THE STORY OF THE EARTH 45 



of Southern Europe is filled with minute organisms 

 which form skeletons or shells of the chalk or lime in it, 

 and as they die their shells sink in countless myriads to 

 the bottom to form the vast beds of chalk that stretch 

 from England to the south of Russia. As the period 

 draws to an end the land rises once more, and the 

 climate becomes colder. Another and greater period of 

 the formation of mountains sets in. In America the 

 Rocky Mountains and the Andes heave slowly upward 

 from the level of the earth, and in Europe the Pyrenees 

 and the earliest Alps appear. 



The whole period occupied about 10 million years 

 (taking 50 millions for the entire geological history), and, 

 if we follow Professor Sollas, we may assume that the 

 millions of tons of sediment deposited in the ample 

 secondary oceans now had its inevitable reaction, and 

 the great mountain chains were pushed upward. This 

 involved a lowering of the climate, first in America, then 

 in Europe. There is a general agreement amongst 

 geologists that the climate was high and fairly uniform 

 in the middle of the period (the Jurassic), when we find 

 sub-tropical trees as far north as Greenland, and that it 

 sank very considerably during the Cretaceous (or chalk) 

 period. We shall see in the next chapter that deciduous 

 trees trees which shed their leaves as summer gives 

 place to winter make their first appearance at that 

 time. There was, therefore, a great lowering of the 

 temperature of the earth, and we shall see that this 

 physical change had an effect of the utmost importance 

 upon its living inhabitants. 



The Tertiary epoch of the geologist (comprising the 

 Eocene, Oligocene, Miocene, and Pliocene strata) brings 

 the story down to within the last million years. Short 

 as the period was, relatively to the preceding, it saw the 

 most remarkable developments of plant and animal life, 



