184 Flowers and their Pedigrees, 



parallel channels on Its naked surface, I wonder ? 

 Any one who has ever looked closely at the rocks 

 about the foot of a glacier in Switzerland will recog- 

 nise at once what \\'as the agency at work on the 

 granite slopes of Mynydd IVIawr. Those arc most 

 undoubtedly ice-marks, caused by the long, slow, 

 grinding action of the superincumbent glaciers. For 

 of course everybody knows nowadays that there was 

 once a time when great glacial sheets spread over 

 the combes and glens of Snowdonia, as they spread 

 to-day over the nants of Chamounix and the buried 

 basin of the Mer dc Glace. 



Dr. Croll's calculations have shown that the astro- 

 nomical conjunction necessary for the production of 

 such a state of things must have occurred some two 

 hundred thousand years since ; and from that date 

 down to eighty thousand years ago our planet kept 

 presenting alternately cither pole to the sun during 

 long cycles of 10,500 years each ; so that, first, the 

 northern hemisphere enjoyed a long summer, while 

 the southern was enveloped for a vast distance from 

 the Antarctic Circle in a single covering sheet of ice ; 

 and then again the southern hemisphere had its 

 lengthened spell of tropical weather, M'hile the north 

 was turned into one enormous Greenland down as 

 far as the British Isles. Eighty thousan 1 years ago, 



