FIRE, ROCK, AND SEA 27 



Ocean, for the northern part of South America and much of 

 Central America was under water, and the central sea of North 

 America, the ancestral Gulf, broke through to the west. Dur- 

 ing all this and later periods of submergence a number of 

 minor movements took place, so that the ocean borders and 

 inland seas were constantly changing. Some say that only the 

 deep Atlantic basin, in common with the other deep oceans, 

 remained unchanged, though this is a question yet to be 

 settled. 



We have written of the Paleozoic, the 300-million-year era 

 of geological time which began when life became abundant in 

 the primeval seas. Two more eras bridge the dim stretches of 

 time to the present day: the Mesozoic, which ran for 100 

 million years or so, and the Cenozoic, the era in which we 

 are at present living. With these as the framework for our 

 story we can follow the great pulses of expansion and contrac- 

 tion which have spread the waters far across the continents, 

 and which have at other times withdrawn them into narrower 

 channels. The slow breathing of the earth giant lifted its crust 

 into mountain chains once more during the 300 million years 

 of the Paleozoic era, and then again at the start of each of the 

 two later eras. After each great uplift the crust relaxed, the 

 mountains were worn down by wind and rain, and streams and 

 sediments were formed. 



The Caledonian mountain chains of the later Paleozoic rose 

 up from Europe to North America across the northern North 

 Atlantic where Greenland now stands, and formed the land 

 bridge of Eria and so cut off the Arctic Seas. Heaving upward 

 millions of years later, at the end of the Paleozoic were the 

 earlier Appalachian Mountains, great ramparts folding the east 

 coast of North America as far as Texas and then rising from 

 the land westward and northward in an earlier Rocky Moun- 

 tain chain. 



Long after the older Appalachian Mountains had subsided. 



