Fisher— Ages of the " Trail " and " WarpJ' 1 97 



and accordingly I have adopted Mr. Hopkins' value of 23° F. for 



Snowdon. The equation then becomes — -^ — o^zf V 



^ 239° + 23° — \l+ej 



I have also added Mr. Croll's values, on the supposition of the Gulf 



Stream being only partially deflected.^ 



My fifth column is calculated from the equation — 



Excess of winter over summer = . ..g x year. 



This is slightly more accurate than the rule I gave in the " Eeader," 

 4th November, 1865, 



Eeferring to the table it will be seen that in the year 1800 the 

 eccentricity was -0168. Hence, on the first preceding occasion of 

 winter occurring in aphelion, the eccentricity was greater than at 

 present by a small amount. On the two next occasions it was less. 

 On the fourth it was considerably greater, and winter fifteen days 

 longer than the summer. On the fifth this difference had increased ; 

 and on the sixth occasion it was larger still ; but it was diminishing 

 rapidly. 



Now it is remarkable that though Sir Charles Lyell notices the 

 high eccentricity occurring about the time of these last two epochs, 

 between which falls his period a ; yet he has not attributed any 

 known Glacial phenomenon to it. Both he and Mr. Croll consider 

 it too recent for the so-called Glacial epoch, and Sir Charles thinks 

 it not recent enough for the Eeindeer period. I would suggest that 

 it is extremely probable that this was the epoch of the formation of 

 the trail, and of the last general denudation of our country. 



Then the period 11020 years before 1800 may be supposed to have 

 been that of the reindeer. For the eccentricity was at that time 

 appreciably larger than it is at present, and the winter nine days 

 longer than the summer. This, with the winter in aphelion, might 

 well have produced the change of climate necessary for the south- 

 ward migration of tjie reindeer, though not sufScient to envelope 

 these latitudes in a sheet of ice. The objection, that the summers 

 would then have been too hot for the reindeer, may perhaps be met 

 by observing, that the Southern range of that animal in Northern 

 Asia at present reaches almost to latitude 50°, which is within the 

 limit of the summer isotherm of 63° ; while, on the other hand, the 

 localities, in which its remains have been found in Southern Europe, 

 must have been within the influence of the Atlantic, whose waters 

 were at that time cooler than at present. Hence we may suppose 

 that those countries would not have been heated, even by a nearer 

 summer's sun, more intensely than the plains of Asia, which the 

 reindeer inhabits, are heated at the present time. Still further, we 

 inust not forget Mr. Croll's arguments for cold and cheerless summers 

 under a bigh condition of eccentricity. I am inclined to think that 

 this was also the period of the formation of the warp, when, as I have 

 shown, the winter frosts were more severe than they are at present,^ 

 The submergence of our lower valleys, beneath the Scrohicularia mud, 



» Phil. Mag. Feb. 1867, p. 3. 2 Geol. Journal, toI. xxii. p. 364. 



