THE STORY OP THE EARTH 47 



to turn rather to mountain formation, and it was in the 

 last part of the Tertiary that the vast ranges of the Alps 

 and the Himalayas were reared. The old phrase of 

 the "everlasting hills" is sadly astray. The greater 

 mountains are quite young in the general story of the 

 earth : the greatest of them hardly more than two 

 million years old. 



Into the details of the last or actual geological period 

 we cannot enter here. The deposits are so recent and 

 superficial that it is impossible even to summarise the 

 results of their study ; nor is it necessary for our purpose. 

 The outstanding phenomenon is the great ice-age that 

 supervened. From six to eight million square miles of 

 Europe and North America were buried under a sheet of 

 ice that seems to have been 10,000 feet thick in Scandi- 

 navia and thinned gradually down to the valleys of the 

 Thames and the Danube. From every mountain great 

 glaciers spread over the country, and flowed together 

 into a vast uneven ocean of ice. Nearly the whole of 

 England (then part of the continent) down to the Thames 

 is scarred and worn by the moving ice-sheet, and vast 

 gravel-beds bear witness to the swollen rivers that bore 

 away the waters as it melted. 



The existence of this great mantle of ice over the 

 upper part of the northern continents within recent 

 geological times is now beyond question, but the ques- 

 tions as to the date and the causes of it are still under 

 discussion; nor is it at all settled in particular places 

 whether the scoring of the rocks was done by floating or 

 by land-ice, nor how many times the ke-sheet crept 

 down from the north and retreated, with temperate 

 " interglacial " periods. Dr. Geikie claims six successive 

 ice-sheets and five interglacial periods for Europe, and 

 Professor Chamberlin gives the same for America ; and 

 some of the leading German geologists admit four or 



