GEOLOGICAL PRINCIPLES i» 



from those that we have known since childhood. Hence when we 

 hear of the remains of bread fruit and camphor-trees in latitude 

 73° north, or coal seams formed by the accumulation of plant 

 remains in both the Arctic and the Antarctic regions it might seem 

 as though the poles must have been somewhere else at the time 

 when Greenland was the land of verdure that its name would seem 

 to indicate. There have been students who subscribed to the 

 theory of wandering poles but it can not be said to have many 

 adherents at the present time and physicists assure us that it is 

 imppssible. The truth seems to be that geological climates in 

 general were much more uniform than they are today — milder in 

 the polar regions and less torrid in the equatorial regions. This 

 may seem to be an exception to the principle of uniformity just 

 enunciated, but this is only apparent. Through a combination 

 of causes such as extension and elevation of the land surface of 

 the earth and a consequent restriction of the oceanic areas, and 

 other causes not yet satisfactorily formulated there have been, at 

 widely separated intervals, periods of glaciation which have inter- 

 rupted the normal more uniform climates that were the rule. One 

 has been recognized in the pre-Paleozoic, another is thought to 

 have occurred in the early Paleozoic but this may be confused with 

 the earlier one just mentioned. A third was ushered in many 

 milHons of years later near or at the close of the Paleozoic, and the 

 fourth and most widely known glaciation is the one which after 

 the lapse of many more millions of years immediately preceded the 

 historic period and witnessed the radiation of the men of the Old 

 Stone Age in Europe. Men of this Old Stone Age were witnesses 

 of the great Rhone glacier and of the ice sheet that covered Britain, 

 Scandinavia and the north German plain. 



This last great glacial period of which the ice cap of Greenland 

 is probably a surviving remnant continued for many thousands of 

 years and was marked by at least four periods of prevalent ice 

 sheets and mountain glaciers and by intervening interglacial periods 

 of long duration during which the climate was no more severe than 

 it is at present. These were much longer than the time that has 

 elapsed since the last ice sheet dammed the valley of the St. Law- 



