106 THE POPULAR SCIENCE MONTHLY. 



giving you a distinct answer. He knows that the chalk is older than 

 the London clay, and the oolite than the chalk, and the red marl than 

 the oolite ; and he knows also that each of them took a very long time 

 indeed to lay down, but exactly how long he has no notion. If you 

 say to him, " Is it a million years since the chalk was deposited ? " he 

 will answer, like the old lady of Prague, whose ideas were excessively 

 vague, " Perhaps." If you suggest five millions, he will answer oracu- 

 larly once more, " Perhaps " ; and if, you go on to twenty millions, 

 "Perhaps," with a broad smile, is still the only confession of faith that 

 torture will wring out of him. But in the matter of the Glacial 

 Epoch, a comparatively late and almost historical event, geologists 

 have broken thi-ough their usual reserve on this chronological question, 

 and condescended to give us a numerical determination. And here is 

 how Dr. Croll gets at it. 



Every now and again, geological evidence goes to show us, a long 

 cold spell occurs in the northern or southern hemisphere. During 

 these long cold spells the ice-cap at the poles increases largely, till it 

 spreads over a great part of what are now the temperate regions of 

 the globe, and makes ice a mere drug in the market as far south as 

 Covent Garden or the Halles at Paris. During the greatest extension 

 of this ice-sheet in the last glacial ej)och, in fact, all England except 

 a small southwestern corner (about Torquay and Bournemouth) was 

 completely covered by one enormous mass of glaciers, as is still the 

 case with almost the whole of Greenland. The ice-sheet, grinding 

 slowly over the hills and rocks, smoothed, and polished, and striated 

 their surfaces in many places till they resembled the roches moutonnees 

 similarly ground down in our own day by the moving ice-rivers of 

 Chamouni and Grindelwald. Now, since these great glaciations have 

 occurred at various intervals in the world's past history, they must de- 

 pend upon some frequently recurring cause. Such a cause, therefore, 

 Dr. Croll began ingeniously to hunt about for. 



He found it at last in the eccentricity of the earth's orbit. This 

 world of ours, though usually steady enough in its movements, is at 

 times decidedly eccenti'ic. Not that I mean to impute to our old and 

 exceedingly respectable planet any occasional aberrations of intellect, or 

 still less of morals (such as might be expected from Mars and Venus) ; 

 the. word is here to be accepted strictly in its scientific or Pickwickian 

 sense as implying merely an irregularity of movement, a slight wob- 

 bling out of the established path, a deviation from exact circularity. 

 Owing to a combination of astronomical revolutions, the precession 

 of the equinoxes and the motion of the aphelion (I am not going to 

 explain them here ; the names alone will be quite sufficient for most 

 people ; they will take the rest on trust) owing to the combination 

 of these profoundly interesting causes, I say, there occur certain periods 

 in the world's life when for a very long time together (10,500 years, 

 to be quite precise) the northern hemisphere is warmer than the south- 



