

 the till of the Norfolk cliffs (though the general opinion has been 

 that the Norfolk till belongs to the earlier part of the Glacial epoch), 

 it may be fairly asked, To what Glacial period is it attributable ? 

 Is it conceivable that arctic conditions can have obtained at so modern 

 a date as the trail evidently belongs to ? To this question I endea- 

 voured to give some reply in the concluding portions of my paper 

 on the warp, as originally communicated to the Geological Society. 

 But it was decided to abridge the paper before publication. The 

 subsequent appearance of the first volume of Sir Charles Lyell's 

 new edition of " The Principles," containing a full discussion of the 

 present state of the theories upon Climatal Changes, now enables 

 me to offer my views on this difficult question with somewhat greater 

 confidence. It also relieves me from the necessity of describing the 

 theories of M. Adhamar and Mr. CroU, which are clearly epitomised 

 by our great geologist. Suffice it to say that the chief point in 

 which the published views of Mr. Croll differ from those of M. 

 Adhemar is, that M. Adhemar considers the present value of the 

 eccentricity of the earth's orbit sufficiently great to produce a marked 

 difference of climate in places having the same latitude in the two 

 hemispheres. Mr. Croll, on the other hand, has stated (though I 

 have reason to believe his opinions are modified) that the present 

 value of the eccentricity is not sufficient for that effect.^ If I under- 

 stand Sir C. Lyell rightl}'', he does not deny that the present state 

 of the eccentricity affects the climates of the two hemispheres, so as 

 to render the northern warmer than it would be if our winter oc- 

 curred in aphelion.^ He only states that the mean temperature of 

 the whole globe is warmer in our summer than in the summer 

 of the antipodes, owing to the greater extent of land exposed to the 

 sun's rays in the northern than in the southern hemisphere. But it 

 seems to follow from the same cause, that since excess of land favors 

 extreme climates, our winters would be colder than at present were 

 the conditions reversed ; though the shorter summers would be 

 hotter as far as regarded the absolute power of the sun. 



But, no doubt, with our present contour of land, the chief point 

 for consideration is the course of the Gulf Stream. Mr. Croll has 

 well suggested that if the southern summer occurred in perihelion 

 during a state of high eccentricity, the Gulf Stream would be de- 

 flected into the Pacific instead of into the Atlantic.^ Part of the 

 equatorial current even now takes that course. It is conceivable 

 that a moderate increase in the mean temperature of the southern 

 hemisphere would so far weaken the force of the S.E. trade winds 

 as to cause the equatorial current to impinge wholly on the southern 

 side of Cape S. lioque, and to be deflected to the south instead of 

 to the north. 



If M. Adhemar is correct in his view, that the present extension 



1 Eeader, 2nd December, 1865. 2 Elements, vol. i. p. 275, cd. 1867. 



3 Phil. Mag., Aug. 1864, and Feb. 1867. 



